evaluation of virtual planning as a tool for prosthodontic treatment · evaluation of virtual...

Evaluation of Virtual Planning as

a Tool for Prosthodontic Treatment

Jaafar Abduo

BDS (Otago), DClinDent (Otago), MRACDS

This thesis is presented for the degree of

Doctor of Philosophy

of The University of Western Australia

School of Anatomy, Physiology and Human Biology

School of Computer Science and Software Engineering

2015

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Abstract

Background: Any prosthodontic treatment should be preceded with thorough planning to

ensure its viability. In the era of digital dentistry, there has been interest to execute the

planning procedure digitally. In order to accept the digital planning, it should provide an

outcome that is, at least, similar to that produced by the conventional methods.

Objective: To compare digital planning, in the form of digital wax-up, with the outcome of

conventional wax-up in relation to precision, axial contour, occlusion and aesthetics.

Materials and Methods: A total of 25 dental arch models of 15 patients were collected. Each

set of models was duplicated twice. One set received conventional wax-up and the other was

used for the digital wax-up. The pre-treatment models and the conventional wax-up models

were converted to digital models after scanning by a micro-CT scanner. This allowed for a

direct digital comparison between all the models. In order to evaluate the impact of each

diagnostic wax-up on precision, axial contour, occlusion and aesthetics, the following digital

tools were implemented: image registration and virtual measurements.

Results: After the wax-up modifications, the dentitions were returned to a more natural

status. The conventional and digital wax-ups were similar in relation to precision, contour,

occlusion and aesthetics. At the gingival level, the digital wax-up appeared to be slightly more

accurate than the conventional wax-up. On the contrary, the accuracy of the occlusal contacts

for the digital wax-up was slightly inferior to the occlusal contacts of the conventional wax-up.

The axial contour increase was greater for the digital wax-up; however, the actual difference

was minimal. In terms of occlusal contact number and area, and lateral occlusal relationship;

the two wax-ups yielded similar outcomes. The two wax-ups had equally altered the aesthetic

value to the teeth; however, the digital wax-up appeared to have an advantage of providing

more natural and symmetrical appearance.

Conclusion: Digital wax-up appears to be very promising in planning for prosthodontic

treatment. In general the outcomes of the two wax-ups were comparable.

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Table of Contents

Abstract .......................................................................................................................................... ii

Table of Contents .......................................................................................................................... iii

List of Tables ................................................................................................................................. ix

List of Figures ................................................................................................................................. x

List of Abbreviations .................................................................................................................... xv

Acknowledgements ..................................................................................................................... xvi

Statement of Candidate Contribution ....................................................................................... xvii

Publications Arising from Thesis ............................................................................................... xviii

Chapter One.................................................................................................................................. 1

1. Introduction and Literature Review ...................................................................................... 1

1.1. Introduction .................................................................................................................. 2

1.2. Literature Review .......................................................................................................... 4

1.3. Prosthesis Requirements .............................................................................................. 6

1.4. The Rationale of Digital Dentistry ................................................................................. 7

1.5. Diagnostic Wax-Up ........................................................................................................ 9

1.5.1. Selecting the most suitable treatment ............................................................... 10

1.5.2. Controlling the tooth preparation ...................................................................... 11

1.5.3. Provisional restoration ........................................................................................ 13

1.5.4. Enhanced communication .................................................................................. 14

1.6. Requirements of Ideal Wax-Up ................................................................................... 15

1.6.1. Precision .............................................................................................................. 15

1.6.2. Aesthetic ............................................................................................................. 16

1.6.3. Contour ............................................................................................................... 21

1.6.4. Intercuspal occlusal contacts .............................................................................. 24

1.6.5. Lateral occlusion scheme .................................................................................... 27

1.6.6. Vertical dimension of occlusion .......................................................................... 30

1.7. Conventional Wax-Up Protocol .................................................................................. 32

1.8. Digital Wax-Up Protocol .............................................................................................. 35

1.9. Contributions of the thesis ......................................................................................... 37

Chapter Two ............................................................................................................................... 39

2. Safety of Increasing Vertical Dimension of Occlusion: A Systematic Review ..................... 39

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2.1 Abstract ....................................................................................................................... 40

2.2. Introduction ................................................................................................................ 41

2.3. Materials and Methods ............................................................................................... 41

2.4. Results ......................................................................................................................... 43

2.4.1. Study search ........................................................................................................ 43

2.2.1. Description of studies ......................................................................................... 43

2.2.2. Studies classification ........................................................................................... 44

2.2.3. Studies summary ................................................................................................. 45

2.3. Discussion .................................................................................................................... 50

2.3.1. Magnitude of VDO increase ................................................................................ 50

2.3.2. Adaptation period ............................................................................................... 52

2.3.3. Method of increasing VDO .................................................................................. 53

2.3.4. Occlusion scheme ............................................................................................... 54

2.4. Conclusions ................................................................................................................. 55

Chapter Three ............................................................................................................................. 56

3. Lateral Occlusion Schemes in Natural and Minimally Restored Permanent Dentition: A

Systematic Review ...................................................................................................................... 56

3.1. Abstract ....................................................................................................................... 57

3.2. Introduction ................................................................................................................ 58


3.3.1. Search strategy and selection criteria ................................................................. 59

3.3.2. Literature assessment ......................................................................................... 60

3.3.3. Study classification .............................................................................................. 60

3.3.4. Qualitative analysis ............................................................................................. 61

3.4. Results ......................................................................................................................... 61

3.4.1. Literature search ................................................................................................. 61


3.4.3. Studies outcome ................................................................................................. 63

3.5. Discussion .................................................................................................................... 73

3.5.1. Magnitude of excursion ...................................................................................... 73

3.5.2. Age effect ............................................................................................................ 74

3.5.3. Static occlusal relationship.................................................................................. 75

3.5.4. TMD relationship................................................................................................. 76

3.5.5. Further considerations ........................................................................................ 77

3.6. Conclusions ................................................................................................................. 78

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Chapter Four ............................................................................................................................... 79

4. Impact of Lateral Occlusion Schemes: A Systematic Review .............................................. 79

4.1. Abstract ....................................................................................................................... 80

4.2. Introduction ................................................................................................................ 81


4.3.1. Search strategy and selection criteria ................................................................. 81

4.3.2. Studies classification ........................................................................................... 82

4.4. Results ......................................................................................................................... 83

4.4.1. Literature search ................................................................................................. 83


4.4.3. Studies’ outcome ................................................................................................ 84

4.5. Discussion .................................................................................................................... 95

4.5.1. Physiological implications of lateral occlusion scheme ...................................... 95

4.5.2. Long-term effect of lateral occlusion scheme .................................................... 98

4.5.3. Ideal lateral occlusion scheme .......................................................................... 100

4.6. Conclusions ............................................................................................................... 102

Chapter Five .............................................................................................................................. 103

5. Aims of the Study and Hypotheses ................................................................................... 103

5.1. Aims........................................................................................................................... 104

5.2. Hypotheses ............................................................................................................... 105

Chapter Six ................................................................................................................................ 106

6. Materials and Methods ..................................................................................................... 106

6.1. Patient Recruitments ................................................................................................ 107

6.2. Pre-Treatment Models .............................................................................................. 107

6.3. Conventional Wax-Up ............................................................................................... 109

6.4. Virtual Articulation .................................................................................................... 111

6.5. Digital Wax-Up .......................................................................................................... 111

6.6. Analysis ..................................................................................................................... 113

6.6.1. Image Registration ............................................................................................ 113

6.6.2. Virtual measurements ....................................................................................... 114

Chapter Seven .......................................................................................................................... 116

7. Precision of Digital Prosthodontic Planning for Oral Rehabilitation ................................. 116

7.1. Abstract ..................................................................................................................... 117

7.2. Introduction .............................................................................................................. 118

7.3. Materials and Methods ............................................................................................. 119

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7.3.1. Conventional wax-up ........................................................................................ 119

7.3.2. Digital wax-up ................................................................................................... 120

7.3.3. Analysis ............................................................................................................. 121

7.3.4. Image registration ............................................................................................. 121

7.3.5. Gingival margin measurements ........................................................................ 123

7.3.6. Statistical analysis ............................................................................................. 124

7.4. Results ....................................................................................................................... 124

7.4.1. Image Registration ............................................................................................ 124

7.4.2. Gingival Margins................................................................................................ 125

7.5. Discussion .................................................................................................................. 128

7.6. Conclusions ............................................................................................................... 131

Chapter Eight ............................................................................................................................ 132

8. Influence of Conventional and Digital Wax-Ups on Axial Tooth Contour ......................... 132

8.1. Abstract ..................................................................................................................... 133

8.2. Introduction .............................................................................................................. 134



8.3.2. Digital wax-up ................................................................................................... 136

8.3.3. Analysis ............................................................................................................. 137


8.4. Results ....................................................................................................................... 138

8.4.1. Inter-arch location (maxillary vs. mandibular teeth) ........................................ 139

8.4.2. Intra-arch location (anterior vs. posterior) ....................................................... 139

8.4.3. Tooth location (mid-tooth vs. line angle) .......................................................... 141

8.5. Discussion .................................................................................................................. 144

8.6. Conclusions ............................................................................................................... 146

Chapter Nine ............................................................................................................................. 147

9. Effect of Prosthodontic Planning on Intercuspal Occlusal Contacts: Comparison of Digital

and Conventional Planning ....................................................................................................... 147

9.1. Abstract ..................................................................................................................... 148

9.2. Introduction .............................................................................................................. 149



9.3.2. Digital wax-up ................................................................................................... 151

9.3.3. Analysis ............................................................................................................. 152

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9.4. Results ....................................................................................................................... 155

9.4.1. Contact number ................................................................................................ 155

9.4.2. Contact area ...................................................................................................... 156

9.4.3. Contact accuracy ............................................................................................... 157

9.5. Discussion .................................................................................................................. 158

9.6. Conclusions ............................................................................................................... 162

Chapter Ten .............................................................................................................................. 163

10. Effect of Prosthodontic Planning on Lateral Occlusion Scheme: A Comparison between

Conventional and Digital Planning ............................................................................................ 163

10.1. Abstract ................................................................................................................. 164

10.2. Introduction .......................................................................................................... 165

10.3. Materials and Methods ......................................................................................... 166


10.3.2. Digital wax-up ................................................................................................... 168

10.3.3. Virtual simulation of lateral movement ............................................................ 168

10.3.4. Analysis ............................................................................................................. 169

10.4. Results ................................................................................................................... 170

10.4.1. Prevalence of lateral occlusion scheme ............................................................ 170

10.4.2. Number of contacting teeth ............................................................................. 171

10.4.3. Percentage of each contacting tooth ................................................................ 173

10.5. Discussion .............................................................................................................. 176

10.6. Conclusions ........................................................................................................... 180

Chapter Eleven ......................................................................................................................... 181

11. Impact of Prosthodontic Planning on Dental Aesthetics: An Objective Evaluation of

Aesthetic Parameters ................................................................................................................ 181

11.1. Abstract ................................................................................................................. 182

11.2. Introduction .......................................................................................................... 183

11.3. Materials and Methods ......................................................................................... 184


11.3.2. Digital wax-up ................................................................................................... 185

11.3.3. Analysis ............................................................................................................. 186

11.3.4. Statistics ............................................................................................................ 188

11.4. Results ................................................................................................................... 189

11.4.1. Perceived frontal proportion ............................................................................ 189

11.4.2. Actual dimensions ............................................................................................. 189

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11.4.3. Perceived symmetry ......................................................................................... 190

11.4.4. Actual symmetry ............................................................................................... 191

11.5. Discussion .............................................................................................................. 193

11.6. Conclusions ........................................................................................................... 197

Chapter Twelve ......................................................................................................................... 198

12. General Discussion and Conclusions ............................................................................. 198

12.1. Research Methodology ......................................................................................... 199

12.2. Tooth Surface Alteration ....................................................................................... 200

12.2.1. Axial surface ...................................................................................................... 200

12.2.2. Occlusal Surface ................................................................................................ 203

12.3. Accuracy ................................................................................................................ 208

12.3.1. Gingival accuracy............................................................................................... 208

12.3.2. Occlusion accuracy ............................................................................................ 209

12.3.3. Digital processing precision .............................................................................. 209

12.4. Future Research .................................................................................................... 216

12.5. Conclusions ........................................................................................................... 218

References ................................................................................................................................ 219

Appendix ................................................................................................................................... 237

Appendix A ............................................................................................................................ 238

Appendix B ............................................................................................................................ 240

Appendix C ............................................................................................................................ 249

Appendix D ............................................................................................................................ 261

Appendix E ............................................................................................................................ 276

Appendix F ............................................................................................................................ 288

Appendix G ............................................................................................................................ 297

Appendix H ............................................................................................................................ 304

Appendix I ............................................................................................................................. 319

Appendix J ............................................................................................................................. 329

Appendix K ............................................................................................................................ 337

Appendix L ............................................................................................................................. 347

Appendix M ........................................................................................................................... 355

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List of Tables

Table 1—1 Determinants of crown preparation ........................................................................................ 12

Table 2—1 Selection criteria used in the review ........................................................................................ 42

Table 2—2 Summary of studies increasing the VDO by removable method and partial arch coverage ... 47

Table 2—3 Summary of studies increasing the VDO by removable method and complete arch coverage

........................................................................................................................................................... 48

Table 2—4 Summary of studies increasing the VDO by fixed method and partial arch coverage ............. 48

Table 2—5 Summary of studies increasing the VDO by fixed method and complete arch coverage ........ 49

Table 3—1 Summary of the included studies ............................................................................................ 69

Table 3—2 Summary of the studies that included static occlusal relationship ......................................... 72

Table 4—1 Inclusion criteria....................................................................................................................... 82

Table 4—2 Implications of lateral occlusion scheme on muscle EMG activity .......................................... 89

Table 4—3 Implications of lateral occlusion scheme on mandibular movement ...................................... 91

Table 4—4 Summary of studies that established the lateral occlusion scheme by composite restorations

........................................................................................................................................................... 92

Table 4—5 Summary of studies that established the lateral occlusion scheme by fixed dental and

implant prostheses ............................................................................................................................ 93

Table 6—1 Selection criteria .................................................................................................................... 107

Table 8—1 The mean and standard deviation (SD) for the maxillary and mandibular teeth after each

diagnostic wax-up ............................................................................................................................ 139

Table 8—2 The mean and standard deviation (SD) for the anterior and posterior teeth after each

diagnostic wax-up ............................................................................................................................ 140

Table 8—3 The mean and standard deviation (SD) for the maxillary anterior and posterior teeth, and

mandibular anterior and posterior teeth ........................................................................................ 140

Table 9—1 CNT mean and standard deviation (SD) for the pre-treatment, conventional wax-up and

digital wax-up casts ......................................................................................................................... 155

Table 9—2 CAT mean and standard deviation (SD) for the pre-treatment, conventional wax-up and

digital wax-up casts ......................................................................................................................... 156

Table 9—3 Contact accuracy mean and standard deviation (SD) for the pre-treatment, conventional

wax-up and digital wax-up casts...................................................................................................... 158

Table 10—1 Inclusion criteria................................................................................................................... 166

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List of Figures

Figure 1-1 Examples of dental conditions that indicate fixed prosthodontic treatment. (A) Teeth

discoloration, (B) deficient dental morphology, (C) failed and unaesthetic restorations, and (D)

worn dentition. .................................................................................................................................... 4

Figure 1-2 Clinical images illustrating the amount of tooth preparation required for prosthodontic

treatment. (A) Pre-treatment situation, and (B) prepared dentition. ................................................. 7

Figure 1-3 Diagnostic wax-up was used to alter the teeth with composite restorative material. (A) Pre-

treatment presentation. (B) Diagnostic wax-up. (D) The modified dentition by composite

restorative material. The new contours can subsequently control the tooth preparation. ............. 11

Figure 1-4 Comparison between the traditional crown preparation and restorative-driven crown

preparation. (A) The initial tooth situation. (B) The crown preparation can be executed according

the initial tooth surface. (C) Alternatively, a diagnostic wax-up can be established on a dental

model. (D) On the diagnostic wax-up model, a silicone index is fabricated. (E) This silicone index is

used intra-orally to dictate the tooth preparation, which might be more conservative. (F)

Eventually, the final tooth preparation and crown design is objectively determined according to the

wax-up. .............................................................................................................................................. 13

Figure 1-5 An example of the usefulness of the diagnostic wax-up in fabrication of provisional

restorations. (A) A pre-treatment situation. (B) The diagnostic wax-up planned to improve the

overall dental condition. (C) According to the diagnostic wax-up, provisional restorations were

fabricated and inserted. In this situation, the provisional restorations restored patient comfort,

aesthetic and function. In addition, they allow the patient the critique the anticipated treatment.

(D) definitive prostheses were fabricted according to the approved provisional restorations. ....... 14

Figure 1-6 From the wax-up, silicone indices can be produced (A, B) and used by the manufacturing

technician to control the final prostheses contour (C). ..................................................................... 15

Figure 1-7 An example of aesthetic improvement by the diagnostic wax-up. (A) Pre-treatment models.

(B) Wax-up models. ........................................................................................................................... 17

Figure 1-8 Frontal image that illustrates the PFP. As the tooth moves distally, it is perceived to be

smaller. .............................................................................................................................................. 18

Figure 1-9 An example of a compromised clinical presentation that requires gingival tissues restoration.

(A) A diagnostic wax-up that incorporates gingival tissues. (B) Definitive prosthesis with pink

porcelain that replaces the missing gingival tissues was constructed according to the wax-up. ..... 20

Figure 1-10 (A) A straight profile in the gingival third facilitates establishing a properly contoured

prosthesis. (B) Widening the profile gingivally is associated with over-contoured prostheses. ....... 22

Figure 1-11 (A) Frontal and occlusal views of pre-treatment anterior teeth that clearly shows deficient

tooth morphology. (B) Similar views after the wax-up indicate establishment of natural

morphology. ...................................................................................................................................... 33

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Figure 1-12 (A) Prior to the treatment, the teeth can be worn, fractured or heavily restored. (B) the wax-

up establishes natural occlusal anatomy such as cusps, grooves and fossae. .................................. 34

Figure 1-13 (A) The adjacent teeth should exhibit similar axial contour. (B) In situations where a tooth

deviate from the contour of the adjacent teeth, there might be aesthetic, functional and hygienic

implications. ...................................................................................................................................... 35

Figure 1-14 Examples of average virtual teeth that can be used in digital designing of the dentition. ..... 36

Figure 2-1 (A) A dentition that suffers from tooth wear. (B) As a result, the teeth are short and in edge-

to-edge relationship. (C) The definitive prostheses involved 3 mm increase of the VDO. Increasing

the VDO allowed for significant aesthetic improvement, correction of anterior tooth relationship,

establishment of a natural overjet and overbite, and lengthening the anterior teeth. .................... 50

Figure 2-2 The impact of tooth wear on the anterior tooth relationship. (A) Natural relationship of

anterior teeth with intact crowns. (B) Tooth wear resulting in the development of a class III (edge-

to-edge) incisal relationship. (C) Increasing the VDO allowed for restoring an adequate anterior

tooth relationship. ............................................................................................................................. 51

Figure 3-1 The relationship between the prevalence of each lateral occlusion scheme and age after

complete excursion (A) and partial excursion (B). The lines represent the age range of each study.

........................................................................................................................................................... 65

Figure 3-2 The relationship between the prevalence of each lateral occlusion scheme and age for Class I

occlusion. ........................................................................................................................................... 66

Figure 3-3 The relationship between the prevalence of each lateral occlusion scheme and age for Class II

occlusion. ........................................................................................................................................... 67

Figure 3-4 The relationship between the prevalence of each lateral occlusion scheme and age for Class III

occlusion. ........................................................................................................................................... 68

Figure 6-1 STL image construction from DICOM images. (A) An example of single slice DICOM image. (B)

The process of STL image construction ........................................................................................... 108

Figure 6-2 (A) Actual pre-treatment maxillary and mandibular casts. (B) Virtual pre-treatment models.

......................................................................................................................................................... 108

Figure 6-3 The micro-CT scanner validation process. (A) A maxillary model scanned by the laser scanner.

(B) The same model after scanning by the micro-CT scanner. (B) A colour-coded map generated

after registering the two STL images, which confirms the similarity between the two images...... 109

Figure 6-4 Examples of conventional wax-up. (A) Pre-treatment situation illustrating irregular and

rotated teeth. (B) Wax-up of the two central incisors. (C) Pre-treatment situation of generalized

tooth wear. (D) Wax-up of the whole maxillary teeth. ................................................................... 110

Figure 6-5 (A) Completed conventional wax-up model. (B) Virtual conventional wax-up model. ........... 110

Figure 6-6 (A) The articulation process. The maxillary and mandibular virtual models before articulation.

(B) The virtual silicone registration indices that can fit on the buccal aspects of articulated models.

(C) The maxillary and mandibular models were repositioned according to the silicone indices by the

process of image registration. (D) The articulated maxillary and mandibular models after the

removal of silicone indices. ............................................................................................................. 111

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Figure 6-7 Examples of the available virtual teeth. As the anterior maxillary teeth are key teeth in

obtaining ideal aesthetics, many teeth shapes are available for clinician use. ............................... 112

Figure 6-8 Series of images that illustrate the digital teeth fitting. (A) Pre-treatment model. (B)

Commencement of the digital wax-up. (C) Completed digital wax-up of the anterior maxillary teeth.

......................................................................................................................................................... 112

Figure 6-9 An example of the process of image registration. (A) A pre-treatment model. (B) The model

after the dental modifications. (C) The models were superimposed by the process of image

registration. As the soft tissues were not altered, they were used as a reference to control the

registration process. (D) Colour-coded map can be implemented to quantify the differences

between the two models. ............................................................................................................... 114

Figure 6-10 Images illustrating the use of the software for virtual measurements. (A) The virtual ruler

can be implemented to measure the distance between the different coordinates that represent

tooth dimension. (B) An example of occlusal area quantification. ................................................. 114

Figure 7-1 Conventional wax-up: (A) Pre-treatment models. (B) Conventional wax-up models. (C)

Scanned conventional wax-up models. ........................................................................................... 120

Figure 7-2 Digital wax-up. (A) Scanned pre-treatment models. (B) Scanned physiological teeth. (C) Digital

wax-up models. ............................................................................................................................... 121

Figure 7-3 The segmentation process that yielded soft tissue model and tooth-gingiva junction model.

(A) Original model. (B) Selected soft tissue. (C) Selected tooth-gingiva junction. (D) Final soft tissue

model. (E) Final junction model....................................................................................................... 123

Figure 7-4 Example of locating the points of measurement around the gingival margin of a lateral incisor.

The black point is located on the mid-tooth area and the red points are on the proximal areas... 124

Figure 7-5 Colour-coded maps of each diagnostic wax-up after fitting on the pre-treatment model. (A)

Conventional wax-up. (B) Magnified section of conventional wax-up. (C) Digital wax-up. (C)

Magnified section of digital wax-up. ............................................................................................... 125

Figure 7-6 The box plot diagrams of the gingival margins for each tooth category. (A) Mid-tooth area of

the maxillary teeth. (B) Proximal area of the maxillary teeth. (C) Mid-tooth area of the mandibular

teeth. (D) Proximal area of the mandibular teeth. .......................................................................... 128

Figure 8-1 Conventional wax-up procedure: (A) Pre-treatment models. (B) Conventional wax-up models.

(C) Scanned conventional wax-up models....................................................................................... 136

Figure 8-2 Digital wax-up procedure: (A) Scanned pre-treatment models. (B) Scanned physiological

teeth. (C) Digital wax-up models. .................................................................................................... 137

Figure 8-3 (A) An image illustrating the extracted three labial planes. (B) A magnified image outlining the

five vertical measurements on each plane. (C) A cross sectional view of an extracted plane on the

pre-treatment model (black line) and the corresponding plane on the post-treatment model (red

line). ................................................................................................................................................. 138

Figure 8-4 Bar diagrams illustrating the contour alteration of each tooth category after each wax-up: C =

conventional wax-up and D = digital wax-up. (A) Maxillary mid-tooth region. (B) Maxillary line angle

region. (C) Mandibular mid-tooth region. (D) Mandibular line angle region. ................................. 143

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Figure 9-1 Example of the virtual pre-treatment (A), conventional wax-up (B) and digital wax-up (C)

casts. ................................................................................................................................................ 152

Figure 9-2 Colour-coded map illustrating the contact number and contact area for the pre-treatment (A),

conventional wax-up (B) and digital wax-up (C) casts. The calculation of the CNT and CAT will

compensate the increase of the number of teeth after the wax-ups. ............................................ 153

Figure 9-3 Determination of the contact number and area according to the colour-coded map. The

number of occlusal contacts was established by counting the areas coloured with yellow or a

warmer colour. The same areas were extracted and measured to quantify the occlusal area. ..... 153

Figure 9-4 Measurement of the occlusal discrepancies. If the contact surfaces are overlapping the (A),

the maximal distance is measured which indicates a positive error (occlusal interferences). In a

situation where the surfaces are not contacting (B), the minimal distance between the surfaces are

measured and reflect a negative error (non-contacting surfaces). ................................................. 154

Figure 9-5 Box plot diagram of the CNT values for the anterior and posterior teeth of pre-treatment,

conventional wax-up and digital wax-up casts. ............................................................................... 156

Figure 9-6 Box plot diagram of the CAT values (mm2) for the anterior and posterior teeth of pre-

treatment, conventional wax-up and digital wax-up casts. ............................................................ 157

Figure 9-7 Box plot diagram of the contact accuracy values (mm) for the anterior and posterior teeth of

pre-treatment, conventional wax-up and digital wax-up casts. ...................................................... 158

Figure 10-1 Example of the evaluated virtual models. (A) Pre-treatment model. (B) Conventional wax-up

model. (C) Digital wax-up model. .................................................................................................... 167

Figure 10-2 An example of virtual simulation of lateral movement. (A) Maximal intercuspation. (B) 0.5

mm excursion. (C) 1.0 mm excursion. (D) 2.0 mm excursion. (E) 3.0 mm excursion. The red colour

indicates the existing contacts. ....................................................................................................... 169

Figure 10-3 Proportion of each lateral occlusion scheme in each excursive position. (A) Pre-treatment

models. (B) Conventional wax-up models. (C) Digital wax-up models. ........................................... 171

Figure 10-4 The mean number of contacting teeth for all the models in each excursive position. (A)

Maxillary arch. (B) Mandibular arch. ............................................................................................... 172

Figure 10-5 Percentage of the contacting teeth in each excursive position for all the models. (A) Pre-

treatment maxillary arch. (B) Pre-treatment mandibular arch. (C) Conventional wax-up maxillary

arch. (D) Conventional wax-up mandibular arch. (E) Digital wax-up maxillary arch. (F) Digital wax-up

mandibular arch. ............................................................................................................................. 175

Figure 11-1 An example of the evaluated models: A, Actual pre-treatment model. B, Actual conventional

wax-up model. C, Virtual pre-treatment model. D, Virtual conventional wax-up model. E, Digital

wax-up model. ................................................................................................................................. 186

Figure 11-2 A frontal image illustrating the separation of the anterior teeth for both sides. The horizontal

lines represent the perceived width of each tooth. ........................................................................ 187

Figure 11-3 Measurement of the W:H ratio: A, Central incisor. B, Lateral incisor. C, Canine. The vertical

line is the height and the horizontal line is the width. .................................................................... 188

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Figure 11-4 PFP of the lateral incisors and canines for the pre-treatment, conventional wax-up and

digital wax-up models. The GP values were added for comparison. .............................................. 189

Figure 11-5 W:H ratio of all the teeth for the central incisors, lateral incisors and canines of all the

models. ............................................................................................................................................ 190

Figure 11-6 Perceived asymmetry percentage of central incisors, lateral incisors and canines of the pre-

treatment, conventional wax-up and digital wax-up models. ......................................................... 191

Figure 11-7 Actual asymmetry percentage of the central incisors, lateral incisors and canines for the pre-

treatment, conventional wax-up and digital wax-up models. A, The actual width asymmetry. B, The

actual height asymmetry. ................................................................................................................ 193

Figure 12-1 Frontal virtual images of (A) conventional wax-up model and (B) digital wax-up model. It is

clear that the teeth of the digital wax-up model exhibited more defined features which might

enhance the overall aesthetics. ....................................................................................................... 203

Figure 12-2 Examples of digital (A) maxillary and (B) mandibular posterior teeth that illustrate the well-

defined occlusal anatomy. ............................................................................................................... 207

Figure 12-3 An example of the effect of dental model simplification. (A) The originally scanned model is

composed of dense points. (B) The model after 50% decimation. (C), 25% decimation. (D) 12.5%

decimation. (E) 6.75% decimation model. The decimation primarily affects flat surfaces. Excessive

decimation causes greater the loss in the resolution...................................................................... 211

Figure 12-4 The effect of bur diameter in line angle production. (A) Sharp virtual line angle cannot be

produced by rounded bur. Therefore, surface inaccuracy will occur on the milled restoration in the

form of (B) negative error after over-milling of the sharp corner, or (C) positive error after under-

milling of the sharp corner. ............................................................................................................. 213

Figure 12-5 The effect of layered production on the surface accuracy. (A) Smooth surface is ideal for

dental models. (B) Thick layers will increase the prominence of surface stepping. (C and D) As the

layers thickness is reduced the surface accuracy will increase. The corrugated surface (occlusal

surface) is more affected by the steps than the vertical surfaces. .................................................. 215

Figure 12-6 (A) An example of a maxillary model produced from the conventional method. (B) A virtual

image of the same model. (C) The same model after production by 3D printing. (D) A magnified

image of the buccal surface of the 3D printed model illustrating the model layers that may

influence the surface quality. .......................................................................................................... 216

xv

List of Abbreviations

3D Three Dimensional

BBO Bilaterally balanced occlusion

CAD Computer-aided design

CAT Contact area per tooth

CAM Computer-aided manufacturing

CGO Canine-guided occlusion

CNC Computer numeric control

CNT Contact number per tooth

CR Centric relation

DICOM Digital imaging and communication medicine

ED Euclidean distance

EMG Electromyography

GFO Group function occlusion

GP Golden proportion

mm Millimetre

µm Micrometre

MPO Mutually-protected occlusion

Micro-CT Micro-computed tomography

PFP Perceived frontal proportion

STL Stereolithography

TMD Temporomandibular joint disorder

TMJ Temporomandibular joint

VDO Vertical dimension of occlusion

W:H Width-to-height ratio

xvi

Acknowledgements

First, the completion of this thesis could not have been possible without the help of many

people. I was fortunate to receive great guidance from my supervisors, Professors Mohammed

Bennamoun, Marc Tennant and John McGeachie. Upon my arrival to Western Australia, I was

fortunate to have made contact with Mohammed who was happy to work with me. Although

we are in different disciplines, a common interest was established which had led to the

development of this research project. He was an excellent mentor. It was a pleasure to work

with Marc and John who had provided insightful ideas in executing the research. They were

very approachable throughout the research period and were a significant source of support

and encouragement.

I wish to express my gratitude to my external supervisor, Dr Patrick Henry for his vital role in

sharing his clinical and technical experience and providing invaluable assistance in conducting

this research. It was a privilege to receive feedback from a world-class clinician.

I am thankful to the technical team of the Oral Health Centre of Western Australia for their

laboratory support. They were very efficient and contributed to a high quality laboratory work.

I am grateful to the staff of the Centre for Microscopy, Characterization & Analysis of the

University of Western Australia for facilitating the micro-CT access, training and technical

support.

I am grateful to my colleagues and mentors at the University of Western Australia, the

University of Otago and Melbourne University. I appreciate the advices of Assistant Professor

Syed Shamsul Islam, Professor Michael Swain, Professor Karl Lyons, Professor Peter Parashos

and Associate Professor Roy Judge. They had provided brilliant ideas and support throughout

my PhD journey.

I would like to acknowledge the financial support of the following grants: Research

Development Award from the University of Western Australia, the Australian Prosthodontic

Society and the Raine Medical Research Foundation.

My sincere gratitude goes to my parents for their infinite care and support. I am grateful to my

wife, for her understanding and believing in me. Special thanks for my daughters Aminah and

Maryam.

xvii

Statement of Candidate Contribution

I hereby declare that all of the included work in this thesis is entirely my own, except some

laboratory work performed by the dental technicians at the Oral Health Centre of Western

Australia, which I have indicated in the thesis accordingly. The research was conducted under

the supervision of Professor Mohammed Bennamoun, Professor Marc Tennant, Professor John

McGeachie and Dr Patrick Henry. For the co-authored published work, my contribution was

80%. Contributions with other colleagues are mentioned accordingly and listed as co-

authorships in the published papers.

xviii

Publications Arising from Thesis

Abduo J, Lyons K. Clinical considerations for increasing occlusal vertical dimension: a

review. Australian Dental Journal. 2012; 57:2-10. (Part of Chapter 1)

Abduo J. Safety of increasing vertical dimension of occlusion: a systematic review.

Quintessence International. 2012; 43:369-380. (Chapter 2)

Abduo J, Tennant M, McGeachie J. Lateral occlusion schemes in natural and minimally

restored permanent dentition: a systematic review. Journal of Oral Rehabilitation. 2013;

40:788-802. (Chapter 3)

Abduo J, Tennant. Impact of lateral occlusion schemes: a systematic review. Journal of

Prosthetic Dentistry. 2015; 114:193-204. (Chapter 4)

Abduo J. Virtual prosthodontic planning for oral rehabilitation: a pilot study. CI Health.

2012; 34-42. (Part of Chapter 6)

Abduo J, Bennamoun M. Three-dimensional image registration as a tool for forensic

odontology: a preliminary investigation. American Journal of Forensic Medicine and

Pathology. 2013; 34:260-266. (Part of Chapter 6)

Abduo J, Bennamoun M, Tennant M, McGeachie J. Precision of virtual prosthodontic

planning for oral rehabilitation. British Journal of Applied Sciences and Technology. 2014;

4:3915-3929. (Chapter 7)

Abduo J, Bennamoun M, Tennant M. Influence of conventional and digital wax-ups on axial

tooth contour. International Journal of Periodontics and Restorative Dentistry. 2015;

35:e50-e59. (Chapter 8)

Abduo J, Bennamoun M, Tennant M, McGeachie J. Effect of prosthodontic planning on

intercuspal occlusal contacts: comparison of digital and conventional planning. Computers

in Biology and Medicine. 2015; 60:143-150. (Chapter 9)


lateral occlusion scheme: a comparison between conventional and digital planning. Journal

of Applied Oral Science. 2015; 23:196-205. (Chapter 10)

xix

Abduo J, Bennamoun M, Tennant M, McGeachie J. Impact of digital prosthodontic

planning on dental esthetics: biometric analysis of esthetic parameters. Journal of

Prosthetic Dentistry. 2015; Accepted. (Chapter 11)

Abduo J, Lyons K, Bennamoun M. Trends in computer-aided manufacturing in

prosthodontics: a review of the available streams. International Journal of Dentistry. 2014;

Accepted. (Part of Chapter 12)

1

Chapter One

1. Introduction and Literature Review

Part of this chapter was published in the following article:

Abduo J, Lyons K. Clinical considerations for increasing occlusal vertical dimension: a review.

Australian Dental Journal. 2012; 57:2-10. (Appendix B)

2

1.1. Introduction

Gradually, dentistry is moving towards the digital era, where computerized technologies are

becoming an integral part of dental treatment. Disciplines, such as prosthodontics,

orthodontics, and oral surgery, were significantly influenced by digital dentistry. In industry,

the benefits of computerized engineering technologies include high precision, simpler

fabrication protocol and minimal human intervention. These advantages make digital

technologies ideal for quality assurance, precision production and cost effective manufacturing

(van Noort, 2012). Because of this, it is no surprise that the digital technology has been

adopted in dentistry (Miyazaki and Hotta, 2011). Today, digital dentistry is the only means of

producing durable tooth-coloured and metal-free components in dental practice. Further, it

provides the option of chair-side fabrication of indirect restorations.

One of the recent applications of digital dentistry is the virtual prosthodontic planning in the

form of diagnostic wax-up. In prosthodontics, diagnostic wax-up simulates the proposed

treatment and allows for visualization of the final outcome. Therefore, it is a useful tool to

decide on the most suitable treatment (Magne and Belser, 2004; Gurel, 2007). Subsequently,

the diagnostic wax-up will dictate the definitive treatment. Conventionally, wax-up involves

altering the tooth morphology on actual diagnostic dental model. Although this method has

been used for many years and the profession is very familiar with it, it suffers from some

limitations. Further, it is common for the clinician to omit the diagnostic wax-up step or to

leave it to the dental technicians to decide. The reason behind this is that wax-up is a time

consuming process and requires special training and artistic abilities. The technicians might

have the expertise in developing aesthetically looking dental morphology; however, they

commonly lack the visualization of the biological parameters. This is mainly important in

relation to the fact that they are not the operators who are directly dealing with patients.

Therefore, the predictable application of the diagnostic wax-up is limited to clinicians with

special level of training.

Today’s patients are aesthetically conscious and have high dental expectations. In the era of

multimedia expansion, clinical procedures in dentistry appear to be increasingly market-

driven. In addition, there is continuous release of new dental materials and fabrication

techniques. As a result, the clinician is confronted with a plethora of treatment options that

can address a specific dental problem. Failure of meeting patient aesthetic and functional

demands can result in major patient’s disappointment, or even legal litigations. Therefore,

more emphasis should be placed on the diagnostic wax-up to allow the patient to visualize and

appraise the final outcome prior to any irreversible treatment. On the basis of accurate

3

planning, the patient will be able to provide consent after being fully informed by observing

the diagnostic wax-up outcome.

Recently, with the advent of laser scanning, virtual planning, rapid prototyping, and computer-

aided design and computer-aided manufacturing (CAD/CAM), digital dentistry was proposed as

a tool for virtual wax-up (Beuer et al., 2008; Probst and Mehl, 2008). It involves digital tooth

contour alteration and obtaining natural tooth contour and dimensions. It is expected that

digital wax-up can be accomplished in a time-efficient and well-controlled fashion. In addition,

it is likely that the digital wax-up will overcome the problems of the conventional wax-up such

as time consumption and the requirement of high technical skills. Further, it is a valid tool for

precise alteration quantification (Abduo et al., 2014b). As the virtual models can be transferred

through the internet, dentists will have greater access for diagnostic wax-ups, even if the

computerized centre location is distant. However, currently, there has been very minimal

research to evaluate the validity and feasibility of digital diagnostic wax-up. For the digital wax-

up to be recommended for routine use, it should provide an outcome that is at least

comparable to the conventional wax-up method. Therefore, qualitative and quantitative

comparisons between conventional wax-up and digital wax-up are necessary.

The research experiments presented in this thesis are early in the digital wax-up field. A

method of executing a digital wax-up is proposed. The obtained digital wax-up outcome is

compared against the conventional wax-up outcome. This was applied to the variables that

can be affected by the diagnostic wax-up, namely: precision, contour, occlusion and aesthetics.

The obtained outcome will improve the current understanding of digital wax-up and will be

used to recommend further refinements of this technique.

4

1.2. Literature Review

To date, with a dentally conscious society and with prolonged life expectancy and preservation

of teeth, the dentition tends to deteriorate for various reasons. Subsequently, many patients

suffer from dental problems such as missing or deficient teeth, disorganized dental arches, or

unacceptable tooth colour (Figure 1-1). Consequently, dentists and dental specialists

commonly encounter such challenging patients from the prosthodontic perspective (Eckert,

2009; Zitzmann et al., 2009; Zitzmann et al., 2010).

A B

C D

Figure 1-1 Examples of dental conditions that indicate fixed prosthodontic treatment. (A) Teeth discoloration, (B) deficient dental morphology, (C) failed and unaesthetic restorations, and (D) worn dentition.

Prosthodontics is the dental specialty that deals with the diagnosis, treatment planning,

rehabilitation and maintenance of the oral function, comfort, appearance and health of these

patients (The Glossary of Prosthodontic Terms, 2005). The complexity of the treatment ranges

from single or few teeth restorations, to complete occlusal rehabilitation. The prosthodontic

treatment can involve altering the tooth morphology, altering the vertical dimension of

occlusion, reorganizing the occlusion and restoring all the teeth of at least one dental arch

(Ambard and Mueninghoff, 2002; Keough, 2003b; Besimo and Rohner, 2005; Mizrahi, 2008;

Abduo, 2011).

5

Any prosthodontic treatment should enhance patient comfort, function and aesthetics. Equally

important, treatment should not induce damage to the remaining natural tissues. Historically,

a major emphasis was placed on the mechanical features related to the prosthesis. Although

this has been shown to be beneficial in several laboratory studies, a great portion of the

clinical complications in fixed prosthodontics has been biological in nature such as caries and

periodontal diseases (Gavelis et al., 1981; Felton et al., 1991; Jacobs and Windeler, 1991).

Further, it appears that as the complexity of the prosthodontic treatment increases, there is an

increase in biological complications (De Backer et al., 2008). Many of the encountered

problems are avoidable if proper case selection and treatment planning were followed (Steele

et al., 2002). As a result, the contemporary literature pertaining to fixed prosthodontic

treatment acknowledges the importance of outlining feasible objectives of prosthodontic

treatment prior to the commencement of the treatment (Ahmad, 2010). This will insure

patient satisfaction of the treatment.

With the ever-increasing public aesthetic demands, many patients request elective dental

treatment (Ahmad, 2010; Kelleher, 2012). On the other hand, many new materials and

products are continuously released that are highly aesthetics, such as composites and

ceramics. The combination of increasing patient demand and the persistent marketing of

aesthetic products had caused a shift of treatment modality from being genuinely justifiable to

purely elective and centred on aesthetic enhancement (Ahmad, 2010; Kelleher, 2012). Dental

conditions such as diastema, chipping, tooth wear, black triangles and irregular teeth position

in otherwise healthy dentition might constitute deviation from what is perceived to be natural

and aesthetic. Some clinicians advocate fixed prosthodontic treatment due to its profitability,

high aesthetic impact and reliability. Although elective treatment is justifiable, the clinician

should balance the aesthetic advantages against the biological and mechanical costs. It is not

uncommon for patients to request prosthodontic treatment, such as veneers and crowns,

purely for aesthetic enhancement. While prosthodontic treatment is ideal for many patients

with these problems, it entails irreversibly preparing many of the visible teeth in the smile (loss

of up to 30% - 70% of natural tooth structure) (Hancocks, 2011). As a consequence of tooth

structure reduction, the rate of biological complications might arise, such as pulpal damage,

tooth fracture, periodontal complications, tooth loss or infection (Goodacre et al., 2003a).

Ethically, the advantages and disadvantages of dental treatment should be well illustrated to

the patients before embarking into the definitive treatment (Ahmad, 2010; Kelleher, 2012). In

situations where the benefits of prosthodontic treatment are not clearly outweighing the

complications, it should be validated by a diagnostic wax-up (Magne and Douglas, 1999a; b;

Magne and Belser, 2004). A diagnostic wax-up permits the visualization of what is achievable

6

by the fixed prosthodontic treatment. It allows the clinician to formulate the balance of all the

achievable factors from the biological, mechanical and aesthetic perspectives.

1.3. Prosthesis Requirements

In order for prosthesis to fulfil its function, it should be durable, aesthetic, cleansibale and

maintainable by the patient. Many prostheses fail because of poor case selection,

inappropriate treatment techniques and inadequate oral hygiene.

Like most of the oral diseases, fixed prostheses failure is accentuated by the deficient hygiene

practice (Lang, 1995; Donovan and Cho, 2001; Gracis et al., 2001). In many instances, the

patient might fail maintain the cleanliness of the prosthesis. However, it is not uncommon for

biological complications to arise as a result of non-cleansible prosthesis design, which can

hinder regular home care practice by the patient (Becker and Kaldahl, 1981; Croll, 1989). On

the other hand, excessive hard tissue reduction should be avoided as this has been shown to

contribute to pulpal complications. Following invasive tooth preparation, additional

periodontal and/or endodontic therapy might become necessary.

Mechanical durability can be ensured by considering factors related to space provided for the

prosthesis components and cementation. Prosthesis material durability is obtained from

controlled abutment tooth preparation to accommodate sufficient material bulk for the final

prosthesis (Goodacre et al., 2001). This will also avoid the risk of over-contouring of the

prosthesis. Uncontrolled abutment tooth reduction will not only compromise tooth vitality,

but will also render the abutment tooth susceptible to mechanical failure. Abutment tooth

preparation with minimal taper and adequate length ensures that retention and resistance

forms are established which will enhance the cementation integrity. In addition, durability of

the prosthesis is optimised by the choice of material for the final prosthesis (Wataha, 2001;

2002). In general, a material that produces a rigid prosthesis is ideal to resist fracture and

deformation in thin section.

In relation to aesthetics, the prosthesis should blend inconspicuously within the surrounding

oral and facial structures. Several dental features will determine the aesthetics of a prosthesis

including tooth dimension, colour, morphology, proportions, symmetry and emergence profile.

In addition, the prosthesis should also integrate naturally with the gingival features (Gracis et

al., 2001; Jorgensen and Nowzari, 2001). Situations such as teeth malalignment, edentulous

areas and compromised dento-gingival dimensions will pose significant challenges to the

restoration of natural aesthetics.

7

In many instances, to achieve the aforementioned objectives, prosthodontic treatment

necessitates irreversible alterations to the remaining hard and soft tissues (Figure 1-2). In

order to justify such alterations, significant benefits of the treatment should be apparent.

A B

Figure 1-2 Clinical images illustrating the amount of tooth preparation required for prosthodontic treatment. (A) Pre-treatment situation, and (B) prepared dentition.

1.4. The Rationale of Digital Dentistry

With the continuous development of computerized technologies, digitized treatment

modalities are becoming an integral approach in prosthodontics, orthodontics and oral and

maxillofacial surgery. The exponential increase in the application of digital dentistry is

attributed to the ease of use, greater ability for quality control, parallel material development,

and the possibility virtual evaluation (Mormann et al., 1989; Duret and Preston, 1991;

Andersson et al., 1996; Miyazaki and Hotta, 2011; van Noort, 2012). Currently, digital dentistry

is utilized as diagnostic and manufacturing tool.

Initially, digital dentistry was introduced to produce dental restorations. In comparison to the

conventional fabrication methods, digital manufacturing is thought to have the advantage of

reducing human intervention and omitting multiple error-introducing steps such as

impression, waxing and casting (Beuer et al., 2008; Miyazaki and Hotta, 2011; Abduo and

Lyons, 2013). This is assumed to reduce the error sources and increase the precision of the

prosthesis. Furthermore, since modelling and production are automated procedures, there is

an overall reduction of fabrication time and cost. However, as with any new technology, the

utilization of digital manufacturing was not without impediments. The early application was

crude and associated with compromised quality and precision of the prosthesis (Beuer et al.,

2008; Miyazaki and Hotta, 2011). Positively, the more recent literature reflects a tendency for

continuous improvements of digital dentistry streams and a gradual shift towards wider

acceptance of the new technology as a mainstream for prosthesis fabrication.

8

Alongside computer-aided systems development, new materials have emerged for prostheses

fabrication. Modern machines can utilize a broad array of metals, ceramics and resins. Of most

interest in prosthodontics are the high-strength ceramics (alumina and zirconia) that

constitute a durable metal-free restoration material and can only be produced by digital

manufacturing (Sadan et al., 2005; Denry and Kelly, 2008). Prior to digital manufacturing,

metal-free restorations were prone to fracture and primarily reliable for single anterior tooth

restorations. To date, with the aid of digital manufacturing, high-strength ceramics prosthesis

can be produced, and the indications of ceramic restorations were expanded to include multi-

unit prostheses and posterior teeth restorations.

Digital manufacturing systems have subsequently been developed to fabricate implant

components and prostheses (Kollar et al., 2008; Abduo and Lyons, 2013). Today, utilizing

computerized technologies to fabricate implant components and prostheses is an acceptable

treatment modality. Materials such as ceramics, metals (base metal alloys and titanium),

resins, and waxes can be processed by the available systems (Beuer et al., 2008; Kollar et al.,

2008; Miyazaki et al., 2009).

A recent application of digital manufacturing is the fabrication of removable prostheses.

Removable partial denture metal frameworks can be produced directly from metal (Eggbeer et

al., 2005; Williams et al., 2006; Han et al., 2010) or, alternatively, a resin pattern framework

can be formed and then cast using conventional fabrication methods (Williams et al., 2004; Wu

et al., 2012). Different computerized protocols have been proposed for the fabrication of

complete denture bases (Maeda et al., 1994; Sun et al., 2009; Kanazawa et al., 2011; Goodacre

et al., 2012; Kattadiyil et al., 2013), and are very useful for fabricating facial prosthesis (Davis,

2010), as the morphologies can be easily obtained by mirror image or average face form

(Ciocca and Scotti, 2004; Feng et al., 2010b) so that a more realistic and natural prosthesis can

be manufactured (Feng et al., 2010a). A map of the surface morphology and colour can be

saved virtually which facilitates future prosthesis fabrication. As extra-oral scanning is a

possibility, the whole experience will also be much more comfortable to the patient (Runte et

al., 2002; Feng et al., 2010a). The automated process will significantly reduce the reliance on

technical skill and human variation. With the available systems, facial prosthesis can be

produced from resin or wax (Chen et al., 1997; Runte et al., 2002; Feng et al., 2010a).

Subsequently, it is invested and transformed to surgical grade silicone.

More recently, digital dentistry is utilized for diagnostic purposes. For example, computerized

technologies are used to plan and idealize surgical implant treatment (Cassetta et al., 2012;

Farley et al., 2013; Vieira et al., 2013). Due to the precision that can be achieved with the aid of

9

information from 3D digital radiographs, implant dimensions and placement locations can be

determined using planning software without violation of critical anatomical features. Further,

the need for bone and/or soft tissue grafting can be established.

Due to versatility of computer-aided design (CAD), digital diagnostic wax-up has been

hypothesized to reduce the difficulties of prosthodontic treatment planning. In this field the

virtual pre-treatment model can be altered digitally to simulate the prosthodontic treatment.

Once the digital wax-up is completed by a computer-aided design (CAD) software, the data is

transferred to a computer-aided manufacturing (CAM) software that controls the production

unit. The aim of CAM process is to produce an accurate physical model as designed by the CAD

software. Although, virtual systems to modify natural teeth are available commercially, their

use has been restricted to design and fabricate provisional restorations (Lin et al., 2013).

Further, very minimal research was conducted on that field (Abduo, 2012). In addition, several

authors have described dental model’s analysis and surveying for removable partial denture

framework design (Williams et al., 2006; Han et al., 2010). This feature will locate the ideal

path of insertion and abutment tooth undercuts and subsequently, the ideal location of

components and prosthesis design will be selected.

An application of digital dentistry that is currently under investigation is the quantification of

the effect of the proposed prosthodontic treatment prior to the active treatment phase. This

takes advantage of the software precision in measurements and quantification. On the 3D

models, volumes and distances can be precisely measured (Davis et al., 2012), and in the

dental practice, analysis of tooth preparation can occur prior to prosthesis fabrication. In some

instances, where the tooth preparation is not ideal or restoration thickness is minimal,

modifications of the tooth preparation can be recommended to reduce the risk of mechanical

failure of the prosthesis. Such feature can be coupled with a digital wax-up to ensure any tooth

preparation will facilitate the planned restoration (Abduo, 2012).

1.5. Diagnostic Wax-Up

According to the Glossary of Prosthodontic Terms (2005), diagnostic wax-up is defined as

waxing of intended restorative contours on dental casts for the purpose of evaluation and

planning restorations. Eventually, optimal clinical and laboratory procedures that can achieve

the desired aesthetic and function can be determined. Normally, wax addition technique is

implemented to develop a wax pattern through organized sequential addition of wax to shape

the individual components to the desired anatomic form (The Glossary of Prosthodontic

10

Terms, 2005). In the dental laboratory, the diagnostic wax-up alters the shape and form of

teeth by preparing dental models, reducing part of the teeth and building the contours with

wax (Davies et al., 2001; Jacobs et al., 2002; Abduo, 2011). The outcome of this “trial”

treatment can be demonstrated to the patient for approval or suggestion of any further

modifications. In this manner, the patient will be more informed of the final outcome.

Subsequently, the diagnostic wax-up will facilitate the “outcome-based treatment” which

implies that the magnitude of irreversible alteration to hard or soft tissues is dictated by the

final outcome rather than the initial patient presentation (Magne and Belser, 2004; Gurel,

2007). This is accomplished clinically by preparing the tooth according to the anticipated final

restoration design as determined by the diagnostic wax-up. In addition, provisional

restorations can be fabricated following the diagnostic wax-up contour and, should the

provisional restorations outcome satisfy the patient, the definitive restorations will be

fabricated to resemble the diagnostic wax-up (Magne and Belser, 2004; Gurel, 2007; Reshad et

al., 2008). In fixed prosthodontics, the diagnostic wax-up is used for the following purposes:

1.5.1. Selecting the most suitable treatment

During the process of diagnostic wax-up, it is important for the operator to objectively

consider what can be achieved in a given clinical scenario. Multiple treatment options can be

considered. As all the process is conducted on dental models via diagnostic wax, it has the

advantage of being reversible. The treatment that can be incorporated with the dental wax-up

can be alteration of tooth shape and form, occlusion alterations, replacement of missing teeth

and orthodontic tooth movement (Morgan et al., 1975; Tarantola and Becker, 1993; Simon and

Magne, 2008). In some situations, tooth extraction and soft tissue alterations can be

considered (Simon and Magne, 2008). It is recommended that through the process of wax-up,

the operator should aim for feasible and conservative treatment, yet without compromising

the patient’s needs.

Some authors had suggested using the information from the wax-up for a direct mock-up

technique, where the shape of the teeth is temporarily altered by direct application of

composite restorative material on unprepared teeth with the aid of silicone matrix (Reshad et

al., 2008). The aim of this step is to allow better 3D visualization of the outcome by the

clinician and the patient, even over a period of time (Figure 1-3). Therefore, the diagnostic

wax-up will be better correlated to the patient dentition and avoid an outcome that appears

ideal on casts but not corresponding optimally to the patient’s appearance (Simon and Magne,

2008).

11

A B C

Figure 1-3 Diagnostic wax-up was used to alter the teeth with composite restorative material. (A) Pre-treatment presentation. (B) Diagnostic wax-up. (D) The modified dentition by composite restorative material. The new contours can subsequently control the tooth preparation.

1.5.2. Controlling the tooth preparation

Whenever a tooth is prepared, it is important to achieve enough clearance to accommodate a

durable and physiological restoration, without over sacrificing natural tooth structure. Ideal

tooth preparation is achieved by controlled tooth surface reduction, maintaining occlusal

surface morphology, obtaining minimal preparation taper and preserving vertical preparation

height (Preston, 1976; Henry, 1978; Goodacre et al., 2001). Implementation of these features

ensures mechanical durability through even prosthesis material thickness and adequate

retention and resistance form in the abutment tooth preparation. Adequate material thickness

also allows for enhanced aesthetics of the prosthesis. Following strict preparation protocol will

also minimise unnecessary tooth reduction and maintain the greatest distance possible

between the external preparation surface and the pulp space. To provide an optimal crown

preparation, a combination of techniques can be applied (Table 1—1).

12

Table 1—1 Determinants of crown preparation

Determinant Description Initial tooth morphology

The final preparation is dependent on the external tooth surface. Controlled reduction can be enhanced by

Depth groove

Indices Opposing dentition Used as a guide to ensure sufficient occlusal clearance exists in the

centric and eccentric mandibular position Applied restorative material

Different restorative materials require different reduction

Metal: - Functional surface: 1.5 mm - Non-functional surface: 1 mm - Margin: 0.3-0.5 mm

Ceramic: - Functional surfaces: 2 mm - Non-function: 1.5 mm - Margin: 1-1.5 mm

Metal-ceramic: - Functional: 2 mm - Non-functional: 1.5 mm - Margin: 1.3-1.5 mm

Prosthesis design Partial coverage prosthesis requires less coverage than full coverage prosthesis

Planned morphological alterations

The external definitive prosthesis surface dictates the final preparation. Accurate reduction can be enhanced by

Diagnostic wax-up

Indices

Prosthodontic treatment driven by diagnostic wax-up is advantageous by selecting the

treatment on the basis of addressing patient concerns rather than on the existing dental

situation (Davies et al., 2001; Magne and Belser, 2004). The possible implication of this

approach is deviation from the classical recommendation for reducing the tooth surfaces prior

to crown restoration. In this situation, the amount of reduction is dictated by the wax-up

contour (Morgan et al., 1975; Simon and Magne, 2008). This means, some surfaces will be

under-prepared, while other surfaces might be over-prepared. This will ensure adequate

thickness of restoration materials can be used without compromising the dental appearance.

Hence the term “outcome-based treatment” which implies that the magnitude of irreversible

tooth preparation is dictated by the planned final outcome rather than the initial tooth

morphology (Magne and Belser, 2004; Gurel, 2007). Clinically, this can be accomplished by

using indices, direct mock-up technique, and temporary alteration of the tooth shape with

composite or provisional restoration materials (Reshad et al., 2008). This approach potentially

allows for more tooth structure preservation, which will lead to more predictable

biomechanics and bonding (Figure 1-4).

13

A B C D E F

Figure 1-4 Comparison between the traditional crown preparation and restorative-driven crown preparation. (A) The initial tooth situation. (B) The crown preparation can be executed according the initial tooth surface. (C) Alternatively, a diagnostic wax-up can be established on a dental model. (D) On the diagnostic wax-up model, a silicone index is fabricated. (E) This silicone index is used intra-orally to dictate the tooth preparation, which might be more conservative. (F) Eventually, the final tooth preparation and crown design is objectively determined according to the wax-up.

1.5.3. Provisional restoration

After final tooth preparation for fixed prosthesis, provisional restoration has to be fitted on the

teeth to protect the tooth and temporarily restore function and aesthetics (Preston, 1976;

Henry, 1978). Given that the diagnostic wax-up is completed according to high standards, it

will be used to fabricate the provisional restoration. The details from the diagnostic wax-up

can be transferred intra-orally via silicone index. Thus, the diagnostic wax-up will act as a

template for the provisional restoration (Tarantola and Becker, 1993). The additional

advantage of this process is allowing the patient to further critique the planned treatment

(Reshad et al., 2008). At this stage, amendments of the provisional restorations are possible,

and the definitive prostheses will be fabricated to mimic the approved provisional restoration

(Figure 1-5).

14

A B

C D

Figure 1-5 An example of the usefulness of the diagnostic wax-up in fabrication of provisional restorations. (A) A pre-treatment situation. (B) The diagnostic wax-up planned to improve the overall dental condition. (C) According to the diagnostic wax-up, provisional restorations were fabricated and inserted. In this situation, the provisional restorations restored patient comfort, aesthetic and function. In addition, they allow the patient the critique the anticipated treatment. (D) definitive prostheses were fabricted according to the approved provisional restorations.

1.5.4. Enhanced communication

As the whole treatment can be visualized physically, the patient is better informed about the

final treatment outcome. From the legal perspective, the patient has to be fully informed

about the likely treatment outcome (Tarantola and Becker, 1993; Reshad et al., 2008; Simon

and Magne, 2008). This is very important since the prosthodontic treatment will cause an

irreversible alteration to the remaining natural tooth structure. Further, the dental technician

can follow the contour of the diagnostic wax-up when fabricating the definitive prosthesis

(Reshad et al., 2008; Simon and Magne, 2008) (Figure 1-6).

15

A B C

Figure 1-6 From the wax-up, silicone indices can be produced (A, B) and used by the manufacturing technician to control the final prostheses contour (C).

Therefore, it is clear from what has been mentioned earlier that diagnostic wax-up is a useful

tool in the contemporary prosthodontic treatment and the clinician should consider

implementing it routinely to increase the predictability of the treatment and achieve a

mutually satisfactory outcome (Reshad et al., 2008; Simon and Magne, 2008). In addition, it

has to be completed according to high standards. However, the diagnostic wax-up is a time

consuming procedure and requires high artistic abilities which might prevent its incorporation

in busy dental practices (Tarantola and Becker, 1993). In most of the situations, the diagnostic

wax-up is completed by a dental technician who is experienced in manipulating wax (Simon

and Magne, 2008). However, the technician tends to complete the wax-up without seeing the

patient, which might render the wax-up a guess work (Simon and Magne, 2008). As a result,

the relationship between the wax-up and the extra-oral features (lips, facial structures) will not

be taken in account. Thus, even if it looks good on the cast, there might be some uncertainty

(Simon and Magne, 2008).

1.6. Requirements of Ideal Wax-Up

From the technical aspects, several wax-up methods were proposed. All the methods were

centered on addition of inlay wax and carving it to the ideal shape (Tarantola and Becker,

1993). The axial surface, occlusal surface and tooth gingiva relationship may be altered by the

waxing procedure. Regardless, of the method of the wax-up, there are few requirements that

should be fulfilled in relation to precision, contour, occlusion and aesthetics:

1.6.1. Precision

In order to ensure applicability of diagnostic wax-up, it should be accurately transferable to the

mouth (Romeo and Bresciano, 2003). In many cases, unprepared teeth or the surrounding soft

16

tissues act as a reference landmark. This mandates that the wax extension should be confined

to areas that will be modified. Unless the soft tissues will be modified, the wax extension

should terminate to the junction between hard and soft tissue (gingival margin). As stated

earlier, indices are used to transfer the wax-up information to the mouth and to the working

models. These indices are used as an aid for controlling the preparation, provisional

restoration fabrication and definitive prosthesis construction. Unnecessary overflowing of wax

will not only affect the final appearance of the diagnostic wax-up, but will significantly affect

the seating of the indices, and subsequent provisional and definitive prostheses fabrication.

Hence, the usefulness of the diagnostic wax-up is reduced (Magne and Belser, 2004).

In addition, further inaccuracy could occur through materials manipulation. The process of

producing pre-treatment models or casts is based on intra-oral full-arch impression and

pouring this impression. Normally for diagnostic procedure, an alginate impression with stock

tray is taken. Due to the presence of many steps and several materials, there will be an

inevitable degree of distortion. For example, the dimensional changes of alginate impressions

was reported to be in the range of 1% to 6 % (Nassar et al., 2012; Todd et al., 2013). Further,

the recorded surface and dimensional accuracy are subjected to changes following the

disinfection protocol (Walker et al., 2010; Nassar et al., 2011; Rentzia et al., 2011). Likewise,

following pouring, the dental stone suffers from additional dimensional distortion (Michalakis

et al., 2012). The final discrepancy is the accumulated error of all the steps and the materials.

Greater accuracy can be achieved by using different materials and techniques, such as rubber

impression materials and special trays (Faria et al., 2008; Schaefer et al., 2012). However, the

greater accuracy from these techniques justifies their application for working model for

definitive prosthesis fabrication. From the clinical aspect, diagnostic models obtained from

alginate and type III dental stone are completely acceptable for the diagnostic purpose (Davies

et al., 2001), and the familiarity of the clinicians with the process can offset the problem of

distortion. Therefore, any newly proposed wax-up protocol should exhibit at least similar

accuracy to currently used wax-up protocol.

1.6.2. Aesthetic

It is acknowledged that prosthodontic treatment will improve aesthetics. Restoring anterior

teeth can optimise their dimensions, display and shade (Raj, 2013). The existence of teeth

appearance in harmony is associated with greater aesthetic perception (Lombardi, 1973; Levin,

1978). Therefore, the wax-up should enhance the dental aesthetics on the diagnostic model

(Figure 1-7).

17

A B

Figure 1-7 An example of aesthetic improvement by the diagnostic wax-up. (A) Pre-treatment models. (B) Wax-up models.

Achieving appropriate dental aesthetics is based on the understanding of the operator of the

aesthetic parameters. Although the acceptable aesthetic appearance is known for its

subjectivity, many authors recommended following objective guidelines. These guidelines are

thought to reflect harmony of dentition in the aesthetic zone and were established from

naturally perceived dental aesthetics (Raj, 2013). Some of the commonly discussed variables

are the perceived frontal proportion (PFP), tooth morphology, the relationship between teeth

and soft tissue and tooth colour (Gillen et al., 1994; Raj, 2013).

Consistent PFP was proposed to infer that there is a relationship between teeth aesthetics and

mathematical proportion (Levin, 1978). The rationale was repeated proportion between the

maxillary anterior teeth is associated with greater harmony and aesthetics (Lombardi, 1973). In

addition, the presence of known recurrent values may simplify achieving good aesthetic

outcome of the restorative treatment. According to the PFP of the maxillary anterior teeth, the

apparent size of the teeth becomes progressively smaller from the midline distally. Levin had

postulated that the ideal PFPs of the lateral incisors and canines to central incisors are 62%

and 38% respectively (Levin, 1978). He named that proportion as the golden proportion (GP).

However, this assumption has not been confirmed by other studies which had shown lack of

coincidence with GP and the generally wider perceived maxillary lateral incisors and canines

has been observed by several earlier studies on natural dentition (Hasanreisoglu et al., 2005;

Ali Fayyad et al., 2006; Persson et al., 2006). Preston had found that in relation to the central

incisors, the PFP of the lateral incisors and canines were 66.2% and 55.6% respectively

(Persson et al., 2006). Similarly, Hasanreisoglu et al. found that the lateral incisors and canines

proportions were 65.9% and 52.3% respectively (Hasanreisoglu et al., 2005). Ali Fayyad et al.

reported that the GP had only existed in 31.3-27.1% of their evaluated population (Ali Fayyad

et al., 2006). Even after treatment, the evidence of development of a recurrent ratio is

minimal. Pini et al. had analysed the existence of GP following prosthodontic and orthodontic

18

treatment of lateral incisors agenesis (Pini et al., 2012). They did not confirm the existence of

GP for the majority of their treated patients. Thus, it could be speculated there is no recurrent

mathematical proportion, and the lateral incisors and, mainly, the canines tend be larger than

what has been proposed by GP (Ward, 2007).

Figure 1-8 Frontal image that illustrates the PFP. As the tooth moves distally, it is perceived to be smaller.

Interestingly, several authors had critiqued the aesthetic value of GP after observing that the

GP did not coincide with the majority of beautiful smiles. Rosenstiel et al. had altered frontal

image by software to incorporate GP. They found that dentists tend to rank frontal images

with GP as less attractive (Rosenstiel et al., 2000). In a follow-up study, they found that lay

persons had minimal preference for images coinciding with GP (Rosenstiel and Rashid, 2002).

Similarly, Mahshid et al. found no relation between GP and what is perceived to be aesthetics

(Mahshid et al., 2004). Basting et al. found that dentists preferred greater proportions than

what has been proposed in GP (Basting et al., 2006). Further, Ward had confirmed that

dentists tend to prefer larger proportions than GP (Ward, 2007). Among the limitations of GP is

the appearance of excessively wide central incisors, which is equivalent to the perceived width

of the lateral incisors and canines (Ward, 2007). Therefore, not only GP rarely exists naturally,

the utilization of GP is not a reliable method to achieve desirable aesthetics of the anterior

maxillary teeth. Like many aspects in nature, instead of being mathematically determined, the

acceptable proportion for the anterior teeth appears to fit within a range.

The tooth morphology is frequently determined by the width-to-height (W:H) ratio, embrasure

appearance, edges roundness and incisal edge location. Deficient or worn down teeth are

expected to have greater W:H ratio than intact teeth (Magne et al., 2003). On the contrary,

lengthening the teeth allows increasing teeth display, restoring anterior teeth relationship and

restoring natural anatomy (Abduo and Lyons, 2012). In a study on natural non-restored young

19

dentition, Gillen et al. found the W:H ratio of the central incisors, lateral incisors and canines

to be 90.2%, 83.9% and 82.5% respectively (Gillen et al., 1994). Likewise, Sterret et al. found

the ranges to be 85-86%, 76-79% and 77-81% (Sterrett et al., 1999). Hasanreisoglu et al. had

found very similar W:H values (89-91%, 82-83% and 83-87% respectively) (Hasanreisoglu et al.,

2005). Zlateri et al. had found slightly less W:H ratio (82.9%, 78.1% and 81.2%) respectively

(Zlataric et al., 2007). Similar outcome was observed by Pini et al, who evaluated the W:H ratio

of the restored anterior dentition that suffered from agenesis of the lateral incisors (Pini et al.,

2013). Due to the minor variation in the outcome of these studies, strict adherence to a

specific proportion should be avoided.

Dental symmetry has significant impact on dental aesthetics. After evaluation of several

smiles, Durgekar et al. had found that lay persons tend to prefer symmetrical smiles (Durgekar

et al., 2010). Similarly, Machado et al. had established that minor unilateral vertical

discrepancies can be perceived as unaesthetic (Machado et al., 2013). Naturally, there should

be relative similarity between contralateral teeth (Mavroskoufis and Ritchie, 1980). The most

symmetrical teeth are the central incisors (Ward, 2001; Hasanreisoglu et al., 2005). The

symmetry tends to reduce as the tooth becomes away from the central incisor (Hasanreisoglu

et al., 2005). The importance of central incisors symmetry was emphasized by several

investigations. In a web-based survey, Brunzel et al. had proved that symmetrical position of

the central incisors is crucial while minor discrepancy in lateral incisors position can be

tolerated (Brunzel et al., 2006). Yet, in the natural dentition, absolute symmetry of the central

incisors is very unlikely (Mavroskoufis and Ritchie, 1980; Gillen et al., 1994). Mavroskoufis and

Ritchie had found that 60% of young individuals had an accumulated central incisors

discrepancy of more than 0.2 mm (Mavroskoufis and Ritchie, 1980). In an aesthetic appraisal

by lay persons and orthodontists, it was found that minor perceived vertical discrepancy of the

incisal edges (0.5 mm) within the central incisors was detected by different investigator

groups. However, vertical discrepancy in the lateral incisor was acceptable by the orthodontist

up to 0.5 mm, while the lay persons had accepted discrepancy of up to 1 mm (Machado et al.,

2013). On the contrary to the incisors, Pinho et al. had found that even 2 mm perceived

asymmetry of the canine cusp tip was acceptable by orthodontists and lay persons (Pinho et

al., 2007). Therefore, through the wax-up, planning of a uniform symmetry will improve the

dental aesthetics. Achieving symmetry is more critical for teeth closer to the midline. Yet,

reasonable deviation from absolute symmetry is not necessary perceived as unaesthetic.

The gingival morphology is more significant for patients with average to high lip line (Sonick,

1997; Camargo et al., 2001; Mehta and Lim, 2010). The dental appearance is likely to be more

aesthetic if the gingival contour is symmetrical and follows the contour of the upper lip (Kois,

20

1994; Sonick, 1997; Donovan and Cho, 2001). The gingival height of the central incisors should

be similar to the canines, while the gingival height of the lateral incisors should more incisal

than the central incisors. On the labial aspect of maxillary anterior teeth, the peak convexity of

the gingival margin should be positioned distal to the long axis of the tooth. The interdental

papilla should exhibit knife-edge morphology and occupy the interdental embrasure gingival to

the contact point. In most of the situations, the definitive prosthodontic treatment has

minimal influence on the gingival aesthetics, unless surgical gingival re-contouring is planned.

In these situations, diagnostic wax-up can include gingival modifications (Kois, 1996; Kois and

Phillips, 1997; Lee and Jun, 2000). Gingival morphological modifications can be incorporated

into the wax-up by extending the wax teeth to the anticipated postsurgical gingival margin. An

index produced from the wax-up can be used by the surgeon to indicate the planned gingival

margin (Lee and Jun, 2000; Lee, 2004). From this margin, a biologic width of 2 mm should be

preserved (Gargiulo et al., 1961). Where the gingival tissues are deficient, gingival

augmentation procedure can be considered (Nowzari, 2001). Alternatively, in some cases, pink

porcelain is incorporated in the prosthesis and can be evaluated by diagnostic wax-up (Vryonis,

1981) (Figure 1-9).

A B

Figure 1-9 An example of a compromised clinical presentation that requires gingival tissues restoration. (A) A diagnostic wax-up that incorporates gingival tissues. (B) Definitive prosthesis with pink porcelain that replaces the missing gingival tissues was constructed according to the wax-up.

Overall, although the literatures do not support a specific recipe for dental aesthetics, the use

of combination of variables as guides to establish dental aesthetics is reasonable (Gillen et al.,

1994). It is clear that dental aesthetic is perceived differently between different individuals.

This emphasizes the significance of incorporating diagnostic wax-up in contemporary dental

practice. As each patient can provide feedback about the anticipated dental aesthetics, the

challenge of subjectivity is reduce. Thus, a mutual satisfaction between the clinician and

patient can be reached (Magne and Belser, 2004). In addition, further aesthetic evaluation of

21

the wax-up can take place in the form of provisional restorations or composite restorations

(Magne and Belser, 2004; Gurel, 2007).

1.6.3. Contour

The contour and profile of a prosthesis contributes to whether the prosthesis will blend

harmoniously with the adjacent teeth and gingival tissues. The tooth contour has significant

relevance on the invasiveness of tooth preparation, the cleanliness of the prosthesis, and the

overall aesthetics. In the process of prosthodontic treatment, a specific amount of tooth

structure on different surfaces should be removed to provide sufficient space for the crown

restoration. This space is necessary to ensure that there is enough bulk of the prosthesis

material for aesthetics, mechanical durability and adequate contour of the crown (Becker and

Kaldahl, 1981; Goodacre et al., 2001). It has been speculated that, the ideal prosthesis contour

should follow the contour of the remaining tooth structure without prominent convexity. This

feature will ensure that the prosthesis blends harmoniously with the adjacent teeth (Becker

and Kaldahl, 1981).

The emergence profile, which is the axial contour of the prosthesis from the base of the

gingival sulcus to the supragingival aspect, should exhibit a straight profile (Croll, 1989) (Figure

1-10). Such a contour will render the tooth more cleansable at the critical regions (Sackett and

Gildenhuys, 1976; Becker and Kaldahl, 1981; Sorensen, 1989). On the other hand, excessive

over-contouring of the gingival third of the crown can alter the biologic relationship between

the tooth and the periodontium, impede adequate cleaning and contribute to gingival

inflammation (Sackett and Gildenhuys, 1976; Sorensen, 1989; Broadbent et al., 2006).

However, it is not uncommon for tooth contour to be altered following prosthetic treatment

(Meijering et al., 1998; Vasconcelos et al., 2009) and, in many instances, altering the tooth

contour may be desirable and is perceived as an objective of the treatment.

22

A B

Figure 1-10 (A) A straight profile in the gingival third facilitates establishing a properly contoured prosthesis. (B) Widening the profile gingivally is associated with over-contoured prostheses.

In the literature, three concepts for establishing axial tooth contour were described:

duplication of original tooth anatomy, under-contouring, and over-contouring. Preserving the

original tooth contour is thought to be biocompatible with its surrounding environment (Croll,

1989; Vasconcelos et al., 2009). The drawback of this concept is tooth modifications following

the treatment are restricted to the pre-treatment tooth contour. Under-contouring the crowns

is another method based on the clinical observation that the gingival tissues will likely to be

maintained as they are self-cleansible (Perel, 1971; Tjan et al., 1980; Becker and Kaldahl,

1981). However, this method is impractical as it could affect the appearance, crown thickness

and preparation invasiveness (Tjan et al., 1980). This could explain why this method is not

favoured. The third method is contouring the restoration according to pre-treatment planning

in the form of diagnostic wax-up (Magne et al., 2003; Magne and Belser, 2004; Gurel, 2007).

The rationale behind this method is the tooth preparation is dictated by the final volume of

crown restoration rather than the existing tooth contour. Therefore, there is a tendency for

this method to be more conservative, but could result in over-contoured surfaces.

Several studies have confirmed the contour alteration following the restorative treatment

(Meijering et al., 1998; Vasconcelos et al., 2009). With the aid of contact scanner and image

registration, Meijering et al. found the dimensions of the veneered teeth were unintentionally

increased, resulting in over-contoured labial surfaces (Meijering et al., 1998). In the same

study, the increase in the labial contour was in the range of 0.33 mm to 0.59 mm. Further, they

found that the increase tends to be greater incisally. Similarly, a contact profilometer study by

Vasconcelos et al. found the contour of all the veneered teeth significantly increased from the

original contour by 0.37 mm to 0.44 mm. They also found that the increase of the contour was

23

directly related to the distance from the gingival margin (Vasconcelos et al., 2009). They

indicated that there is a tendency for the manufacturing technician to produce over-contoured

restorations. From the technical perspective, producing over-contoured restorations might be

desirable since it will facilitate tooth shape improvement, provide more material space for

natural shade matching, and increase restoration bulk and durability (Goodacre et al., 2001).

It has been postulated that an enlarged tooth contour will inhibit adequate home care and

hinder self-cleansing abilities, which will inevitably contribute to increased gingival

inflammation, periodontal complications and subsequent caries (Sackett and Gildenhuys,

1976; Sorensen, 1989; Broadbent et al., 2006). From an anatomical perspective, Burch et al.

recommended that the maximal convexity should occur on the gingival third of the crown of

the restored tooth and should not exceed 0.5 mm (Burch and Miller, 1973). In a dog study,

Perel increased the contour on the labial aspect 0.5 mm above the gingival margin. The over-

contoured restorations resulted in gingival inflammation (Perel, 1971). However, this outcome

was refuted by another dog study which found that as long as professional oral hygiene is

properly executed, periodontal health is minimally influenced by the over-contoured crowns

(Kohal et al., 2003). A follow-up study by the same investigators confirmed that the microbial

composition was slightly affected by the over-contoured crown (Kohal et al., 2004). In a human

split-mouth study, Ehlrich and Hochman provided up to 1 mm over-contoured crowns on one

side and under-contoured crowns on the other side. After a period of 4 months, the

periodontal status between the two sides was found to be similar (Ehrlich and Hochman,

1980). Likewise, Sundh and Kohler (Sundh and Kohler, 2002), on 6 patients, tried three

experimental crowns with different emergence profiles. After one week of normal oral hygiene

practice, all the experimental crowns exhibited similar plaque quantity and quality. Further,

the plaque quality was similar to the contralateral control teeth. The conclusion was that an

increased emergence profile did not contribute to increased plaque accumulation. Therefore,

it appears that as long as adequate oral hygiene can be maintained, reasonable over-

contouring (up to 1 mm) will not contribute to periodontal complications.

It is likely that over-contouring the teeth will increase their aesthetics. This could be attributed

to the greater aesthetic demand and the attempt to improve the tooth dimensions (Magne

and Belser, 2004). This has been confirmed by Ehlrich and Hochman who reported that 3 out

of 4 participants preferred the over-contoured crowns over the under-contoured crowns

(Ehrlich and Hochman, 1980). Slight over-contouring of anterior teeth might be advantageous

from the conservative perspective. Since the anterior teeth have less tooth structure available

for the preparation than posterior teeth, there is a clinical preference to minimize the amount

of tooth reduction (Goodacre et al., 2001). For example, the cervical dentine thickness for the

24

mandibular incisors in some regions is about 1.5 mm (Katz and Tamse, 2003). Therefore, a

shoulder or chamfer of 1 mm width will sacrifice a major proportion of the remaining teeth

and can lead to pulpal complications. Alternatively, under-preparation and slight over-

contouring of the crown restoration might be a conservative approach. Consequently,

diagnostic wax-up is strongly recommended in situations where significant tooth contour

alteration is to take place. An index constructed according to the diagnostic wax up can be

utilized to ensure the preparation is restoratively-driven and adequate for the final crown

volume, rather than the initial tooth contour (Magne and Belser, 2004; Gurel, 2007). From the

biological aspect, maintaining the cleanliness of the anterior teeth is very feasible. Therefore, it

could be envisaged that slightly over-contoured anterior teeth will not hinder regular patient

home care, and the advantages of reasonable over-contouring of anterior crown restorations

exceeds the potential risk.

On the other hand, it is advantageous for the posterior teeth, especially molars, to have

minimal contour increase. From the aesthetic perspective, since most of the posterior teeth

are not in the aesthetic zone, there is minimal merit in significantly altering their labial

contour. Further, there will be no clinical advantage of increasing the contour of the posterior

teeth to realign them (Bryant, 2003). In addition, for posterior teeth, it is more important to

avoid prominent convexity as they are more difficult to clean than anterior teeth. The

importance of a straight profile for posterior teeth is even greater in situations where the

furcation is exposed. The fabricated crown form should have a flat emergence profile coronally

so that there is no undercut to trap food or plaque (Yuodelis et al., 1973; Becker and Kaldahl,

1981), and the crown should re-create the contours of the furcation, to merge or blend with

the coronal aspects of the crown to minimise cleaning difficulty in these areas.

Therefore, the contour of the prosthesis should be carefully considered during the diagnostic

wax-up. Through the wax-up procedure, the contour is easily changed on the waxed surfaces.

The best combination of prosthesis durability, cleansibility and aesthetics should be selected

by the treating clinician.

1.6.4. Intercuspal occlusal contacts

Occlusal contacts in maximal intercuspation position are thought to be essential for

maintaining teeth alignment, stability of the mandible and efficiency of mastication, since

maximal teeth contacts occur in this position (Yurkstas and Manly, 1949; Becker and Kaiser,

1993; Wiskott and Belser, 1995; Hidaka et al., 1999; Owens et al., 2002; Wang and Mehta,

2013). As many patients requiring prosthodontic treatment have deficient occlusal surface due

25

to wear, fracture and old restorations, it is important to idealise the occlusal contacts through

the wax-up procedure. The occlusal surface is comprised of positive features, such as cusp tips

and ridges, and negative features such as grooves and fossae. This morphology dictates the

quality and quantity of occlusal contacts. Several variables were commonly used to describe

the quality of occlusal contacts at maximal intercuspation. For example, pattern of contacts,

number of contacts and surface area of the occlusal contacts (Julien et al., 1996; Owens et al.,

2002).

In terms of occlusal contact pattern, in normal dentitions, the mandibular buccal cusp should

fit into the central fossae and embrasures of the maxillary teeth and the maxillary palatal cusps

into the central fossae and embrasures of the mandibular teeth (Stuart, 1964; Lundeen, 1971;

Wiskott and Belser, 1995). Historically, several authors stated the desire to attain cusp-to-fossa

contact as opposed to cusp-to-marginal ridge contact (Lucia, 1962; Stuart, 1964). The rationale

behind this preference is the cusp-to-fossa relationship is more likely to maintain the stability

of teeth by directing the occlusal forces vertically along the long axis of the tooth, as the cusps

are nested in the fossae (Wang and Mehta, 2013). Further, this arrangement will reduce the

risk of food impaction. On the contrary, cusp-to-marginal ridge relationship is thought to be

associated with greater lateral contacts elements, and the food is likely to be packed

proximally. Thus, teeth will be less stable and might be vulnerable to proximal contacts

opening (Lucia, 1962). However, although this appears to be mechanically sound (Lucia, 1962;

Stuart, 1964), there is no clinical evidence supporting this claim (Wang and Mehta, 2013).

In designing the cusp-to-fossa relationship, two conflicting theories were discussed. The first

theory is the cusp tripodization which states that the cusp-to-fossa contacts are limited to

three points (tripodal contacts) on the sides of the cusps but with no contact on the cusp tip

(The Glossary of Prosthodontic Terms, 2005). This theory was proposed by the gnathologist for

the hope of maintaining the cusp tip by protecting it from wear, yet without compromising the

direction of vertical forces (Lucia, 1962). Eventually, stable cusp-to-fossa relationship is

achieved. However, although such relationship will facilitate producing very natural anatomy,

it is very difficult to establish artificially (Wang and Mehta, 2013). The alternative method is

the cusp-to-fossa contact through the cusp tip which is favoured by Pankey-Mann-Schuyler

(PMS) theory. According to this philosophy, it is acceptable for cusp tip to occlude against

widened fossa. The rationale behind this technique is allowing a degree of freedom in

eccentric movement uninfluenced by tooth inclines. Thus, the risk of interferences

development is reduced while loading the teeth vertically (Schuyler, 1963; Davies and Gray,

2001). Further, it is easier to produce teeth with such simpler scheme than the gnathology

theory (Schuyler, 1963; Davies and Gray, 2001). This explains why this view took preference

26

over the gnathology theory (Darveniza, 2001). As the cusp tip occludes against central fossa,

the design ensures that contacts on inclines do not occur, which will further enhance the

stability (Darveniza, 2001). Although what has been proposed by these philosophies is based

on sound mechanical understanding, there lack of actual clinical evidence (Koyano et al., 2012;

Wang and Mehta, 2013). Since natural dentition has combination of contacting surfaces and

inclines has been observed (Hochman and Ehrlich, 1987; Wiskott and Belser, 1995), it might be

that the cusp-to-fossa relationship has been overrated in the earlier literature. Currently, cusp-

to-fossa relationship is preferred, without negating the use of cusp-to-marginal ridge

relationship (Becker and Kaiser, 1993; Koyano et al., 2012).

As the amount of contacting surfaces indicates the active chewing surfaces (Wiskott and

Belser, 1995), the contact number and contact area have been used frequently as a measure of

the adequacy of dental occlusion. In the literature, the contact number (McNamara and Henry,

1974; Korioth, 1990b; Ciancaglini et al., 2002; Delong et al., 2002) and area (Hidaka et al.,

1999; Owens et al., 2002; Iwase et al., 2011) were heavily investigated and quantified. Most of

the studies had evaluated the contact number and area of intact natural dentition in young

individuals. However, occlusion features are dynamic and subject to change with aging and

dental treatment. Restoration, tooth wear and fracture will inevitably affect the tooth

morphology and subsequently the contact number and area will change in the intercuspal

position overtime (Yi et al., 1996). To date, most of the research was centred on the contact

number and area of the natural dentition. However, evaluation of the occlusal changes over

time was minimally investigated.

In relation to the contact number, Becker and Kaiser recommended that each tooth should

have at least one contact (Becker and Kaiser, 1993). On natural dentition of young individuals,

DeLong et al. and Korioth found that each tooth had about 1.5 – 1.75 contacts per tooth

(Korioth, 1990a; Delong et al., 2002). On the other hand, earlier studies on occlusal contact

area revealed significant variations (Hidaka et al., 1999; Delong et al., 2002; Alkan et al., 2006;

Iwase et al., 2011). The variation of the occlusal contact area measurements is very likely to be

due to differences of the methods applied for area quantification (Owens et al., 2002). Slight

vertical discrepancies between the opposing models can lead to prominent alterations in the

contact area (Wilding et al., 1992; Hidaka et al., 1999). A commonly applied method for

recording the contact area is the use of occlusal medium. The drawback of this method is the

ease of introduction of minimal vertical displacement, which can cause significant

underestimation of the contact area. On the contrary, virtual quantification of occlusal contact

area can potentially cause slight overlapping of the models (Iwase et al., 2011), which will

overestimate the contact area.

27

The location of the tooth in the arch appears to be a strong determinant of the occlusal

contact number and area for the pre-treatment and post-treatment models. The posterior

teeth exhibit greater contact number and area than anterior teeth (Ehrlich and Taicher, 1981;

McDevitt and Warreth, 1997). On natural young individual dentition, McNamara and Henry

found 8 times more contact on posterior teeth compared to the anterior teeth (McNamara

and Henry, 1974). Similarly, another study found that the posterior teeth had 3 times more

contacts than the anterior teeth (Ciancaglini et al., 2002). On the restored dentition (Yi et al.,

1996), it has been found that the posterior teeth had twice the contact number than the

anterior teeth. In relation to the contact area, the posterior teeth were found to consistently

have greater contact area than the anterior teeth (Yurkstas and Manly, 1949; Owens et al.,

2002). Similarly to what has been mentioned earlier, it is likely that the implemented

methodologies influence the area outcome. The more profound contacts on the posterior

teeth are due to greater area, cuspal morphology and interdigitation of the opposing teeth.

The anterior teeth, on the other hand, have more confined surfaces and incisal edges. This

observation fits with the mutually-protected occlusion concept, where the posterior teeth

prevent excessive contact of the anterior teeth in maximal intercuspation (Beyron, 1964; The

Glossary of Prosthodontic Terms, 2005).

Nevertheless, the true impact of contact number and area is still to be determined. After more

than 10 years of prosthetic treatment, Yi and Carlsson found an average of 1 contact per tooth

(Yi et al., 1996), yet no abnormal physiological consequences were observed. This was further

supported by several studies on shortened dental arch that confirmed that although the

number of total occlusal contacts is less than the complete dentition contact, the patient can

function within normal physiological abilities (Kanno and Carlsson, 2006).

Therefore, after the wax-up process, adequate intercuspal occlusal contacts should be present.

According to the available literature, this involves accurate interdigitation at maximal

intercuspation, posterior teeth with cuspal morphology and at least single contact on each

opposed tooth.

1.6.5. Lateral occlusion scheme

Among the conjectured principles for the prosthodontic treatment is the selection of the

lateral occlusion scheme that can be implemented in prosthesis design. Through prosthodontic

treatment, the lateral occlusion scheme can be controlled by altering teeth morphologies,

alignments and orientations. For dentate patients, the available lateral occlusion schemes are

canine-guided occlusion (CGO), group function occlusion (GFO) and bilaterally balanced

28

occlusion (BBO) (Thornton, 1990). CGO is defined as a mutually-protected articulation, in

which the vertical and horizontal overlap of the canine teeth disengages the posterior teeth in

the lateral movement of the mandible (The Glossary of Prosthodontic Terms, 2005). On the

other hand, GFO is distinguished by the existence of multiple contacts between the maxillary

and mandibular teeth in lateral movement on the working side (The Glossary of Prosthodontic

Terms, 2005). The simultaneous, anterior and posterior occlusal contact of teeth in centric and

eccentric positions is called BBO (The Glossary of Prosthodontic Terms, 2005). Anecdotally,

several claims have been made supporting each lateral occlusion philosophy. For example,

CGO will protect posterior teeth laterally while the anterior teeth are protected in the centric

position; hence the term “mutually-protected occlusion”. The canines were considered ideal

guidance teeth because of their strategic location, anatomy and proprioceptive properties

(Rinchuse et al., 2007). Conversely, GFO has been assumed to facilitate a wide distribution of

occlusal forces over many teeth instead of single tooth; therefore, a more comfortable,

efficient and functional occlusion can be established (Thornton, 1990). Nevertheless, there is a

lack of compelling evidence indicating the superiority of any philosophy (Becker and Kaiser,

1993; Turp et al., 2008).

Through the diagnostic wax-up procedure it is possible to plan for lateral occlusion scheme

alterations. This can be done by altering the morphologies of the canines and posterior teeth.

However, among the limitations of proposing rigid criteria for the lateral occlusion scheme is

the possible discrepancy between the classical definitions and what is considered to be a

physiological occlusion. According to the Glossary of Prosthodontic Terms (2005), any

occlusion that is in harmony with functions of the masticatory system is deemed physiological.

This could potentially mean that as long as the lateral occlusion scheme is not contributing to

mechanical, biological or aesthetic problems, it can be deemed as physiological occlusion.

A recent systematic review on natural dentition indicated a clear heterogeneity of lateral

occlusion scheme prevalence between the available studies on lateral occlusion scheme of the

natural dentition (Abduo et al., 2013). Due to the variation of each lateral occlusion scheme

prevalence after partial and complete excursive movements, it could be postulated that true

CGO or GFO hardly, if ever, exists in nature and the classical criteria might not be applicable.

However, there is an overall consistency on the factors associated with lateral occlusion

schemes, such as magnitude of excursion, static occlusal relationship and age effect.

All the studies that compared different degrees of excursion found that the CGO prevalence is

lower after partial excursion than after complete excursion (Yaffe and Ehrlich, 1987; Ingervall

et al., 1991; Ogawa et al., 1998; Al-Nimri et al., 2010). Likewise, the opposite was observed for

29

GFO. This observation was attributed to the greater number of sole canine contacts on the

working side after complete excursion; while the prevalence of premolar and molar contacts,

in addition to canine contacts, are greater on the working side after partial excursion (Ogawa

et al., 1998).

With aging, there is a decline of CGO prevalence. On the contrary, the prevalence of GFO

followed an opposite pattern. Such alterations support that occlusion is dynamic, adaptive and

subjected to changes with time (Beyron, 1954; Panek et al., 2008). The aging effect can be

primarily attributed to tooth wear, which is a physiological phenomenon and can increase the

number of lateral tooth contacts (Ingervall, 1972; Ingervall et al., 1991).

In terms of masticatory function, there is some evidence of the cross sectional studies that the

lateral occlusion scheme has influence on chewing cycle, chewing velocity, condylar loading

and EMG activities (Belser and Hannam, 1985; Jemt et al., 1985; Okano et al., 2002; Okano et

al., 2005; Okano et al., 2007). However, significant long-term functional difference cannot be

established. The clinical studies revealed that patients have the capacity to adapt to CGO or

GFO as new lateral occlusion scheme (Dahl and Krogstad, 1985; Gross and Ormianer, 1994;

Ormianer and Gross, 1998; Ormianer and Palty, 2009). The applied lateral occlusion scheme

appears to have a minimal impact on patient’s comfort, and biological or mechanical

complications (Yi et al., 1996; Hemmings et al., 2000; Redman et al., 2003; Poyser et al., 2007;

Schmidlin et al., 2009; Attin et al., 2012; Al-Khayatt et al., 2013). Instead, mechanical

complications are associated with other risk factors such as bruxism, restorative material

properties and implant prosthesis occluding against implant prosthesis.

Historically, temporomandibular joint disorder (TMD) was commonly attributed to occlusal

factors; however, the available studies revealed that there is no causative relationship

between lateral occlusion schemes and TMD development (Weinberg, 1964; Ingervall et al.,

1991; Donegan et al., 1996; Kahn et al., 1999). Therefore, the current state of evidence

indicates that occlusal treatment will not prevent or treat TMD. Instead, non-occlusal

treatment is considered more justifiable and conservative (De Boever et al., 2000a; b; Koh and

Robinson, 2003; Liu et al., 2012; Turp and Schindler, 2012).

Since the long-term studies have confirmed that patients with CGO or GFO can function

comfortably, a bench mark lateral occlusion scheme cannot be proposed, as stated by earlier

reports (Becker and Kaldahl, 1981; Turp et al., 2008). Consequently, it is recommended to

implement flexibility and broader principles in occlusion design (Becker and Kaldahl, 1981;

Turp et al., 2008; Carlsson, 2010). Therefore, for the diagnostic wax-up, instead of adhering to

a preconceived occlusion scheme when complex restorative treatment is indicated, the

30

clinician should consider an occlusion scheme that is practical, simple, conservative and allows

aesthetic treatment (Becker and Kaiser, 1993; Wiskott and Belser, 1995; Bryant, 2003;

Carlsson, 2010). Therefore, when executing a diagnostic wax-up, CGO and GFO are acceptable.

The lateral interferences should be avoided, as they might contribute to increased patient

awareness and excessive prosthesis loading (Becker and Kaiser, 1993; De Boever et al., 2000a;

b).

1.6.6. Vertical dimension of occlusion

The Glossary of Prosthodontic Terms (2005) defines the vertical dimension as the distance

between two selected anatomic points. The vertical dimension when the mandibular teeth are

occluding with the maxillary teeth is defined as the vertical dimension of occlusion (VDO). The

VDO for dentate individuals is mainly determined by the remaining dentition, hence loss of

tooth substance might influence the VDO. A loss of VDO can significantly influence patient

function, comfort and aesthetics (Turner and Missirlian, 1984).

In some cases, vertical dimension alteration can be incorporated within the diagnostic wax-up

(Morgan et al., 1975). Increasing the VDO from the clinical perspective has been reported to

facilitate the treatment of patients presenting with generalized and complex dental

abnormalities such as generalized tooth wear and significant occlusal irregularities (Johansson

and Omar, 1994; Keough, 2003b; Johansson et al., 2008). However, prior to VDO alterations,

series of comprehensive extra-oral and intra-oral assessments should be followed. Extra-orally,

features such as magnitude of VDO loss, facial profile and aesthetics, and status of the

temporomandibular joint (TMJ) should be considered.

Intra-oral assessment is more relevant to the prosthodontic treatment and involves examining

the remaining tooth structure and occlusion status. The prognosis of a dental restoration is

directly determined by the amount of remaining tooth structure (Goodacre et al., 2001). For

generalized loss of vertical tooth height, the clinician is faced with the dilemma of limited

remaining tooth structure that can retain the restoration. The original tooth height determines

the active preparation height, which can be defined as the vertical distance between the

preparation margin and the occlusal-axial line angle. In order to avoid compromising the

preparation height, increasing the VDO should be considered to provide adequate space to

accommodate the restorative material. The merit behind this technique is more prominent in

generalized loss of tooth height manifested from tooth wear

31

The final preparation height is a critical determinant of the need and the magnitude of the

VDO increase. It has been recommended that the minimal tooth preparation height should be

3 to 4 mm (Maxwell et al., 1990; Parker et al., 1993; Goodacre et al., 2001). If this dimension is

not available, adjunctive treatment, such as VDO increase, should be considered.

Patients with a worn anterior dentition suffer from a loss of clinical crown height and the

possibility of development of an edge-to-edge incisal relationship (Crothers and Sandham,

1993; Johansson et al., 2008). As a result, the aesthetic appearance is affected and the anterior

tooth guidance is lost (Sarita et al., 2003). In these situations, increasing the VDO rectifies the

anterior tooth relationship, by re-establishing an overjet and overbite, and facilitating the

establishment of anterior tooth guidance (Keough, 2003a; Vence, 2007). Anterior tooth

guidance is desirable as it is believed to protect the posterior teeth in eccentric movements

(Becker and Kaiser, 1993; Pokorny et al., 2008; Carlsson, 2009).

Increasing VDO has been considered by some authors to be a hazardous procedure that can

violate a patient’s dental physiology and adaptation (Tench, 1938; Schuyler, 1939). The basis of

such claims is the thought that VDO occurs at a specific level that should be maintained

through an individual’s life (Turner and Missirlian, 1984). The anticipated consequences of

increasing the VDO are hyperactivity of the masticatory muscles, elevation in occlusal forces,

bruxism and TMD (Tench, 1938; Schuyler, 1939; Turner and Missirlian, 1984). On the other

hand, multiple articles have challenged the hypothesis of the negative implications of

increasing VDO (Carlsson et al., 1979; Dahl and Krogstad, 1982; 1985; Gross and Ormianer,

1994; Ormianer and Gross, 1998; Ormianer and Palty, 2009). In general, their outcomes reflect

the safety, patient adaptation and predictability of increasing the VDO. This is true in relation

to TMJ and masticatory muscle health.

In relation to the magnitude of increasing the VDO, an increase of up to 5 mm inter-incisally is

a feasible alteration (Carlsson et al., 1979; Dahl and Krogstad, 1982; 1985; Gross and Ormianer,

1994; Ormianer and Gross, 1998; Ormianer and Palty, 2009). Such outcomes support the

assumption of other investigations that physiological VDO occurs at a range, commonly known

as the comfort zone, rather than a specific constant level. Subsequently, it could be expected

that the patient can adapt to an alteration in VDO as long as it is confined to this zone. The

possible adaptation mechanisms to an increased VDO could be masticatory muscle

lengthening and relaxation, dentoalveolar maturation, or a combination of these two

mechanisms (Ormianer and Gross, 1998). However, from the clinical perspective, it is wise to

keep the VDO increase to minimal, as this will simplify the restorative treatment. This is more

32

important when arch discrepancy exists, such as in Angle Class II and III occlusions (Jensen,

1990a; c).

Currently, the clinical techniques to assess VDO loss are of limited predictability and reliability.

Therefore, they cannot be used reliably to estimate the magnitude of increasing VDO (Rivera-

Morales and Mohl, 1991; 1992). Instead, the factors that should be considered as

determinants for increasing the VDO are the remaining tooth structure, the space available for

the restoration, occlusal variables and aesthetics. All of these factors can be evaluated by the

diagnostic wax-up and further validated by intra-oral provisional restorations. Since any

restorative material can be applied on the occlusal surface in a space of 2 mm (Hemmings et

al., 2000; Goodacre et al., 2001), a 4 mm interarch space will be adequate for comprehensive

rehabilitation. Subsequently, a VDO increase greater than 5 mm inter-incisally is rarely

indicated from the clinical perspective. From the available studies, the negative consequences

of increased VDO are of a minimal nature and most of the signs and symptoms resolve within

two weeks. Therefore, it is wise to consider a probationary increase of the VDO, with a fixed

provisional restorations or composite build-ups, fabricated according to the wax-up, for a

period of a few weeks before the provision of the definitive prostheses.

1.7. Conventional Wax-Up Protocol

To complete the conventional diagnostic wax-up, the following steps should be followed

(Magne et al., 1993):

1. Obtaining diagnostic models: impressions for the diagnostic models are normally taken by

alginate impression material and stock tray (Romeo and Bresciano, 2003). The impressions

are poured by dental stone. The diagnostic models should be accurate, and represent the

patient’s dental arches.

2. Mounting diagnostic models on dental articulator: generally, semi-adjustable or fully-

adjustable articulator is used (Romeo and Bresciano, 2003). Dental facebow can be used to

mount the maxillary model; however, its use is debatable (Carlsson, 2009). Alternatively,

due to the lack of evidences, arbitrary articulation is assumed to be sufficient.

3. Altering the tooth form: the tooth form is executed by addition of dental wax and/or

trimming external surfaces (Magne et al., 1993; Cutbirth, 2001; Kois et al., 2008). The

tooth alteration tends to be restricted to area of treatment (Chen and Raigrodski, 2008).

Further, missing teeth can be replaced by wax or acrylic teeth.

33

Currently, the conventional wax-up protocol has been considered the gold standard for many

years. However, all the involved steps require considerable human intervention and

manipulation of materials that may also exhibit inherent processing shrinkage and/or

expansion (Wataha and Messer, 2004; Sadan et al., 2005). This can translate to increased

processing errors, and inaccuracies, as well as increased time and cost. Further, considerable

skill is required to produce a wax-up of good quality (Pietrobon and Paul, 1997; Romeo and

Bresciano, 2003). The problems of the conventional protocol are however offset by the

familiarity of the processes by the operators.

For the anterior teeth, two tooth alteration methods are available: additive waxing on the

labial surface (Magne and Douglas, 1999b) and using prefabricated wax pattern (Magne et al.,

1996). The former method is more suitable for veneers or full coverage crown restorations

planning, while the other method is best suited to replace missing teeth. Prior to the wax

addition, the magnitude of tooth alterations should be determined. This involves locating the

incisal edge and the transitional line angle between the facial and proximal surfaces (Magne

and Douglas, 1999a; Curry, 1998; Romeo and Bresciano, 2003). The initial contour is obtained

by addition of vertical and horizontal lobes that dictate the overall tooth outlines (Romeo and

Bresciano, 2003). At the last stage, tooth outline, final contour, interproximal contacts and

surface texture are refined (Magne and Douglas, 1999a; Pietrobon and Paul, 1997) (Figure 1-

11).

A B

Figure 1-11 (A) Frontal and occlusal views of pre-treatment anterior teeth that clearly shows deficient tooth morphology. (B) Similar views after the wax-up indicate establishment of natural morphology.

34

For posterior tooth form alteration, it is recommended to develop positive and negative

features such as cusps, ridges, grooves and fossae (Figure 1-12). On the contrary to simple and

flat surfaces, these features will establish natural tooth appearance and ideal pattern of

occlusal contacts. When the cusps are properly formed, they should be convex on all the

surfaces and contact the opposing teeth in several and well-distributed locations. On the other

hand, flat opposing surfaces will produce fewer and large contact areas (Shillingburg et al.,

2000).

A B

Figure 1-12 (A) Prior to the treatment, the teeth can be worn, fractured or heavily restored. (B) the wax-up establishes natural occlusal anatomy such as cusps, grooves and fossae.

Shillingburg et al. discussed their method in executing the wax-up (Shillingburg et al., 2000).

Prior to waxing, the occlusal surface can be reduced for about 1.5 mm. This will provide space

for the wax addition. On the occlusal surface, a wax cone is added on the position of each

cusp, starting with the functional cusps. The cones are added with the aid of heated

instrument. This is followed by adding a triangular ridge that extends from the cusp tip to the

central groove. The triangular ridge is narrow at the cusp and wide at its base in the central

groove. After the addition of the triangular ridges, the axial contours are finalized (Shillingburg

et al., 2000). In general, the contour of the adjacent teeth and the occlusal contacts with the

opposing tooth are used as a guide (Figure 1-13). An alternative method is the addition of wax

in excess and carving the desired morphology from the wax to remove the unwanted wax

(Davies et al., 2001).

35

A B

Figure 1-13 (A) The adjacent teeth should exhibit similar axial contour. (B) In situations where a tooth deviates from the contour of the adjacent teeth, there might be aesthetic, functional and hygienic implications.

1.8. Digital Wax-Up Protocol

More recently, with the advent of laser scanning, virtual planning, rapid prototyping and

computer-aided design and manufacturing, it is hypothesized that diagnostic planning can be

accomplished in time-efficient and well-controlled fashion (Abduo, 2012). With the aid of a

scanning system, the actual pre-treatment models can be converted to virtual models (Yoshida

et al., 2011). Multiple alterations can be executed and assessed virtually with 3D designing

software (Bootvong et al., 2010). After attaining the ideal tooth morphology, new actual

models can be printed by rapid prototyping technology. In addition, natural dental staining can

be applied to the printed model to mimic the anticipated final outcome. As an alternative way,

the diagnostic wax-up can be used directly to produce provisional restorations (Guth et al.,

2012; Lin et al., 2013).

The digital diagnostic wax-up protocol is assumed to exhibit several advantages (Abduo, 2012;

Abduo and Lyons, 2013). As the process is completely computerized, the need of technician

involvement and high technical skills is minimal. As a result, minimal time will be consumed

and there will be no material wastage. Eventually, multiple treatment proposals can be

evaluated with no additional costs. Most of the 3D softwares allow quantification of tooth

alteration which enhances the precision of the prosthodontic treatment. As all the models are

digitized, the wax-up can be conducted in distant centers and transferred electronically to the

treatment clinician. This feature will also allow distant consultation.

Several algorithmic approaches were proposed to model natural teeth (Rekow et al., 1991;

Mehl et al., 2005b; Ender et al., 2011). The techniques available can be summarized as follows:

occlusal generated path, approximation of defect margins, fitting normalized intact tooth

surfaces, fitting average tooth, scanning manually waxed tooth, and mirror image of the

contralateral tooth.

36

Olthoff et al. discussed the occlusal generated path (Olthoff et al., 2000). This procedure

involves registering the static and dynamic movements of the arch and constructing tooth

anatomy that fits in harmony with the existing occlusion. Subsequently, this movement

determines the boundary of the restoration. A relative accuracy can be obtained for 1 to 2 unit

alterations given that a clear occlusal relationship is present (Olthoff et al., 2000). A similar

concept was applied by different researchers who selected the opposing teeth occlusal surface

through a movement simulation as a reference to design the occlusal form of the altered teeth

(Hikita et al., 2002). However, due to the variation of opposing teeth morphology, this

technique solely will not generate a natural anatomy.

The approximation of the cavity margins is more applicable for the partial and simple forms of

tooth alterations (Masek, 2003; Reiss, 2003; Reich et al., 2004), due to the presence of a more

intact anatomical tooth structure that can be used as a reference (Yuan et al., 2010). However,

as this approach does not account for opposing dentition morphology. Subsequently occlusal

irregularities can be expected which require manual adjustments (Zheng et al., 2011). The

reported accuracy of the approximation was 50 µm and the recommendation is to be used for

simple form of restorations (Yuan et al., 2010).

A normalized intact tooth surface can be obtained from a small number of digitized intact

teeth to replace the missing tooth surface (Paulus et al., 1999) (Figure 1-14). The software has

the ability to automatically adapt the selected tooth surfaces to the patient’s tooth anatomy

(Rekow et al., 1991). Through an automated process, the reconstructed surfaces are adjusted

according to the aesthetic principles and to ensure an adequate fit within the arch and against

the opposing arch. The limitation of this step is the lack of the customization of the tooth

anatomy to each patient.

Figure 1-14 Examples of average virtual teeth that can be used in digital designing of the dentition.

To overcome this problem, several authors described a biogeneric tooth model which

mathematically represents each tooth by reference to a number of specific parameters (Mehl

et al., 2005a; b; Ender et al., 2011). Following the scanning of a large number of defect-free

37

teeth, an “average tooth” was statistically calculated which essentially represent a number of

parameters commonly present in any tooth, such as fossae, grooves and cusp tips. Therefore,

these parameters can be used to compare the average tooth to any natural tooth. Through the

reconstruction process, the differences between each individual tooth and the average tooth

were measured to obtain a list of typical deviations from the average. Tooth reconstruction

was then accomplished by matching the specific features of the average tooth and the list of

the calculated deviations. This method was proven to be efficient in reconstructing the full

anatomic tooth design (Mehl et al., 2005a; b; Ender et al., 2011). Ender et al. found that the

biogeneric method provided a more natural and a faster determination of the surface

morphology compared to a simple selection from a range of teeth (Ender et al., 2011). In

addition, recent studies found that combining the biogeneric methodology in conjunction with

the opposing teeth anatomy resulted in more accurate occlusal contacts (Fasbinder and

Poticny, 2010).

For anterior teeth, where the morphology is much less complex, a mirror image can be

obtained from the contralateral tooth, rotated and translated to fit the locations of the defect

(Probst and Mehl, 2008). However, this technique requires an acceptable contralateral tooth

and assumes a perfect symmetry which is not necessarily a natural appearance.

Similar to the conventional wax-up protocol, the teeth morphology of the opposing arch

greatly affects the restoration anatomy. Since the final restoration anatomy is dependent on

the opposing arch, the scanning accuracy of the opposing arch is mandatory.

Despite the clear merit of digital diagnostic wax-up, the amount of research on this subject is

very minimal and this technology remains in its infancy. In order to justify the routine use of

digital diagnostic wax-up, it should be at least as reliable to the conventional wax-up in terms

of accuracy, aesthetics and practicality.

1.9. Contributions of the thesis

Since this thesis is a thesis by publications, it is composed of series of coherent papers. There

are three types of generated papers: literature review, methods validation and actual

experimentation.

Part of Chapter 1 was published as a clinical literature review that summarizes the factors that

should be considered prior to complex prosthodontic treatment. Chapters 2 to 4 are

systematic reviews that outline the current state of evidence about some variables that can be

38

influenced by the diagnostic wax-up of complex prosthodontic treatment. For example, the

vertical dimension of occlusion (Chapter 2) and the lateral occlusion scheme. The later was

investigated at two levels: lateral occlusion scheme of natural and minimally restored dentition

(Chapter 3) and lateral occlusion scheme of the restored dentition (Chapter 4).

The materials and methods section (Chapter 6) was published in two papers. The first

publication is a conference paper which was a pilot study. It summarized the tests that were

implemented for the rest of the thesis and allowed for technique refinement. An additional

paper was published strictly on the image registration feature of the methodology. To

demonstrate the potential accuracy of the image registration process, it was implemented as a

forensic odontology tool. The same method was used to compare the digital wax-up to the

conventional wax-up. The outcome of this paper confirmed the validity of the digital

comparison of virtual models.

Chapters 7 to 11 list the experiments that compared the digital and the conventional wax-ups.

The comparison was based on the features that can be influenced by the diagnostic wax-up.

Therefore, wax-up precision (Chapter 7), tooth contour (Chapter 8), intercuspal occlusal

contacts (Chapter 9), lateral occlusion scheme (Chapter 10), and dental aesthetics (Chapter

11). As such experiments have not been conducted in earlier literature; the outcome of this

thesis is expected to be an original contribution to the field.

The general discussion section (Chapter 12) critiqued the outcome of the study and discussed

the methodology limitations. Further, the future applications and modifications were

proposed. The manufacturing procedure of digital dentistry was also discussed and published

in a narrative review.

39

Chapter Two

2. Safety of Increasing Vertical Dimension of

Occlusion: A Systematic Review

This chapter was published in the following article:

Abduo J. Safety of increasing vertical dimension of occlusion: a systematic review.

Quintessence International. 2012; 43:369-380. (Appendix C)

40

2.1 Abstract

Objective: The purpose of this study is to review all the literature investigating the implications

of increasing the vertical dimension of occlusion (VDO).

Materials and methods: A comprehensive electronic search was conducted through PubMed

(MEDLINE) with the aid of Boolean operators to combine the following key words: ‘occlusal

vertical dimension,’ ‘increasing vertical dimension,’ ‘bite raising,’ ‘occlusal space,’ ‘resting

vertical dimension,’ ‘rest position,’ ‘altered vertical dimension,’ ‘mandibular posture,’

‘temporomandibular joint’ and ‘masticatory muscles’. The search was limited to peer-reviewed

articles written in English and published up to August 2011. Further, the literature search was

endorsed by manual searching through peer-reviewed journals and reference lists of the

selected articles.

Results: A total of 902 studies were initially retrieved but only 9 of them met the specified

inclusion criteria for the review. From the selected studies, four variables were identified to be

relevant to the topic of VDO increase: magnitude of VDO increase, method of increasing VDO,

occlusion scheme and the adaptation period.

Conclusions: Considering the limitations of this review it could be concluded that whenever

indicated, permanent increase of the VDO is a safe and predictable procedure. Intervention

with fixed restoration is more predictable and results in higher adaptation level. Negative signs

and symptoms were identified, but they were self-limiting. Due to the lack of a well-designed

study, further controlled and randomized studies are needed to confirm the outcome of this

review.

Key words: occlusal vertical dimension, patient adaptation, occlusion, occlusal splint, muscle

relaxation

41

2.2. Introduction

According to the Glossary of Prosthodontic Terms (2005), the vertical dimension is defined as

the distance between two selected anatomic or marked points. For dentate individuals, the

vertical dimension of occlusion (VDO) is largely determined by the occluding dentition (The

Glossary of Prosthodontic Terms, 2005). Subsequently, loss of tooth substance will directly

affect the VDO leading to alteration in facial morphology, function, comfort and aesthetics

(Turner and Missirlian, 1984). Although the loss of VDO is possible clinically, the original VDO

can be maintained by dentoalveolar compensatory mechanism that involves the over-eruption

of worn teeth. This dynamic nature of the stomatognathic system is considered by several

authors as adaptation mechanism of the masticatory system in response to progressive loss in

tooth substance (Murphy, 1959; Berry and Poole, 1976; Richards, 1985; Varrela, 1992;

Crothers and Sandham, 1993). However, for generalized loss of crown height due to tooth

wear, from the clinical perspective, it is advantageous to consider increasing the VDO as it will

provide space for restorative material, enhance the aesthetic tooth display, rectify anterior

teeth relationship, allow for re-establishing physiological occlusion and minimize the need for

biologically invasive clinical procedures such as crown lengthening surgery and elective

endodontic treatment (Rivera-Morales and Mohl, 1991; Johansson and Omar, 1994; Keough,

2003b; Johansson et al., 2008).

Empirically, many authors claimed that the VDO is a constant dimension through individual

life. Subsequently, they expressed concerns and reservations regarding altering the VDO

through dental rehabilitative treatment (Tench, 1938; Schuyler, 1939; Turner and Missirlian,

1984). The expected consequences of increasing the VDO are hyperactivity of masticatory

muscles, elevation of bite force and temporomandibular disorder (TMD). However, to date,

there is no compelling evidence supporting the pathological consequences of altering the VDO.

The purpose of this study is to systematically review all the clinical studies that assessed the

implications of increasing the VDO, and to identify the factors associated with patient

adaptation.

2.3. Materials and Methods

A comprehensive electronic literature search was conducted through MEDLINE (PubMed) with

the aid of Boolean operators. The outcomes of the following key words were combined:

‘occlusal vertical dimension,’ ‘increasing vertical dimension,’ ‘bite raising,’ ‘occlusal space,’

‘resting vertical dimension,’ ‘rest position,’ ‘altered vertical dimension,’ ‘mandibular posture,’

42

‘temporomandibular joint’ and ‘masticatory muscles’. No publication year limit was applied.

The purpose of the search was to obtain all the clinical studies that assessed the effect of

increasing the vertical dimension of occlusion. The search included articles published up to

August 2011 that contained all or part of the key words in their headings. The electronic search

was endorsed with manual searching through the following journals: Journal of Oral

Rehabilitation, Journal of Prosthetic Dentistry, Journal of Prosthodontics, International Journal

of Prosthodontics, International Journal of Periodontics and Restorative Dentistry, Journal of

Dentistry, Quintessence International and Journal of Prosthodontic Research. Further, the

references of each selected article were reviewed for possible inclusion. Initially, the potential

studies were selected on the basis of the relevance of the titles and abstracts. Subsequently,

the full text of the article was reviewed and cross-matched against the predefined selection

criteria (Table 2—1). The inclusion criteria were as follows: human clinical studies on dentate

and asymptomatic individuals; a minimum of 5 participants followed for at least 5 days; the

increase of VDO must be established by clinically relevant methods that might include full or

partial arch coverage. The study was excluded if it was an animal study, a study on edentate or

symptomatic individuals, or a case report.

Table 2—1 Selection criteria used in the review

Inclusion criteria Human clinical study For dentate individuals Including follow-up of at least 5 days Increasing the VDO by full or partial arch coverage On asymptomatic individuals At least 5 participants Involving permanent or temporary increase of VDO Peer reviewed journal article In English Exclusion criteria

Animal study For edentate individuals On symptomatic individuals Case report

43

2.4. Results

2.4.1. Study search

The electronic search had initially retrieved 902 articles. The analysis of titles and abstracts

excluded 838 articles, leaving only 64 articles eligible for inclusion. Following the application of

the inclusion criteria, 26 articles were considered to be suitable for full-text analysis which

then revealed that only 6 articles were acceptable for inclusion (Carlsson et al., 1979; Dahl and

Krogstad, 1985; Burnett and Clifford, 1992; Gross and Ormianer, 1994; Ormianer and Gross,

1998; Ormianer and Palty, 2009). Searching manually and through the references of the

selected articles, 3 additional articles were disclosed (Christensen, 1970; Dahl and Krogstad,

1982; Gough and Setchell, 1999). Two of the studies (Dahl and Krogstad, 1985; Ormianer and

Gross, 1998) are follow-ups of the same participants of previous experiments (Dahl and

Krogstad, 1982; Gross and Ormianer, 1994). As they provide information regarding the long-

term effect of increasing the VDO, they were included. Therefore, a total of 9 articles

(Christensen, 1970; Carlsson et al., 1979; Dahl and Krogstad, 1982; 1985; Burnett and Clifford,

1992; Gough and Setchell, 1999; Gross and Ormianer, 1994; Ormianer and Gross, 1998;

Ormianer and Palty, 2009) were considered acceptable for this systematic review (Tables 2—2,

2—3, 2—4 and 2—5).

2.2.1. Description of studies

The selected studies show significant heterogeneity in relation to study design. Therefore,

qualitative analysis of the studies was applied. One of the possible sources of this variation is

the discrepancy in the inclusion of participants. The participants ranged from healthy

individuals (Christensen, 1970; Carlsson et al., 1979; Burnett and Clifford, 1992), where no

treatment is indicated, to individuals with worn dentition (Dahl and Krogstad, 1982; 1985;

Gross and Ormianer, 1994; Gough and Setchell, 1999; Ormianer and Gross, 1998) or missing

teeth (Ormianer and Palty, 2009), where intervention is indicated. The difference between the

studies is even more prominent in relation to the technique of patient adaptation assessment.

The applied assessment techniques were:

1. Evaluation of subjective patient symptoms such as headache, clenching, grinding, muscle

and joint fatigue, soreness of teeth, cheek biting, and difficulties in chewing and speech

(Christensen, 1970; Carlsson et al., 1979; Dahl and Krogstad, 1982; 1985; Gross and

Ormianer, 1994; Gough and Setchell, 1999).

44

2. Masticatory muscles tenderness to palpation (Christensen, 1970; Carlsson et al., 1979;

Gross and Ormianer, 1994; Ormianer and Gross, 1998).

3. Electromyography (EMG) (Carlsson et al., 1979).

4. Objective Speech and closest speaking space evaluation (Burnett and Clifford, 1992).

5. Interocclusal space measurement (Gross and Ormianer, 1994; Ormianer and Gross, 1998).

6. Radiographic measurement of the vertical dimension with the aid of tantalum implants

inserted in the mandible and maxilla (Dahl and Krogstad, 1982; 1985).

7. Evaluation of mechanical and biological complications associated with restored teeth or

implants (Gough and Setchell, 1999; Ormianer and Palty, 2009).

2.2.2. Studies classification

For the purpose of uniformity, the studies were classified in to the two broad categories,

according to the prosthetic concept for increasing the VDO; fixed (Tables 2—2 and 2—3) or

removable (Tables 2—4 and 2—5) method. From the identified studies, the fixed method is

comprised from provisional restorations, composite build-ups, onlays and definitive fixed

restorations. The removable method involved increasing the VDO by an occlusal splint or

removable partial denture. Alternatively, by the experimental studies, the removable occlusal

splint was temporarily cemented on one of the arches to ensure continuous splint wearing.

For each category, the increase in the VDO was accomplished by either fully or partially

covering the arch. The partial arch coverage was further divided into anterior teeth coverage

or posterior teeth coverage. The anterior teeth coverage was based on treatment concept

where the partial increase of the VDO intended to orthodontically extrude the posterior teeth

and intrude the anterior teeth commonly known as the “Dahl concept” (Dahl and Krogstad,

1982).

In addition, the following variable were reported from each study; magnitude of the VDO

increase, duration of follow-up after increasing the VDO, occlusion scheme, adaptation level

and adaptation period.

Wherever possible, the exact magnitude of the VDO increase was recorded from each study.

The duration of treatment follow-up after increasing the VDO was discretely classified into the

following:

45

1. Experimental duration: up to 1 week

2. Short-term duration: up to 1 month

3. Medium-term duration: from one month to 2 years

4. Long-term duration: over 2 years

The occlusion scheme was classified as follows:

1. Static relationship: the maxilla-mandibular relationship after increasing the VDO.

2. Dynamic relationship: the form of guidance after increasing the VDO. In general, from the

selected studies, the dynamic occlusal relationship can be mutually-protected occlusion,

group function occlusion or bilaterally balanced occlusion.

3. The adaptation level is defined as the proportion of the participants who adapted to the

increase in the VDO. The adaptation period is the time required for the VDO increase-

related symptoms to resolve.

2.2.3. Studies summary

In general, the VDO increase range was from 2 mm to 5 mm. The studies clearly stated that the

static occlusal relationship after increasing the VDO was according to centric relation (CR). In

relation to the dynamic occlusal relationship, three studies established bilaterally balanced

occlusion (Christensen, 1970; Carlsson et al., 1979; Burnett and Clifford, 1992), four studies

established mutually-protected occlusion (Dahl and Krogstad, 1982; 1985; Gross and

Ormianer, 1994; Ormianer and Gross, 1998), and one study established unilateral group

function on premolars and molars (Ormianer and Palty, 2009). One study did not clarify the

dynamic occlusal relationship (Gough and Setchell, 1999). Regarding the duration of the

studies, three studies were of experimental nature and followed the participants for up to 1

week (Christensen, 1970; Carlsson et al., 1979; Burnett and Clifford, 1992). One study was a

short-term study that followed the participants for up to 1 month (Gross and Ormianer, 1994).

Two studies were classified as medium-term studies and followed the participants in average

for less than 2 years (Dahl and Krogstad, 1982; Gough and Setchell, 1999). The other studies

were long-term studies and followed the participants for more than 2 years (Dahl and

Krogstad, 1985; Ormianer and Gross, 1998; Ormianer and Palty, 2009).

46

Most of the studies agreed that patient adaptation can be obtained after increasing the VDO.

Only one study reported no adaptation to VDO increase (Christensen, 1970). For the other

studies, the adaptation level was 86-100% for the removable method and 100% for the fixed

method. The adaptation period ranged from 2 days to 3 months.

47

Table 2—2 Summary of studies increasing the VDO by removable method and partial arch coverage

Study (year) Study details Method Main findings Design N Duration VDO

increase (mm)

Occlusion Assessment method

Adaptation rate (%)

Adaptation period

Further comments

Static Dynamic

Posterior teeth coverage Christensen (1970) P 20 7 days 4 CR BBO -Subjective symptoms

-Muscles tenderness 0 No adaptation -Development of TMD signs and symptoms

-Development of clenching, grinding, soreness of teeth, cheek biting, speech difficulties and chewing limitations -Muscle and joint fatigue

Carlsson et al. (1979)

P 6 7 days 4 CR BBO -Subjective symptoms -Muscles tenderness -Radiographic evaluation -EMG

86 1-2 days -Development of clenching, speech difficulties and discomfort -No implication on muscles tenderness -Reduction of EMG activities -New interocclusal distance was established -One participant could not adapt to the intervention

Anterior teeth coverage Dahl and Krogstad (1982)

P 20 6-14 months 1.8-4.7 CR MPO -Radiographic evaluation of inserted tantalum implants -Subjective symptoms

100 2 weeks -Development of speech difficulties and chewing limitations with lisping being the most prominent -No symptoms of dysfunction or pain -Teeth over-eruption was more prominent than intrusion especially for younger participants -The average increase in VDO after the completion of the treatment was 1.9 mm

Gough and Setchell (1999)

R 11 5.9 months and up to 4.1 years

Variable CR NA -Subjective symptoms -Patient compliance -Biological complications

91% NA -One patient could not wear the appliance -Minimal signs of functional discomfort -Minimal pulpal and periodontal symptoms and vitality loss

NA, not available; P, prospective; R, retrospective; CR, centric relation; BBO, bilaterally balanced occlusion; MPO, mutually-protected occlusion.

48

Table 2—3 Summary of studies increasing the VDO by removable method and complete arch coverage

Study (year)

Study details Method Main findings Design N Duration VDO

increase (mm)

Occlusion Assessment method Adaptation rate (%)

Adaptation period

Further comments

Static Dynamic Burnett and Clifford (1992)

P 6 5 days 4 CR BBO -CSS NA NA

-Up to 1 mm reduction in CSS -Significant reduction of CSS after increasing VDO

NA, not available; P, prospective; CR, centric relation; BBO, bilaterally balanced occlusion; CSS, closest speaking space.

Table 2—4 Summary of studies increasing the VDO by fixed method and partial arch coverage

Study (year)


increase (mm)

Occlusion Assessment method Adaptation rate (%)

Adaptation period

Further comments

Static Dynamic

Anterior teeth coverage Dahl and Krogstad (1985)

P 20 67 months and up to 5.5 years

1.8-4.7 CR MPO -Radiographic evaluation of inserted tantalum implants

100 NA -Variable long-term individual response to adaptation -Reduction of the increased VDO through the treatment period (1.73 mm after 6 months and 1.52 mm after 67 months)

Gough and Setchell (1999)

R 39 5.9 months and up to 4.1 years

Variable CR NA -Subjective symptoms -Patient compliance -Biological complications

100 NA -Greater patient compliance with fixed appliance than removable appliance -Minimal signs of function discomfort -Minimal pulpal and periodontal symptoms and vitality loss

NA, not available; P, prospective; R, retrospective; CR, centric relation; MPO, mutually-protected occlusion.

49

Table 2—5 Summary of studies increasing the VDO by fixed method and complete arch coverage

Study (year)


increase (mm)

Occlusion Assessment method

Adaptation rate (%)

Adaptation period

Further comments

Static Dynamic Gross and Ormianer (1994)

P 8 1 month 3.5-4.5 CR MPO -Subjective symptoms -Muscles tenderness -Interocclusal space measurements

100 2 weeks -Initial development of muscle tenderness, clenching and speech difficulties -Establishment of new interocclusal space after 1 month

Ormianer and gross (1998)

P: intervention

8 2 years 3.5-4.5 CR MPO -Interocclusal space measurements -EMG -Muscles tenderness

100 NA -No effect on EMG -Consistent interocclusal space after 1 month, 1 year and 2 years

P: control group 8 MI NA -No significant difference for the interocclusal space or EMG through the study

Ormianer and Palty (2009)

R: tooth-supported FDP in both arches

10 3 years up to 11 years

3-5 CR GFO -Subjective symptoms -Radiographic assessment of alveolar bone around teeth and implants -Complications assessment

100 2-3 months -Adaptation to new VDO -Average bone loss was 2.3mm -Few cases of porcelain fracture

R: tooth-supported FDP in one arch and implant-supported FDP in the other arch

10 -Adaptation to new VDO -More bone loss around teeth than implants -Average bone loss was 2 mm -Two patients reported grinding that resolved within 2-3 months

R: implant-supported FDP in both arches

10 -Adaptation to new VDO -Average bone loss was 2 mm -No screw loosening or fracture -Few cases of porcelain fracture -Four patients reported grinding that resolved with occlusal device after 3 months

NA, not available; P, prospective; R, retrospective; FDP, fixed dental prosthesis; CR, centric relation; MPO, mutually-protected occlusion; GFO, group function

occlusion.

50

2.3. Discussion

Although the included articles provide information regarding patient adaptation to increased

VDO, they suffer from lack of randomization and control. In addition, the therapy was applied

to a limited number of participants and there is a lack of agreeable subjective and objective

signs and symptoms assessments. Therefore, the results should be interpreted with caution.

In general, the outcomes of the studies reflect the adaptation of the masticatory system after

increasing VDO in a time-dependent fashion. The emphasis of the discussion is placed on

potential factors influencing the adaptation to the increase in the VDO; namely the magnitude

of VDO increase, adaptation period, method of increasing the VDO and occlusion scheme.

2.3.1. Magnitude of VDO increase

Several authors mentioned the merit of increasing the VDO as a method to facilitate the

restorative treatment and enhance dental aesthetics (Rivera-Morales and Mohl, 1991; Keough,

2003b). These advantages are even more obvious for a dentition suffering from prominent

tooth wear (Figure 2-1) (Johansson and Omar, 1994; Johansson et al., 2008). However, to date,

there are no clear objective guidelines that determine the ideal increase of the VDO which can

be physiologically accepted by the patient (Turner and Missirlian, 1984; Rivera-Morales and

Mohl, 1991). A commonly measured clinical variable is the freeway space (FWS) that is the

difference in vertical dimension between when the mandible is at rest and when the mandible

is in occlusion (The Glossary of Prosthodontic Terms, 2005). The rationale behind measuring

the FWS is to determine how the VDO can be altered. An FWS of 2 mm has been suggested as

the physiological space, and therefore an FWS of more than 2 mm indicates that the VDO can

be safely increased (Turner and Missirlian, 1984).

A B C

Figure 2-1 (A) A dentition that suffers from tooth wear. (B) As a result, the teeth are short and in edge-to-edge relationship. (C) The definitive prostheses involved 3 mm increase of the VDO. Increasing the VDO allowed for significant aesthetic improvement, correction of anterior tooth relationship, establishment of a natural overjet and overbite, and lengthening the anterior teeth.

51

Interestingly, several of the included studies in this systematic review reported patients’

adaptation even after increasing the VDO beyond the FWS (Carlsson et al., 1979; Gross and

Ormianer, 1994; Ormianer and Gross, 1998; Ormianer and Palty, 2009). Therefore, this

systematic review supports the observation of many authors that concluded the physiological

posture of the mandible occurs at a zone commonly named as the “comfort zone” rather than

a specific constant location (Tryde et al., 1977; Rivera-Morales and Mohl, 1991; Abekura et al.,

1996).

Despite the selected studies revealed that patients can adapt to an increase of VDO of up to 5

mm, it is impossible to determine the upper limit since there is a lack of evidence in relation to

a greater increase in the VDO. Nevertheless, from the clinical perspective, it is difficult to

recommend a greater increase in the VDO due to its significant impact on the horizontal

relationship of the teeth (Keough, 2003b; Johansson et al., 2008). As a consequence, greater

clinical expertise is necessary to manage these cases. The emerging complexities are mainly

related to loss of anterior teeth guidance, excessive increase in the overjet and loss of lip

competence (Keough, 2003b). Such complexities are, however, advantageous in the case of

severely worn dentition where a class III incisal relationship or collapsed lower third of the face

might be evident (Figure 2-2) (Turner and Missirlian, 1984; Johansson et al., 2008).

A B C

Figure 2-2 The impact of tooth wear on the anterior tooth relationship. (A) Natural relationship of anterior teeth with intact crowns. (B) Tooth wear resulting in the development of a class III (edge-to-edge) incisal relationship. (C) Increasing the VDO allowed for restoring an adequate anterior tooth relationship.

Therefore, until clear guidelines are established in relation to the ideal magnitude of increasing

the VDO, empirical clinical procedures should be employed and is largely variable between

individual patients. It is also wise to consider increasing the VDO to the minimal level required

to address patient functional and aesthetic needs.

52

2.3.2. Adaptation period

In general, the short-, medium- and long-term studies reported resolution of signs and

symptoms of maladaptation throughout the period of the studies. However, the experimental

studies disclosed less level of adaptation (Christensen, 1970; Carlsson et al., 1979; Burnett and

Clifford, 1992). This is anticipated from the short follow-up period (5-7 days) and the nature of

studies, where the occlusal splint is temporary cemented on the remaining teeth. Nonetheless,

the outcome of the experimental studies indicated that the immediate acceptance to

increasing the VDO can be related to masticatory muscles lengthening and relaxation. This

statement is supported by Carlsson et al. who found reduction of EMG activities after

increasing the VDO (Carlsson et al., 1979). After a period of 1 month, the short-term study

(Gross and Ormianer, 1994) obtained high adaptation level after increasing the VDO. The

clinical significance of this observation is that permanent restoration can be predictably

delivered after a period of 1 month. Likewise, the medium-term studies further proved the

stability of increased VDO and the dentoalveolar maturation (Dahl and Krogstad, 1982; Gough

and Setchell, 1999). In addition, the long-term study that partially covered the anterior arch

segment reported that occlusal stability was achieved due to orthodontic movement

manifested as intrusion of the occluding segments of the arch and over-eruption of the non-

occluding segments of the arch (Dahl and Krogstad, 1985). Although complete relapse of the

altered VDO did not occur, an average of 0.4 mm reduction of the increased VDO was

observed (Dahl and Krogstad, 1985). On the contrary, the long-term study that covered the

whole arch found that the relapse of VDO to its original value was minimal (Ormianer and

Gross, 1998). This indicated that muscle relaxation and increase in muscle length were the

primary adaptation mechanisms rather than alterations in dentoalveolar dimensions. This is

even endorsed by the finding of Ormianer and Palty that reported patient adaptation even

when the implant support was utilized (Ormianer and Palty, 2009). Therefore, it could be

speculated that the VDO increase after partial coverage of the arch will lead to dentoalveolar

alterations, while the complete coverage will immediately establish the occlusion with minimal

alterations in the dentoalveolar complex. The clinical significance of this finding is that

complete coverage of the arch will manage the patient in a more predictable and time

controlled fashion.

Since the majority of the studies reported resolution of signs and symptoms within 1-2 weeks,

it is wise to consider a probationary period of a few weeks before the placement of complex

definitive restorations. Throughout this period, the patient can be thoroughly reviewed and

the restoration adjusted accordingly.

53

2.3.3. Method of increasing VDO

Since the studies (Christensen, 1970; Carlsson et al., 1979; Dahl and Krogstad, 1982; Gough

and Setchell, 1999) that increased the VDO by removable method reported development of

signs and symptoms, it could be speculated that the removable method, suffered from a

greater level of complications and limited patient compliance. After covering the mandibular

molars only, Christensen reported development of multiple complications that led him to the

conclusion that increasing VDO can lead to joint and muscle derangement (Dahl and Krogstad,

1982). However, due confining the occlusal coverage to mandibular molars only, the

intervention protocol in this study seems more similar to creating an occlusal interferences

rather than increasing the VDO. This is in accordance with other investigations that found

experimental introduction of occlusal interferences caused short-term clinical signs and

symptoms (Seligman and Pullinger, 1991; Christensen and Rassouli, 1995a; b). Carlsson et al.

anticipated that the subjective signs and symptoms after increasing the VDO are associated

with the discomfort from wearing the splint rather than a direct effect of the VDO increase

(Carlsson et al., 1979). Likewise, the phonetic difficulties reported by Burnett and Clifford could

be due to covering the incisal surfaces of mandibular anterior teeth, which is significantly

associated with phonetics (Burnett and Clifford, 1992). Although the removable splint provided

by Dahl and Krogstad achieved high level of acceptance, lisping was the most commonly

reported complaint, which can be the result of covering the palatal surfaces of the maxillary

anterior teeth (Dahl and Krogstad, 1982; 1985). However, the complaints associated with their

metal splint were limited in comparison with the previously mentioned studies that applied

acrylic splints (Christensen, 1970; Carlsson et al., 1979; Burnett and Clifford, 1992). Due to the

better fit and smoother finish, the metal splint contributes to greater comfort, adaptation and

less interference with patient function.

After comparing fixed and removable methods for increasing the VDO, Gough and Setchell

found that the fixed method was more predictable and comfortable to the patient (Gough and

Setchell, 1999). Consequently, for the rehabilitation procedure where the VDO increase is

indicated, it is wise to reconsider the benefit of wearing the removable splint, since it does not

provide a predictable indication for patient acceptance or adaptation. In general, the

significant splint limitations are patient discomfort, interference with speech and the lack of

aesthetic assessment. Nevertheless, the splint should still be considered when the patient

presents with TMD signs and symptoms before embarking into definitive rehabilitation (Dao

and Lavigne, 1998; Al-Ani et al., 2005).

In relation to the fixed method, all the studies reported consistent and predictable patient

adaptation. Where the restorations are tooth-supported, the most commonly reported

54

symptoms are the subjective grinding and clenching, which has the tendency to resolve within

1-2 weeks. For implant-supported prostheses, extended adaptation period (2-3 months) was

reported (Ormianer and Palty, 2009). A possible explanation of this finding is that patients

were initially edentulous and suffered from considerable reduction in the occlusal force, even

with conventional complete dentures (Zarb, 1983). However, several authors established that

after the replacement of the conventional complete dentures by implant-supported

prostheses, the occlusal force increased dramatically (Lindquist and Carlsson, 1985; Carr and

Laney, 1987). Subsequently, these patients might experience immediate improvement of the

occlusal force that can manifest clinically as increased grinding and clenching. Another

explanation of increased grinding and clenching is the lack of sensory input from the

periodontal ligament that hinders rapid patient adaptation after increasing the VDO. Similar

findings were observed by few studies (Gartner et al., 2000; Weiner et al., 2004; Hsieh et al.,

2010), however, the clinical significance of this statement is doubtful. Therefore, when

implant-supported prosthesis is used to increase the VDO, it adds further variables that can

influence patient adaptation. In the same study, the authors (Ormianer and Palty, 2009)

reported more mechanical failure for implant-supported prostheses in comparison to tooth-

supported prostheses which supports the implication of the lack of sensory input from the

periodontal ligament.

After comparing the fixed and removable methods of increasing the VDO, it seems the fixed

method is more predictable. The main advantages of the fixed method are the

reestablishment of original tooth morphology and the fixed nature of the restoration. As a

result, minimal interference will be introduced to patient comfort and function. Subsequently,

it is more feasible to assess patient function, aesthetics and phonetics.

2.3.4. Occlusion scheme

At the increased VDO, the included studies achieved a static occlusal relationship in the CR

position which is in accordance with all the studies pertaining to occlusion reestablishment

(Becker and Kaiser, 1993; Turp et al., 2008; Carlsson, 2009). CR establishment has been

advocated since it is a reproducible position and indicated for cases that require extensive

occlusal rehabilitation as might occur after increasing the VDO (Becker and Kaiser, 1993;

Keshvad and Winstanley, 2001). Therefore, whenever increasing the VDO, it is wise to consider

CR reestablishment, even if there is a lack of compelling evidence.

In relation to the dynamic occlusion relationship, mutually-protected occlusion and group

function occlusion were considered as acceptable elements of healthy occlusion (Becker and

55

Kaiser, 1993; Turp et al., 2008). In general, for the mutually-protected occlusion and group

function occlusion, the studies revealed the possibility of safe application of such schemes.

Despite the limitation of sound evidences, bilaterally balanced occlusion was discouraged due

to the possible risk of inducing parafunctional activities. This was supported by EMG studies

that revealed increased muscle activities with the introduction of balanced contacts

(MacDonald and Hannam, 1984; Wood and Tobias, 1984). The included studies in this review

that applied the bilaterally balanced occlusion reported greater incidence subjective symptoms

(Christensen, 1970; Carlsson et al., 1979). However, with the lack of a controlled group, it is

difficult to state that the symptoms were associated with the occlusal scheme.

2.4. Conclusions

Within the limitations of this systematic review, the following can be concluded:

1. Whenever indicated, permanent increase of VDO of up to 5 mm is a safe and predictable

procedure without detrimental consequences. According to the included studies, the

associated signs and symptoms were self-limiting with tendency to resolve within two

weeks.

2. Increasing VDO with a form of fixed restorations is preferable since it enhances patient

function, acceptance, and adaptation and allows for aesthetic evaluation. A removable

splint provoked more signs and symptoms that appear to be associated with the appliance

rather than the actual VDO increase. The signs and symptoms are more prominent with

acrylic splints than metal splints.

3. Because of the limited number of available studies and the significant heterogeneity of the

experimental design, well-controlled and robustly designed clinical studies are needed to

validate the outcome of this review.

56

Chapter Three

3. Lateral Occlusion Schemes in Natural and

Minimally Restored Permanent Dentition: A

Systematic Review


Abduo J, Tennant M, McGeachie J. Lateral occlusion schemes in natural and minimally restored

permanent dentition: a systematic review. Journal of Oral Rehabilitation. 2013; 40:788-802.

(Appendix D)

57

3.1. Abstract

Objective: Clinicians commonly encounter the dilemma of which lateral occlusion scheme is

most suitable for a specific patient. The aim of this review is to evaluate the prevalence of the

lateral occlusion schemes that exist naturally.

Materials and methods: An electronic search was completed through PubMed (MEDLINE),

Google Scholar and Cochrane Library. The search was confined to peer-reviewed studies

published in English, up to April 2013. The literature search was supplemented by manual

searching through the bibliography lists of the selected studies.

Results: The initial search retrieved a total of 575 studies. After applying the selection criteria,

only 12 studies were suitable for inclusion. The Critical Appraisal Skills Programme (CASP) tools

were utilized to appraise the quality of the studies. The prevalence of canine-guided, group

function and balanced occlusions was reported. Overall, there was a clear variability between

the studies. The prevalence of the lateral occlusion scheme appears to be influenced by the

following factors: (1) the magnitude of excursion, (2) an individual’s age and (3) the static

occlusal relationship.

Conclusions: During complete excursion, the canine-guided occlusion tends to be more

frequently observed. After partial excursion, the most prevalent lateral occlusion scheme was

group function occlusion. With aging, the prevalence of canine-guided occlusion tends to be

reduced and the prevalence of group function occlusion is increased. Dentition that is closer to

Class II occlusion exhibits mainly canine-guided occlusion, while for Class III occlusion, group

function occlusion is more prevalent. The studies revealed no relationship between the lateral

occlusion scheme and TMD development.

Key words: lateral occlusion, canine-guided occlusion, group function occlusion, balanced

occlusion, Angle’s classification, excursion

58

3.2. Introduction

For patients requiring comprehensive prosthodontic treatment, many rehabilitation

philosophies have been proposed. Each philosophy aims to produce functional, comfortable

and aesthetic occlusion. Among the conjectured principles for the prosthodontic treatment is

the selection of the lateral occlusion scheme that can be implemented in prosthesis design.

Through prosthodontic treatment, the lateral occlusion scheme can be controlled by altering

teeth morphologies, alignments and orientations. For dentate patients, the available lateral

occlusion schemes are canine-guided occlusion, group function occlusion and balanced

occlusion (Thornton, 1990). The canine-guided occlusion is defined as a mutually-protected

articulation, in which the vertical and horizontal overlap of the canine teeth disengages the

posterior teeth in the lateral movement of the mandible (The Glossary of Prosthodontic Terms,

2005). On the other hand, group function occlusion is distinguished by the existence of

multiple contacts between the maxillary and mandibular teeth in lateral movement on the

working side (The Glossary of Prosthodontic Terms, 2005). The simultaneous, anterior and

posterior occlusal contact of teeth in centric and eccentric positions is called bilaterally

balanced occlusion (The Glossary of Prosthodontic Terms, 2005). Anecdotally, several claims

have been made supporting each lateral occlusion philosophy. For example, canine-guided

occlusion will protect posterior teeth laterally while the anterior teeth are protected in the

centric position; hence the term “mutually-protected occlusion”. The canines were considered

ideal guidance teeth because of their strategic location, anatomy and proprioceptive

properties (Rinchuse et al., 2007). Conversely, group function occlusion has been claimed to

facilitate a wide distribution of occlusal forces over many teeth instead of single tooth;

therefore, a more comfortable, efficient and functional occlusion can be established

(Thornton, 1990). Nevertheless, there is a lack of compelling evidence indicating the

superiority of any philosophy (Becker and Kaiser, 1993; Turp et al., 2008).

Among the limitations of proposing rigid criteria for the lateral occlusion scheme is the

possible discrepancy between the classical definitions and what is considered to be a

physiological occlusion. According to the Glossary of Prosthodontic Terms (2005), any

occlusion that is in harmony with functions of the masticatory system is deemed physiological.

This could potentially mean that as long as the lateral occlusion scheme is not contributing to

mechanical, biological or aesthetic problems, it can be deemed as physiological occlusion. It is

expected that the level of variation in lateral occlusion schemes can be obtained by observing

the occlusion features of natural dentition.

59

Therefore, it is the aim of this systematic review to evaluate the prevalence of lateral occlusion

schemes that exist in natural physiological dentition, and the possible influencing factors.

Consequently, this systematic review attempts to formulate guidelines for clinicians to

consider when they are rehabilitating the dentition. The hypotheses are: (1) the prevalence of

lateral occlusion schemes is variable, (2) there are influencing factors that can affect the

prevalence of lateral occlusion schemes, and (3) there is no implication of the lateral occlusion

scheme on abnormal physiology.


3.3.1. Search strategy and selection criteria

A comprehensive literature search was completed in April 2013. The search was conducted by

the first author through PubMed (MEDLINE), Cochrane Central Register of Controlled Trials

and Google Scholar. The search strategy through PubMed database was performed through

the Boolean operator and involved the following combinations of key words: (“lateral

occlusion” OR “dynamic occlusion” OR “excursive occlusion”) AND (“canine guided” OR “group

function” OR “balanced”) AND (“dental”) NOT (“implant”). All the articles pertaining to dental

occlusion were retrieved in Cochrane Database. The Google Scholar search engine was used to

obtain additional articles by combining the following key words: ‘dental occlusion,’ ‘lateral,’

‘dynamic,’ ‘excursive,’ ‘guidance,’ ‘canine,’ ‘group function,’ and ‘balanced.’ The electronic

search aimed to obtain all the clinical studies that evaluated the prevalence of the lateral

occlusion scheme in natural permanent dentition. The inclusion criteria were peer-reviewed

journal article or abstract proceeding, human clinical study, study evaluating natural or

minimally restored permanent dentition, study involving adult participants, study’s

participants are representative of today’s population and English language study.

Studies selection was performed in three consecutive stages; (1) articles selection according to

title relevance, (2) screening abstracts and filtering irrelevant articles, and (3) full text analysis

of the remaining articles and cross-matching against predefined inclusion criteria. The

literature search was supplemented with manual searching of the bibliographies of all the

included studies to retrieve further potentially related studies.

60

3.3.2. Literature assessment

The methodological quality of the selected studies was appraised according to the Critical

Appraisal Skills Programme (CASP) (Critical Appraisal Skills Programme (CASP)). The following

CASP tools for cross-sectional studies were implemented:

Is the sample representative of its target population?

Does the study achieve a good response rate (≥ 80%)?

Were valid and reliable outcome tools were implemented?

For every study, each CASP tool was rated as present, not present or unclear. A score mark was

allocated if the answer to the tool is present. Subsequently, on the basis of this assessment,

the quality of the studies can be rated as high (score = 3), moderate (score = 2) or low (score =

1).

3.3.3. Study classification

Since the definition of each lateral occlusion scheme varies among the studies, standardized

criteria were applied according to the Glossary of Prosthodontic Terms (2005). The applied

definitions were as follows:

Canine-guided occlusion: The vertical and horizontal overlap of the canine teeth

disengages the posterior teeth in excursive movements of the mandible.

Group function occlusion: Multiple contact relations between the maxillary and

mandibular teeth in lateral movements on the working side. As there is no specification

regarding the amounts of the present contact, two or more simultaneous contacts on the

working side were considered as group function.

Balanced occlusion: Bilateral, simultaneous, anterior, and posterior occlusal contact of

teeth in centric and eccentric positions. As this definition is almost non-existing in natural

dentition, balancing contacts, that do not interfere with smooth mandibular movements,

were considered as indicators of balanced occlusion.

Non-specified occlusion: If there is no clarification of the existing lateral occlusion scheme.

The prevalence percentage of each lateral occlusion scheme was recorded from each included

study. However, since it is expected that many patients presented with mixed lateral occlusion

61

schemes (e.g. canine-guided occlusion on one side and group function occlusion on the other

side), the prevalence of the lateral occlusion scheme in each side was considered rather than

the prevalence in the whole mouth.

Whenever possible, from each study, the three potentially influencing parameters were

recorded: the magnitude of lateral mandibular movement, the individual’s age, and the static

occlusal relationship according to Angle’s classification. The rationales behind the selection of

these parameters are as follows:

Magnitude of lateral excursion: Due to the complexity of occlusal morphology, the location

and magnitude of tooth contacts can be affected with the level of excursion (Ogawa et al.,

1998). The degree of excursion was classified in to two categories: partial (0.5-1.5 mm),

and full (2-3 mm or cusp-to-cusp position).

Age: As the patient age increases there is greater possibility of tooth wear. Subsequently,

tooth wear was found to influence the location and magnitude of tooth contacts at the

static and dynamic positions (Beyron, 1954; Panek et al., 2008).

Static occlusal relationship: Since several studies considered the anterior teeth and arch

size to contribute to the lateral occlusion schemes (Schwartz, 1986; Jensen, 1990b),

whenever possible, Angle’s classification was documented.

3.3.4. Qualitative analysis

Only the studies that illustrated the age of the participants and clearly outlined the influencing

parameters were included in the qualitative analysis. The aim of this step is to delineate the

presence or absence of a linear relationship between the prevalence of lateral occlusion

schemes and the different parameters: magnitude of excursion, age and static occlusal

relationship. Scatter diagrams were employed accordingly. Due to the lack of age average in

most of the studies, the age range was represented in lines within the scatter diagrams.

3.4. Results

3.4.1. Literature search

A total of 575 articles were obtained after the initial electronic search. From this literature

pool, 452 articles were excluded after the analysis of the titles’ relevance. Screening the

abstracts excluded an additional 72 articles, leaving 51 articles suitable for full text analysis.

62

Following cross-matching against the inclusion criteria, 9 articles were considered for inclusion.

Manual searching through bibliography lists of the selected articles disclosed an additional 3

articles suitable for inclusion. Therefore, a total of 12 articles were deemed suitable for

inclusion for this review. Since some studies evaluated several parameters, the relevant

information about the prevalence of the lateral occlusion scheme was extracted.


According to CASP tools, the studies’ appraisal scores ranged from high to moderate. Nine

studies evaluated the prevalence of lateral occlusion schemes for individuals with no

significant restorative work or missing teeth, other than third molars (Tables 3—1 and 3—2)

(Weinberg, 1964; Scaife and Holt, 1969; Ingervall, 1972; Guevara and Ismail, 1976; Yaffe and

Ehrlich, 1987; Ogawa et al., 1998; Al-Hiyasat and Abu-Alhaija, 2004; Panek et al., 2008; Al-

Nimri et al., 2010). One study included participants with few missing teeth (less than 2 teeth),

history of restorative work and orthodontic treatment (Ingervall et al., 1991). Two studies

measured the prevalence of lateral occlusion schemes for temporomandibular disorder

patients (TMD) and TMD-free patients (Donegan et al., 1996; Kahn et al., 1999). Two studies

evaluated the prevalence of TMD in the evaluated participants (Weinberg, 1964; Ingervall et

al., 1991).

The selected studies evaluated the lateral occlusion scheme by eccentric movement in the

range of 0.5 mm to 3 mm from the maximal intercuspation position. Eight studies (Weinberg,

1964; Scaife and Holt, 1969; Ingervall, 1972; Guevara and Ismail, 1976; Donegan et al., 1996;

Kahn et al., 1999; Al-Hiyasat and Abu-Alhaija, 2004; Panek et al., 2008) applied complete

excursion and four studies applied complete and partial excursion (Yaffe and Ehrlich, 1987;

Ingervall et al., 1991; Ogawa et al., 1998; Al-Nimri et al., 2010).

Five studies specified the static occlusal relationship (Scaife and Holt, 1969; Guevara and

Ismail, 1976; Yaffe and Ehrlich, 1987; Al-Hiyasat and Abu-Alhaija, 2004; Al-Nimri et al., 2010).

Three of them included all Angle’s classifications (Scaife and Holt, 1969; Al-Hiyasat and Abu-

Alhaija, 2004; Al-Nimri et al., 2010), while the other two were primarily for Class I occlusion

(Guevara and Ismail, 1976; Yaffe and Ehrlich, 1987).

Nine studies stated the participants’ age (Scaife and Holt, 1969; Ingervall, 1972; Guevara and

Ismail, 1976; Yaffe and Ehrlich, 1987; Ingervall et al., 1991; Ogawa et al., 1998; Al-Hiyasat and

Abu-Alhaija, 2004; Panek et al., 2008; Al-Nimri et al., 2010). In general, the age of the

63

participants of all the studies ranged from 14 to 63 years. Only one study clearly outlined four

age categories (Panek et al., 2008).

The evaluation approaches applied by the studies to assess the lateral occlusion schemes

were:

Clinical evaluation of patient occlusion with the aid of a disclosing medium (e.g.

articulating paper, articulating wax or alginate indicator material).

Clinical evaluation of patient occlusion with the aid of an articulating medium (e.g. dental

floss, maylar strip or shimstock).

Visualization of tooth contacts (e.g. visual observation or cinematic motion picture).

Evaluation of articulated models on adjustable articulators.

3.4.3. Studies outcome

Overall, there is a clear heterogeneity between the included studies due to the differences in

methodology and the selection criteria of the participants. This heterogeneity reflects on the

significant variation in the prevalence of each lateral occlusion scheme. The prevalence of

canine-guided, group function and balanced occlusions were 6-74%, 26-74% and 3-16%

respectively. The outcome of the studies will be listed according to the following headings:

magnitude of excursion (Table 3—1), age effect, static occlusal relationship (Table 3—2 ), and

TMD relationship.

Magnitude of excursion. The studies confirmed the variation in the lateral occlusion pattern

with different degrees of excursion. With partial excursion, there is a greater tendency for

more tooth contact on the working and non-working sides. While complete excursion caused a

reduction of tooth contacts except on the canines (Yaffe and Ehrlich, 1987; Ingervall et al.,

1991; Ogawa et al., 1998; Al-Nimri et al., 2010). After complete excursion, the prevalence of

canine-guided occlusion was in the range of 17-74%, and the prevalence of group function

occlusion was in the range of 26-68%. After partial excursion, the canine guidance prevalence

ranged from 6% to 26% and the group function occlusion ranged from 45-74%. Similar

prevalence of balanced occlusion was observed following complete (3-16%) and partial

excursions (3-14%).

64

All the studies that compared different degrees of excursion found that the canine-guided

occlusion prevalence is lower (by 20-70%) after partial excursion than the prevalence after

complete excursion (Yaffe and Ehrlich, 1987; Ingervall et al., 1991; Ogawa et al., 1998; Al-Nimri

et al., 2010). Likewise, the opposite was observed for group function occlusion (the prevalence

after complete excursion was 40-80% of the prevalence after partial excursion). This

observation was attributed to the greater number of canine contacts on the working side after

complete excursion; while the prevalence of premolar and molar contacts, in addition to

canine contacts, are greater on the working side after partial excursion (Ogawa et al., 1998).

Overall, there is a discrepancy between reporting the prevalence of balanced occlusion and

non-working side interferences. The studies that evaluated specifically the prevalence of

balanced occlusion found that there is no major difference in prevalence after partial or

complete excursion. However, the studies that measured the prevalence of non-working side

interferences found the prevalence to be high. Ingervall found that 20% of the participants had

unilateral non-working side contacts and 64% had bilateral non-working side contacts after

complete excursion (Ingervall, 1972). After partial excursion, Ingervall et al., in another study,

found that half of their participants exhibited non-working side contacts, which was greater

than the prevalence with complete excursion (Ingervall et al., 1991). Ogawa et al. found the

frequency of non-working side interferences decreased after complete excursion (about 50%

less) (Ogawa et al., 1998). The most frequent non-working side contacts existed more

commonly on the 2nd molar, followed by the 1st molar (Ingervall et al., 1991; Ogawa et al.,

1998).

Age effect. A pattern of occlusion schemes alteration was detected as an individual’s age

increases. With aging, there is a decline of canine-guided occlusion prevalence (Figure 3-1).

This has been clearly demonstrated by Panek et al. after evaluating four age categories (Panek

et al., 2008). In general, canine-guided occlusion tends to be a dominant occlusion scheme (60-

70%) for adolescents and young adults. Two studies slightly deviated from that tendency and

reported a lower prevalence of canine-guided occlusion for young adults (about 33.5%)

(Ingervall, 1972; Ingervall et al., 1991). Nevertheless, these studies revealed that canines were

still the most commonly involved teeth in lateral occlusion. For adolescents, the prevalence of

canine-guided occlusion was 61.5%. 32-73.4% of young adults had canine-guided occlusion.

For middle aged individuals, the canine-guided occlusion prevalence was 38.4%. Elders had the

lowest canine guidance prevalence (17.9%).

65

A

B

Figure 3-1 The relationship between the prevalence of each lateral occlusion scheme and age after complete excursion (A) and partial excursion (B). The lines represent the age range of each study.

On the contrary, the prevalence of group function occlusion followed an opposite pattern.

Overall, there is an increase of group function occlusion prevalence with aging. The

adolescents had the least prevalence of group function occlusion (15.5%) while the elders had

66

the greatest (67.9%). The group function occlusion for young adults was in the range of 25.5%

to 63.5%. Middle aged individuals had a group function occlusion prevalence of 48.9%.

The prevalence of balanced occlusion did not appear to be affected by age, and it was in the

range of 3-13.6% for all age categories.

Static occlusal relationship. In general, there is a relationship between the static and dynamic

occlusions after complete excursion (Scaife and Holt, 1969; Al-Hiyasat and Abu-Alhaija, 2004;

Al-Nimri et al., 2010). However, after partial excursion, no relationship was established

between static and dynamic occlusions (Al-Nimri et al., 2010).

For Class I occlusion (Figure 3-2), the canine guidance prevalence was 48.5-67.6% following

complete excursion. This was more than the prevalence for group function occlusion (23.5-

51.5%). The prevalence of balanced occlusion was low (1-9.2%). Following partial excursion,

the prevalence of canine-guided occlusion was much lower than for complete excursion (16.1-

30.6%). The group function occlusion was prominently more prevalent (36.7-83.9%). The

prevalence of balanced occlusion was similar to that of complete excursion (11.2%). However,

Al-Nimri et al. study found a greater prevalence of non-specified occlusion schemes after

partial excursion (Al-Nimri et al., 2010).

Figure 3-2 The relationship between the prevalence of each lateral occlusion scheme and age for Class I occlusion.

67

For Class II occlusion (Figure 3-3), the canine guidance prevalence was consistently higher than

for Class I (71.3-85.6%) following complete excursion. The prevalence of group function

occlusion was less than for Class I (10-24.1%). The prevalence of balanced occlusion was similar

to Class I (0-13.5%). The two studies that differentiated between Class II Division 1 or Class II

Division 2 did not reveal a significant difference in the prevalence of canine and group function

occlusion (Al-Hiyasat and Abu-Alhaija, 2004; Al-Nimri et al., 2010). Following partial excursion,

the prevalence of canine guidance was prominently less (11.8-39.3%), the prevalence of group

function occlusion was higher (46.4-55.9%) and the prevalence of balanced occlusion was

similar (3.5-17.6%) (Al-Nimri et al., 2010) to the prevalence after complete excursion.

Figure 3-3 The relationship between the prevalence of each lateral occlusion scheme and age for Class II occlusion.

For Class III occlusion (Figure 3-4), the prevalence of canine guidance was about half than for

Class I (16.7-32.5%) after complete excursion. The prevalence of group function occlusion was

higher than Class I (31.3-76.7%). In addition, the prevalence of balanced occlusion was more

than for Class I (19-36.2%). After partial excursion, the prevalence of canine-guided occlusion

was 11.1%, the prevalence of group function was 47.2%, and the prevalence of balanced

occlusion was 22.2%. Thus, it appears that the lateral occlusion is not markedly affected by the

degree of excursion for Class III occlusion (Al-Nimri et al., 2010).

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Figure 3-4 The relationship between the prevalence of each lateral occlusion scheme and age for Class III occlusion.

Therefore, canine-guided occlusion tends to be dominant for Class II occlusion, followed by

Class I occlusion. Class III occlusion exhibited mainly group function occlusion.

TMD relationship. The included studies revealed that there is no relationship between lateral

occlusion schemes and TMD incidence. The two studies that evaluated the relationship

between lateral occlusion schemes and TMD incidence did not find a clear relationship

(Weinberg, 1964; Ingervall et al., 1991). Further, the two studies that evaluated the prevalence

of lateral occlusion scheme for TMD symptomatic and asymptomatic participants also did not

reveal a clear relationship (Donegan et al., 1996; Kahn et al., 1999). Donegan et al. did not find

a relationship between the incidence of TMD symptoms and presence or absence of canine-

guided occlusion (Donegan et al., 1996). Although Kahn et al. detected that canine-guided

occlusion tends to be more prevalent for TMD patients, this association was not statistically

significant (Kahn et al., 1999).

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Table 3—1 Summary of the included studies

Study (year) Quality rating

Participant (description) Sample source

Age (years)

Degree of excursion Method of examination

Lateral occlusion schemes

Prevalence percentage (%)

Relevant information

Weinberg (1964) High 100 (at least 28 teeth are present; no previous dental treatment) Not specified sample source

Not specified -Cusp tip to cusp tip -Cinematic and motion picture -TMJ examination (pain, mandibular movement and sound)

Canine-guided

19 -No relationship between TMD and lateral occlusion scheme

Group function

65

Balanced 16 Scaife and Holt (1969) High 1200 (at least 28 teeth; no extensive

restorations) Students’ sample

17-25 -Cusp tip to cusp tip -Visual inspection

Canine-guided

73.4 -More wear facets as canine guidance occlusion prevalence decreases and group function occlusion prevalence increases Group function 26.6

Ingervall (1972) Moderate 50 (at least 28 teeth; no extensive restorations; no crossbite) General population sample

12-29 -3 mm -Alginate indicator

Canine-guided

32 -No gender difference -On the working side, canines were most commonly involved (40%) followed by first premolars (20%) -64% had bilateral non-working occlusal contacts and 20% had unilateral non-working occlusal contacts

Not specified 68

Guevara and Ismail (1976)

Moderate 466 (at least 28 teeth) General population sample

23-37 -3 mm -Models evaluation on adjustable articulator

Canine-guided

58.4

Group function

27.3

Balanced 9.2 Yaffe and Ehrlich (1987)

Moderate 72 (complete dentition; no tooth wear; no TMD) Young population sample

19-35 -1 mm -2 mm -3 mm -Articulating paper

Canine-guided 1 mm: 16.1 2 mm: 32.3 3 mm: 48.5

-Tooth contact in lateral movement is complex and subjected to changes with altering the magnitude of movement -Concluded that applying a specific philosophy to every patient is not justifiable Group function 1 mm: 83.9

2 mm: 67.7 3mm: 51.5

Ingervall et al. (1991)

Moderate 75 (few missing teeth; some restored dentition; half of them had orthodontic treatment) Students’ sample

20-33 -1.5 mm -3 mm -TMJ examination (pain, mandibular movement and sound) -Alginate indicator

Canine-guided

1.5 mm: 25.5 3 mm: 33.5

-Positive correlation was found between abrasion of incisors, canines, premolars and the number of tooth contacts on the working side -With 1.5 mm excursion, half the participants had balancing contacts -Less prevalence of balancing contacts after 3 mm excursion -Canine abrasion was negatively associated with the number of tooth contacts on the non-working side -No relationship between TMD and lateral occlusion scheme

Group function

1.5 mm: 71.5 3 mm: 63.5

Not specified 1.5 mm: 3 3 mm: 3

70

Donegan et al. (1996) Moderate 46 (asymptomatic) Students’ sample

Not specified -Cusp tip to cusp tip -Clinical examination -TMJ examination (sound) -Maylar strip

Canine-guided

30 -No relationship between TMD and lateral occlusion scheme

Absence of canine-guided

70

46 (symptomatic; TMJ sound) General population sample of matched age to the asymptomatic group

Canine-guided

22

Absence of canine-guided 78

Ogawa et al. (1998) Moderate 86 (at least28 teeth; no orthodontic treatment; no cuspal restoration) Students’ sample

20-29 -0.5 mm -1 mm -2 mm -3 mm -Shimstock

Canine-guided 9.3 -On the working side, from 0.5 to 3 mm the frequency of canine contacts increased -On the working side, from 0.5 to 3 mm the frequency of premolars and molars contact decreased -On the non-working side, the contacts were mainly on second molar followed by first molar -On the non-working side, greater contacts after 0.5 mm movement. -On the non-working side, the number of contacts decreased as the lateral movement increased

Group function 45.3

Balanced 41.8 Kahn et al. (1999) Moderate 82 (asymptomatic)

General population sample

Not specified -3 mm -Questionnaire -Clinical examination -Articulating paper and dental floss

Canine-guided 34.4 -Slightly more canine guidance for symptomatic group -No relationship between TMD and lateral occlusion scheme

Group function 64.6

263 (Symptomatic; TMD) General population sample of matched age to the asymptomatic group

Canine-guided 47.5 Group function 52.5

Al-Hiyasat and Abu-Alhaija (2004)

High 447 (at least 28 teeth; no major restorations; no orthodontic treatment) School students’ sample

14-17 -Cusp tip to cusp tip -Clinical examination -Shimstock

Canine-guided

65.1 -The lateral occlusion scheme is affected by incisors relationship

Group function 21.3

Balanced 13.6

71

Panek et al. (2008)

High 834 (complete dentition; no more than single missing tooth; simple restorations; no cuspal involvement; no fixed or removable prosthesis) General population sample

20-29 -2 mm -Clinical examination -Articulation paper

Canine-guided 47.2 -No gender difference -There is an effect of age on lateral occlusion scheme -Recommended canine guidance for young and group function for older patients -The prevalence of balanced occlusion is minimal in all age groups

Group function 35.5 Balanced 9.4

30-39 Canine-guided 47.4 Group function 40.3 Balanced 9.3

40-49 Canine-guided 38.4 Group function 48.9 Balanced 6

50-63 Canine-guided 17.9 Group function 67.9 Balanced

10.7

Al-Nimri et al. (2010) Moderate 94 (at least 28 teeth; no orthodontic treatment; no occlusal adjustment; no major restorations; canines are in the line of the arch) Students’ sample

21-30 -0.5 mm -3 mm -Clinical examination -Shimstock

Canine-guided 0.5: 21.9 3: 59.6

-No gender difference -At 3 mm excursion, there is a relationship between dynamic and different static occlusal relationship -At 0.5 mm excursion, the incisor, canine and molar relationships were not related to dynamic occlusion

Group function 0.5: 45.3 3: 23.9

Balanced 0.5: 13.9 3: 3.2

Not specified 0.5: 13.8 3: 11.7

72

Table 3—2 Summary of the studies that included static occlusal relationship

Study (year) Age Angle’s classification

Degree of excursion

Prevalence of lateral occlusion schemes (percentage)


Canine guidance

Group function

Balanced occlusion

Scaife and Holt (1969)

17-25 Class I 3 mm 63.8 36.2 -The lateral occlusion scheme is influenced by the static occlusal relationship

Class II 75.9 24.1 Class III 23.3 76.7

Guevara and Ismail (1976)

23-37 Class I 3 mm 58.4 27.3 9.2

Yaffe and Ehrlich (1987)

19-35 Class I 1 mm 16.1 83.9 2 mm 32.3 67.7

3 mm 48.5 51.5

Al-Hiyasat and Abu-Alhaijah (2004)

14-17 Class I 3 mm 67.6 23.5 8.9 -The lateral occlusion scheme is affected by incisor relationship -Greater prevalence of canine guidance occlusion for Class II occlusion

Class IIA 71.3 15.2 13.5 Class IIN 85.6 11.8 2.6 Class III 32.5 31.3 36.2

Al-Nimri et al. (2010) 21-30 Class I 0.5 mm 30.6 36.7 11.2 -At 3 mm excursion, there is a relationship between dynamic and static occlusal relationship. Greater prevalence of canine-guided occlusion for Class II occlusion. Class III occlusion had the least prevalence of canine-guided occlusion -At 0.5 mm excursion, the incisor, canine and molar relationship are not related to dynamic occlusion

Class IIA 39.3 46.4 3.6 Class IIB 11.8 55.9 17.6 Class III

11.1 47.2 22.2

Class I 3 mm 64.3 23.5 1 Class IIA 80 10 0 Class IIB 76.5 11.8 0 Class III 16.7 35.7 19

73

3.5. Discussion

It can be observed that there is variation between the included studies in reporting the

prevalence of lateral occlusion schemes. This variation can be attributed to methodological

differences and participant selection (Rinchuse et al., 2007). This heterogeneity prevented the

conduction of proper meta-analysis. Nevertheless, there is an overall consistency on the

factors associated with lateral occlusion schemes, such as magnitude of excursion, static

occlusal relationship and age effect. Therefore, the hypotheses that the prevalence of lateral

occlusion schemes is variable and there are influencing factors on the prevalence of lateral

occlusion schemes were accepted. This review also accepts the hypothesis that there is no

implication of the lateral occlusion scheme on abnormal physiology. Each point will be

discussed separately.

3.5.1. Magnitude of excursion

It is clear from the included studies that the recorded lateral occlusion scheme is influenced by

the degree of excursion. Partial excursion is associated with a greater prevalence of multiple

teeth contacts on the working side and non-working side than for complete excursion (Yaffe

and Ehrlich, 1987; Ingervall et al., 1991; Ogawa et al., 1998; Al-Nimri et al., 2010). The different

effect of degree of excursion on the tooth contact pattern is related to teeth morphology and

positional factors.

After partial excursion, the numerous tooth contacts indicate that the cusp-to-fossa contact

occurs on a range rather than in one distinguished location. This appears to support the earlier

recommendation of Schuylar who proposed the concept of “freedom of movement in centric

occlusion”, where the mandible can move for a short distance (about 1 mm) in the same

horizontal level while maintaining tooth contact. After this short distance, the actual disclusion

will commence. It was envisaged that this design will promote smooth multidirectional

mandibular movement and patient comfort (Schuyler, 1963). Further maturation and wear of

the dentition will widen the area range of cusp-to-fossa contacts which was observed in a

clinical follow-up study by Beyron (Beyron, 1954). Eventually, more tooth contacts will exist

after complete excursion, which will manifest as a greater group function occlusion prevalence

after aging (will be discussed later).

Physiologically, broad occlusal contact areas were found to be helpful in mitigating excessive

occlusal forces on teeth (Hidaka et al., 1999), which might contribute to the dissipation of

sudden lateral forces on teeth. It could also be speculated that the greater contacts at less

lateral movement (1-1.5 mm) enhance the occlusal phase of the chewing cycle (where occlusal

74

contacts occur and the pathway taken by the mandible is determined by the morphology of

the teeth) (Wang and Mehta, 2013). Therefore, it is the posterior teeth that take a

predominant role in guiding the chewing movement and the effect of anterior teeth is more

important at greater lateral movement. Previously, the term progressive occlusion was

discussed, where the initial lateral movement is dictated by a few posterior teeth, in addition

to canines, and the complete lateral movement is controlled primarily by canines (DiPietro,

1977; Goldstein, 1979). Such scheme appears to be more physiologically relevant than genuine

mutually-protected occlusion, where the canine teeth are expected to control the all the

lateral movements. This complex relationship might have a protective role in tolerating lateral

forces (Yaffe and Ehrlich, 1987).

Due to the variation of each lateral occlusion scheme prevalence after partial and complete

excursive movements, it could be postulated that true canine-guided occlusion or group

function occlusion hardly, if ever, exists in nature and the classical criteria might not be

applicable. In support with other investigators, there is a need to propose criteria of what

constitutes an acceptable lateral occlusion scheme (Yaffe and Ehrlich, 1987; Ogawa et al.,

1998).

Although the pattern of tooth contacts depends on the degree of lateral movement, and the

prevalence of group function is relatively high at all movement levels, this systematic review

illustrates the importance of canines in the lateral occlusion scheme. Canines were the most

frequently involved teeth on the working side (40-75%) (Ingervall, 1972; Yaffe and Ehrlich,

1987; Ingervall et al., 1991; Ogawa et al., 1998). The importance of canines can also be

illustrated from the prominent wear facets that existed on the canines of patients with group

function occlusion (Weinberg, 1964; Scaife and Holt, 1969). Further, a positive correlation was

observed between canine wear and the number of tooth contacts on the working side

(Ingervall et al., 1991). This corroborates the protective role of canines on the rest of the

dentition.

3.5.2. Age effect

This review indicates that occlusion is dynamic, adaptive and subjected to changes with time

(Panek et al., 2008). The aging effect can be primarily attributed to tooth wear, which is

commonly a physiological phenomenon. In general, canine-guided occlusion was observed to

be common in adolescence and young age. Although two studies found group function

occlusion to be dominant for this group, the average number of working dental contacts on

individual teeth was only two, and the canine tooth was commonly associated with guidance

75

(Ingervall, 1972; Ingervall et al., 1991). This scheme, despite it being a group function

occlusion, is not significantly deviating from canine guidance. Likewise, if all the contacts were

summed, canine contacts would be the most common (Ingervall, 1972; Yaffe and Ehrlich, 1987;

Ingervall et al., 1991; Ogawa et al., 1998).

Young patients and adolescents tend to have pointy and sharp canines. This will manifest in

steep anterior tooth guidance, which will, subsequently increase the prevalence of canine-

guided occlusion. As tooth wear progresses on the canines, the guidance angle will become

shallower, the exiting contacts will be widened, and the posterior tooth contacts will be

established. Subsequently, the canine guidance is progressively modified during young

adulthood and replaced by group function that becomes the occlusal pattern in middle age.

Scaife and Holt found that 13.8% of their canine-guided participants had visible wear facets

while 52.8% of the group function participants had visible wear facets. This endorses that

group function occlusion is associated with tooth wear more than for canine-guided occlusion.

Likewise, Weinburg found that since canines and premolars are commonly associated with

lateral guidance, they were the most common teeth to be influenced by tooth wear in full

dentition (Weinberg, 1964). The findings of these studies indicate that the teeth controlling

the lateral movement were most susceptible to tooth wear. Clinically, this phenomenon was

observed by Beyron who found after 8 to 12 years of follow up, that the number of lateral

contacts increased which was also relevant to dentition wear (Beyron, 1954).

The acknowledged dynamic nature of occlusion had caused the emergence of several concepts

to restore the dentition. Panek et al. suggested canine-guided occlusion for younger patients

and group function occlusion for older patients (Panek et al., 2008). Others suggested

confirming the existing occlusion scheme (Yaffe and Ehrlich, 1987). Although such concepts are

logical, it is difficult to assume that problems could arise if different schemes are provided

(Becker and Kaiser, 1993; Turp et al., 2008).

3.5.3. Static occlusal relationship

The outcome of this systematic review outlines a correlation between the static and dynamic

occlusion (Scaife and Holt, 1969; Al-Hiyasat and Abu-Alhaija, 2004; Al-Nimri et al., 2010). This

correlation could emerge because of differences in arch sizes and teeth arrangement.

Subsequently, the tooth-to-tooth relationship will be altered, which could manifest as altered

anterior tooth guidance. The steepness or shallowness of the anterior tooth guidance will

eventually dictate the amount of posterior tooth contacts in excursive movements (Schwartz,

1986; Jensen, 1990b). This observation was confirmed by Al-Hiyasat and Abu-Alhaija, who

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found that the most critical static occlusion feature that affects the lateral occlusion scheme

was anterior tooth relationship (Al-Hiyasat and Abu-Alhaija, 2004).

The observation of this review indicates that, after complete excursion, Class II lateral

occlusion tends to be dominated by canine guidance, while the prevalence of canine guidance

tends to be the lowest for Class III occlusion. For Class II occlusion, the anterior teeth exhibit

prominent vertical overlap, which causes steep anterior tooth guidance that hinders the

posterior teeth with minimal contacts laterally, hence a high prevalence of canine-guided

occlusion. On the other hand, Class III dentition tends to exhibit minimal anterior teeth

overlap, an edge-to-edge relationship or cross bite; which reduces the influence of anterior

teeth on lateral occlusion, hence a greater prevalence of group function and balanced

occlusion (Jensen, 1990a).

It appears that the Class II occlusion was the most affected by partial excursion and exhibited a

significant reduction in canine guidance. On the contrary, Class III was not markedly affected

by the degree of excursion (Al-Nimri et al., 2010). This confirms that during partial excursion,

the posterior teeth are playing a dominant role in controlling the occlusion. For Class III

occlusion the role of the posterior teeth will continue after the complete excursion. Therefore,

regardless of the static occlusal relationship, at partial excursion, the posterior teeth will

dictate the lateral movements.

However, one of the limitations of the studies included in this systematic review is a lack in

illustrating the effect of severity of angle classification (extreme Class II or Class III). This was

postulated to be a contributing significant variation in lateral occlusion schemes and each

patient can present with a unique scenario (Jensen, 1990b).

3.5.4. TMD relationship

Historically, TMD was commonly attributed to occlusal factors; however, a causative

relationship between occlusion parameters and TMD development has not been definitely

delineated. In a multiple logistic regression, Pullinger et al. established that the occlusal factors

were related to TMD development in 15% of cases only (Pullinger et al., 1993). This systematic

review illustrates that the incidence of TMD is not related to the lateral occlusion scheme

(Donegan et al., 1996; Kahn et al., 1999). This finding is in accordance with the TMD clinical

research (Seligman and Pullinger, 1991) that found that altering the occlusal variable is not

related to TMD developments.

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Likewise, several studies, including systematic reviews, revealed no strong evidence supporting

the superiority of occlusal treatment over any other treatment modalities (e.g. cognitive

behavioural, pharmacological, or physical therapies) (De Boever et al., 2000a; b; Koh and

Robinson, 2003; Liu et al., 2012; Turp and Schindler, 2012). In addition, altering the lateral

occlusion scheme will not necessarily prevent or reduce the incidence of TMD. Therefore, it

could be established that the lateral occlusion scheme is not an influencing factor to TMD

development and altering the occlusion scheme for the aim of preventing TMD solely is not

justifiable.

3.5.5. Further considerations

Although the topic of lateral occlusion schemes has been a subject of numerous investigations

in the last few decades, this systematic review supports the view that there is no lateral

occlusion scheme that can be considered as a gold standard for every patient (Becker and

Kaiser, 1993; Turp et al., 2008). Further, the review revealed that an occlusion that perfectly

fulfils all of the features of any defined lateral occlusion scheme is rarely observed clinically for

an individual patient. Likewise, the prevalence of the non-working side contacts, which were

thought to be associated with pathological consequences, was high. Therefore, the deviation

from the perfectionist standards of the lateral occlusion scheme does not necessarily mean

that the occlusion is not physiological (The Glossary of Prosthodontic Terms, 2005). This finding

is in accordance with other experts who confirmed that ideal occlusions are seldom observed

in real life (Woda et al., 1979; Jensen, 1990b; Carlsson, 2010). Yet, the dentition is functioning

within its physiological capabilities (Carlsson, 2010).

Similarly, the clinical studies did not reveal differences in the restoration performance

following the establishment of any form of lateral occlusion scheme. Yi et al. found no

difference in patients’ satisfaction if their full-arch prosthesis exhibited canine-guided, group

function or balanced occlusion (Yi et al., 1996). Further, the prospective studies that

implemented canine-guided occlusion (Ormianer and Gross, 1998) or group function occlusion

(Attin et al., 2012) did not find a pathological association with any occlusion scheme.

Therefore, it appears that not only can patients present with variable lateral occlusion

schemes that can still be considered as physiologic occlusion, but also restoring their dentition

according to different schemes can be perfectly acceptable. This indicates that the significance

of the lateral occlusion scheme was heavily overrated in the earlier literature.

Due to the variation between the prevalence of each occlusion scheme, the lack of

pathological association and great individual abilities to adapt to occlusal alterations; flexible

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and practical occlusion practice should be implemented. Likewise, broader definitions of

acceptable, natural or therapeutic occlusion should be accepted (Becker and Kaiser, 1993; Turp

et al., 2008). Therefore, it could be safe to state that there is no benchmark lateral occlusion

scheme that can be applied to every patient. Instead, the acceptable occlusal criteria can

present as a range of variables that incorporate different occlusion parameters. In the context

of this review, canine-guided occlusion or group function can both be biologically acceptable.

As a clinical guide, instead of implementing a specific occlusion scheme, if complex restorative

work is to take place, the clinicians should consider an occlusion scheme that renders the final

treatment conservative, aesthetic, practical and simple (Becker and Kaiser, 1993; Wiskott and

Belser, 1995; Bryant, 2003; Carlsson, 2009). Additional influential factors on the selection of

lateral occlusion schemes that have not been thoroughly investigated clinically are the

condition of the abutment teeth and the ease of construction. The current state of research

provides minimal information about the impact of the lateral occlusion scheme on

compromised abutments from the endodontic and periodontic perspectives. Likewise, the

effect of lateral occlusion on restorative material has not been well delineated. Clearly, these

areas demand further clinical investigations.

3.6. Conclusions

Within the limitations of this systematic review, the following points can be emphasised:

1. A genuine lateral occlusion scheme is rarely occurring in nature. The lateral occlusion is

dynamic and subjected to changes with time. An individual could have different occlusion

at different phases of the life.

2. Canine-guided occlusion or group function occlusion with multidirectional freedom of

occlusal contact movement multidimensional freedom of tooth contact in mandibular

excursion are equally acceptable. The pathological or therapeutic effect of any occlusion

scheme cannot be established. The presence of non-working side contacts is common and

should be preserved in the non-restored dentition.

3. Young individuals tend to have a canine-guided occlusion and older individuals tend to

have group function occlusion. Canine-guided occlusion is more dominant for Class II

occlusion and group function occlusion is more prevalent for Class III occlusion. Although it

is difficult to set a rigid recipe for occlusion rehabilitation, these observations can be

utilized as a treatment guide for therapeutic occlusion.

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Chapter Four

4. Impact of Lateral Occlusion Schemes: A

Systematic Review


Abduo J, Tennant. Impact of lateral occlusion schemes: a systematic review. Journal of

Prosthetic Dentistry. 2015; 114:193-204. (Appendix E)

80

4.1. Abstract

Objective: Although several lateral occlusion philosophies were proposed in the literature,

there is a lack of compelling evidence supporting any scheme. The aim of this systematic

review was to investigate the clinical implications of different lateral occlusion schemes.

Materials and methods: A literature search was completed through PubMed (MEDLINE),

Google Scholar and Cochrane Library, up to January 2014. The literature search aimed to

retrieve two studies categories; Group 1: comparative studies; Group 2: clinical outcome

studies. The inclusion criteria were peer-reviewed human clinical studies published in English.

The search was further supplemented by manual searching through the reference lists of the

selected studies.

Results: The initial search revealed a total of 680 studies; however, after applying the inclusion

criteria, 26 studies were found suitable for the analysis (13 for Group 1 and 13 for Group 2).

The most commonly evaluated lateral occlusion schemes were canine-guided occlusion (CGO)

and group function occlusion (GFO). Group 1 studies evaluated the impact of lateral occlusion

schemes on muscular electromyographic (EMG) activity, condylar displacement, chewing and

mandibular movement. Group 2 studies evaluated the impact of restored occlusion on

longevity, patient’s comfort and pathological consequences. CGO was associated with

narrower chewing and less EMG activity of the masticatory muscles during clenching. GFO was

associated with wider mandibular movement and quicker chewing. During chewing, there was

no difference in EMG activity between the two lateral occlusion schemes. Further, the long-

term studies indicated that there is no difference between the two schemes in patient’s

comfort and restoration longevity.

Conclusions: Although there are immediate differences between the different lateral occlusion

schemes, patients have the capability to successfully adapt to CGO or GFO. The occlusion

scheme might influence the masticatory muscles activity, condylar displacement, chewing and

mandibular movement. However, since the long-term studies reflect patients’ acceptance for

occlusion alteration, the clinical significance of these differences is yet to be determined.

Key words: lateral occlusion; canine-guided occlusion; group function occlusion; balanced

occlusion; excursion

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4.2. Introduction

When restoring a patient’s dentition, the clinician encounters a clinical dilemma of which

occlusion scheme should be established. In the literature, many occlusion rehabilitation

philosophies have been proposed. In general, the available lateral occlusion schemes are

canine-guided occlusion (CGO), group function occlusion (GFO) and bilateral balanced

occlusion (BBO) (Thornton, 1990). Although at maximal intercuspation they might have similar

occlusal contacts, they differ in the amount of contacts during lateral movement. The CGO is

distinguished by the prominent vertical and horizontal overlap of the canine teeth that

prevents posterior tooth contacts in the lateral movement of the mandible (The Glossary of

Prosthodontic Terms, 2005). Dentition with GFO exhibits multiple contacts between the

maxillary and mandibular teeth in lateral movement on the working side (The Glossary of

Prosthodontic Terms, 2005). In addition to the occlusal contacts of GFO, BBO has additional

posterior tooth contact on the non-working side (The Glossary of Prosthodontic Terms, 2005).

Although each lateral occlusion philosophy has its advocates (Thornton, 1990; Rinchuse et al.,

2007), the amount of clinical evidence supporting the superiority of any philosophy is limited

(Becker and Kaiser, 1993; Turp et al., 2008).

Instead of rigidly following a preconceived lateral occlusion philosophy, it is worthy to ask the

question of what the influence of the lateral occlusion scheme is on patient’s comfort,

masticatory system physiology and prosthesis longevity. Therefore, this systematic review

aims to investigate the clinical implications of lateral occlusion schemes on the restored

dentitions. The null hypotheses being: there is no effect of lateral occlusion scheme on

patient’s comfort and masticatory physiology, and there is no effect of lateral occlusion

scheme on restoration longevity.


4.3.1. Search strategy and selection criteria

An electronic literature search was accomplished in January 2014 through PubMed (MEDLINE),

Google Scholar and Cochrane Central Registrar of Controlled Trials. Through the PubMed

database, the Boolean operator was used to combine the following key words: (‘lateral

occlusion’ OR ‘dynamic occlusion’ OR ‘excursive occlusion’) AND (‘canine guided’ OR ‘canine

protected’ OR ‘group function’ OR ‘balanced’) AND (‘dental’) OR (‘implant’). All the articles

related to dental occlusion were retrieved from the Cochrane Database. The Google Scholar

search engine was used to retrieve additional articles by combining the following key words:

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dental occlusion, lateral, dynamic, excursive, guidance, canine, group function, balanced,

implant, restoration, fixed and prosthesis. No publication year limit was applied. The purpose

of the search was to obtain all the clinical studies that evaluated the impact of lateral occlusion

schemes. This involved impact on physiological response, longevity and patient’s acceptance.

Studies selection was performed in three stages: (1) studies selection according to the

relevance of the title; (2) studies selection according to the abstract relevance; (3) full text

analysis and cross-matching against predefined inclusion criteria (Table 4—1). In addition, the

literature search was endorsed with manual searching of the bibliographies of all the included

studies.

Table 4—1 Inclusion criteria

Peer-reviewed journal article Human clinical study Adult participants Asymptomatic participants The occlusion alterations were executed by fixed restoration/prosthesis Cross sectional, retrospective or prospective study English language publication

4.3.2. Studies classification

Two studies categories were considered for this review.

Group 1: Comparative studies. Where the study compares multiple lateral occlusion

schemes.

Group 2: Clinical outcome studies. Where the study describes the applied occlusion

scheme for the restored dentition.

The definition of each lateral occlusion scheme was adopted from the Glossary of

Prosthodontic Terms (2005):

CGO: The vertical and horizontal overlap of the canine teeth disengages the posterior

teeth in excursive movements of the mandible.

GFO: Multiple contact relations between the maxillary and mandibular teeth in lateral

movements on the working side. As there is no specification regarding the amounts of the

present contact, two or more simultaneous contacts on the working side were considered

as GFO.

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BBO: Bilateral, simultaneous, anterior, and posterior occlusal contact of teeth in centric

and eccentric positions. As this definition rarely exists naturally, balancing contacts, that

do not interfere with smooth mandibular movements, were considered as indicators of

BBO.

4.4. Results

4.4.1. Literature search

The electronic search yielded a total of 680 articles. After the analysis of titles’ relevance, 621

articles were excluded. Abstracts’ screening excluded an additional 24 articles. Thus, 35 articles

were suitable for full-text analysis. Cross-matching against the inclusion criteria rendered a

total of 16 articles suitable for inclusion. Manual searching through the references of the

selected articles revealed additional 10 articles suitable for inclusion. Therefore, a total of 26

articles were included in this review. Through all of the studies, only the relevant information

about the lateral occlusion schemes was extracted. Non-physiological occlusions were

excluded from the analysis.


From the 26 studies, 13 studies were comparative studies (Group 1), and 13 of them were

long-term studies (Group 2).

Group 1 studies evaluated the immediate response to alteration of the lateral occlusion

scheme by the following methods:

Electromyography (EMG) (Table 4—2): where the electrical activities of masticatory

muscles were recorded. This method was used to evaluate the effect of the lateral

occlusion scheme on muscle response to different mandibular movements.

Mandibular movement (Table 4—3): evaluated the impact of lateral occlusion on

mandibular movement or condylar position when the teeth are in function.

The participants were requested to undertake the following movements:

Physiological: chewing, sliding from intercuspal position to edge-to-edge position

(eccentric grinding), and sliding from edge-to-edge position to intercuspal position

(concentric grinding).

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Non-physiological: maximal clenching in the intercuspal position, maximal clenching in the

edge-to-edge position and submaximal edge-to-edge clenching. The aim of these

movements was to simulate the muscle reaction to parafunctional activities.

The studies either selected patients with existing occlusion or experimentally altered the

occlusion by bonding occlusal overlay or altering the prosthesis. The evaluated occlusion

schemes were: CGO, GFO and BBO. Some studies considered semi-group function occlusion

(sGFO) in which the canines and the first premolars controlled the lateral movements.

Group 2 studies are long-term studies that reported the applied occlusion scheme in the

prostheses/restorations design. Although not specifically evaluating the impact of the lateral

occlusion scheme, they investigated the patients’ response, restoration longevity and

complications in situations closer to the routine clinical practice. The lateral occlusion scheme

was achieved by composite restorations (Table 4—4), fixed dental prostheses and implant

prostheses (Table 4—5). In several studies, the lateral occlusion scheme was altered in

conjunction with increasing the vertical dimension of occlusion (VDO). The implemented

occlusion schemes were CGO, GFO and BBO.

4.4.3. Studies’ outcome

Group 1: Comparative studies: Five studies evaluated the effect of altering the lateral occlusion

scheme on chewing and mandibular movement (Belser and Hannam, 1985; Jemt et al., 1985;

Okano et al., 2002; Okano et al., 2005; Salsench et al., 2005). One of which was for unaltered

natural occlusions (Salsench et al., 2005), three were for altered occlusion (Belser and

Hannam, 1985; Okano et al., 2002; Okano et al., 2005) and one was for restored dentition with

fixed implant prosthesis (Jemt et al., 1985).

Belser and Hannam found that altering GFO to CGO narrowed the envelop of mandibular

movements, while the muscle coordination during chewing was not altered (Belser and

Hannam, 1985). Likewise, Jemt et al. found CGO was associated with a slightly steeper

movement path than GFO during chewing (Jemt et al., 1985). Further, their participants

reported GFO to be more comfortable than CGO. Salsench et al. demonstrated that

participants with CGO had the steepest lateral guidance angle, while participants with GFO had

less overbite (Salsench et al., 2005).

In terms of chewing speed, Jemt et al. found GFO to be associated with greater mandibular

velocity than CGO (Jemt et al., 1985). Salsench et al. found that the duration of chewing is

influenced by the occlusion scheme, and CGO had a longer chewing cycle than GFO. As the

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mastication height was similar between GFO and CGO, it was indicative that GFO had a greater

velocity than CGO (Salsench et al., 2005).

In relation to the condylar displacement, maximal edge-to-edge clenching caused the condyles

to displace regardless of the lateral occlusion scheme (Okano et al., 2002). However, the

lateral occlusion scheme altered the magnitude and direction of the condyles displacement.

On the working side, there is an insignificant total displacement between the different

occlusion schemes, while on the non-working side, GFO caused the greatest displacement,

followed by sGFO, CGO and BBO respectively. There was an insignificant difference between

CGO and BBO or GFO. The most prominent displacement was vertical on the non-working side.

Interestingly, another study by the same investigators applied submaximal edge-to-edge

clenching and found different results (Okano et al., 2005). CGO caused the greatest condylar

displacement, followed by sGFO, GFO and BBO respectively. On the working side, there was a

significant difference between CGO and GFO or BBO. On the non-working side, the significant

difference was between CGO and BBO. They attributed the difference between the two

studies to the difference in the magnitude of clenching force.

A total of 10 studies evaluated the effect of altering the lateral occlusion scheme on muscles

activities by muscle EMG activity measurements (Belser and Hannam, 1985; Akoren and

Karaagaclioglu, 1995; Okano et al., 2002; Valenzuela et al., 2006; Miralles et al., 2007; Okano et

al., 2007; Campillo et al., 2008; Gutierrez et al., 2010; Rodriguez et al., 2011; Valenzuela et al.,

2012). 7 studies evaluated the effect of natural lateral occlusal scheme on EMG activity

(Akoren and Karaagaclioglu, 1995; Valenzuela et al., 2006; Miralles et al., 2007; Campillo et al.,

2008; Gutierrez et al., 2010; Rodriguez et al., 2011; Valenzuela et al., 2012), while the other

studies experimentally altered the occlusion (Belser and Hannam, 1985; Okano et al., 2002;

Okano et al., 2007). The evaluated muscles were the masseter, anterior temporalis, posterior

temporalis, suprahypoid, infrahyoid and sternocleidomastoid muscles.

On unaltered dentition, Akoren and Karaagaclioglu found no difference in the masseter and

anterior temporalis muscles EMG activities between participants with CGO and GFO during

chewing (Akoren and Karaagaclioglu, 1995). However, during eccentric clenching, the anterior

temporalis muscle had greater EMG activity with group occlusion, while the masseter EMG

activity was not influenced. Campillo et al. confirmed that the masseter muscle was minimally

affected during maximal intercuspal clenching, eccentric grinding, maximal edge-to-edge

clenching and concentric grinding (Campillo et al., 2008). Rather, the mandibular position was

the most influencing where clenching in the intercupsal position induced the greatest EMG

activity. Gutierrez et al. found GFO caused a significantly higher EMG activity for the anterior

86

temporalis muscle than CGO during eccentric and concentric grinding, and edge-to-edge

clenching (Gutierrez et al., 2010). For the suprahyoid and infrahoid muscles, Valenzuela et al.

found GFO caused insignificantly greater activities than CGO during eccentric, concentric

grinding and edge-to-edge clenching (Valenzuela et al., 2006; Valenzuela et al., 2012). A similar

outcome was confirmed for the sleeping position by Miralles et al. (Miralles et al., 2007) For

the sternocleidomastoid muscle, Rodriguez et al. found significantly lower activity for CGO

than GFO during eccentric grinding, concentric grinding and edge-to-edge clenching (Rodriguez

et al., 2011). The aforementioned studies found significant effects of the type of activity,

where clenching generally caused more EMG activity than grinding. These studies suggested

that jaw stability is more critical to the reduction of muscle EMG activity than the lateral

occlusion scheme.

The studies that deliberately altered the occlusion scheme found similar outcomes to the

studies on unaltered dentition. Belser and Hannam confirmed that CGO caused a significant

reduction of temporalis and masseter muscles EMG activity (by 50%) during clenching, while

mastication did not alter the EMG activity (Belser and Hannam, 1985). Okano et al. also found

less combined temporalis and masseter muscles EMG activities for CGO followed by sGFO, GFO

and BBO during maximal clenching in edge-to-edge position (Okano et al., 2002). CGO

produced significantly less activity than for all other occlusion schemes. In addition, with sGFO,

the EMG activity was found to be significantly less than BBO. In another study, Okano et al.

confirmed their previous observation (Okano et al., 2007). However, no statistical difference

was observed for the masseter muscle EMG activity for any scheme. For anterior and posterior

temporalis muscles, no significant difference on the working side was observed. On the non-

working side, the CGO caused significantly less EMG activity than GFO and BBO for the anterior

temporalis muscle. For the posterior temporalis muscle, CGO caused significantly less EMG

activity than BBO.

From the included studies, it appears that CGO is associated with a narrowing chewing cycle

laterally, and steeper mandibular motion. GFO appears to increase the velocity of chewing.

There are indications that the presence of more tooth contacts can reduce the loads on the

condyle on the working and non-working sides during clenching. There is a tendency for CGO

to cause less EMG activity than other occlusion schemes. In addition, the EMG activity tends to

increase with increasing the number of posterior tooth contacts and the cross-arch contacts.

However, during physiological movements like chewing and grinding, this difference appears

to be minimal. The most commonly influenced muscle is the anterior temporalis muscle.

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Group 2: Long-term clinical studies: Seven studies evaluated the long-term outcome of the

modified occlusion scheme by fixed prostheses (Dahl and Krogstad, 1985; Gross and Ormianer,

1994; Yi et al., 1996; Ormianer and Gross, 1998; Kinsel and Lin, 2009; Ormianer and Palty,

2009; Sierpinska et al., 2013). Four of them were solely for fixed prostheses supported by

teeth (Dahl and Krogstad, 1985; Gross and Ormianer, 1994; Yi et al., 1996; Ormianer and

Gross, 1998; Sierpinska et al., 2013). One study was for implant prostheses (Kinsel and Lin,

2009) and one study included tooth- and implant-supported prostheses (Ormianer and Palty,

2009). Six studies evaluated the long-term outcome of the modified occlusion scheme by

composite restorations (Hemmings et al., 2000; Redman et al., 2003; Poyser et al., 2007;

Schmidlin et al., 2009; Attin et al., 2012; Al-Khayatt et al., 2013). Only two studies attempted to

find the implications of the existing lateral occlusion scheme (Yi et al., 1996; Kinsel and Lin,

2009).

Three studies provided CGO (Dahl and Krogstad, 1985; Gross and Ormianer, 1994; Ormianer

and Gross, 1998) by fixed prosthesis and four studies by composite restorations (Hemmings et

al., 2000; Redman et al., 2003; Poyser et al., 2007; Al-Khayatt et al., 2013). None of the studies

reported biological or mechanical complications associated with the provided occlusion

schemes. Further, patient acceptance of the new occlusion scheme was reported (Gross and

Ormianer, 1994; Ormianer and Gross, 1998). The composite restorations suffered from some

mechanical degradation in the form of wear, chipping and margin deterioration over a period

of 3 to 6 years. However, the failure cannot be attributed to the applied lateral occlusion

scheme. Instead, material properties and bruxism appear to be more influential on the success

of composite restorations (Hemmings et al., 2000; Redman et al., 2003).

For GFO, two studies restored the occlusion by fixed prosthesis (Ormianer and Palty, 2009;

Sierpinska et al., 2013) and two studies altered the occlusion by composite (Schmidlin et al.,

2009; Attin et al., 2012). Similar to CGO, all the studies reported patient acceptance to the new

occlusion, and biological or mechanical complications were not attributed to the occlusion

scheme. The composite restoration studies reported surface deterioration and mechanical

complications which were not related to the occlusion scheme (Schmidlin et al., 2009; Attin et

al., 2012). Sierpinska et al. reported that following prostheses insertion, the muscle EMG

activity during maximal clenching within the temporalis, masseter and digastric muscles had

decreased, while the EMG activity for sternocleidomastoid muscle had increased. After 3

months of function, the EMG activity was restored to pre-treatment levels (Sierpinska et al.,

2013).

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One study evaluated the incidence of lateral occlusion schemes for periodontally-treated

restored dentition with cross-arch fixed dental prosthesis (Yi et al., 1996). They found that 51%

of their patients had GFO, 16% had CGO and 14% had mixed occlusion. The prevalence of BBO

was 19%. They noted that most of the patients were satisfied with their function in terms of

mastication, aesthetics, phonetics and comfort. They found that patients with limited

supporting tissues (less than 70%) had more difficulties chewing hard foods. No relationship

was observed between the occlusion scheme and dysfunction or subjective function.

Interestingly, they reported that CGO tended to be the dominant scheme if the dentition is

ideal. When there is no mobility, and more than 50% of the supporting periodontium remains,

GFO tends to be more frequent. BBO was found to be associated with very mobile teeth. As

the prostheses were stable, they postulated that the occlusal variable cannot contribute to the

long-term result. Another study evaluated the lateral occlusion as a factor for implant-

supported prosthesis complications (Kinsel and Lin, 2009). The most commonly applied

occlusion scheme was CGO (82.2%). Patients with GFO were found to have about three times

more mechanical complications than those with CGO. However, the most prominent risk

factors for mechanical complications were the presence of bruxism and an implant prosthesis

opposing another implant prosthesis. Similarly, Ormianer and Palty reported that more

mechanical complications occurred for implant prosthesis opposing another implant prosthesis

when compared to implant prosthesis opposing restored natural dentition, or when all

prostheses are supported by natural teeth (Ormianer and Palty, 2009).

From the previous studies, it appears that patients have the capacity to adapt to CGO or GFO

as new lateral occlusion scheme. The selected lateral occlusion scheme appears to have a

minimal impact on patient’s comfort, and biological or mechanical complications. Instead,

mechanical complications are associated with other risk factors such as bruxism, restorative

material properties and implant prosthesis occluding against implant prosthesis.

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Table 4—2 Implications of lateral occlusion scheme on muscle EMG activity

Study (year)

Study design

Participant (description) Existing occlusion scheme

Evaluated movements

Evaluated muscles

Occlusion alteration method

Lateral occlusion scheme

Duration of alteration


Belser and Hannam (1985)

Cross-sectional

12 (intact dentition, no masticatory dysfunction) Natural GFO

-Chewing -Maximal vertical clenching and eccentric grinding

-Anterior temporalis -Posterior temporalis -Masseter

-Bonded metal overlays -No VDO increase

CGO GFO

NA -CGO significantly reduced EMG activity during clenching, but not during mastication

Akoren and Karaagaclioglu (1995)

Cross-sectional

30 (intact dentition, no masticatory dysfunction, no previous treatment) 15 CGO 15 GFO

-Chewing -Eccentric grinding

-Anterior temporalis -Masseter

-No alteration CGO GFO

NA -No significant difference between the occlusion schemes during chewing -During eccentric clenching, GFO increased EMG activity of anterior temporalis, but there was no effect on masseter

Okano et al. (2002)

Cross-sectional

20 (intact dentition, no masticatory dysfunction) Natural CGO

-Maximal edge-to-edge clenching



CGO GFO sGFO BBO

NA -Statistically significant effect of lateral occlusion scheme on EMG activity, on the working and non-working side -CGO caused lowest EMG activity. Increasing posterior teeth contacts resulted in increased total EMG activity -Balancing contacts caused greater EMG activity than other occlusion schemes

Valenzuela et al. (2006)

Cross-sectional


-Eccentric grinding -Edge to edge clenching -Concentric grinding

-Supra-hyoid -Infra-hyoid


NA -GFO is associated with insignificantly more EMG activity -The location of the jaw and the function is more influential on EMG activity than the occlusion scheme. More EMG activity for clenching than grinding

Miralles et al. (2007)

Cross-sectional


-Edge to edge clenching -Concentric grinding -Eccentric grinding



NA -GFO is associated with insignificantly more EMG activity -The location of the jaw and the function is more influential on EMG activity than the occlusion scheme. More EMG activity for clenching than grinding

Okano et al. (2007)

Cross-sectional


-Maximal edge-to-edge clenching



CGO GFO sGFO BBO

NA -There is significant difference between the different occlusion schemes -Masseter activities remained the same -Significant increase for the anterior temporalis EMG for GFO and BBO -Increasing posterior teeth contacts resulted in increased total EMG activity

90

Campillo et al. (2008)

Cross-sectional


-Maximal intercuspal clenching -Eccentric grinding -Maximal edge-to-edge clenching -Concentric grinding

-Masseter -No alteration CGO GFO

NA -No significant difference between the occlusion schemes -The location of the jaw and the function is more influential on EMG activity than the occlusion scheme. More EMG activity for clenching than grinding

Gutierrez et al. (2010)

Cross-sectional


-Eccentric grinding -Maximal edge-to-edge clenching -Concentric grinding

-Anterior temporalis -No alteration CGO GFO

NA -CGO was associated with significantly less EMG activity than GFO

Rodriguez et al. (2011)

Cross-sectional



-Sternocleido-mastoid


NA -Significantly lower activity was observed with CGO than GFO

Valenzuela et al. (2012)

Cross-sectional





NA -No significant difference between the occlusion schemes

CGO, canine-guided occlusion; GFO. Group function occlusion; sGFO, semi-group function occlusion; BBO, bilaterally balanced occlusion; VDO, vertical dimension of occlusion; EMG, electromyography; NA, not applicable.

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Table 4—3 Implications of lateral occlusion scheme on mandibular movement

Study (year)

Study design

Participant (description) Existing occlusion scheme

Method of evaluation





Belser and Hannam (1985)

Cross-sectional

30 (intact dentition, no masticatory dysfunction, no previous treatment) Natural GFO

-Jaw movement during chewing


CGO GFO

NA -CGO is associated with narrower chewing movement

Jemt et al. (1985)

Cross-over 5 (fixed maxillary implant prosthesis opposed by natural dentition)

-Jaw movement during chewing

-Occlusion alteration of the maxillary implant prosthesis

CGO GFO

4 months 5 months

-Slightly steeper movement path was noted for CGO than GFO GFO was associated with more chewing cycle variation, lateral mandibular displacement and mandibular velocity. -All the participants found GFO to be more comfortable

Okano et al. (2002)

Cross-sectional


-3D condylar displacement during maximal edge-to-edge clenching


CGO GFO sGFO BBO

NA -On the working side, the condylar displacement was similar for all the occlusion schemes -Clenching with GFO caused significantly greater superior displacement of the non-working side condyle -Clenching with BBO caused significantly less superior displacement on the non-working side -On the non-working side, there was similarity between CGO and sGFO

Okano et al. (2005)

Cross-sectional


-3D condylar displacement during submaximal edge-to-edge clenching


CGO GFO sGFO BBO

NA -On the working side, the condylar displacement with CGO was greater than for GFO or BBO. This difference was insignificant with sGFO -For the non-working side, BBO was associated with the least condylar displacement followed by GFO. The statistical difference was between BBO and CGO

Salsench et al. (2005)

Cross-sectional

53 (intact dentition, no masticatory dysfunction, no previous treatment) 36 CGO or anterior protected occlusion 17 GFO

-Duration of masticatory cycle during chewing


NA -CGO had highest lateral guidance angle and greatest chewing cycle duration -GFO had less total duration of mastication -The masticatory height for CGO and GFO was similar

CGO, canine-guided occlusion; GFO. Group function occlusion; sGFO, semi-group function occlusion; BBO, bilaterally balanced occlusion; VDO, vertical dimension of occlusion; NA, not applicable.

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Table 4—4 Summary of studies that established the lateral occlusion scheme by composite restorations

Study (year)

Study design

Participant (description)






Hemmings et al. (2000)

Prospective 16 (dentition with tooth wear)

-Evaluated restoration longevity -Subjective patient evaluation

-Composite build-up -VDO increase

CGO 30 months -Success rate of composite restoration was 89.4% -The restorative material has impact on the survival of the restoration -Good patient satisfaction

Redman et al. (2003)




CGO Up to 6 years -No restoration failure in the 1st

year -Half of the failures occurred in the 5th year -The failure is related to bruxism and material properties -Bulk fracture was not common -80% had evidence of wear

Poyser et al. (2007)




CGO 2.5 years -6% complete loss of restoration -High level of patient satisfaction -Material loss was due to wear

Schmidlin et al. (2009)




GFO 3 years -Most of the restorations maintained anatomic form (97%) -All patients demonstrated good to excellent acceptance to the treatment

Attin et al. (2012)




GFO 5.5 years -86% of the restorations had good anatomic form -All restorations were adequate -No signs of masticatory dysfunction -After 3 years, no deterioration of surface texture -After 5 years, 28% of the restorations had some surface deterioration

Al-Khayatt et al. (2013)




CGO 7 years -The approximate survival rate of the restoration was 85% -The patients were satisfied with the treatment

CGO, canine-guided occlusion; GFO. Group function occlusion; VDO, vertical dimension of occlusion.

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Table 4—5 Summary of studies that established the lateral occlusion scheme by fixed dental and implant prostheses

Study (year)

Study design

Participant (description)




Prosthesis support



Dahl and Krogstad (1985)


-Clinical evaluation -Subjective patient evaluation

-Anterior teeth crowns according to Dahl concept -VDO increase

CGO Teeth Up to 5.5 years -None of the restored teeth had endodontic complications -No crown had to be replaced due to excessive wear -No development of masticatory dysfunction symptoms

Gross and Ormianer (1994)

Prospective 8 (dentition with tooth wear and no signs of masticatory dysfunction)

-Subjective patient evaluation -Masticatory system evaluation

-Provisional restorations -VDO increase

CGO Teeth 1 month -All participants adapted to the new occlusion scheme

Yi et al. (1996) Retrospective 34 (patients history of periodontal disease)

-Evaluation of the incidence of each occlusion scheme -Subjective patient evaluation

-Cross-arch prostheses -Flattened occlusal morphologies

CGO: 16% GFO: 51% GFO and CGO: 14% BBO: 19%

Teeth and implants

More than 10 years

-None of the examined occlusal variables were related to the long-term results -The great majority of patients were satisfied with the function of their prostheses -Subjective function was not significantly influenced by occlusal variables

Ormianer and Gross (1998)

Prospective 8 (dentition with tooth wear and no signs of masticatory dysfunction)

-Masticatory system evaluation

-Definitive prostheses -VDO increase

CGO Teeth 2 years -All participants adapted to the new occlusion scheme

Kinsel and Lin (2009)

Retrospective 152 (single and multiple-unit implant prostheses)

-Evaluated the incidence of ceramic chipping

-Single implant crown or fixed dental prosthesis

CGO: 82% GFO: 18%

Implants Variable -The ceramic chipping were significantly associated with opposing implant prostheses, bruxism and not wearing occlusal device -For CGO, 15.9% of the patients experience ceramic fracture and 5.3% of implants had ceramic fracture -For GFO, 51.9% of patients experienced ceramic fracture and 16.1% of implants had ceramic fracture -At patient level, significantly more complications with GFO that CGO. At implant level, there was no significant difference between the two schemes

Ormianer and Palty (2009)

Retrospective 30 (natural dentition and whole arch implant prosthesis) 10 (natural dentition in both arches) 10 (natural dentition in one arch against implant prosthesis in the opposing arch) 10 (implant prostheses in both arches)

-Subjective patient evaluation -Radiographic assessment of alveolar bone around teeth and implants -Evaluated restoration longevity


GFO Teeth Implants

2-3 months -All participants adapted to the new occlusion scheme -More bone loss and tooth failure with prosthesis supported by natural dentition in the two arches -More mechanical complications such as veneer fracture was for patients with implant prosthesis in the two arches

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Sierpinska et al. (2013)


-EMG (anterior temporalis, superficial masseter, anterior digastric, sternocleidomastois) -Digital occlusion examination


GFO Teeth

3 months -The mean value of functional EMG activity during clenching immediately post-treatment decreased compared to pre-treatment -After 3 months, no side effects in the form of masticatory dysfunction, chewing, and comfort -After 3 months period of adaptation, the post-treatment EMG activity had increased to levels similar to pre-treatment levels

CGO, canine-guided occlusion; GFO. Group function occlusion; BBO, bilaterally balanced occlusion; VDO, vertical dimension of occlusion.

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4.5. Discussion

It is observed from this systematic review that the lateral occlusion scheme has an impact on

muscle activity, chewing and mandibular movements. CGO appears to exhibit some protective

role, while GFO results in quicker chewing. Therefore, the hypothesis that there is no effect of

lateral occlusion scheme on the patient’s comfort and masticatory physiology is rejected.

However, long-term studies have revealed an equivocal outcome in relation to the long-term

effects of lateral occlusion. Thus, the hypothesis that there is no effect of the lateral occlusion

scheme on restoration longevity is accepted. Consequently, the observed difference between

the lateral occlusion schemes could be of significance at an experimental level, while clinically,

the significance of the difference is questionable.

4.5.1. Physiological implications of lateral occlusion scheme

Uncontrolled dynamic occlusion was classically believed to precipitate pathological

consequences. As the mandible slides along cuspal inclines, the forces are distributed to the

teeth, supporting structure, muscles of mastication and the temporomandibular joint (TMJ)

(Suit et al., 1976; Ogawa et al., 1996). Therefore, uncontrolled forces due to the occlusion

scheme or parafunctional activities might affect the physiological balance (Ramfjord, 1961).

From the included studies, there are indications that CGO exhibits some protective roles for

posterior teeth, masticatory muscles and the TMJ complex. On the other hand, it was observed

that GFO is more efficient for chewing and is more comfortable.

The narrowness of the chewing cycle of CGO could be attributed to the presence of more

overbite between the anterior teeth (Jemt et al., 1985; Salsench et al., 2005). This feature

could translate clinically to less magnitude of lateral mandibular movement and, as a result,

less posterior tooth contact laterally (Wang and Mehta, 2013). Eventually, with CGO, the

posterior teeth will be subjected to less oblique forces and tensile stresses which are more

traumatic to tooth structure (Palamara et al., 2000; Palamara et al., 2006). Instead, the

posterior teeth will receive vertical forces primarily (Akoren and Karaagaclioglu, 1995). This

could potentially support the protective role of canines in mutually-protected occlusion.

On the other hand, although the muscle coordination is similar to CGO (Belser and Hannam,

1985), GFO was distinguished by the wider range of lateral movement in the occlusal phase of

chewing. This was attributed to the less overbite observed for GFO as a result of canine wear

(Salsench et al., 2005). It could be speculated that the posterior teeth will be subjected to

more stresses laterally. However, some authors have proposed that as tooth wear proceeds,

the surface contact area between teeth increases as well, which might dissipate the occlusal

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forces, rendering them less susceptible to future wear in comparison with pointy cusps

(Seligman and Pullinger, 1995; Hidaka et al., 1999).

The included studies evaluated the risk of temporomandibular joint disorder (TMD)

development by condylar displacement and muscle EMG activity. The condylar displacement

has been investigated to estimate TMJ loading where a larger upward displacement of the

condyle could be associated with a larger compressive load within the TMJ (Okano et al.,

2002). The outcome of this review indicates that clenching contributes to upward condylar

movements, which coincides with other investigations (Ito et al., 1986; Kuboki et al., 1996). In

addition, the occlusion scheme influences the magnitude of condylar displacement (Okano et

al., 2002; Okano et al., 2005) when the mandible is in edge-to-edge position. Regardless of the

clenching level, BBO caused the least vertical condylar displacement. Such an observation

could be related to the upward mandibular movement being resisted by the bilateral posterior

tooth contacts (Baba et al., 2000; Seedorf et al., 2009). It has been postulated that balancing

contacts might protect against compressive TMJ loading, causing fewer incidence of joint

noises (Minagi et al., 1997; Okano et al., 2002). However, during maximal clenching, CGO

caused less non-working condylar displacement than GFO, and was similar to BBO (Okano et

al., 2002). This could be due to the inability to clench with the heavy occlusal force in the edge-

to-edge position with CGO in comparison to GFO. As the canine is the primary tooth in contact

laterally, the occlusal loads will be concentrated on the canines, leading to the excessive

stimulation of the mechanoreceptors (Ottenhoff et al., 1992; Wang and Mehta, 2013) which

will reduce muscle contraction (Hayasaki et al., 2002). On the contrary, with GFO, due to the

additional contacts laterally, participants were able to induce more occlusal loads during

maximal clenching. Such an observation is confirmed by all the included EMG studies, that

indicated participants were able to produce greater EMG activity during edge-to-edge

clenching (Okano et al., 2002; Okano et al., 2007; Gutierrez et al., 2010). However, for

submaximal clenching, CGO caused a more superior condylar displacement than GFO (Okano

et al., 2005). The difference between the magnitudes of clenching could be related to

mandibular deformation after maximal clenching on the non-working side and subsequent

condylar elevation (Korioth and Hannam, 1994). As the participants were able to exert more

maximal clenching with GFO, mandibular deformation might occur leading to more upward

non-working condylar displacement. Despite the statistically significant difference between the

two schemes, the clinical impact is yet to be determined. Since the maximal displacement is

about 0.6-0.8 mm, attributing adverse TMJ consequences to such displacement is questionable

(Okano et al., 2002; Okano et al., 2005).

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EMG activity is a commonly used as an indicator of muscle activity in research (Gonzalez et al.,

2011; Hugger et al., 2012); where large EMG activity induced by clenching or parafunctional

activities can be indicative of muscle fatigue (Lerman, 1973; Christensen, 1981). Selective

occlusion alteration is thought to alleviate signs and symptoms of TMJ (Belser and Hannam,

1985), which is based on the assumption that certain occlusal interferences may act as triggers

in the development of bruxism or cause pain in the masticatory muscles by disturbing their

pattern of activity; however, the physiologic processes that ensue are poorly understood

(Hannam et al., 1977). Still, providing an occlusion scheme that can reduce the EMG activity of

muscles of mastication during function or parafunction is desirable. This systematic review

indicates that it is very likely that altering the lateral occlusion scheme influences the EMG

activity during parafunctional activities, while normal physiological function (chewing) was

found to minimally influence the muscles activity. In addition, masticatory muscles were

influenced by the lateral occlusion scheme differently, with the anterior temporalis muscle

being the most affected. In support to the protective role of CGO, several studies found that

CGO is associated with less EMG activity during parafunctional activities (Okano et al., 2002;

Okano et al., 2007; Gutierrez et al., 2010), while BBO was clearly associated with the greatest

EMG activity (Okano et al., 2002; Okano et al., 2007). Overall, as the number of contacts

increased on the working side, the magnitude of EMG activity also increased (Okano et al.,

2002; Okano et al., 2007). This could be due to the inability of patients to exert excessive

clenching forces where there are fewer teeth contacts. Interestingly, the significant effect of

different occlusion schemes on muscle activities was not always observed from all the studies.

Some muscles (anterior temporalis and sternocleidomastoid) (Gutierrez et al., 2010; Rodriguez

et al., 2011) appear to be more affected than others (masseter, infrahyoid and suprahyoid)

(Valenzuela et al., 2006; Miralles et al., 2007; Campillo et al., 2008; Valenzuela et al., 2012).

Further, the dynamic position appears to play a significant role for muscles activities. For

example, parafunctional activities, primarily edge-to-edge, were responsible for the greatest

increase in EMG activities. Clinically, muscle activities will be more complex as there is no

standardized position for parafunctional activities. Further, a genuine specific lateral occlusion

scheme is not commonly occurring. For example, many people have GFO for the first 1-2 mm

excursion followed by established CGO in the edge-to-edge position (Abduo et al., 2013). In

addition, balancing contacts were found to be very common within the normal population

(Abduo et al., 2013), which could lead to a deviation from the outcome of the experiments

included that are based on ideal scenarios or experimental set-up.

In terms of function, there are some signs that GFO facilitates quicker chewing. This could be

attributed to the greater tooth contacts during lateral movements, and greater freedom in

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lateral movement. Similar observations were made from the chewing experiment by Buschang

et al, who found participants with a deep bite had slower chewing and a narrowing chewing

pattern (Buschang et al., 2007).

For natural dentition, there is no evidence that lateral occlusion scheme influences patient’s

satisfaction (Yi et al., 1996). However, for implant-supported prostheses, GFO might be more

comfortable to patients with fixed maxillary implant-supported prosthesis over a period of a

few months (Jemt et al., 1985). This could be related to the lack of proprioception for the

implant-supported prosthesis, and the greater freedom for mandibular movement. Such

findings could support the recommendation of a greater freedom of movement.

4.5.2. Long-term effect of lateral occlusion scheme

From the long-term studies, this systematic review supports that there is no relationship

between lateral occlusion scheme and dysfunction. According to Yi and Carlsson, CGO, GFO,

and BBO were not related to dysfunction development (Yi et al., 1996). The other studies

confirmed that for asymptomatic individuals, the lateral occlusion scheme can be altered

without causing patient discomfort or dysfunction development. This applies to CGO and GFO

and for tooth- and implant-supported prosthesis. Although the patient might be aware of the

occlusion alteration, patient adaptation was reported after a brief period following prostheses

insertion (a few weeks to a few months) (Dahl and Krogstad, 1985; Gross and Ormianer, 1994;

Ormianer and Gross, 1998; Ormianer and Palty, 2009). However, patient awareness of the

alteration has been primarily attributed to the increase of the VDO. This has been clearly

demonstrated by Sierpinska et al. who found that following an increase of VDO, the EMG

activity of masticatory muscles had reduced immediately. Following a 3 months period, the

EMG activity was restored to similar levels to the pre-treatment EMG activity (Sierpinska et al.,

2013). This also supports that the altered EMG activity detected by the earlier studies is due to

experimental design with no true clinical significance, and the EMG activity might be restored

to closer to base line record.

The outcome of this review supports that there is no causative relationship between the

lateral occlusion scheme and TMD development. This is in accordance with the multiple logistic

regression analysis that found that only for 15% of cases, the occlusal factors were related to

TMD development (Pullinger et al., 1993). Further, Seligman and Pullinger established that

altering the occlusal variable is not necessarily associated with TMD developments (Seligman

and Pullinger, 1991). Likewise, the current state of evidence indicates that occlusal treatment

will not prevent or treat TMD. Instead, non occlusal treatment is considered more justifiable

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and conservative (De Boever et al., 2000a; b; Koh and Robinson, 2003; Liu et al., 2012; Turp

and Schindler, 2012). Therefore, altering the occlusion scheme by restoration to solely prevent

or treat TMD is not acceptable according to the current level of evidence.

In addition, this review cannot find a clear relationship between the lateral occlusion scheme

and mechanical complications of the restorative treatment for tooth supported restoration or

prosthesis. CGO and GFO appear to have a satisfactory outcome for composite restorations

and for fixed prostheses for up to 5 years. For composite, restoration degradation was

observed in relation to wear, surface roughness, marginal integrity and staining (Hemmings et

al., 2000; Redman et al., 2003; Poyser et al., 2007; Schmidlin et al., 2009; Attin et al., 2012; Al-

Khayatt et al., 2013). However, this is not necessarily related to the occlusion scheme. Instead,

it appears to be related to the restorative material which is in accordance with other clinical

trials (Kopperud et al., 2012; Pallesen et al., 2013). Hemmings et al. found the success was

dependent on the composite brand (Hemmings et al., 2000). Likewise, Redman et al. found the

marginal fracture was affected by the composite materials used (Redman et al., 2003). Other

than the occlusion scheme, bruxism appears to contribute to the failure rate of the composite

restorations (Redman et al., 2003). Therefore, a relationship between restoration/prosthesis

longevity and lateral occlusion scheme cannot be established at this stage. Preferably, this

relationship should further be validated by a comparative long-term clinical trial.

Biomechanically, Implant-supported prosthesis differs from tooth prosthesis in the lack of the

periodontal ligament and its proprioceptive capabilities (Kim et al., 2005). Several authors

confirmed that teeth are more sensitive in detecting occlusal interferences than implants

(Jacobs and van Steenberghe, 1993; Mericske-Stern et al., 1995). In addition, the cushioning

effect of the periodontal ligament will render the tooth twenty times more mobile more than

the implant (Kim et al., 2005). Therefore, it is expected that the risk of overloading is greater

for implants than teeth. However, whether altering the occlusion scheme will cause a

significant overloading of the implants is yet to be answered. Further, it is not yet known if the

occlusion on oral implants should be different from that of teeth (Carlsson, 2009).

For fixed dental prosthesis supported by implants, there are some signs that GFO is associated

with greater mechanical complications in the form of ceramic chipping than CGO (Kinsel and

Lin, 2009). However, the authors did not attribute the increased rate of ceramic chipping to

the GFO. In general, for implant prosthesis, it is recommended to alleviate the implant

prosthesis from lateral contacts during excursion and maintaining contacts on natural teeth

(Taylor et al., 2000; Kim et al., 2005), which is envisaged to minimize the non-axial loading of

implant components, which puts them at greater risk for mechanical failure through micro

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movement or flexure (Taylor et al., 2000). Thus, CGO would be beneficial to alleviate the

contacts on posterior implant prosthesis. On the contrary, with more extensive implant

prosthesis, it is rather difficult to alleviate all of the implant prosthesis from lateral contacts

during excursion, which mandates GFO. However, systematic reviews revealed that multi-unit

implant prosthesis has a tendency to have more mechanical complications than single implant

prosthesis (Goodacre et al., 2003b). For example, over a period of 5 years, the incidence of

ceramic veneer fracture was 3.5% (Jung et al., 2012) for single implant prosthesis and 13.5%

for fixed dental prosthesis (Pjetursson et al., 2012). The effect of the prosthesis extension

could contribute to the observed difference in incidence of ceramic chipping. Still, the

implication of the lateral occlusion scheme on major complications such as screw

complications, implant fracture, and components fracture is yet to be investigated. Therefore,

the current evidence does not answer the question of whether the implant occlusion should

differ from natural teeth occlusion or not (Carlsson, 2009). More critically contributing factors

to veneer complications are the presence of bruxism and the opposing occlusion. There is an

agreement between the two included studies that bruxism and opposing prosthesis supported

by implants contribute to the increased incidence of veneer chipping (Kinsel and Lin, 2009;

Ormianer and Palty, 2009). Therefore, a recommendation was made to wear an occlusal splint

for protection against the parafunctional activities. It is very likely that the lateral occlusion

scheme is a less critical factor for implant overloading than the parafunctional activities or

opposing dentition.

4.5.3. Ideal lateral occlusion scheme

This systematic review revealed that although the different lateral occlusion schemes illicit

different immediate responses, the long-term effect of any scheme cannot be confirmed. Since

the long-term studies have confirmed that patients with CGO or GFO can function comfortably,

a bench mark lateral occlusion scheme cannot be proposed, as stated by earlier reports

(Becker and Kaiser, 1993; Turp et al., 2008). This also fits with a recent systematic review that

reports a significant variation of the lateral occlusion scheme for the physiological non-

restored dentition, and that a genuine lateral occlusion scheme rarely occurs naturally (Abduo

et al., 2013). This supports the view that the impact of the lateral occlusion schemes was

overrated in the earlier literature (Carlsson, 2010). Consequently, in accordance with several

investigators, it is recommended to implement flexibility and broader principles in occlusion

design (Becker and Kaiser, 1993; Turp et al., 2008; Abduo et al., 2013). Therefore, as a clinical

guide, instead of adhering to a preconceived occlusion scheme when complex restorative

treatment is indicated, the clinician should consider an occlusion scheme that is practical,

101

simple, conservative and allows aesthetic treatment (Becker and Kaiser, 1993; Wiskott and

Belser, 1995; Bryant, 2003; Carlsson, 2010).

The current state of evidence does not provide information about the effect lateral occlusion

has on different restorative parameters. For example, the knowledge is scarce about the effect

of lateral occlusion scheme on periodontally and endodontically compromised abutment

teeth. Further, there is no information about the response of restorative materials to different

lateral occlusion schemes. Several authors have mentioned that there is no implant-specific

occlusion, and the lateral occlusion scheme for implant prostheses should not necessarily

deviate from the occlusion scheme on tooth supported prostheses (Taylor et al., 2000; Kim et

al., 2005). It is very desirable that the effects of the lateral occlusion scheme on different

restorative variables are evaluated by randomized controlled trials.

Nevertheless, after scrutinizing the current evidences, it might be possible to confirm some

reasonable guidelines. Three key features should be incorporated in any lateral occlusion

scheme for prosthesis design: (1) long centric (2) disclusion induced by morphologies and (3)

lack of balancing side contacts and interferences.

A feature of the natural dentition is maintenance of numerous tooth contacts after a partial

excursion of 1-1.5 mm. Therefore, it was stated that the incidence of GFO is much higher than

CGO after partial excursion (Abduo et al., 2013). This fits with the recommendation of several

authors about the freedom of movement of centric occlusion, where teeth contacts are

maintained with mandibular lateral movements of 1 mm (Schuyler, 1963; DiPietro, 1977;

Goldstein, 1979). The proposal was that such a design will allow smooth and multidirectional

freedom of mandibular movement and enhanced patient comfort (Jemt et al., 1985). In

addition, such morphology will centralize the occlusal forces vertically to the apical direction

and reduce the lateral forces and bending moments (Weinberg, 1964). Further, the possibility

of introducing premature occlusal contacts is reduced. On the contrary, constricted movement

and immediate disclusion is expected to manifest in greater patient awareness and an increase

of lateral forces on the dentition (Schuyler, 1963; Weinberg, 1964; Belser and Hannam, 1985).

From the long-term studies included in this review, it appears that CGO and GFO are equally

acceptable. CGO has a practical advantage of being easier to produce than GFO. Further, CGO

might be a useful option if the canines are in excellent condition (Yi et al., 1996) which will

allow them to cope with heavy lateral forces. However, it is also acknowledged that occlusion

is rather dynamic and has a tendency to change with time. With aging and tooth wear, the

prevalence of GFO is increased accordingly whether the dentition is natural (Panek et al., 2008)

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or restored (Yi et al., 1996). Therefore, it might be useful to design the occlusion to allow GFO

development following canine wear.

In line with many recommendations, balancing contacts are better to be avoided (Becker and

Kaiser, 1993; Kim et al., 2005). Although they frequently occur naturally and clinical evidence

about their negative consequences is still lacking (Carlsson, 2010), achieving evenly balancing

contacts is rather impractical and difficult without introducing interferences. Further, there is

no genuine advantage that can justify their introduction, although they might develop after

settling of the prosthesis (Yi et al., 1996).

4.6. Conclusions

Within the limitations of this review, the following can be concluded:

1. There are some differences between the different lateral occlusion schemes in relation to

parafunctional muscle activities, and the magnitude of mandibular movement. However,

physiological function and patient’s acceptance appear to be minimally influenced by the

lateral occlusion scheme. Nevertheless, the clinical significance of the reported differences

cannot be established since the long-term studies have confirmed the suitability of CGO

and GFO.

2. CGO or GFO on are equally acceptable. The degree of multidirectional freedom of

mandibular movement appears to be physiological. The evidence supports a flexible

principle of occlusion rather than a preconceived occlusion theory.

3. To date, similar lateral occlusion principles can be considered for implant prosthesis.

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Chapter Five

5. Aims of the Study and Hypotheses

104

5.1. Aims

The central aim of this thesis is to compare the outcome of conventional and digital wax-ups

for prosthodontic treatment in relation to precision, aesthetics, contour, and occlusion. These

parameters were included because they are the critical features in any diagnostic wax-up.

The specific aims of this research:

Develop a form of digital wax-up.

Develop a tool for comparison.

Actual comparison between digital and conventional.

105

5.2. Hypotheses

The research has the following null hypotheses:

Hypothesis 1

There is no difference between conventional and digital wax-ups in relation to precision.

Hypothesis 2

There is no difference between conventional and digital wax-ups in relation to contour

alterations.

Hypothesis 3

There is no difference between conventional and digital wax-ups in relation to the maximal

intercuspal occlusal relationship (static occlusion).

Hypothesis 4

There is no difference between conventional and digital wax-ups in relation to dynamic

occlusal relationship.

Hypothesis 5

There is no difference between conventional and digital wax-ups in relation to aesthetics.

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Chapter Six

6. Materials and Methods

The methodology described in this chapter was described in the following articles:

Abduo J. Virtual prosthodontic planning for oral rehabilitation: a pilot study. CI Health. 2012;

34-42. (Appendix F)

Abduo J, Bennamoun M. Three-dimensional image registration as a tool for forensic

odontology: a preliminary investigation. American Journal of Forensic Medicine and Pathology.

2013; 34:260-266. (Appendix G)

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6.1. Patient Recruitments

A human research ethics approval was obtained from the Human Research Ethics Committee

of The University of Western Australia (RA/44/1/5079). Models of 15 patients receiving fixed

prosthodontic treatment were collected according to pre-determined selection criteria (Table

6—1). From all the models, a total of 238 teeth required fixed prostheses. For all the patients,

diagnostic wax-up prior to the prosthetic treatment was performed. The treatments were

provided at the Oral Health Centre of Western Australia.

Table 6—1 Selection criteria

Fixed prosthodontic treatment in the form of crowns or bridges Diagnostic wax-up is needed The planned prostheses were completely supported by natural teeth No removable or implant prostheses

6.2. Pre-Treatment Models

For each patient, maxillary and mandibular impressions were taken by irreversible hydrocolloid

impression material (Alginate, GC America, IL, USA). Whenever indicated, an intra-oral occlusal

relation record was taken by polyvinyl siloxane registration material (GC Exabite, GC America,

IL, USA). The impressions were poured by dental stone (Buff Stone, Adelaide Moulding &

Casting Supplies, South Australia, Australia). These models comprised the pre-treatment dental

situations. All the models were duplicated twice by reversible hydrocolloid material (Magafeel,

MKM System, Haanova, Slovakia). One set of models were treated by conventional wax-up and

the other by digital wax-up.

The pre-treatment models were scanned by a micro-CT scanner (SkyScan, Bruker micro-CT,

Kontich, Belgium) (12 µm resolution, 360o scanning, 70 KV source voltage, 1.0 mm Al filtration).

The micro-CT scanner was used in this study as an alternative to 3D surface scanner due to its

availability in The University of Western Australia. The advantage of micro-CT scanning is the

ability to produce an accurate image that exhibits a dimensional error of 0.1% (Waring et al.,

2012). Virtual 3D Stereolithography (STL) images of the maxillary and mandibular models were

constructed from the Digital Imaging and Communication Medicine (DICOM) images with the

aid of a DICOM viewing program (CTvox, Bruker micro-CT, Kontich, Belgium) (Figure 6-1). The

STL images of the pre-treatment models were used for the digital wax-up and the subsequent

analysis (Figure 6-2).

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A B

Figure 6-1 STL image construction from DICOM images. (A) An example of single slice DICOM image. (B) The process of STL image construction

A

B

Figure 6-2 (A) Actual pre-treatment maxillary and mandibular casts. (B) Virtual pre-treatment models.

To ensure the STL images obtained from micro-CT scanning are at least equivalent to the

images provided by a commercial scanner, the micro-CT scanner precision was validated. A

single model was sent to a commercial dental laboratory for scanning by a laser surface

scanner with an accuracy of 20 µm (3Shape D-640, Wieland-Imes, Pforzheim, Germany). The

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laser surface scanner was used as it is the benchmark scanner for dental models (Persson et

al., 2006). The same model was scanned by the micro-CT scanner, and the two STL images

were compared. The comparison revealed a similar outcome by the two scanners, which

confirms the reliability of the micro-CT scanner (dimensional error of less than 0.0 mm) (Figure

6-3).

A B C

Figure 6-3 The micro-CT scanner validation process. (A) A maxillary model scanned by the laser scanner. (B) The same model after scanning by the micro-CT scanner. (B) A colour-coded map generated after registering the two STL images, which confirms the similarity between the two images.

6.3. Conventional Wax-Up

The actual models of each patient were articulated on a semi-adjustable articulator (Whip Mix,

Louiseville, KY, USA) using the intra-oral occlusal record. Average values were used for the

articulator setting. To facilitate the subsequent digital evaluation, silicone material putty

(Dental Speedex Putty, Coltene/Whaledent AG, Altstatten, Switzerland) was applied on the

buccal aspects on the posterior teeth of the articulated models. The silicone registration

indices were scanned by the micro-CT scanner to obtain virtual registration indices.

The conventional wax-up was completed by the additive waxing technique, where the inlay

wax (VITA Zahnfabrik, Bad Sackingen, Germany) was applied to modify tooth morphology. In

some situations, the external surfaces of the teeth were trimmed. The wax-up aimed to

replace the missing tooth structures, establish natural tooth morphology, achieve symmetry

between the two sides, and obtain even bilateral occlusal contacts and a physiological lateral

occlusal scheme (canine-guided or group function occlusions) (Figure 6-4). All the conventional

wax-ups were completed by an experienced dental technician with fixed prosthodontics (more

than 10 years of experience). One clinician was responsible for approving the diagnostic wax-

ups. Following the completion of the conventional wax-up, silicone putty was applied on the

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buccal aspects of the modified models. As per the pre-treatment models, the conventional

wax-up models and the registration indices were scanned by the micro-CT scanner to obtain

STL images (Figure 6-5).

A B

C D

Figure 6-4 Examples of conventional wax-up. (A) Pre-treatment situation illustrating irregular and rotated teeth. (B) Wax-up of the two central incisors. (C) Pre-treatment situation of generalized tooth wear. (D) Wax-up of the whole maxillary teeth.

A B

Figure 6-5 (A) Completed conventional wax-up model. (B) Virtual conventional wax-up model.

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6.4. Virtual Articulation

With the aid of the virtual silicone indices, the pre-treatment and conventional wax-up models

were articulated through the process of image registration. A 3D rendering software package

(Geomagic Studio, Raindrop Geomagic Inc., Research Triangle Park, NC, USA) was used for the

registration process (Delong et al., 2002). Eventually, the silicone indices were deleted digitally,

and the two models were digitally articulated (Figure 6-6).

A B

C D

Figure 6-6 (A) The articulation process. The maxillary and mandibular virtual models before articulation. (B) The virtual silicone registration indices that can fit on the buccal aspects of articulated models. (C) The maxillary and mandibular models were repositioned according to the silicone indices by the process of image registration. (D) The articulated maxillary and mandibular models after the removal of silicone indices.

6.5. Digital Wax-Up

The maxillary and mandibular virtual pre-treatment models of each patient were used for the

digital wax-up procedure. The virtual pre-treatment models were articulated according to the

articulated conventional wax-up models. This was achieved by superimposing the virtual pre-

treatment models on the virtually articulated conventional wax-up models. The unaltered

tissues were used for the registration. The Geomagic Studio software was used to complete

the digital wax-up. To obtain aesthetic tooth morphology, physiological teeth moulds

(Phonares Teeth, Ivoclar Vivadent AG, Schaan, Liechtenstein) were scanned by the micro-CT

scanner (Figure 6-7).

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Figure 6-7 Examples of the available virtual teeth. As the anterior maxillary teeth are key teeth in obtaining ideal aesthetics, many teeth shapes are available for clinician use.

Each tooth was fitted virtually on the model with the aim of obtaining ideal teeth

arrangement, emergence profile, symmetry and aesthetics. Since the gingival-tooth junction

demarcates the most apical extension of tooth modification, it was marked on the virtual pre-

treatment models. The virtual tooth alignment involved size alteration, rotation and

translation. This was followed by ensuring that ideal occlusal contacts existed. This was

achieved by locating the cusps within the opposing fossae. For each case, a similar occlusion

scheme was established to what would have been implemented in the conventional wax-up

(Figure 6-8). After the completion of the wax-up, the scanned model and the virtual teeth were

merged to formulate a single model, which improves the computation speed of the

subsequent analysis.

A B C

Figure 6-8 Series of images that illustrate the digital teeth fitting. (A) Pre-treatment model. (B) Commencement of the digital wax-up. (C) Completed digital wax-up of the anterior maxillary teeth.

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6.6. Analysis

Since all the models (pre-treatment, conventional wax-up and digital wax-up models) are

digital, all the analyses were completed digitally. The details of each analysis are presented

within the relevant experimental chapter. However, the mandatory steps for digital analysis

are discussed in this section: image registration and virtual quantification.

6.6.1. Image Registration

Several authors have discussed the image registration process which implements algorithms to

automatically align and estimate similarities between two 3D images in a process commonly

called image registration (Williams et al., 1999; Williams and Bennamoun, 2001; Mian et al.,

2006; Abduo and Bennamoun, 2013). For this project, a similar approach was used to compare

and align the different models. The virtual image registration of the different models was

performed by Geomagic Studio software. The alignment process is composed of three

sequential steps: point-to-point registration, global registration, and the calculation of the 3D

euclidean distances (EDs) between the two models. The point-to-point registration determines

the initial approximate orientation of the two images by manually locating common

anatomical landmarks, such as cusp tip, fossa, groove or gingival margin. A correspondence of

at least three points was selected. These points are selected on unaltered surfaces. The global

registration is based on the Iterative Closest Point algorithm (Besl and McKay, 1992), and it

aims to align the meshes according to the best-fit principle. As a function of the software, the

deviations between two images were represented as the average 3D EDs of 2000 random

corresponding points on the common surfaces of the two meshes. The absolute deviation

values were used in the study to solely indicate the magnitude of the deviations. Therefore,

the less mean distance between the meshes, the better the fit between the two meshes. This

quantitative measure provides an estimate of the similarity between the 2 images.

Qualitatively, the discrepancy distribution between the different models can be illustrated in

colour-difference maps to locate the dimensional positive and negative differences (Figure 6-

9). The threshold value was set at 1 mm. The warm colours represent positive deviations,

whereas the cold colours represent negative deviations. The green colour indicates an optimal

match.

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A B

C D

Figure 6-9 An example of the process of image registration. (A) A pre-treatment model. (B) The model after the dental modifications. (C) The models were superimposed by the process of image registration. As the soft tissues were not altered, they were used as a reference to control the registration process. (D) Colour-coded map can be implemented to quantify the differences between the two models.

6.6.2. Virtual measurements

A key advantage of digital measurements is the accurate quantification. This involves the

measurements of the distance between two spatial points and calculation of specified surface

area (Figure 6-10). As the measurements occur on 3D virtual models, the models can be

magnified. This will facilitate accurate location of the area of interest. The distance and area

measurements were conducted using the feature of 3D rendering software.

A B

Figure 6-10 Images illustrating the use of the software for virtual measurements. (A) The virtual ruler can be implemented to measure the distance between the different coordinates that represent tooth dimension. (B) An example of occlusal area quantification.

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Several authors reported that the advantages of digital measurements over physical

measurements are accuracy, convenience, consistency and less chance of errors (Redlich et al.,

2008; Dalstra and Melsen, 2009; Prasad and Al-Kheraif, 2013). Redlich et al. demonstrated that

3D virtual measurements were, at least, similar to manual measurements (Redlich et al., 2008).

Furthermore, Dalstra et al. reported that virtual model measurements suffered from less

variability than manual model measurements (Dalstra and Melsen, 2009). Prasad and Al-

Kheraif compared measurements obtained from travelling microscope and micro-CT. They

found that the virtual measurements of the micro-CT slices were more consistent than

travelling microscope measurements (Prasad and Al-Kheraif, 2013). This superior accuracy

could be attributed to the ability of enlarging the models to locate the points of interest

precisely, and the software ability to measure the distance between coordinates accurately

(Quintero et al., 1999; Kusnoto and Evans, 2002). This will eventually overcome the difficulties

of measuring the dimensions of extremely fine features.

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Chapter Seven

7. Precision of Digital Prosthodontic Planning for

Oral Rehabilitation


Abduo J, Bennamoun M, Tennant M, McGeachie J. Precision of virtual prosthodontic planning

for oral rehabilitation. British Journal of Applied Sciences and Technology. 2014; 4:3915-3929.

(Appendix H)

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7.1. Abstract

Objective: Recently, digital wax-up is proposed as a tool to aid prosthodontic planning.

Accuracy is a requirement of the diagnostic wax-up in order for its information to be

transferrable clinically. The purpose of this study is to evaluate the precision of digital

prosthodontic planning in the form of digital wax-up.

Materials and Methods: Twenty-five dental arch models of 15 patients were collected. The

models were duplicated twice to allow for the execution of conventional wax-up and digital

wax-up. The conventional wax-up involved tooth modification with inlay wax. The digital wax-

up was based on fitting average tooth forms on virtual pre-treatment models. For the analysis,

the conventional wax-up models were converted to digital models. All the wax-up models

were segmented to yield soft tissue and tooth-gingiva models. With the aid of the 3D image

registration process, the segmented models were superimposed on the pre-treatment models

to evaluate the accuracy of fit. Further, the gingival margin discrepancies of each wax-up

protocol were evaluated.

Results: The image registration process revealed less discrepancies for the digital wax-up (soft

tissue = 0.11mm, tooth-gingiva junction = 0.11mm) than the conventional wax-up (soft tissue =

0.18mm, tooth-gingiva junction = 0.20mm). Similarly, the gingival margin discrepancies were

less for the digital wax-up. However, the patterns of discrepancies were similar for the two

wax-up protocols.

Conclusion: In terms of accuracy and transferability, the digital wax-up is comparable to the

conventional wax-up.

Key words: wax-up; digital; micro-CT; dental model; image registration

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7.2. Introduction

Prosthodontic treatment is frequently indicated to manage aesthetic and functional problems.

In many instances, the treatment necessitates irreversible alterations to the existing teeth. In

order to justify such alterations, a significant benefit of the treatment should be apparent. In

an aesthetically conscious society, the patient’s aesthetic needs should be predictably

incorporated into the prosthodontic treatment. Therefore, to reach a satisfactory outcome,

comprehensive diagnostic planning and wax-up should be conducted before embarking on the

definitive prosthodontic rehabilitation.

Diagnostic wax-up simulates the prosthodontic treatment and allows for visualization of the

outcome and helps in deciding on the most adequate treatment plan for a specific case

(Magne and Belser, 2004; Gurel, 2007). The outcome of this “trial” treatment can be

demonstrated to the patient for approval or suggestion of any further modifications. In this

manner, the patient will be more informed of the final outcome. Subsequently, the diagnostic

wax-up will facilitate the “outcome-based treatment” which implies that the magnitude of

irreversible alteration to the teeth is dictated by the final outcome rather than the initial

patient presentation (Magne and Belser, 2004; Gurel, 2007). In addition, the provisional

restorations can be fabricated according to the diagnostic wax-up, and should the provisional

outcome satisfy the patient, the definitive prostheses will be fabricated to resemble the

diagnostic wax-up (Magne and Belser, 2004; Gurel, 2007).

The wax-up involves altering the teeth of diagnostic models to improve tooth morphology,

contour, vertical dimension and horizontal tooth width. The ideal diagnostic wax-up should be

precise, applicable and transferable. In order for the wax-up information to be precisely

transferrable intra-orally, the surrounding soft tissues should act as a reference landmark. This

mandates that the soft tissues are not to be altered through the wax-up process. Instead, the

modifications should be confined to the hard tissues. However, since the clinical crown

emerges from the gingival tissues, the tooth contour should be minimally affected on the

cervical area. Subsequently, the contour alteration should increase gradually towards the

occlusal surface.

More recently, with the advent of laser scanning, virtual planning, rapid prototyping and

computer-aided design and manufacturing, it is hypothesized that digital prosthodontic

planning can be accomplished in a time-efficient and well-controlled fashion. Digital wax-up

was proposed as a tool to plan for prosthodontic treatment instead of conventional wax-up

(Abduo, 2012). It is envisioned that the digital wax-up will overcome the problems of the

conventional wax-up such as time consumption and the requirement of high technical skills.

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This study aims to evaluate the accuracy of digital diagnostic wax-up. The null hypothesis is

that the two wax-up protocols exhibit similar accuracy.


Human research ethics approval was obtained from the Human Research Ethics Committee of

The University of Western Australia (RA/4/1/5097). Twenty five dental arch models of 15

patients requiring prosthodontic treatment were collected. From the models’ pool, a total of

238 teeth required fixed prosthodontic treatment. Consequently, diagnostic wax-up prior to

the prosthetic treatment was indicated.

Maxillary and mandibular impressions were taken by irreversible hydrocolloid impression

material (Alginate, GC America, IL, USA). Whenever indicated, an occlusal relation record was

taken by polyvinyl siloxane material (GC Exabite, GC America, IL, USA). The impressions were

poured by dental stone (Buff Stone, Adelaide Moulding & Casting Supplies, South Australia,

Australia). These models comprised the pre-treatment situation. Each model was duplicated

twice by reversible hydrocolloid material (Magafeel, MKM System, Haanova, Slovakia). One set

of models were treated by conventional wax-up and the other by digital wax-up.

7.3.1. Conventional wax-up



articulator setting. The conventional wax-up was completed by the addition of inlay wax (VITA

Zahnfabrik, Bad Sackingen, Germany) on the teeth contour. In some cases, the external

surfaces of the teeth were trimmed. The wax-up aimed to replace the missing tooth structures,

establish natural tooth morphology, achieve symmetry between the two sides, and obtain

even bilateral occlusal contacts and a physiological lateral occlusal scheme (canine-guided or

group function occlusions) (Figure 7-1). The conventional wax-ups were completed by an

experienced dental technician.

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A B C

Figure 7-1 Conventional wax-up: (A) Pre-treatment models. (B) Conventional wax-up models. (C) Scanned conventional wax-up models.

The conventional wax-up models were scanned by a micro-CT scanner (SkyScan, Bruker micro-

CT, Kontich, Belgium). Virtual 3D Stereolithography (STL) images of the maxillary and

mandibular models were constructed from the Digital Imaging and Communication Medicine

(DICOM) images with the aid of a DICOM viewing program (CTvox, Bruker micro-CT, Kontich,

Belgium). The STL image of the conventional wax-up was used for comparison purpose with

the digital wax-up.

7.3.2. Digital wax-up

The maxillary and mandibular pre-treatment models were scanned by the micro-CT scanner

and STL images were constructed. A 3D rendering software package (Geomagic Studio,

Raindrop Geomagic Inc., Research Triangle Park, NC, USA) was used to complete the digital

wax-up. The maxillary and mandibular models were virtually articulated by using the point-to-

point alignment feature of Geomagic Studio. To obtain aesthetic tooth morphology,

physiological teeth moulds (Phonares Teeth, Ivoclar Vivadent AG, Schaan, Liechtenstein) were

scanned by the micro-CT scanner. Each virtual tooth was fitted manually on the model with the

aim of obtaining ideal teeth arrangement, emergence profile, symmetry and aesthetics. The

virtual tooth alignment involved size alteration, rotation and translation. This was followed by

ensuring that ideal occlusal contacts existed. For each case, a similar occlusion scheme was

established to mimic what would have been implemented in the conventional wax-up (Figure

7-2). After the completion of the wax-up, the scanned model and the virtual teeth were

merged to formulate a single model. Merging the models improves the computation speed of

the subsequent analysis.

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A B C

Figure 7-2 Digital wax-up. (A) Scanned pre-treatment models. (B) Scanned physiological teeth. (C) Digital wax-up models.

7.3.3. Analysis

For the direct comparison between the conventional and digital wax-ups, all the models were

remeshed with a density of 0.1 mm, and the base of the models where trimmed to a similar

extension. All the measurements were completed digitally (DeLong et al., 2003; Redlich et al.,

2008; Abduo and Bennamoun, 2013). In order to compare the conventional wax-up and the

digital wax-up, the following variables were considered: image registration of the soft tissues,

image registration of tooth-gingiva junctions and measurements of the gingival margin

extensions.

The purpose of the image registration of the soft tissue was to indicate the accuracy of the

unaltered structures that can be used as a reference for intra-oral application. The tooth-

gingiva junctions’ evaluation will quantify the effect of the wax-up on tooth emergence profile.

Such information is relevant to the hygienic feature of the proposed treatment. The

measurements of gingival margin extensions aimed to locate and quantify the possible impact

of each wax-up on the gingival margins. This information reflects the precision of each wax-up

of the different regions in the arch. For all the measurements, absolute values between the

images were used. This was to avoid any under-estimation of the mean discrepancy between

the images by combining positive and negative values. Therefore, the less the mean distance

between the wax-up and the pre-treatment model, the better the accuracy of the wax-up.

7.3.4. Image registration

As mentioned by several authors, image registration involves the automatic alignment of two

3D images to estimate any similarities or discrepancies (DeLong et al., 2003; Abduo and

Bennamoun, 2013). In this study, the 3D image of each wax-up was registered against the 3D

pre-treatment image. The registration process was performed by Geomagic Studio Software

through three sequential steps: (1) point-to-point registration, (2) global registration and (3)

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calculation of the 3D Euclidean distance (ED). The point-to-point registration determines the

initial approximate orientation of the pre-treatment and the wax-up images by manually

locating at least three common landmarks. The global registration is based on the Iterative

Closest Point algorithm (Abduo and Bennamoun, 2013), and it aims to align the meshes

according to the best-fit principle. The deviation between two aligned points was determined

by the 3D ED value. The following equation was utilized to calculate the ED between a pre-

treatment image point (AP) and a corresponding wax-up image point (AW) in 3D coordinates

(x, y and z).

𝐸𝐷(𝐴𝑃, 𝐴𝑊) = √(𝐴𝑃𝑥 − 𝐴𝑊𝑥)2 + (𝐴𝑃𝑦 − 𝐴𝑊𝑦)2 + (𝐴𝑃𝑧 − 𝐴𝑊𝑧)2 (1)

As a function of the software, the final deviation between the pre-treatment and wax-up

images were represented as an average 3D EDs of 2000 random corresponding points of the

common surfaces of the two images. This quantitative measurement provides an estimate of

the similarity between the two images.

In order to register the wax-up images to pre-treatment images, the segmentation function of

the Geomagic Studio Software was employed. The segmentation of each wax-up model

yielded two structures: (1) soft tissue and (2) tooth-gingiva junction (Figure 7-3). To obtain the

images of the soft tissues, the segmentation was executed at the junction between the teeth

and the gingiva. The tooth-gingiva junction image was comprised of the 0.5 mm cervical teeth

portion and the 0.5 mm coronal gingival portion. Subsequently, each segmented structure was

registered against the corresponding pre-treatment model. The segmentation ensured that

the registration process was restricted to the soft tissues and the tooth-gingiva junction

without the influence of the altered teeth.

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A B C

D E

Figure 7-3 The segmentation process that yielded soft tissue model and tooth-gingiva junction model. (A) Original model. (B) Selected soft tissue. (C) Selected tooth-gingiva junction. (D) Final soft tissue model. (E) Final junction model.

The accuracy of the segmented soft tissue of each wax-up was qualitatively evaluated against

the pre-treatment model by the colour-difference map. This map allows the determination of

the locations of the dimensional positive and negative deviations from the pre-treatment

model. The threshold value was set at 1 mm. The warm colours represent positive deviations,

whereas the cold colours represent negative deviations. The green colour indicates an optimal

match.

7.3.5. Gingival margin measurements

Following the image registration of the segmented soft tissues, the extensions of the gingival

margins were measured from the wax-up model to the gingival margins of the pre-treatment

model. For each tooth, the margins were measured at six locations: mesio-buccal, mid-buccal,

disto-buccal, mesio-lingual, mid-lingual and disto-lingual (Figure 7-4). For each location, a

digital point was located on the pre-treatment model and the corresponding location of the

wax-up model. Subsequently, the distance between the points was measured, which reflects

the accuracy of the gingival margins.

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Figure 7-4 Example of locating the points of measurement around the gingival margin of a lateral incisor. The black point is located on the mid-tooth area and the red points are on the proximal areas.

7.3.6. Statistical analysis

The difference of image registration and gingival margin measurements of the conventional

wax-up and digital wax-up was evaluated by the t-test analysis (P value = .01). In relation to the

gingival margins, the following variables were evaluated: location on the arch (anterior vs.

posterior), and location on the tooth (mid-tooth vs. proximal).

In box plot diagrams, the gingival margin measurements of incisors, canines, premolars and

molars of the maxillae and mandibles were plotted. Further, the differences between the wax-

ups were compared for each tooth type.

7.4. Results

7.4.1. Image Registration

Overall, after global registrations, the conventional wax-up exhibited a greater ED than digital

wax-up. In relation to the soft tissues accuracy, the conventional wax-up had an average ED of

0.18 mm (SD = 0.04) and the digital wax-up had an average ED of 0.11 mm (SD = 0.01mm). The

difference between the two wax-ups was statistically significant (P < .001). The ED for tooth-

gingiva junction was 0.20 mm (SD = 0.05mm) for conventional wax-up and 0.11 mm (SD =

0.04mm) for digital wax-up. Likewise, there was a significant statistical difference (P < .001).

The qualitative evaluation of colour maps revealed consistency in all the included cases (Figure

7-5). The conventional wax-ups generally exhibited a good fit on the soft tissues distant from

the gingival margins. Overall, some islands of distortion were observed but confined to

0.25mm. It appears that the discrepancies are associated with the corrugated areas, while the

smooth surfaces had better fit. However, most of the discrepancies occurred on the gingival

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margins. The most affected areas were the interdental papillae. Conversely, the soft tissues of

the digital wax-ups exhibited an optimal fit on the pre-treatment models. The areas of

distortion were confined to the gingival marginal area.

A B

C D

Figure 7-5 Colour-coded maps of each diagnostic wax-up after fitting on the pre-treatment model. (A) Conventional wax-up. (B) Magnified section of conventional wax-up. (C) Digital wax-up. (C) Magnified section of digital wax-up.

7.4.2. Gingival Margins

The mean gingival margin discrepancies for all locations was slightly higher for the

conventional wax-up (mean = 0.45mm, SD = 0.16mm) than for the digital wax-up (mean =

0.40mm, SD = 0.17mm). This difference was statistically insignificant. A comparison of the

proximal margins revealed that the conventional wax-up (mean = 0.59mm, SD = 0.20mm) had

insignificantly greater gingival margin discrepancies than the digital wax-up (mean = 0.51mm,

SD = 0.23mm). On the contrary, the mid-tooth measurements revealed insignificantly greater

values for digital wax-up (mean = 0.20mm, SD = 0.10mm) than the conventional wax-up (mean

= 0.16mm, SD = 0.13mm).

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Comparing the gingival margin discrepancies around the anterior teeth revealed insignificantly

greater discrepancies for the conventional wax-up (mean = 0.47mm, SD = 0.40mm) than for

the digital wax-up (mean = 0.43, SD = 0.43mm). Mid-tooth comparison, revealed insignificantly

greater values for conventional wax-up (mean 0.30 mm, SD = 0.26mm) than digital wax-up

(mean = 0.27mm, SD = 0.23mm). Likewise, the proximal gingival margins of the conventional

wax-up (mean = 0.62mm, SD = 0.45mm) had insignificantly greater discrepancy than digital

wax-up (mean = 0.58 mm, SD = 0.51mm). For both of the wax-ups, the proximal gingival

margin values were significantly greater than mid-tooth values (P < .001).

Posteriorly, the conventional wax-up (mean = 0.38mm, SD = 0.45mm) had greater

discrepancies than the digital wax-up (mean = 0.33, SD = 0.43mm). This difference was

statistically insignificant. At the mid-tooth, the digital wax-up (mean = 0.18mm, SD = 0.13mm)

was not statistically different from the conventional wax-up (mean = 0.13, SD = 0.13mm). At

the proximal areas, the conventional wax-up (mean = 0.56mm, SD = 0.43mm) had significantly

greater discrepancies (P < .001) than digital wax-up (mean = 0.45mm, SD = 0.44mm). Similar to

the anterior segments, both of the wax-ups showed significantly greater proximal gingival

margin values than the mid-tooth values (P < .001).

For the different teeth, the box plot diagrams illustrate that there is similarity in the pattern of

the gingival margin discrepancies magnitude for the conventional and digital diagnostic wax-

ups (Figure 7-6). After comparing the two wax-up protocols for the different maxillary and

mandibular teeth there was no statistically significant difference. The exception was the

proximal areas of the maxillary and mandibular molars which were highly significant (P < .001).

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Figure 7-6 The box plot diagrams of the gingival margins for each tooth category. (A) Mid-tooth area of the maxillary teeth. (B) Proximal area of the maxillary teeth. (C) Mid-tooth area of the mandibular teeth. (D) Proximal area of the mandibular teeth.

7.5. Discussion

Several authors proposed methods for CAD modelling of dental restorations. In general, the

modelling process is composed of restoration space calculations and algorithmic

approximation of the virtual restoration that exhibits a natural anatomy (Rekow et al., 1991).

The estimation of the virtual restorations can take the form of fitting an average tooth (Paulus

et al., 1999; Mehl et al., 2005a; b; Ender et al., 2011) and mirror imaging of the contralateral

tooth (Probst and Mehl, 2008). In this study, an average tooth mould was used to modify the

pre-treatment tooth contour. To enhance the applicability of the digital wax-up, actual models

can be produced by rapid prototyping technology. The printed model is envisioned to provide

direct guidance to the involved dental clinician and technician.

In the presence of multiple treatment options to manage complex patient presentations, it is

critical for the clinician and the patient to be fully aware of the treatment outcome prior to

commencement. This becomes even more important with today’s high aesthetic expectations.

Consequently, an accurate diagnostic wax-up will facilitate the clinician-patient

communication and allow the patient to provide an informed consent prior to the treatment.

Since the diagnostic wax-up is time consuming and requires special training and artistic

abilities, many clinicians provide prosthodontic treatment without an accurate and

representative wax-up. Thus, it is speculated that the introduced digital diagnostic wax-up will

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alleviate many of the difficulties associated with conventional wax-up. In addition, the digital

wax-up exhibits several advantages that justify its application. For example, the process is

completed digitally and requires no physical materials which present a significant economical

benefit (Miyazaki and Hotta, 2011; van Noort, 2012). The pre-treatment models are not

altered and can be preserved with the patient’s record (Hans, 2002; Abduo and Bennamoun,

2013). This will alleviate the difficulties of preserving records, which should be kept for 5 to 10

years (Abduo and Bennamoun, 2013). In addition, there are no restrictions on the

transferability of the virtual models, which will facilitate rapid and remote consultations

between clinicians (Hans, 2002; Mayers et al., 2005; Redlich et al., 2008). With the aid of

software analysis, it is possible to evaluate the implications of the proposed diagnostic wax-up

on oral tissues, which is a useful feature to analyse the feasibility of the proposed treatment

(Davis et al., 2012). It is expected that this feature will enhance patient communication, and

the reviewing of the proposed dental treatment with relative ease. Further, from the practical

perspective, the adequacy of tooth preparation can be quantitatively evaluated by the

software and any additional modifications can be proposed (Hans, 2002; Mayers et al., 2005;

Redlich et al., 2008; Davis et al., 2012).

In this study, all the measurements were completed digitally. Several authors reported that the

advantages of digital measurements over physical measurements are accuracy, convenience,

consistency and less chance of errors (Redlich et al., 2008; Dalstra and Melsen, 2009; Prasad

and Al-Kheraif, 2013). Redlich et al. demonstrated that 3D virtual measurements were similar

to manual measurements (Redlich et al., 2008). Furthermore, Dalstra et al. reported that

virtual model measurements suffered from less variability than actual model measurements

(Dalstra and Melsen, 2009). Prasad and Al-Kheraif compared measurements obtained from

travelling microscope and micro-CT. They found that the virtual measurements of the micro-CT

slices were more consistent than travelling microscope measurements (Prasad and Al-Kheraif,

2013). This superior accuracy could be attributed to the ability of enlarging the models to

locate the points of interest precisely, and the software ability to measure the distance

between coordinates accurately (Quintero et al., 1999; Kusnoto and Evans, 2002). This will

eventually overcome the problems of measuring the dimensions of extremely fine features.

ED measurements in this study (about 0.20 mm) were within the expected range found by

other investigators using a similar experimental set-up. For example, superimposing duplicated

dental models revealed that the ED ranged from 0.15 to 0.17 mm in one study (Abduo and

Bennamoun, 2013). Additionally, Bell et al. found the average ED between actual and digital

models was 0.27 mm (Bell et al., 2003). Hirogaki et al. found the difference between actual and

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digital models was within 0.30 mm (Hirogaki et al., 2001). Ayoub et al. found the models

digitization accuracy to be 0.20 mm (Ayoub et al., 1997).

The overall outcome of this study reflects the feasibility of digital wax-up. This is clear from its

comparability to the conventional wax-up protocol, which is the standard practice. This applies

to all the evaluated parameters of the study such as the anterior and posterior arch segments,

and the different locations around each tooth. Therefore, it could be expected that the

transferability and accuracy of the two wax-ups are similar. Both of them will produce

relatively similar restoration contours. This might potentially indicate that to a similar

situation, each wax-up will yield a similar outcome.

In terms of accuracy and transferability to the mouth, there are mathematical indications that

the digital wax-up is more accurate than the conventional wax-up. This is illustrated by the

lower ED values for the soft tissue and tooth-gingiva junction images, and the generally lower

discrepancies at the gingival margin. Although the null hypothesis is rejected, the outcome of

this study should be interpreted with caution as the base of the digital wax-up models was

composed from the pre-treatment model, while the conventional wax-up required additional

steps such as model duplication and second scanning. The duplication process involves using

duplicating material and pouring with stone model, which inevitably accumulates errors in the

form of expansion and shrinking (DeLong et al., 2003). The other source of inaccuracies is the

scanning that involves rendering and surface noise control to eliminate poorly located points.

Although rendering improves the quality of the image, it will inevitably influence the accuracy

of the final image (DeLong et al., 2003). However, from the clinical perspective, this difference

is most likely to be insignificant as the EDs for both wax-ups are minimal, which might be

acceptable for diagnostic wax-up.

The two wax-ups revealed that the proximal areas are more susceptible to discrepancies than

the mid-tooth areas (about twice the discrepancies). This was clearly observed from the

gingival margin measurements and the qualitative map analysis. Such findings could be

associated with more contour modifications and increased tooth emergence profile at the

proximal area compared with the middle of the tooth. An additional contributing factor is the

potential discrepancies in the impression procedure when the proximal area is recorded by

impression, due to the presence of excessive undercuts. Further, discrepancies can be caused

by the experimental design. For example, it is more difficult to virtually segment and quantify

corrugated regions compared to smooth regions (Redlich et al., 2008). On linear surfaces, such

as the mid-tooth area, it is easier to locate the points of interest (Zilberman et al., 2003), while

on the corrugated regions, such as the proximal areas, it might be difficult to locate the points

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of interest, resulting in more inconsistencies of measurements (Santoro et al., 2003; Zilberman

et al., 2003). Still, such discrepancies can be of minimal clinical relevance as the wax-up is

primarily used for provisional restoration fabrication, which can be predictably modified

clinically in the proximal areas (Magne and Belser, 2004).

The presented digital wax-up protocol did not incorporate the alteration of the gingival

architecture, which might be indicated prior to periodontal surgery (Nowzari, 2001).

Therefore, the results of this study cannot be generalized to such clinical presentations.

However, with the increased popularity of 3D digital imaging of hard tissues (Misch et al.,

2006), it is a possibility to integrate bone and soft tissue morphologies with the pre-treatment

model. With such information, the root anatomy and bone-soft tissue relationship can be

clearly visualized (Pinsky et al., 2006). Subsequently, in addition to tooth contour

modifications, the bone and the soft tissues can be altered by the digital wax-up. The obtained

information could further increase the accuracy of the computer-guided surgery.

In the future, in addition to the accuracy and transferability of the digital wax-up, the occlusion

contacts and the contour modifications should be quantified. In addition, the implications on

the dental aesthetics should be evaluated. Although there are some good indications that

digital prosthodontic planning is a valid procedure, prior to its routine application, this

approach should be refined. The applicability and efficiency of this approach can further

improve through the automation of the tooth modification process.

7.6. Conclusions

Within the limitations of this study, the following can be concluded that the digital

prosthodontic planning in the form of digital wax-up appears to be a promising treatment

planning tool in fixed prosthodontics. The outcome of the digital wax-up is comparable to the

outcome of the conventional wax-up in terms of accuracy and transferability.

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Chapter Eight

8. Influence of Conventional and Digital Wax-Ups

on Axial Tooth Contour


Abduo J, Bennamoun M, Tennant M. Influence of conventional and digital wax-ups on axial

tooth contour. International Journal of Periodontics and Restorative Dentistry. 2015; 35:e50-

e59. (Appendix I)

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8.1. Abstract

Objective: The aim of this study was to evaluate the impact of conventional and digital

diagnostic wax-up on the axial tooth contour.

Materials and Methods: Dental models of fifteen patients were collected. Each model

received conventional wax-up and digital wax-up. The conventional wax-up was based on

tooth modification with dental wax on actual models. The digital wax-up involved fitting an

average tooth form on virtual pre-treatment models. Each wax-up model was digitally

superimposed on the corresponding pre-treatment model. For each modified tooth, analysis

planes were extracted at three locations: mesial line angle, mid-tooth and distal line angle. The

impacts of the following variables were evaluated: inter-arch location (maxilla vs. mandible),

intra-arch location (anterior vs. posterior), tooth category (incisors, canines, premolars and

molars) and tooth location (mid-tooth vs. line angle).

Results: The axial contour of the modified teeth increased following each wax-up, and this

increase was directly proportional to the distance from the gingival margin. There is a clear

tendency for the digital wax-up to cause a greater contour increase than the conventional wax-

up. The anterior teeth were associated with a greater tooth contour increase than posterior

teeth and the contour of the molars was the least affected.

Conclusion: Although the conventional wax-up contour alteration was significantly less than

for the digital wax-up, the actual difference is minimal.

Key words: prosthodontics, planning, wax-up, profile, micro-CT

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8.2. Introduction

Fixed prosthodontic treatment is useful in improving tooth colour and morphology. However, a

significant drawback of the treatment is the necessity of irreversible tooth preparation. Ideally,

the prosthodontic treatment benefits should outweigh the disadvantages. One of the tools

used to evaluate the usefulness of fixed prosthodontic treatment is the diagnostic wax-up

(Magne and Belser, 2004), which aims to simulate the possible prosthodontic treatment on

dental models. Consequently, the modified models can be demonstrated to the patient for

prior approval. This tool is highly recommended in this age, where the population is very

aesthetically conscious (Gurel, 2007). In addition, the diagnostic wax-up facilitates the

“outcome-based treatment,” which implies the amount of tooth preparation is dictated by the

final outcome rather than the existing tooth morphology. Traditionally, the diagnostic wax-up

is executed on pre-treatment models and involves tooth modification and addition of wax until

the correct tooth shape is obtained.

It has been recommended that whenever a tooth is restored, the restoration should blend,

harmoniously with the existing tooth contour (Becker and Kaldahl, 1981; Croll, 1989), in a way

that the restoration cleanliness, durability and aesthetics are achieved (Goodacre et al.,

2003b). Three concepts of restoring the axial tooth contour were discussed in the literature:

preserving the original tooth contour, under-contouring and over-contouring. The rationale

behind maintaining the original contour is that the existing contour tends to be more

physiological and less likely to interfere with regular cleaning (Becker and Kaldahl, 1981; Croll,

1989). However, maintaining the original contour means the final shape modifications will be

subtle. Some authors described the merit of under-contouring which might render gingival

tissues more self-cleansible and maintainable (Perel, 1971; Tjan et al., 1980). However, this

approach is impractical as it could dramatically affect the appearance, crown thickness and

preparation invasiveness (Tjan et al., 1980). Over-contouring can occur if the final restoration

volume is greater than the initial tooth volume (Magne and Belser, 2004; Gurel, 2007). It has

the advantage of allowing for significant crown aesthetic improvement, especially if major

dentition irregularities existed (Cohen, 1995). Therefore, since diagnostic wax-up is

implemented prior to complex treatment, it could be associated with over-contouring the

prostheses. However, we do not yet have data about the impact of diagnostic wax-up on axial

tooth contour.

Recently, digital dentistry is growing in prosthodontics and has been utilized to execute a

diagnostic wax-up (Abduo, 2012). The procedure involves scanning, virtual modelling and

CAM. When compared to conventional wax up, the digital wax-up is an attractive option as it

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can be reversible, time-efficient, and cost-effective. This study aims to evaluate and compare

the impact of conventional and digital wax-ups on axial tooth contour. The null hypotheses are

that the wax-ups will increase the tooth contour, and the digital wax-up alterations will be

similar to the conventional wax-up alterations.


A human research ethics approval was obtained from the Human Research Ethics Committee

of The University of Western Australia (RA/44/1/5079). Models of 15 patients receiving fixed

prosthodontic treatment were collected. For all the patients, diagnostic wax-up was indicated.

From all the models, a total of 238 teeth required fixed prostheses. For all the patients,

diagnostic wax-up prior to the prosthetic treatment was performed. The treatments were

provided at the Oral Health Centre of Western Australia.

An impression of each arch was taken by irreversible hydrocolloid impression material

(Alginate, GC America, IL, USA). Whenever indicated, an occlusal relation record was taken by

polyvinyl siloxane registration material (GC Exabite, GC America, IL, USA). The impressions

were poured by dental stone (Buff Stone, Adelaide Moulding & Casting Supplies, South

Australia, Australia). These models comprised the pre-treatment dental situation. All the

models were duplicated twice by reversible hydrocolloid material (Magafeel, MKM System,

Haanova, Slovakia). One set of models were treated by conventional wax-up and the other by

digital wax-up.




articulator setting. The conventional wax-up was completed by the additive waxing technique,

where the wax was applied to modify tooth morphology. In some situations, the external

surfaces of the teeth were trimmed. The wax-up aimed to replace the missing tooth structures,

establish natural tooth morphology, achieve symmetry between the two sides, and obtain

even bilateral occlusal contacts and a physiological lateral occlusal scheme (canine-guided or

group function occlusions) (Figure 8-1). All the conventional wax-ups were completed by an

experienced dental technician.

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A B B

Figure 8-1 Conventional wax-up procedure: (A) Pre-treatment models. (B) Conventional wax-up models. (C) Scanned conventional wax-up models.

The conventional wax-up models were scanned by a micro-CT scanner (SkyScan, Bruker micro-

CT, Kontich, Belgium). Virtual 3D Stereolithography (STL) images of the maxillary and

mandibular models were constructed from the Digital Imaging and Communication Medicine

(DICOM) images with the aid of a DICOM viewing program (CTvox, Bruker micro-CT, Kontich,

Belgium). The STL image of the conventional wax-up was used for the subsequent analysis.


The maxillary and mandibular pre-treatment models were scanned by the micro-CT scanner

and STL images were constructed. A 3D rendering software package (Geomagic Studio,

Raindrop Geomagic Inc., Research Triangle Park, NC, USA) was used to complete the digital

wax-up. The maxillary and mandibular models were virtually articulated by using the point-to-

point alignment feature of Geomagic Studio. To obtain aesthetic tooth morphology,

physiological teeth moulds (Phonares Teeth, Ivoclar Vivadent AG, Schaan, Liechtenstein) were

scanned by the micro-CT scanner. As mentioned by earlier investigation (Abduo, 2012), each

virtual tooth was fitted manually on the model with the aim of obtaining ideal teeth

arrangement, emergence profile, symmetry and aesthetics. Since the gingival-tooth junction

demarcates the most apical extension of tooth modification, it was marked on the virtual pre-

treatment models. The virtual tooth alignment involved size alteration, rotation and

translation. This was followed by ensuring that ideal occlusal contacts existed. For each case, a

similar occlusion scheme was established to mimic what would have been implemented in the

conventional wax-up (Figure 8-2). After the completion of the wax-up, the scanned model and

the virtual teeth were merged to formulate a single model, which improves the computation

speed of the subsequent analysis.

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A B C

Figure 8-2 Digital wax-up procedure: (A) Scanned pre-treatment models. (B) Scanned physiological teeth. (C) Digital wax-up models.

8.3.3. Analysis

For the direct comparison between the pre-treatment and the wax-ups models, all the models

were remeshed with a density of 0.1 mm. All the measurements were completed digitally. In

order to quantify the tooth contour alteration, each wax-up model was overlapped on the

corresponding pre-treatment model. The overlapping of models involved the automatic

alignment of two 3D images to ensure exact 3D orientation (Bell et al., 2003; Abduo and

Bennamoun, 2013). The overlapping process was performed by a 3D rendering software

package (Geomagic Studio, Raindrop Geomagic Inc., Research Triangle Park, NC, USA) through

two sequential steps: (1) point-to-point registration, and (2) global registration. The point-to-

point registration determines the initial approximate orientation of the pre-treatment and the

wax-up model images by manually locating at least three common landmarks. The points were

selected on unaltered portions of the models. The global registration is based on the Iterative

Closest Point algorithm (Bell et al., 2003), and it aims to align the meshes according to the

best-fit principle.

Following the image overlapping, on each crowned tooth, three virtual planes were located on

the labial aspect: (1) mid-tooth, (2) mesial line angle, and (3) distal line angle (Figure 8-3A).

Only the labial surfaces were evaluated as they are the most influenced surfaces by the

prosthodontic treatment. In addition, the lingual surface of many crowns was located

supragingivally which does not critically alter the tooth contour. The discrepancy between the

models was measured in the most apical 3 mm of the clinical crown. This dimension was

selected as it tends to exhibit the greatest convexity on the labial surfaces (Becker and Kaldahl,

1981; Croll, 1989). Five levels were selected in relation to the gingival margin: 0.0, 0.5, 1.0, 2.0,

and 3.0 mm, where 0.0 mm level is the gingival margin (Figure 8-3B and 3C). For each location,

a digital point was located on the pre-treatment model and the corresponding location of the

wax-up model. Subsequently, the distance between the points was measured. A minimal

magnitude indicates a close match between the models, while a great magnitude indicates

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significant surface alteration following the wax-up modifications. Positive values indicate that

the wax-up modification increased the tooth contour while negative values indicate that the

wax-up modification reduced the tooth contour.

A B C

Figure 8-3 (A) An image illustrating the extracted three labial planes. (B) A magnified image outlining the five vertical measurements on each plane. (C) A cross sectional view of an extracted plane on the pre-treatment model (black line) and the corresponding plane on the post-treatment model (red line).


At each level, the average contour alteration and standard deviation was calculated. The

effects of the following dentition variables on tooth contour were evaluated: inter-arch effect

(maxillary vs. mandibular arches), intra-arch location (anterior vs. posterior location), tooth

category (incisors, canines, premolars and molars) and tooth surface location (mid-tooth vs.

line angle). To evaluate the presence of a significant difference, a series of t-tests were

conducted (P value = .01). The difference between the different vertical locations was

evaluated by the one-way ANOVA test. In addition, the contour alterations for each tooth

category were plotted in bar diagrams.

8.4. Results

Following each wax-up, the modified models showed wider labial tooth contours compared

with the pre-treatment models. A consistent pattern was observed for all the wax-ups where

the contour increase was directly proportional to the distance from the gingival margin. For

every wax-up, the difference between all the levels was statistically significant. For the

conventional wax-up, at 0.0, 0.5, 1.0, 2.0 and 3.0 mm levels, the mean values (standard

deviation) were 0.05 mm (0.14), 0.14 mm (0.24), 0.20 mm (0.29), 0.30 mm (0.36) and 0.37 mm

(0.43) respectively. The values for the digital wax-up were 0.08 mm (0.14), 0.18 mm (0.40),

0.25 mm (0.27), 0.33 mm (0.33), and 0.39 mm (0.41) from the gingival margin. The digital wax-

139

up caused a significantly greater contour increase than the conventional wax-up at 0.0, 0.5 and

1.0 mm levels.

8.4.1. Inter-arch location (maxillary vs. mandibular teeth)

Table 8—1 summarizes the outcome for the maxillary and mandibular teeth after each wax-

up. With respect to the conventional wax-up, the maxillary teeth generally exhibited a greater

contour increase than the mandibular teeth. The difference was significant for the 0.0 and 0.5

mm levels. Similarly, for the digital wax-up, the maxillary teeth had a greater contour increase

than the mandibular teeth. This difference was significant at all levels. After comparing the two

wax-ups, the maxillary teeth of the digital wax-up exhibited a significantly greater contour

increase than the conventional wax-up. However, the mandibular teeth were very similar

between the two wax-ups, except at level 0.0 mm where the conventional wax-up caused

significantly lower contour changes compared with the digital wax-up.

Table 8—1 The mean and standard deviation (SD) for the maxillary and mandibular teeth after each diagnostic wax-up

Level (mm)

Conventional wax-up Digital wax-up Conventional vs. digital wax-ups

Maxillary teeth

Mandibular teeth

Difference Maxillary teeth

Mandibular teeth

Difference Maxillary teeth

Mandibular teeth

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

0.0 0.06 0.15 0.02 0.12 Significant 0.10 0.15 0.04 0.10 Significant Significant Significant 0.5 0.16 0.25 0.10 0.20 Significant 0.24 0.25 0.11 0.53 Significant Significant Insignificant 1.0 0.22 0.31 0.17 0.26 Insignificant 0.30 0.29 0.19 0.24 Significant Significant Insignificant 2.0 0.31 0.38 0.30 0.34 Insignificant 0.38 0.34 0.27 0.32 Significant Significant Insignificant 3.0 0.35 0.43 0.40 0.42 Insignificant 0.43 0.40 0.35 0.42 Significant Significant Insignificant

8.4.2. Intra-arch location (anterior vs. posterior)

For the two wax-ups, the anterior teeth had greater contour increase than the posterior teeth

(Table 8—2). The significant statistical difference was present at all levels except at level 0.5

mm for the digital wax-up. There was a trend for the anterior teeth of the digital wax-up to

have a greater labial contour than the conventional wax-up. This difference was significant at

0.0, 0.5 and 1.0 levels. Similarly, for the posterior teeth, the digital wax-up caused more labial

contour increase than the conventional wax-up. However, only at the 0.0 mm level, there was

a significant statistical difference.

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Table 8—2 The mean and standard deviation (SD) for the anterior and posterior teeth after each diagnostic wax-up

Level (mm)


Anterior teeth

Posterior teeth

Difference Anterior teeth

Posterior teeth


Posterior teeth

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

0.0 0.06 0.14 0.03 0.14 Significant 0.09 0.15 0.06 0.12 Significant Significant Significant 0.5 0.19 0.23 0.08 0.23 Significant 0.21 0.51 0.15 0.22 Insignificant Significant Insignificant 1.0 0.27 0.28 0.12 0.28 Significant 0.31 0.28 0.20 0.26 Significant Significant Insignificant 2.0 0.39 0.36 0.21 0.35 Significant 0.41 0.34 0.24 0.30 Significant Insignificant Insignificant 3.0 0.45 0.43 0.28 0.40 Significant 0.50 0.41 0.27 0.37 Significant Insignificant Insignificant

Table 8—3 illustrates that for the two arches, the anterior teeth had greater contour increase

than the posterior teeth at all levels. Except at the 0.0 mm level of conventional mandibular

wax-up and levels 0.0 mm and 0.5 mm of digital mandibular wax-ups, the difference between

the anterior and posterior teeth was significant. Regarding the maxilla, the digital wax-up was

associated with a greater labial contour increase of the anterior teeth than the conventional

wax-up. This difference was significant at 0.0, 0.5 and 3.0 levels. A similar relationship was

observed for the posterior teeth. This difference was statistically significant at all levels except

at 3.0 mm.

Table 8—3 The mean and standard deviation (SD) for the maxillary anterior and posterior teeth, and mandibular anterior and posterior teeth

Maxillary teeth Level (mm)


Anterior teeth

Posterior teeth


Posterior teeth


Posterior teeth

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

0.0 0.09 0.15 0.04 0.15 Significant 0.12 0.17 0.08 0.13 Significant Significant Significant 0.5 0.23 0.25 0.09 0.24 Significant 0.29 0.27 0.18 0.21 Significant Significant Significant 1.0 0.31 0.31 0.12 0.29 Significant 0.36 0.30 0.24 0.26 Significant Insignificant Significant 2.0 0.41 0.36 0.20 0.37 Significant 0.45 0.33 0.29 0.32 Significant Insignificant Significant 3.0 0.44 0.43 0.26 0.41 Significant 0.52 0.39 0.32 0.39 Significant Significant Insignificant

Mandibular teeth Level (mm)


Anterior teeth

Posterior teeth


Posterior teeth


Posterior teeth

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

Mean (mm)

SD (mm)

0.0 0.03 0.11 0.02 0.13 Insignificant 0.05 0.10 0.03 0.11 Insignificant Insignificant Insignificant 0.5 0.13 0.19 0.06 0.22 Significant 0.11 0.69 0.12 0.22 Insignificant Insignificant Insignificant 1.0 0.21 0.24 0.12 0.26 Significant 0.23 0.23 0.14 0.25 Significant Insignificant Insignificant 2.0 0.37 0.35 0.22 0.33 Significant 0.35 0.34 0.17 0.27 Significant Insignificant Insignificant 3.0 0.47 0.44 0.31 0.39 Significant 0.48 0.44 0.20 0.34 Significant Insignificant Significant

With regard to the mandible, there was great similarity between the anterior teeth contours

from the two wax-ups. This was supported by the lack of statistically significant difference at

any level. Likewise, the posterior teeth labial contours were similar for the two wax-ups. At

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level 3.0 mm, the conventional wax-up caused a significantly greater labial contour increase

than the digital wax-up.

8.4.3. Tooth location (mid-tooth vs. line angle)

Figure 8-4 illustrates the contour alterations for each tooth category for the mid-tooth and line

angle locations. Regardless of the arch and the tooth category, the graphs showed consistent

pattern of contour increase with the increasing level of measurement. Further, the two wax-

ups showed similar pattern of contour alterations for all the teeth.

A

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

C D C D C D C D


Axi

al C

on

tou

r A

lte

rati

on

Tooth Category

0

0.5

1

2

3

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B

C

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

C D C D C D C D


Axi

al C

on

tou

r A

lte

rati

on

Tooth Category

0

0.5

1

2

3

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

C D C D C D C D


Axi

al C

on

tou

r A

lte

rati

on

Tooth Category

0

0.5

1

2

3

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D

Figure 8-4 Bar diagrams illustrating the contour alteration of each tooth category after each wax-up: C = conventional wax-up and D = digital wax-up. (A) Maxillary mid-tooth region. (B) Maxillary line angle region. (C) Mandibular mid-tooth region. (D) Mandibular line angle region.

For the maxilla (Figure 8-4A and 4B), all the teeth were less affected at the gingival margin

level (0.0 mm level) at all the locations. The molars were generally the least affected by the

wax-up at all levels. The incisors, premolars and molars appeared to be more affected at the

line angle location than at the middle of the tooth. For these teeth categories, there was an

insignificant difference between the mid tooth and line angle locations after the conventional

wax-up was applied. However, for the digital wax-up, the molars revealed significantly more

contour increase for the line angle location at all levels. The canines showed similar effect on

the middle of the tooth and line angle locations. The differences between the two locations

were insignificant for both wax-ups.

The mandibular teeth showed a steeper contour increase compared with the maxillary teeth.

In general, at the gingival margin, the teeth were less affected than for the maxilla (0.05 mm

alterations or less). Similarly to the maxilla, the contour of the molars was least affected by the

wax-ups at all levels, while the incisors were the most affected. For the conventional wax-up,

the incisors contour increase was similar for the two locations on the teeth. However, for the

digital wax-up, greater contour increase occurred at the line angle location. Statistical

difference was observed at the 2.0 mm level. The canines had similar contour for the two

locations after the conventional wax-up was applied. However, for the digital wax-up there

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

C D C D C D C D


Axi

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on

tou

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on

Tooth Category

0

0.5

1

2

3

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was a clear increase for the line angle labial contour. Statistically, the significant difference was

at level 1.0 mm. The premolars had similar contours at the two locations for the two wax-ups.

However, there was a steeper increase of the contour for the conventional wax-up. The molars

of the conventional wax-up exhibited a similar contour alteration at the two tooth locations.

For the digital wax-up, the contour is larger at the line angle than at the middle of the tooth.

However, this difference was insignificant.

8.5. Discussion

This study suggests that regardless of the wax-up method applied, there is a tendency for the

axial tooth contour to increase. This increase appears to be directly proportional to the

distance from the gingival margin. Therefore, the hypothesis that wax-ups increase the axial

contour is accepted. A similar outcome was observed by studies evaluating the effect of fixed

prosthesis on tooth contour (Meijering et al., 1998; Vasconcelos et al., 2009). Meigering et al.

found that veneering discoloured teeth increased the labial contour and resulted in over-

contoured restorations (Meijering et al., 1998). Likewise, Vasconcelos et al. concluded that all

the veneered teeth exhibited an increase of the labial contour. In support to the current study,

they found the increase of the contour was directly related to the distance from the gingival

margin (Vasconcelos et al., 2009). Practically, the increase of labial contour could be

intentional as it will allow tooth shape improvement and will increase the restorative material

thickness for aesthetics and durability (Goodacre et al., 2001).

Overall, there is similarity between the two wax-ups; however, the clear finding was that the

digital wax-up was associated with greater contour increase, even at the gingival margin. Thus,

the hypothesis that there is no difference between the two wax-ups is rejected. The tendency

for the digital wax-up to cause greater labial contour could be due to the difficulty in locating

the gingival margin digitally on the scanned pre-treatment model. Locating the restoration

margin on virtual mesh was reported to invariably lead to a slight deviation from the exact

gingival margin (Abduo et al., 2010). Some investigators reported that a possible consequence

of this limitation is the marginal discrepancy of the digitally produced crowns in comparison to

the conventionally produced crowns (Tan et al., 2008; Han et al., 2011). On the contrary, the

conventional wax-up is based on actual and tactile feeling of the gingival margin which could

lead to a more accurate outcome. However, although for some regions the difference between

the two wax-ups is statistically significant, the actual difference is minimal and might not be of

clinical significance. On the other hand, for the digital wax-up to be applicable, it should be

transferrable to the clinic. This can be accomplished by 3D printing or milling of a physical

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digital wax-up or production of provisional restorations (Kasparova et al., 2013). Due to the

additional processing steps, it is likely that the actual accumulated discrepancies will be greater

to what has been reported in this study. For example, dimensional distortion of 3D printed

objects could be well above 100 µm (Inokoshi et al., 2012). The impact of the total

discrepancies should be evaluated in an additional study.

In general, increasing tooth contour was discouraged by several literatures as it can impact

home care and self-cleansing abilities, which will eventually induce gingival inflammation,

periodontal complications and dental caries (Sackett and Gildenhuys, 1976; Sorensen, 1989;

Broadbent et al., 2006). According to Burch et al, the maximal convexity should exist on the

gingival third of the anatomical crown of the restored tooth and, preferably, should not exceed

0.5 mm (Burch and Miller, 1973). In an attempt to evaluate the effect of increasing tooth

contour, Perel augmented the labial contour of dogs’ teeth. The teeth with the over-contoured

restorations suffered from gingival inflammation (Perel, 1971). On the contrary, more recent

animal studies indicated that so long as professional oral hygiene is regularly maintained, the

periodontal health is likely to be preserved, even with over-contoured crowns (Kohal et al.,

2003; Kohal et al., 2004). Ehlrich and Hochman conducted a split-mouth study on four

participants. On one side of the mouth, the crowns were over-contoured by 1 mm and on the

side the crowns were under-contoured by 1 mm. After 4 months, there was no significant

difference in periodontal status between the two sides (Ehrlich and Hochman, 1980). Similarly,

on 6 patients, Sundh and Kohler evaluated three experimental crowns with different contours.

They found that after one week of regular oral hygiene practice, none of the crowns were

associated with increased plaque deposition (Sindel et al., 1999). Therefore, it could be

speculated that as long as adequate oral hygiene is maintainable, reasonable over-contouring

of up to 1 mm is not necessarily associated with periodontal complications. Since the maximal

contour recorded in this study, by the two wax-ups was less than 1 mm the modified tooth

contours are less likely to induce pathological consequences. Further, for the two wax-ups, the

linear increase of the modified teeth coronally means that the contour increase is likely to

blend smoothly with the unaltered tooth surface. This is further supported by the

measurements at the gingival margin being the least (less than 0.2 mm). Therefore, potential

implications on oral health from the two wax-ups are very unlikely.

An interesting observation in this study was the digital wax-up showing greater differences

between the mid-tooth and the proximal locations compared with conventional wax-up. This

indicates that the conventional wax-up is more consistent in providing a similar outcome

around the teeth than the digital wax-up. The most likely explanation of this observation is the

placement of a tooth with average morphology on the pre-treatment model. Thus, it is more

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likely for the digitally modified teeth to exhibit exaggerated anatomical features (line angles,

cusp tips and marginal ridges) (Paulus et al., 1999; Mehl et al., 2005a). However, the advantage

of the high anatomical definition is the digital wax-up might be perceived as more natural and

aesthetic. Although the accuracy of the digital wax-up is slightly less than for the conventional

wax-up, for a purely diagnostic purpose, it can still be viable tool to plan for the definitive

prostheses.

Regardless of the wax-up, the maxillary teeth tend to have greater contour increase than

mandibular teeth. Likewise, the anterior teeth were more prominently affected by the wax-ups

than the posterior teeth. This could be related to the need to improve tooth aesthetics by

increasing the labial contour, (Magne and Belser, 2004) which has been perceived to be more

aesthetic by patients (Ehrlich and Hochman, 1980). From the conservative perspective, the

anterior teeth might benefit from being over-contoured which would mean less tooth

reduction is needed. Overall, there is a clinical preference to minimize the amount of tooth

reduction which could reduce the pulpal complications (Goodacre et al., 2001). Reports

pertaining to aesthetic dentistry recommended preparing the tooth according to the final

crown volume as determined by diagnostic wax-up rather than the existing tooth contour

(Magne and Belser, 2004; Gurel, 2007). One of the advantages of this approach is ensuring a

conservative tooth preparation. Further, since the anterior teeth have the advantage of being

accessible for cleaning, it is less likely for the oral health to be affected by over-contouring. On

the other hand, the posterior teeth will be advantaged by having less contour increase. This

will ensure the ease of cleanliness for less accessible areas (Becker and Kaldahl, 1981). In

addition, it is less likely for the appearance of posterior teeth to benefit from over-contouring

as they are not in the aesthetic zone.

8.6. Conclusions

Within the limitations of this study, it appears that there is an overall similarity in the pattern

of tooth modifications between the two wax-ups. The conventional wax-up was associated

with a significantly less increase of axial tooth contour than the diagnostic wax-up. However,

the actual difference was minimal. As the wax-ups are used purely for diagnostic purposes, this

difference appears to be of minimal clinical significance. The axial contour of all the modified

teeth with diagnostic wax-ups was increased. This increase was clearly proportional to the

distance from the gingival margin. The anterior teeth were much more affected by the

increase of the contour than the posterior teeth.

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Chapter Nine

9. Effect of Prosthodontic Planning on Intercuspal

Occlusal Contacts: Comparison of Digital and

Conventional Planning



intercuspal occlusal contacts: comparison of digital and conventional planning. Computers in

Biology and Medicine. 2015; 60:143-150. (Appendix J)

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9.1. Abstract

Objective: Adequate occlusal contacts are critical for masticatory function. The aim of this

study is to evaluate the intercuspal occlusal contacts following conventional and digital wax-

ups.

Materials and Methods: Stone casts of fifteen patients undergoing prosthodontic treatment

were gathered. Each cast was duplicated twice, so that conventional and digital wax-ups could

be performed. To assess the occlusion, the following variables were evaluated: contact

number per tooth (CNT), contact area per tooth (CAT) and contact accuracy. Further, the

impact of tooth location in the arch was assessed.

Results: The CNT and CAT after the wax-ups increased significantly following each wax-up, and

this increase was more prominent for the posterior teeth than the anterior teeth. The

conventional wax-up was associated with lower CNT than the digital wax-up, especially for the

posterior teeth. On the other hand, the CAT was greater for the conventional wax-up than the

digital wax-up for the anterior and posterior teeth. In terms of accuracy, the two wax-ups

showed greater discrepancies than the pre-treatment casts, however, the magnitude of

discrepancy was greater for the digital wax-up.

Conclusions: The two wax-ups improved the contact number and area. Despite the statistical

variation between the wax-ups, the actual difference was minimal. Therefore, it could be

speculated that the two wax-ups produced a similar outcome.

Key words: digital dentistry, wax-up, contact number, contact area, articulation

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9.2. Introduction

In many instances, diagnostic wax-up is advocated to determine the viability of any proposed

prosthodontic treatment (Magne and Belser, 2004; Ahmad, 2010). The wax-up is a useful tool

for selecting the ideal treatment, enhancing communication with the patient and for

provisional restoration construction. Subsequently, a good wax-up will dictate the final

prosthesis fabrication (Magne and Belser, 2004). Any diagnostic wax-up should be accurate,

aesthetic and feasible. Traditionally, the tooth surface is designed with conventional wax-up

technique as part of prosthodontic planning. The tooth contour and occlusion are modified by

the addition of wax on the external tooth surface (Ahmad, 2010). Recently, virtual designing of

the tooth surface with digital techniques was proposed as an alternative method for

prosthodontic planning (Abduo, 2012). It entails altering the tooth contour of a virtual stone

cast. To facilitate the contour alterations, several authors had proposed applicable approaches

and algorithms (Paulus et al., 1999; Mehl et al., 2005a; b; Ender et al., 2011). To ensure the

usability of the digital wax-up, the alteration process is followed by production of a physical

cast by subtractive of additive CAM (Abduo et al., 2014b). Alternatively, provisional

restorations can be produced according to the digital wax-up (Lin et al., 2013).

The digital wax-up has the advantages of not permanently altering the stone cast, quantifying

the dental modifications, simplicity of execution, and the possibility of trying different

treatments. Further, as the digital wax-up is performed using specialised software, more

clinicians can provide a wax-up, even without artistic technical abilities. However, the digital

wax-up should at least exhibit a similar accuracy to conventional wax-up.

Regardless of the type of diagnostic wax-up, the static and dynamic occlusal contacts should

be of adequate quality and accuracy. The occlusal contacts will eventually contribute to the

functional benefit and comfort of the prosthesis (Owens et al., 2002; Koyano et al., 2012).

Experimentally, the quality of the occlusal contacts can be determined by the number of

contacts, the area of the contacts and the accuracy of the contacts. Therefore, the aim of this

study is to evaluate the effect of conventional and digital wax-ups on occlusal contacts in

terms of number, area and accuracy. The null hypotheses are that the wax-ups will alter the

occlusal contacts and that there is no difference in occlusal contacts between the conventional

and virtual techniques in designing of the occlusal surface.

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A human research ethics approval was granted from the Human Research Ethics Committee of

The University of Western Australia (RA/44/1/5079). Stone casts of 15 patients who required

fixed prosthodontic treatment were retrieved for the study. The inclusion criterion was

necessity of diagnostic wax-up prior to prosthodontic treatment that will influence the dental

occlusion. The patients were under care at the Oral Health Centre of Western Australia.

For each arch, an irreversible hydrocolloid impression (Alginate, GC America, IL, USA) was

made. An occlusal relation record was obtained using polyvinyl siloxane registration material

(GC Exabite, GC America, IL, USA) according to the centric relation position. The impressions

were poured by type III dental stone (Buff Stone, Adelaide Moulding & Casting Supplies, South

Australia, Australia). These casts comprised the pre-treatment casts. All the casts were

duplicated twice by reversible hydrocolloid duplicating materials (Magafeel, MKM System,

Haanova, Slovakia). One cast received conventional wax-up and the other cast was used for

the digital wax-up (Figure 9-1).


Semi-adjustable articulator (Whip Mix, Louiseville, KY, USA) with pre-determined values was

used in this study for the wax-up as advised by some authors (Hobo and Takayama, 1997). One

set of casts were articulated according to the maximal intercuspation position and the other

set of casts were articulated according to centric relation position with the aid of the intra-oral

record. The maximal intercuspation position was used to relate the pre-treatment casts. This

position will reflect the habitual relationship which is more relevant for function prior to the

treatment (Becker et al., 2000). On the contrary, the centric relation position was used to

relate the arches prior to the wax-ups. Therefore, after the wax-up, there will be a coincidence

between centric relation and maximal intercuspation positions (Becker et al., 2000). Following

the articulation, silicone material putty (Dental Speedex Putty, Coltene/Whaledent AG,

Altstatten, Switzerland) was applied on the buccal aspects of the posterior teeth of the

mounted casts. This silicone index was used for the digital articulation of the pre-treatment

casts.

The conventional wax-up was completed by inlay wax (VITA Zahnfabrik, Bad Sackingen,

Germany) addition on the external tooth surface. In some areas, the external tooth surface

was modified by trimming. The wax-up aimed to rectify the defective tooth structure, establish

natural and aesthetic tooth morphology, and achieve symmetry between the two sides. As the

occlusion of all the teeth were altered, the alteration mechanism involved obtaining even

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bilateral occlusal contacts and a physiological lateral occlusal scheme (canine-guided or group

function occlusion) (Becker et al., 2000; Turp et al., 2008). All the conventional wax-ups were

completed by an experienced dental technician. On the completed wax-up casts, silicone

indices were formed on the buccal aspect of the posterior teeth.

The pre-treatment and conventional wax-up casts and the associated silicone indices were

scanned by a micro-CT scanner (SkyScan, Bruker, micro-CT, Kontich, Belgium) (12 μm

resolution, 360o scanning, 70 KV source voltage, 1.0 mm Al filtration). The reported advantage

of micro-CT scanning is the possibility of producing an accurate image that exhibits a

dimensional error of 0.1% (Waring et al., 2012). Subsequently, virtual 3D Stereolithography

(STL) images of the maxillary and mandibular casts were constructed from the Digital Imaging

and Communication Medicine (DICOM) images using a DICOM viewing program (CTvox, Bruker

microCT, Kontich, Belgium). The construction procedure was based on model surface

extraction from the stacked series of DICOM images. The virtual image of the conventional

wax-up was used for the subsequent analysis (Figure 9-1B).

The virtual silicone indices were used to articulate the pre-treatment and conventional wax-up

casts digitally, using the process of image registration as illustrated by DeLong et al. (Delong et

al., 2002). A 3D rendering software package (Geomagic Studio, Raindrop Geomagic Inc.,

Research Triangle Park, NC, USA) was used for the registration process. The purpose of the

registration process is to precisely align the models that share common surfaces. As discussed

by several authors (Bell et al., 2003), the registration process involved two sequential steps: (1)

point-to-point registration and (2) global registration. The point-to-point registration is based

on coarse registration between two similar surfaces. This step was completed manually by

selecting points on common surfaces of the virtual cast and the silicone index. As a result, the

virtual models translate spatially until they reach a reach position. In this experiment, the cusp

tips and the most cervical gingival margins were selected. After the initial superimposition of

the two meshes, an automated global registration was completed. This step aimed to

approximate the best alignment of virtual cast against the virtual silicone index according to

the Iterative Closest Point Algorithm. The registration process was executed between each

arch and the corresponding virtual silicone indices. Subsequently, the virtual silicone indices

were deleted digitally, and the two virtual casts were digitally articulated.


The virtual pre-treatment casts were articulated according to centric relation position. This

was achieved by superimposing the virtual pre-treatment casts on the virtually articulated

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conventional wax-up casts. The unaltered tissues were used for the registration. A 3D

rendering software (Geomagic Studio) was used to execute the digital wax-up. Virtual

physiological tooth (Phonares Teeth, Ivoclar Vivadent AG, Schaan, Liechtenstein) moulds were

used as mentioned by earlier investigation. The virtual tooth alignment involved size

alteration, rotation and translation. Each virtual tooth was fitted manually on the virtual cast

with the aim of obtaining an ideal teeth arrangement, emergence profile, symmetry and

aesthetics. Accurate occlusal interdigitation with the antagonist teeth was planned. This was

achieved by locating the cusps within the opposing fossae. For each set of casts, a similar

occlusion scheme was established to mimic what would have been implemented in the

conventional wax-up. After the completion of the wax-up, the scanned cast and the virtual

teeth were merged to formulate a single model (Figure 9-1C), which improved the

computation speed of the subsequent analysis.

A B C

Figure 9-1 Example of the virtual pre-treatment (A), conventional wax-up (B) and digital wax-up (C) casts.

9.3.3. Analysis

To ensure uniformity of the comparison between the pre-treatment and the wax-up casts, all

the casts were remeshed with a density of 0.1 mm. The casts were imported to mesh

measurement software (Meshlab Software, Visual Computing Lab, University of Pisa, Italy) to

analyse the occlusal contacts. Only the affected teeth or pontics by the prosthodontic

treatment were considered for the analysis. This involved the restored unit and the opposing

unit.

A threshold of 200 µm was selected to visualize the contacting surfaces (Delong et al., 2002;

Iwase et al., 2011). The surfaces that are opposing within a distance of 200 µm are thought to

be critical surfaces for occlusion (Delong et al., 2002). As a function of the Meshlab software,

the opposing cast surfaces were converted to color-coded 3D models according to the

Hausdorff Distance between the two meshes. This feature was used to visualize the occlusal

relationship of the opposing occlusal surfaces according to the inter-occlusal distance, where

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the yellow colour indicated a 200 µm distance and the red colour indicated a 0 µm distance

(Figure 9-2).

A B C

Figure 9-2 Colour-coded map illustrating the contact number and contact area for the pre-treatment (A), conventional wax-up (B) and digital wax-up (C) casts. The calculation of the CNT and CAT will compensate the increase of the number of teeth after the wax-ups.

Therefore, the number of occlusal contacts was based on the number of spots coloured with

yellow (Figure 9-3A). The contact area was quantified by measuring the area with yellow

boundaries (Figure 9-3B).

A B

Figure 9-3 Determination of the contact number and area according to the colour-coded map. The number of occlusal contacts was established by counting the areas coloured with yellow or a warmer colour. The same areas were extracted and measured to quantify the occlusal area.

154

Since only the affected teeth were considered for the analysis, the contact number per tooth

(CNT) and contact area per tooth (CAT) were measured. Therefore, these variables ensured the

comparison between pre-treatment and post-treatment casts would not be influenced by the

alteration of the occlusal unit number. The following equations were implemented:

𝐶𝑁𝑇 = 𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑜𝑛𝑡𝑎𝑐𝑡𝑠

𝑛 𝐶𝐴𝑇 =

𝑇𝑜𝑡𝑎𝑙 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑟𝑒𝑎

𝑛

where n is the number of affected teeth.

For each detected contact, the contact accuracy was measured by the digital ruler of the

software (Figure 9-4). This was executed by measuring the distance between a point of one

mesh and the corresponding point of the opposing mesh. The purpose of measuring the

contact accuracy is to evaluate the relationship of the opposing surfaces. Ideally, the surfaces

should be contacting without overlapping. If the meshes were not contacting, the measured

discrepancy was the least perpendicular distance between the two surfaces, and the value was

labelled with a negative mark. If the meshes were overlapping, the discrepancy was

determined by the greatest perpendicular distance, and the value was labelled positive. Thus

negative values mean that the surfaces are not contacting, while the positive values indicate

contact interference.

Figure 9-4 Measurement of the occlusal discrepancies. If the contact surfaces are overlapping the (A), the maximal distance is measured which indicates a positive error (occlusal interferences). In a situation where the surfaces are not contacting (B), the minimal distance between the surfaces are measured and reflect a negative error (non-contacting surfaces).


For the pre-treatment, conventional and digital wax-up casts, the average CNT, CAT and

accuracy were calculated. The Mann-Whitney test was performed (P = .05) to determine the

significance of the difference between the pre-treatment casts and each wax-up cast, and

between the conventional and digital wax-ups. The anterior and posterior dental units were

separated. For all the variables, the impact of each wax-up procedure on the anterior and

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posterior teeth was evaluated. Further, the difference between the anterior and posterior

teeth was determined.

9.4. Results

A total of 310 units were analysed for the pre-treatment casts of which 112 were anterior unit

and 198 were posterior unit. Following each wax-up procedure, a total of 464 units were

included for the analysis (144 anterior units and 320 posterior units).

9.4.1. Contact number

The CNT differed significantly between the casts. The pre-treatment casts exhibited a

significantly lower CNT (mean = 1.08, SD = 0.55) than the conventional wax-up (P = .03) and

digital wax-up (P = .00) casts. The digital wax-up casts had greater CNT (mean = 1.58, SD =

0.61) than the conventional wax-up casts (mean = 1.37, SD = 0.49), however, this difference

was insignificant (P = .07).

For all the casts, the posterior teeth had a significantly greater CNT than the anterior teeth (P =

.00) (Table 9—1). Figure 9-5 illustrates the influence of tooth location on the CNT. Overall,

there is similarity in the CNT pattern between all the casts. On the anterior teeth, the pre-

treatment casts showed similar CNT to the conventional casts (P = .36). However, the pre-

treatment casts had significantly less CNT than the digital wax-up casts (P = .03). There was no

statistical difference between the conventional and digital wax-up casts (P = .29). For the

posterior teeth, the pre-treatment casts and the two wax-ups for the posterior teeth (P = .00).

The conventional wax-up casts and digital wax-up casts had similar CNT for the posterior teeth

(P = .20).

Table 9—1 CNT mean and standard deviation (SD) for the pre-treatment, conventional wax-up and digital wax-up casts

Pre-treatment Conventional Wax-Up Digital Wax-Up Mean SD Mean SD Mean SD Anterior teeth 0.78 0.37 0.90 0.45 1.03 0.45 Posterior teeth 1.41 0.61 1.89 0.55 2.15 0.59

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Figure 9-5 Box plot diagram of the CNT values for the anterior and posterior teeth of pre-treatment, conventional wax-up and digital wax-up casts.

9.4.2. Contact area

The pre-treatment casts had lower CAT (mean = 2.95 mm2, SD = 1.75) than conventional (mean

= 5.43 mm2, SD = 3.48) or digital wax-ups (mean = 4.26 mm2, SD = 2.86). This difference was

statistically significant between the pre-treatment and conventional wax-up casts (P = .01), but

not between the pre-treatment and digital wax-up casts (P = .07). The two wax-ups had similar

CAT (P = .11).

All the casts had a significantly greater CAT for the posterior teeth than anterior teeth (P = .00)

(Table 9—2), and exhibited generally a similar CAT pattern (Figure 9-6). For the anterior region,

there was an insignificant difference between the pre-treatment casts and the conventional

wax-up (P = .72) and the digital wax-up (P = .28) casts. The conventional wax-up casts had

similar CAT to the digital wax-up (P = .20). The posterior teeth had significantly lower CAT than

the conventional wax-up (P = .00) and digital wax-up (P = .00) casts. The two wax-ups did not

differ significantly at the posterior region (P = .18).

Table 9—2 CAT mean and standard deviation (SD) for the pre-treatment, conventional wax-up and digital wax-up casts

Pre-treatment Conventional Wax-Up Digital Wax-Up Mean

(mm2) SD (mm2) Mean

(mm2) SD (mm2) Mean

(mm2) SD (mm2)

Anterior teeth 1.90 1.42 2.67 2.70 1.51 1.31 Posterior teeth 4.00 2.33 8.75 4.87 7.12 3.47

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Figure 9-6 Box plot diagram of the CAT values (mm2) for the anterior and posterior teeth of pre-treatment,

conventional wax-up and digital wax-up casts.

9.4.3. Contact accuracy

For the whole arch, the mean contact accuracy for pre-treatment casts, conventional wax-up

and digital wax-up casts were 0.01 mm (SD = 0.16), 0.08 mm (SD = 0.15) and 0.11 mm (0.21),

respectively. The pre-treatment casts had significantly more precise contacts than the two

wax-up casts (P = .00). Overall, the two wax-up casts had similar contact precision (P = .70).

However, the digital wax-up casts appeared to have more occlusal overlap that the

conventional wax-up casts (Figure 9-7).

The posterior teeth of the pre-treatment and conventional wax-up casts had significantly less

contact discrepancy than the anterior teeth (P = .01) (Table 9—3). On the contrary, the digital

wax-up casts had significantly more contact discrepancy for the posterior teeth than the

anterior teeth (P = .00). Anteriorly, the pre-treatment casts had significantly less discrepancies

than the conventional wax-up casts (P = .00) but insignificantly less than the digital wax-up

casts (P = .19). The anterior teeth of the conventional wax-up casts had significantly more

discrepancies than the digital wax-up casts (P = .00). For the posterior teeth, the pre-treatment

casts had significantly less discrepancies than conventional and digital wax-up casts s (P = .00),

and the digital wax-up had significantly greater discrepancies than the conventional wax-up (P

= .01).

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Table 9—3 Contact accuracy mean and standard deviation (SD) for the pre-treatment, conventional wax-up and digital wax-up casts

Pre-treatment Conventional Wax-Up Digital Wax-Up Mean

(mm) SD (mm) Mean (mm) SD (mm) Mean

(mm) SD (mm)

Anterior teeth 0.04 0.18 0.11 0.17 0.06 0.19 Posterior teeth -0.01 0.15 0.07 0.14 0.13 0.22

Figure 9-7 Box plot diagram of the contact accuracy values (mm) for the anterior and posterior teeth of pre-treatment, conventional wax-up and digital wax-up casts.

9.5. Discussion

The outcome of this study indicates that regardless of the wax-up method employed, the

planned prosthodontic treatment positively influences the number and area of occlusal

contacts. Therefore the hypothesis that the contact quality is improved by the wax-up is

accepted. The lower occlusal contact number and area for the pre-treatment casts could be

attributed to the pre-treatment dentition’s tendency to suffer from dental problems that

influence the morphology, such as large restoration, tooth wear and chipping. Such

abnormalities can affect the quality of the occlusal contact and contact area.

As this study is early in the field, it is difficult to compare its outcome with the findings of

previous studies. Instead, the obtained outcome of this study was compared to the

observations of the studies that quantified the contact number and area of ideal young

dentitions which might constitute the benchmark (McNamara and Henry, 1974; Korioth,

1990b; Ciancaglini et al., 2002; Delong et al., 2002). The earlier studies on intercuspal occlusal

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contacts evaluated the total contact number and areas for intact dentition of young

individuals. In this study, it was necessary to measure the contact number and area per tooth

to compensate for the effect of missing teeth. To compare the outcome of this study with

earlier studies, approximate contact number and area per tooth was calculated from the

earlier studies by dividing the reported total contact number or area on the number of teeth

per arch.

Overall, the studies on natural and intact dentition revealed that the occlusal contact number

was similar to the contacts obtained following the wax-up treatment of this study (McNamara

and Henry, 1974; Korioth, 1990b; Ciancaglini et al., 2002; Delong et al., 2002). Therefore, it

might be reasonable to assume that the planned prosthodontic treatment will restore the

occlusal anatomy to a more natural anatomy and the occlusal relationship might return to the

baseline relationship. Such an observation is attributed to the more natural dental anatomy

that can be generated following the wax-up. This is in accordance with the clinicians’

recommendations on idealising intercuspal occlusal contacts with prosthodontic treatment

(Wiskott and Belser, 1995; Koyano et al., 2012). The envisioned improvement of occlusal

contacts will potentially contribute to a more stable and functional occlusion (Owens et al.,

2002). Therefore, in addition to aesthetic improvement following the prosthodontic treatment,

the oral function is more likely to improve as well.

There was a marked variation in the reported CAT from previous studies. After manual

evaluation of the contact area, Alkan et al. found less area than what has been reported in this

study for all the casts (Alkan et al., 2006), while Hidaka et al. reported outcome was similar

only to the pre-treatment casts (Hidaka et al., 1999). Conversely, in a digital study, Iwasa et al.

recorded a relatively large CAT (about 5 mm2 per tooth) (Iwase et al., 2011) which was similar

to the wax-up casts of this study. The variation in the outcome of the studies could be related

to the method of area quantification (Owens et al., 2002). It has been acknowledged that a

slight vertical discrepancy of the maxilla-mandibular tooth relationship will cause an

exponential reduction of the recorded area (Wilding et al., 1992; Hidaka et al., 1999; Delong et

al., 2002). Several of the earlier studies have applied occlusal medium to quantify the area, in

such cases, vertical displacement of the jaw will likely have occurred, resulting in an

underestimation of the contact area. On the other hand, in this study and the study by Iwasa

et al, a threshold of 200 µm was applied (Iwase et al., 2011), which was likely to overestimate

the contact area due to the risk of the models overlapping. Nevertheless, it is clear that the

wax-up process increases the CAT, which is indicative that the planned prosthodontic

treatment will increase the contact area at the occlusal phase of chewing, hence resulting in

more efficient chewing.

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For all the evaluated casts, the location of the tooth in the arch appeared to be a strong

determinant of the quality of the occlusal variables evaluated in this study. There was a

dominance of contact number and area on the posterior teeth (two times the anterior teeth),

which corroborates several earlier investigations (Ehrlich and Taicher, 1981; McDevitt and

Warreth, 1997). On natural young individual dentition, McNamara and Henry found 8 times

more contact on posterior teeth than anterior teeth (McNamara and Henry, 1974). Similarly,

Ciancaglini et al. found the number of contacts to be 3 times greater for posterior teeth than

anterior teeth (Ciancaglini et al., 2002). On the restored dentition, Yi and Carlsson (Yi et al.,

1996) found the posterior teeth had twice the contact number than the anterior teeth, which

was similar to the pre-treatment and post-treatment casts of this study. In relation to the

posterior teeth contact area, Yurktas and Manly found that the CAT on the posterior teeth

tended to be 6.96 mm2-per-tooth (Yurkstas and Manly, 1949), which was close to the wax-up

casts of this study. On the other hand, Owens et al. found the contact area on posterior teeth

tended to be less than the wax-up casts of this study (3.16 mm2-per-tooth), and close to the

pre-treatment casts (Owens et al., 2002). In similarity to what has been mentioned earlier, it is

likely that the participant selection and the implemented methodology influenced the area

outcome. The more profound contacts on the posterior teeth are due to greater area, cuspal

morphology and interdigitation of the opposing teeth. The anterior teeth, on the other hand,

have more confined surfaces and incisal edges. Further, this finding fits with the mutually-

protected occlusion concept, where the posterior teeth prevent excessive contact of the

anterior teeth at maximum intercuspation (The Glossary of Prosthodontic Terms, 2005).

Although this finding is correct for the pre-treatment and the wax-up casts, the difference

between the anterior and posterior teeth is greater following the prosthodontic treatment,

which indicates the idealisation of the occlusion scheme following prosthodontic planning.

Thus, it could be speculated that the posterior teeth receive greater benefit in terms of contact

number and area following the prosthodontic treatment. This is advantageous from the

functional perspective, as the posterior teeth are responsible for food chewing and grinding.

Despite the similarity between the two wax-ups, there is tendency for the digital wax-up to

exhibit greater CNT than the conventional wax-ups, while for the CAT, the conventional wax-

up was associated with greater area. Since this difference is not statistically significant, the

hypothesis that there is no difference in occlusal contacts between the two wax-ups is

accepted. The observed slight difference is most likely related to the differences between the

occlusal morphology generated by each wax-up. As the digital wax-up utilizes an average tooth

form, the final tooth morphology tends to exhibit more defined and steeper anatomical

features (Ender et al., 2011). This means greater cuspal angle, more pointy cusps and deeper

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grooves and fossae. The more defined occlusal anatomy could explain the greater contact

number and the lower area of the posterior teeth for the digital wax-up casts that was

observed in the current study. Nevertheless, in terms of occlusal contact number and area, it is

reasonable to state that the two wax-ups generated a similar outcome and the clinical

difference is likely to be insignificant.

The occlusal accuracy after each wax-up was lower than the pre-treatment casts, and this

inaccuracy was more prominent for the digital wax-up. Although the occlusion of the

conventional wax-up casts was established on physical casts, there has been greater occlusal

inaccuracy than for pre-treatment casts. This is likely to be related to the mathematical

determination of the model surface after scanning, which is associated with inevitable

inaccuracies (Persson et al., 2006). The external surface is formed of vertices connected by

accumulating triangles of polygonal mesh. This polygonal mesh is composed of flat triangles

which approximate the curved surfaces of the dental restoration (Pfeiffer, 1999; Luthardt et

al., 2002). As the conventional wax-up casts exhibit more prominent occlusal curvature than

pre-treatment casts, they will be more prone for occlusal discrepancy. On the contrary, the

pre-treatment casts have more flat surfaces, which are easier to represent digitally. As the

digital teeth have more defined occlusal anatomy for the posterior teeth, they might be more

vulnerable to loose surface accuracy. This might explain the slightly greater discrepancy of the

digital wax-up casts than the conventional wax-up casts on the posterior teeth.

Many researchers have developed computer algorithmic systems for tooth surface design. This

includes occlusal generated path, the approximation of cavity margins, fitting normalised

intact tooth surfaces (Paulus et al., 1999), the fitting of an average tooth (Mehl et al., 2005a; b;

Ender et al., 2011), and scanning a manually waxed tooth or the mirror image of the

contralateral tooth. Regardless of the technique, virtual reconstruction generally was found to

cause up to 0.5 mm vertical discrepancies of the completed restoration and in many cases, the

operator is expected to manually adjust the occlusal contacts (Ender et al., 2011). In addition,

as the digital wax-ups are yet to be produced by CAM processes, the accumulated final

inaccuracy will be greater to what has been observed in this study. Nevertheless, since in

clinical practice the wax-ups are used primarily for provisional restorations, minor occlusal

discrepancies can be modified easily in the clinic, with no major consequences on the

definitive prostheses.

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9.6. Conclusions

Within the limitations of this study, it could be concluded that in terms of occlusal contact

number and area, the wax-up procedures significantly improved the intercuspal occlusal

morphology. For all the evaluated casts, the posterior teeth experienced greater benefit in

occlusal contact number and area. Between the two wax-ups, there were some variations in

occlusal contact number and area; however, the obtained figures were very similar. In terms of

occlusal contacts accuracy, the digital wax-up seems to be less accurate than the conventional

wax-up on the posterior teeth. Although the implication of this inaccuracy is yet to be

determined, it is likely that continuous technological improvements will enhance the digital

wax-up outcome. Therefore, the digital wax-up appeared to be comparable to conventional

wax-up. However, the production of physical models from the digital wax-up and the clinical

implications should be subjected to additional investigations.

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Chapter Ten

10. Effect of Prosthodontic Planning on Lateral

Occlusion Scheme: A Comparison between

Conventional and Digital Planning


Abduo J, Bennamoun M, Tennant M, McGeachie J. Effect of prosthodontic planning on lateral

occlusion scheme: a comparison between conventional and digital planning. Journal of Applied

Oral Science. 2015; 23:196-205. (Appendix K)

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10.1. Abstract

Objective: Recently, digital wax-up is proposed as a tool to aid prosthodontic planning.

However, there is no information about the effect of prosthodontic planning on lateral

occlusion scheme. This study aims to evaluate the impact of conventional and digital

prosthodontic planning on lateral occlusion scheme.

Materials and Methods: Dental models of 10 patients were collected. All models had Angle

Class I occlusion and were undergoing prosthodontic treatment that would influence the

lateral occlusion scheme. Each set of models had received both conventional wax-up and

digital wax-up. In relation to the lateral occlusion scheme, the following variables were

evaluated: the prevalence of the different lateral occlusion scheme, number of contacting

teeth and percentage of each contacting tooth. Four excursive positions on the working side

were included: 0.5, 1.0, 2.0 and 3.0 mm from the maximal intercuspation position.

Results: The lateral occlusion scheme of the two wax-up models was subjected to alterations

following excursion. There was a tendency for the prevalence of canine-guided occlusion to

increase and the prevalence of group function occlusion to decrease with increasing the

excursion. The number of contacting teeth was decreasing with the increasing magnitude of

excursion. For the 0.5 mm and 1.0 mm positions, the two wax-ups had significantly greater

contacts than the pre-treatment models, while at the 2.0 mm and 3.0 mm positions, all the

models were similar. For all models, canines were the most commonly contacting teeth,

followed by the teeth adjacent to them. No difference was observed between the two wax-ups

in relation to the number of contacting teeth.

Conclusion: Although the prosthodontic planning had influenced the pattern of the lateral

occlusion scheme and contacts, there was no difference between the conventional and digital

prosthodontic planning.

Key words: wax-up, canine-guided occlusion, group function occlusion, dental model

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10.2. Introduction

The ideal lateral occlusion scheme has been a topic of debate for many years. It has been

postulated that the lateral occlusion scheme impacts masticatory function, comfort and

aesthetics. Several authors discussed the rationale of each lateral occlusion scheme (Thornton,

1990; Rinchuse et al., 2007). It is common throughout prosthodontic treatment, that the

lateral occlusion scheme be altered by controlling morphologies, alignments and orientations

of teeth. The two commonly applied schemes are canine-guided occlusion and group function

occlusion. The canine-guided occlusion is a mutually-protected occlusion where the vertical

and horizontal overlap of the canine teeth causes disengagement of the posterior teeth in the

lateral movement of the mandible (The Glossary of Prosthodontic Terms, 2005). The group

function occlusion is based on multiple contacts between the maxillary and mandibular teeth

in lateral movement on the working side (The Glossary of Prosthodontic Terms, 2005). It has

been speculated that canine-guided occlusion protects the posterior teeth laterally because of

the canines’ strategic location, anatomy and proprioceptive properties (Rinchuse et al., 2007).

On the other hand, group function occlusion might contribute to a wide distribution of occlusal

forces on several teeth instead of a single tooth; thus, the occlusion can be more comfortable

and functional (Thornton, 1990). However, true clinical evidence supporting either scheme is

still lacking, therefore, both schemes are deemed acceptable (Becker and Kaiser, 1993; Turp et

al., 2008).

More recently, there has been a discussion about the limitations of defining each lateral

occlusion scheme, as the occlusal presentation is more complex naturally (Ogawa et al., 1998).

For example, with different degrees of excursion, the lateral occlusion scheme might differ.

Further, any functional occlusion is subjected to changes with time, yet without manifestation

of physiological abnormalities (Abduo et al., 2013). It is also acknowledged that most patients

are comfortable with their existing dentition and occlusion. Thus, the occlusion scheme can be

considered physiological even if it does not fit into any specific category.

Fixed prosthodontic treatment is indicated to alter the tooth morphology, which can

eventually alter the lateral occlusion scheme. To date, the authors are not aware of any study

that evaluated the impact of fixed prosthodontic treatment on the lateral occlusal scheme. The

purpose of this observational study is to evaluate the effect of two forms of fixed

prosthodontic planning (conventional and digital diagnostic wax-ups) on lateral occlusion

scheme. Further the frequency of each tooth contact will be quantified. The null hypotheses

are that the prosthodontic planning will change the lateral occlusion scheme and the

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frequency of teeth contacts, and there will be a difference between the two forms of

prosthodontic planning.


Models of 10 patients who required fixed prosthodontic treatment (crowns or fixed partial

dentures) were collected for this study. Table 10-1 summarizes the inclusion criteria. The

treatment indications were managements of heavily restored dentition, tooth wear, aesthetic

problems and short-span edentulous area. For all models, the diagnostic wax-up had an

influence on the lateral occlusion scheme. The patients received the treatment at the Oral

Health Centre of Western Australia and a human research ethics approval was obtained from

the Human Research Ethics Committee of The University of Western Australia (RA/44/1/5079).

Table 10—1 Inclusion criteria

Requirements of fixed prosthodontic treatment in the form of crowns or fixed partial dentures in at least one arch Diagnostic wax-up is indicated prior to the treatment Angle Class I occlusal relationship Well-distributed occlusal contacts Absence of TMD The planned prosthesis is completely supported by natural teeth No removable or implant prosthesis

An irreversible hydrocolloid impression (Alginate, GC America, IL, USA) was taken for each arch

and an occlusal relation record was obtained by polyvinyl siloxane registration material (GC

Exabite, GC America, IL, USA) according to the centric relation position. All the impressions

were poured by type III dental stone (Buff Stone, Adelaide Moulding & Casting Supplies, South

Australia, Australia). These casts comprised the pre-treatment models. All the models were

duplicated twice by reversible hydrocolloid duplicating materials (Magafeel, MKM System,

Haanova, Slovakia). On one model, the conventional wax-up was executed and the other

model was used for the digital wax-up (Figure 10-1).

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A B C

Figure 10-1 Example of the evaluated virtual models. (A) Pre-treatment model. (B) Conventional wax-up model. (C) Digital wax-up model.


On a semi-adjustable articulator (Whip Mix, Louiseville, KY, USA), one set of duplicated models

were articulated according to maximal intercuspation to represent the pre-treatment

articulation. The other set were used for the conventional wax-up and were articulated

according to centric relation position with the aid of the intra-oral record. Therefore, after the

wax-up, there will be a coincidence between centric relation and maximal intercuspation

positions (Becker et al., 2000). Silicone material putty (Dental Speedex Putty,

Coltene/Whaledent AG, Altstatten, Switzerland) was applied on to the buccal aspects of the

posterior teeth of the mounted models. This silicone index was used for the digital articulation

of the pre-treatment models.

The conventional wax-up procedure involved inlay wax addition on the external tooth surface

with the aim of rectifying the defective tooth structure, obtaining natural and aesthetic tooth

morphology, achieving symmetry between the two sides, and obtaining even bilateral occlusal

contacts. For some patients, an increase in the vertical dimension of occlusion was necessary.

All the conventional wax-ups were completed by an experienced dental technician. The dental

technician was advised to produce group function lateral occlusion scheme for the initial

excursions with no steep occlusion guidance. On the completed wax-up models, silicone

indices were formed on the buccal aspect of the posterior teeth.

A micro-CT scanner (SkyScan, Bruker, micro-CT, Kontich, Belgium) was used to scan the

conventional wax-up models and the associated silicone indices. The generated Digital Imaging

and Communication Medicine (DICOM) image were used to construct virtual 3D

Stereolithography (STL) images of the maxillary and mandibular models by a DICOM viewing

program (CTvox, Bruker micro-CT, Kontich, Belgium). The virtual image of the conventional

wax-up was used for the subsequent analysis.

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In order to digitally articulate the models, the image registration process by 3D rendering

software package (Geomagic Studio, Raindrop Geomagic Inc., Research Triangle Park, NC, USA)

was applied. Initially, corresponding points on common surfaces between the model and the

silicone indices were selected to approximate the positions of the images. This was followed by

implementing the Iterative Closest Point Algorithm which re-orients the models according to

the best fit principles. The same process was repeated for the other silicone index and the

opposing arch. Eventually, the silicone indices were deleted digitally, and the two models were

digitally articulated.


The digital wax-up has been discussed in earlier report (Abduo, 2012). In summary, the pre-

treatment models and the associated silicone indices were scanned and converted to virtual

images. The virtual pre-treatment models were articulated according to the centric relation

position. This was achieved by superimposing the virtual pre-treatment models on the virtually

articulated conventional wax-up models. The unaltered tissues were used for the registration.

A 3D rendering software (Geomagic Studio) was used to execute the digital wax-up. The pre-

treatment models were articulated by the process of Image Registration. Virtual physiological

teeth (Phonares Teeth, Ivoclar Vivadent AG, Schaan, Liechtenstein) were used to alter the

teeth morphologies. The virtual tooth alignment involved size alteration, rotation and

translation. Each virtual tooth was fitted manually on the model with the aim of obtaining ideal

teeth arrangement, emergence profile, symmetry and aesthetics. After the completion of the

wax-up, the scanned model and the virtual teeth were merged to generate a single model,

which enhanced the computation speed of the subsequent analysis.

10.3.3. Virtual simulation of lateral movement

For each set of articulated models, the occlusion scheme was evaluated for each working side

separately. Four horizontal excursive positions were considered: 0.5 mm, 1.0 mm, 2.0 mm and

3.0 mm (Figure 10-2). The lateral movement was simulated virtually by moving the mandibular

arch in the working side horizontally for each specified location. This was followed by gradually

moving the mandible vertically away from the maxilla by 0.05 mm increments. Once all the

contacts on the working side disappeared, the mandible was moved by 0.05 mm vertically

towards the maxilla (DeLong et al., 2003). This process detected the existing working side

contacts that dictate the lateral occlusion scheme. Due to the limitation of this process in

detecting non-working side contacts, only the working side contacts were considered.

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Depending on the detected contacts, three occlusion schemes were considered: (1) canine-

guided occlusion, (2) group function occlusion, and (3) single tooth-guided occlusion. Canine-

occlusion was recorded if the lateral contact occurred on a canine tooth. The occlusion is

considered group function occlusion if more than two teeth in one arch were contacting

laterally. In situations where the lateral contact occurred on a single tooth other than the

canine, the lateral occlusion scheme was considered to be single tooth-guided occlusion.

A B C D E

Figure 10-2 An example of virtual simulation of lateral movement. (A) Maximal intercuspation. (B) 0.5 mm excursion. (C) 1.0 mm excursion. (D) 2.0 mm excursion. (E) 3.0 mm excursion. The red colour indicates the existing contacts.

10.3.4. Analysis

For each position, three variables were evaluated: (1) the prevalence of each lateral occlusion

scheme (2) the average number of the contacting teeth and (3) the percentage of each

contacting tooth. All the variables were blotted in bar diagrams. For the last two variables, the

maxillary and mandibular teeth contacts were distinguished. In addition, for the average

number of contacting teeth, the Kruskall-Wallis test was used to determine the presence of a

statistical difference between the different positions (P value = .05). When a significant

difference was observed, the Mann-Whitney test was used for post-hoc analysis. Further, the

difference in the number of teeth in contacts between the pre-treatment models and each of

the two wax-up models, and between the two wax-up models was evaluated by the Mann-

Whitney test (P value = .05). The same test was applied to evaluate the difference between

maxillary and mandibular arches for each position.

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10.4. Results

10.4.1. Prevalence of lateral occlusion scheme

As the excursion increases, the lateral occlusion scheme changes for all the evaluated models

(Figure 10-3). Overall, the models exhibited similar patterns of lateral occlusion alterations. For

the pre-treatment models, the prevalence of canine-guided occlusion had increased minimally

through the excursion (from 30% to 45%). There was a tendency for the group function

occlusion to reduce with increased excursion (from 65% to 20%). The single tooth occlusion

had a tendency to increase with increased lateral excursion.

A

B

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Figure 10-3 Proportion of each lateral occlusion scheme in each excursive position. (A) Pre-treatment models. (B) Conventional wax-up models. (C) Digital wax-up models.

For the two wax-up models, there was a consistent and gradual increase of canine-guided

occlusion (from 20% to 60% for conventional wax-up, and 0% to 65% for digital wax-up) and

reduction of group function occlusion (from 75% to 15% for conventional wax-up, and 95% to

29% for digital wax-up) with increased excursion. It appears that the single tooth occlusion had

minimally increased as the excursion increased. The overall patterns for the two wax-ups were

similar, except that the conventional wax-up had a slightly more even pattern of alteration,

while the digital wax-up exhibited steeper occlusion alterations.

10.4.2. Number of contacting teeth

Regardless of the evaluated model, there was a clear pattern of reduction of the average

number of teeth in contact with increasing the degree of excursion (Figure 10-4). For the pre-

treatment models, there was a slight reduction of number of teeth in contact with increasing

the degree of excursion, yet there was a statistical difference between all the positions, except

between the 1.0 mm and 2.0 mm positions. For the two wax-ups, there was a clear nominal

reduction of number of teeth in contact with increasing the excursion. A statistically significant

difference was observed between all the positions, except between the 2.0 and 3.0 mm

positions for the digital wax-up. Between the maxillary and mandibular arches, no statistically

significant difference was observed in any position for pre-treatment and the two wax-up

models.

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A

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Figure 10-4 The mean number of contacting teeth for all the models in each excursive position. (A) Maxillary arch. (B) Mandibular arch.

At the 0.5 mm and 1.0 mm excursion position, the pre-treatment models had significantly less

teeth contacting than the two wax-up models. However, at the 2.0 mm and 3.0 mm positions,

there had been no difference between the pre-treatment and the two wax-up models. At all

the positions, there had been no statistical difference between the conventional and digital

wax-ups.

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10.4.3. Percentage of each contacting tooth

For all the arches of all the models, and almost at all positions, the canines had the tendency to

be the dominant contacting tooth. Further, it appears that the contacts decreased gradually

from canines to the more anterior teeth and from the canines to the posterior teeth. However,

there were some differences in the frequency and patterns of contacts (Figure 10-5).

A

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E

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Figure 10-5 Percentage of the contacting teeth in each excursive position for all the models. (A) Pre-treatment maxillary arch. (B) Pre-treatment mandibular arch. (C) Conventional wax-up maxillary arch. (D) Conventional wax-up mandibular arch. (E) Digital wax-up maxillary arch. (F) Digital wax-up mandibular arch.

For the pre-treatment models, the maxillary arch had frequent canine contacts at all the

positions. The lateral incisors, premolars and the first molars had consistent contact at all the

positions. In the mandible, there was a reduction in the percentage of canine contacts. Overall,

there was similarity in contacts’ frequency between the canines and premolars.

For the conventional wax-up, there was a general similarity between the two arches. The

canines maintained the frequency of contacts between the positions. The first premolar was

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the second tooth in the frequency of contacts. For all the remaining teeth, it was clear that the

frequency of contacts had a tendency to decrease with increasing the excursion position. The

mandibular canines were distinguished in the slight reduction of frequency with increased

excursion.

In relation to the digital wax-up, the two arches showed similar patterns and frequency. The

frequency of the canines contacts had increased with excursion, while the remaining teeth

contact frequency had decreased significantly with excursion.

10.5. Discussion

This study indicates that conventionally and digitally planned prosthodontic treatment

influences the lateral occlusion in relation to the prevalence of each scheme at different

positions, and the contacting teeth type and quantity. Therefore, the hypothesis that the

prosthodontic planning will impact the dynamic occlusion was accepted. Such a finding

supports that the lateral occlusion scheme should be carefully considered before and after the

treatment.

In this study, multiple lateral locations were considered in order to provide an insight about

the possible contact pattern from the immediate excursion to the maximal excursion (about 3

mm) (Ogawa et al., 1998). This was found to be more clinically relevant than only evaluating

the lateral occlusion at maximal excursion (Suit et al., 1976; Woda et al., 1979). Guiding

contacts at the 3 mm position might occur primarily during parafunctional activity and bruxism

in the edge-to-edge position. However, occlusal guiding during mastication and physiological

movement tend to occur within 0.5 mm from the maximal intercuspation (Ogawa et al., 1996;

1997). Therefore, the range evaluated in this study covers functional and parafunctional jaw

movement.

In relation to the prevalence of each lateral occlusion scheme, it is clear that with greater

excursion, the prevalence of canine-guided occlusion tends to increase. This was in accordance

with the studies that evaluated the prevalence of each lateral occlusion scheme at different

positions for the natural dentitions of young participants (Yaffe and Ehrlich, 1987; Al-Nimri et

al., 2010). Yaffe and Ehlrich evaluated the prevalence of the different lateral occlusion schemes

at 1.0, 2.0 and 3.0 mm excursions (Yaffe and Ehrlich, 1987). They found that the prevalence of

canine-guided occlusion had increased from 16.1% to 48.6%, and the group function occlusion

had reduced from 83.9% to 51.5%. A similar finding was observed by Al-Nimri et al., when they

evaluated the prevalence of the lateral occlusion scheme at 0.5 and 3 mm positions. They

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found the prevalence of canine-guided occlusion had increased from 21.9% to 59.6%, and the

prevalence of group function had reduced from 45.3% to 23.9% (Al-Nimri et al., 2010).

Although the present study had evaluated the lateral occlusion scheme of restored dentitions,

overall, the figures were supporting to the outcome of the earlier studies. The differences in

documenting the prevalence of each occlusion scheme could be related to the different

classification and recording methods of the lateral occlusion scheme (Ogawa et al., 1998;

Abduo et al., 2013). In addition, it is important to emphasize that a single lateral occlusion

scheme did not exist for any model for the entire excursive path. Therefore, in accordance

with all the published clinical studies, true canine-guided occlusion or group function occlusion

seldom exists clinically (Woda et al., 1979; Becker and Kaiser, 1993; Turp et al., 2008; Abduo et

al., 2013).

The dynamic nature of the lateral occlusion scheme at the different arch positions is attributed

to teeth morphological factors. In the initial phase of excursion, the cusps are articulated

against wider fossa surfaces (Schuyler, 1963). As excursion progresses the total contact area

reduces, thus more teeth will be discluded. This observation supports the concepts of

“progressive occlusion”, where many teeth initially control the occlusion, followed by primarily

the canines during the maximal excursion (DiPietro, 1977; Goldstein, 1979). This occlusion

scheme appears to be more physiologically relevant than a single occlusion scheme that

controls the lateral movement through the entire excursion. This complex relationship might

have a protective role in tolerating lateral forces (Yaffe and Ehrlich, 1987). Physiologically, it is

thought that such an arrangement is advantageous in facilitating smooth multidirectional

movement of the mandibular arch, which might reduce the risk of patient discomfort

(Schuyler, 1963). Further, broad occlusal contact areas were found to be helpful in mitigating

excessive occlusal forces on teeth (Hidaka et al., 1999), which might contribute to the

dissipation of sudden lateral forces on the teeth. In addition, the greater prevalence of group

function occlusion means that greater contacts at less lateral movement can enhance the

occlusal phase of chewing (Wang and Mehta, 2013).

The pattern of lateral occlusion scheme alteration with different excursion was observed for

the pre-treatment and wax-up models. However, the wax-up models had more consistent

gradual occlusion scheme changes. This might be due to the pre-treatment dentitions suffering

from morphological abnormalities, such as tooth wear or failed restorations. Subsequently, a

wider contact area between worn down teeth might be evident and will maintain a greater

number of contacts, even after maximal excursion (Beyron, 1954). Whereas the dentitions of

the wax-up models had restored natural dental morphology and less prominent wear facets,

which can produce steeper articulation and cause a consistent reduction of the total number

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of tooth contacts during excursion. This was evident in the initial phases of excursion (0.5-1.0

mm) where the contacts quantity was significantly greater for the two wax-up models than at

the later stages of excursion. In addition, it was clear that the proportion of canine-guided

occlusion at later stages of excursion was greater for the two wax-ups than the pre-treatment

models. This was also likely to be related to restoring the cuspal morphology of the canines to

the original canine morphologies. The studies that evaluated the implication of individual age

on excursion scheme confirmed that the younger the individual, the greater the prevalence of

canine-guided occlusion (Panek et al., 2008; Abduo et al., 2013). This was attributed to the

prominent dental morphology and less cuspal wear of the young dentitions (Abduo et al.,

2013; Panek et al., 2008). Therefore, it could be speculated that the planned prosthodontic

treatment can return the dentition occlusion scheme closer to its original state. Between the

two wax-ups, some differences were observed, but not to a significant level. Thus, it is difficult

to assume that a different wax-up protocol will produce a different outcome on the final

treatment. Therefore, the hypothesis that there will be a difference between the two

prosthodontic planning procedures was rejected.

It is clear that at all positions, the canines had the greatest number of contacts. The significant

contribution of canines was observed even after partial excursion, where group function

occlusion was dominating. This reflects the importance of canines for lateral occlusion scheme

development. This finding corroborates the observation by several earlier investigations

(Ingervall, 1972; Yaffe and Ehrlich, 1987; Ingervall et al., 1991; Ogawa et al., 1998). Ogawa et

al., on natural and young dentition, found that the canines were contacting in about 70% of

the excursive positions (Ogawa et al., 1998), which was similar to the outcome of our pre-

treatment models of the present study. Such a finding supports the protective role of canines

during the lateral occlusal movements. The dominance of canines is related to two factors: the

canine’s morphology and position. The canines are innately long teeth, which tend to have

more prominent cuspal morphology, which controls the articulation against the opposing

teeth (Rinchuse et al., 2007). As the excursion starts, the canines are in contact, however with

increased excursion, the canines will play a greater role and it is more likely for the other teeth

to disclude. As the working side condyle rotate and the non-working side condyle slides, the

working side canine is positioned in the corner, where interferences control the occlusal

guidance (D'Amico, 1961). The importance of the location in the arch explains the reason why

the teeth immediately adjacent to the canine tend to have frequent occlusal contacts.

Likewise, as the teeth are more distant from the canines, they are less likely to be in contact on

the working side (D'Amico, 1961; Ogawa et al., 1998). Although some investigators had found

group function was more common, for a great portion of their participants, the group function

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occlusion was composed of canine and premolar occlusion, which reinforces the importance of

the location in the arch (Weinberg, 1964; Ingervall, 1972; Ingervall et al., 1991). The strategic

importance of canines is further supported by being the most vulnerable teeth for attrition

(Weinberg, 1964; Rinchuse et al., 2007). Therefore, regardless of the observed lateral occlusion

scheme, it could be stated that the canines are significant teeth to control the lateral occlusion

(Rinchuse et al., 2007).

Following the prosthodontic planning, it was clear that the frequency of the canine contact

was increased, followed primarily by the first premolars. As stated earlier, this was attributed

to restoring the original canine cuspal morphology, while the canines of pre-treatment models

suffered from greater tooth wear (Beyron, 1954). Therefore, after the prosthodontic planning

a greater protective role of canines was more evident. The two wax-ups were very similar in

the prevalence of the lateral occlusion scheme and the pattern of occlusal contacts frequency.

However, for the digital wax-up, there appeared to be an exaggerated role of the canine teeth

from the greater frequency of canine contacts. The most likely explanation is the well-defined

dental morphologies that can be attained digitally by the software (Mehl et al., 2005a). On the

other hand, this means that the digital wax-up can produce a steeper lateral occlusion than the

conventional wax-up, which can result in more restricted lateral movement. Although this can

have clinical implications, like patient’s discomfort (Schuyler, 1963), such assumption should

be confirmed by additional study.

Despite the lack of a significant statistical difference between the two wax-up protocols

applied in this study, it is important to reinforce that the digital wax-up is still in its early phase

of application and requires additional investigation. For example, for the digital wax-up to be

applicable clinically, the models have to be produced by CAM, which will inevitably influence

the accuracy of the lateral occlusal contacts. Further, the methodology of this study is limited

by not considering the non-working side contacts, which could influence the observed lateral

occlusion scheme. The omission of observing the non-working side contacts was necessary, as

virtual modelling of the condylar movement is very imprecise (Schierz et al., 2014). However,

as the digital wax-up is associated with steeper occlusal surfaces, the likelihood of developing

non-working side contacts is higher.

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10.6. Conclusions

Given the limitations of the present study, the following can be concluded:

1. The prosthodontic planning had influenced the pattern of the lateral occlusion scheme.

The influence of the alteration tends to be more prominent at the initial stages of

excursion.

2. For all the models, the initial phase of excursion involved a greater number of contacting

teeth and higher prevalence of group function occlusion than maximal excursion. Canine-

guided occlusion tends to be more prevalent at the later stage of excursion.

3. Overall, the difference in the number and pattern of contacts is very minimal between the

two wax-ups.

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Chapter Eleven

11. Impact of Prosthodontic Planning on Dental

Aesthetics: An Objective Evaluation of Aesthetic

Parameters


Abduo J, Bennamoun M, Tennant M, McGeachie J. Impact of digital prosthodontic planning on

dental esthetics: biometric analysis of esthetic parameters. Journal of Prosthetic Dentistry.

2015; Accepted. (Appendix L)

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11.1. Abstract

Objective: Improving dental aesthetics is one of the main objectives of prosthodontic

treatment. Recently, digital diagnostic wax-up has been proposed as an alternative to

conventional diagnostic wax-up; however, the impact on aesthetics has not been evaluated.

This study aims to evaluate the impact of diagnostic wax-ups on objective dental aesthetic

variables, and to compare the aesthetic outcome achieved by different wax-up procedures.

Materials and Methods. Three objective variables were evaluated: perceived frontal

proportion (PCP), width-to-height (W:H) ratio and symmetry. Maxillary models of thirteen

patients were collected. All of them had maxillary anterior teeth that required prosthodontic

treatment. Two forms of diagnostic wax-ups were executed: conventional and digital wax-ups.

Measurements of the aesthetic variables were conducted digitally. For the PCP, a frontal image

was taken and the width of each tooth was measured. Subsequently, the PCP values of the

lateral incisor to central incisor, and of the canine to central incisor were calculated. In

addition, on the digital model, the height and width of each tooth was measured to calculate

the W:H ratio. Using the previous measurements, the symmetry between the right and left

sides was determined.

Results: No consistent or recurrent PCP was detected for any model. The diagnostic wax-ups

did not alter the PCP of the pre-treatment models. The diagnostic wax-ups had restored the

W:H ratio to what is assumed to be a natural ratio. The symmetry had improved after the

diagnostic wax-ups. There was no significant difference between the two diagnostic wax-ups.

Conclusions. The diagnostic wax-ups had improved the aesthetic variables of the anterior

maxillary teeth. The wax-up procedures had yielded very similar outcomes from the aesthetic

perspective. The digital diagnostic wax-up appears to be a reasonable alternative, but further

investigations are desirable to ensure its practicality.

Key words: diagnostic wax-up, perceived aesthetics, frontal proportion, width-to-height ratio

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11.2. Introduction

Optimizing dental aesthetics is one of the key indications of prosthodontic treatment.

Restoring anterior teeth can improve their dimensions, display and shade (Raj, 2013). Today’s

population is more aesthetically driven than ever, and many of the dental patients request

dental treatment solely to improve their aesthetics. Several dental aesthetic variables were

discussed in the literature. For example, frontal view teeth proportion, symmetry, width-to-

height (W:H) ratio, teeth arrangement, morphology and location (Gillen et al., 1994; Raj, 2013).

Although the literature does not support specific criteria for dental aesthetics, the use of such

variables as guides to establish dental aesthetics has been proposed (Gillen et al., 1994). It is

acknowledged that the maxillary anterior teeth are the most influential teeth for dental

aesthetics, and their presence in harmony is associated with greater aesthetic perception

(Lombardi, 1973; Levin, 1978). Therefore, it is common to analyse dentition aesthetics by using

objective variables that reflect harmony.

To enhance the predictability of the aesthetic outcome, clinicians frequently recommend

diagnostic wax-up prior to the definitive prosthodontic treatment (Magne and Belser, 2004).

The diagnostic wax-up involves altering the tooth morphology by addition of wax on diagnostic

model. The rationales behind the diagnostic wax-up are simulation of the possible dental

treatment, demonstration of what is achievable by the treatment, and improving the

communication with the patient and the dental technician. In addition, the wax-up can be

utilized to dictate the tooth preparation and fabricate provisional restorations (Magne and

Belser, 2004; Gurel, 2007).

Recently, with the advancement of digital dentistry, digital wax-up has been proposed as an

alternative to the conventional wax-up (Abduo, 2012). This recent form of wax-up is based on

virtual alteration of the dental morphologies. Several methods are available to modify the

tooth digitally. This involves the fitting of an average tooth, using a biogeneric library, and

mirror imaging of an intact adjacent tooth (Mehl et al., 2005b; Probst and Mehl, 2008). In

comparison to the conventional method, altering the teeth by digital methods exhibits several

advantages. Digital technologies tend to reduce materials manipulation and the number of the

involved error-introducing steps. This is assumed to reduce the overall execution time and the

accumulated inaccuracies (Beuer et al., 2008; Miyazaki and Hotta, 2011). Since the software is

an integral part of the digital wax-up, many programs allow quantification of the effect of the

proposed treatment prior to the active treatment phase. This feature can be used to

accurately critique the impact of the dental treatment, such as the aesthetic outcome,

material thickness and the potential preparation invasiveness (Abduo, 2012; Davis et al., 2012;

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Abduo et al., 2014b). Despite all of these advantages and the initial promising outcome, a true

comparison between the conventional and digital wax-ups from the aesthetic perspective is

lacking (Mehl et al., 2005b; Probst and Mehl, 2008). The aims of this study are evaluating the

impact of diagnostic wax-ups on dental aesthetics and to compare the dental aesthetics

achieved by different wax-ups. Three objective aesthetic variables are applied: perceived

frontal proportion (PFP), width-to-height (W:H) ratio and symmetry. The null hypotheses are

the wax-up procedure will idealize the dental aesthetics and there is no difference between

the conventional and digital wax-ups.


Thirteen patients had participated in this study. All of them presented with all of their

maxillary anterior teeth and required diagnostic wax-up prior to prosthodontic treatment. The

indications for the treatment were failing restorations, tooth wear, or unaesthetic teeth. A

human research ethics approval was obtained from the Human Research Ethics Committee of

The University of Western Australia (RA/44/1/5079). The treatments were provided at the Oral

Health Centre of Western Australia.

For all of the patients, an impression of each arch was taken by irreversible hydrocolloid

impression material (Alginate, GC America, IL, USA). Type III dental stone (Buff Stone, Adelaide

Moulding & Casting Supplies, South Australia, Australia) was used to pour the impressions. The

obtained models comprised the pre-treatment dental situation (Figure 11-1A). Each model was

duplicated twice by reversible hydrocolloid material (Magafeel, MKM System, Haanova,

Slovakia). One set of models were used to apply the conventional wax-up and the other set

were used for the digital wax-up.


The conventional wax-up was executed after articulating the actual models on semi-adjustable

articulator (Whip Mix, Louiseville, KY, USA). The articulator was set according to average

values. Additive waxing technique was implemented, where an inlay wax was added to modify

the external tooth surface. In some situations, the external surfaces of the teeth were reduced.

The wax-up aimed to restore the deficient tooth structures, establish natural tooth

morphology, restore symmetry, and achieve a physiological occlusion (Figure 11-1B). The

additive waxing was completed by an experienced dental technician.

185

To facilitate the digital analysis, the pre-treatment and conventional wax-up models were

scanned by a micro-CT scanner (SkyScan, Bruker micro-CT, Kontich, Belgium). The virtual 3D

Stereolithography (STL) image of the maxillary model was constructed from the Digital Imaging

and Communication Medicine (DICOM) images by a DICOM viewing program (CTvox, Bruker

micro-CT, Kontich, Belgium). The STL images of the pre-treatment and conventional wax-up

models were used for the subsequent analysis (Figure 11-1C and 1D).


To complete the digital wax-up, a 3D rendering software package (Geomagic Studio, Raindrop

Geomagic Inc., Research Triangle Park, NC, USA) was used. The maxillary and mandibular

models were virtually articulated through the point-to-point alignment feature of the

software. As mentioned by an earlier investigation (Abduo, 2012), digital physiological teeth

moulds (Phonares Teeth, Ivoclar Vivadent AG, Schaan, Liechtenstein) were utilized to alter the

tooth morphology. Each virtual tooth was fitted manually on the model with the purpose of

obtaining ideal teeth arrangement, emergence profile, symmetry and aesthetics. Since the

gingival-tooth junction outlines the most apical extension of tooth modification, it was

demarcated on the virtual pre-treatment models. The virtual tooth alignment process included

size alteration, rotation and translation. This was followed by ensuring that adequate occlusal

contacts existed. After the completion of the wax-up, the scanned model and the virtual teeth

were merged into a single model, which improves the computation speed of the subsequent

analysis (Figure 11-1E).

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A B

C D

E

Figure 11-1 An example of the evaluated models: A, Actual pre-treatment model. B, Actual conventional wax-up model. C, Virtual pre-treatment model. D, Virtual conventional wax-up model. E, Digital wax-up model.

11.3.3. Analysis

The comparison between the pre-treatment models, conventional wax-up models and digital

wax-up models was conducted using three objective aesthetic variables: perceived frontal

proportion (PFP), width-to-height (W:H) ratio and symmetry. All the measurements were

completed digitally.

The PFP measurement allows comparison of the frontal mesio-distal dimension of the lateral

incisor and canine to the central incisor. Earlier studies attempted to propose a consistent PFP

maxillary anterior teeth width to dictate the dental aesthetics. The most famous guide is the

golden proportion (GP), where the perceived width of the lateral incisor is 62% of the width of

the central incisor and the perceived width of the canine is 38% of the central incisor.4 In the

present study, measuring the PFP will provide information about the perceived appearance of

all the teeth after each wax-up. Each maxillary virtual model was oriented with the occlusal

187

plane parallel to the horizon and with the midline located centrally. A snap image was taken

and imported to measurement software with digital calliper CorelDRAW software (version

11.633, Corel, Ottawa, Canada). Vertical lines were added to the image to separate the

different anterior teeth. The zooming feature of the software was used to facilitate the

measurements. Subsequently, the perceived width of each tooth was obtained by measuring

the horizontal distance between the vertical lines (Figure 11-2). Since the measured variable is

a proportion, no specific unit was used. Eventually, the perceived width of the lateral incisor or

canine relative to the central incisor was calculated.

Figure 11-2 A frontal image illustrating the separation of the anterior teeth for both sides. The horizontal lines represent the perceived width of each tooth.

Contrary to the PFP, W:H ratio will provide details about the actual tooth dimension. This

information is necessary to evaluate the actual morphological alteration of each tooth

category. Due to the teeth angulation and positioning in the arch curvature, the actual and the

perceived dimension will differ significantly (Raj, 2013). Each model was repositioned to allow

complete facial view of each tooth. Subsequently, the longest and widest tooth dimensions

were measured in millimetres by the Geomagic Studio software (Figure 11-3). For the length,

the measurement plane was parallel to the long axis of the clinical crown. The width

measurement plane was perpendicular to the long axis of the clinical crown.

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A B D

Figure 11-3 Measurement of the W:H ratio: A, Central incisor. B, Lateral incisor. C, Canine. The vertical line is the height and the horizontal line is the width.

Utilizing the earlier measurements, two forms of symmetry were measured: perceived, and

actual. The perceived symmetry was based on calculation of the asymmetry percentage of

teeth width from the frontal view. The actual symmetry is based on measuring the asymmetry

percentage of the actual height and width of each tooth. The asymmetry percentage is

measured according to the following formula:

𝐴𝑠𝑦𝑚𝑚𝑒𝑡𝑟𝑦 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 = √(𝑅𝑆𝑇𝐷 − 𝐿𝑆𝑇𝐷

𝑅𝑆𝑇𝐷 + 𝐿𝑆𝑇𝐷)

2

× 100

where RSTD is the right side tooth dimension and LSTD is the left side tooth dimension. The

less asymmetry percentage means greater symmetry.

11.3.4. Statistics

For each model, the mean and standard deviation were calculated for the PFP, W:H ratio and

asymmetry. Subsequently, the mean value for each tooth was compared between the models.

The asymmetry percentage for the different teeth was compared within each model. In

addition, the asymmetry percentage for each tooth category was compared between the

different models. To evaluate the presence of a significance difference, the Kruskall-Wallis test

was used (P = .05). The Mann-Whitney test was used as a post-hoc analysis, when a significant

difference was identified.

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11.4. Results

11.4.1. Perceived frontal proportion

The average PFP of the lateral incisors and canines to the central incisors for all the models

were presented in Figure 11-4. The reported average lateral incisor proportion for the pre-

treatment, conventional wax-up and digital wax-up models was 65.6%, 60.8% and 60.5%

respectively. The canine proportion was 49.4%, 46.5% and 45.8% respectively. An exact match

between any model and GP (lateral incisors = 68%, canines = 38%) was not observed. However,

the PFP pattern was relatively comparable to GP. There has not been a significant difference

for neither lateral incisors not canines between the pre-treatment models and conventional

wax-up models (P = .08) and digital wax-up models (P = .08). Further, the two wax-ups

presented similar proportions statistically (P = .81).

Figure 11-4 PFP of the lateral incisors and canines for the pre-treatment, conventional wax-up and digital wax-up models. The GP values were added for comparison.

11.4.2. Actual dimensions

The W:H ratio of all the maxillary anterior teeth for all the models were presented in Figure 11-

5. For all the teeth, the pre-treatment models had the greatest W:H ratio (central incisors =

106.8%, lateral incisors = 92.2%, canines = 88.6%), followed by the conventional wax-up

models (central incisors = 88.5%, lateral incisors = 78.5%, canines = 81.2%) and the digital wax-

up models (central incisors = 82.8%, lateral incisors = 74.1%, canines = 74.5%). For the central

incisors, the pre-treatment models had a significantly greater W:H ratio than the conventional

wax-up models (P = .04) and digital wax-up models (P = .00). The lateral incisors and canines of

the pre-treatment models had significantly greater W:H ratio than the digital wax-up (P = .00

0

10

20

30

40

50

60

70

80

Pre-treatment model Conventional wax-upmodel

Digital wax-up model Golden proportion

Pe

rce

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(%

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190

and P = .03). However, the W:H ratio of the lateral incisors and canines did not differ

significantly between the pre-treatment and conventional wax-up models (P = .07 and P = .50).

Between the two wax-ups, there was no significant difference in the W:H ratio for the central

incisors (P = .07), lateral incisors (P = .15) or canines (P = .07).

Figure 11-5 W:H ratio of all the teeth for the central incisors, lateral incisors and canines of all the models.

11.4.3. Perceived symmetry

Figure 11-6 illustrates the asymmetry percentage of the PFP for all the anterior teeth of each

model. For all the teeth, the pre-treatment models generally exhibited greater asymmetry

than for the wax-up models. For all the models, the central incisors were the most symmetrical

followed by the lateral incisors. The canines had the greatest asymmetry percentage. For the

pre-treatment models, there has been an insignificant difference in the symmetry discrepancy

between all the teeth (P = .09). The conventional wax-up models had significant discrepancy

only between the central incisors and the canines (P = .02). The digital wax-up models revealed

a significant difference in the discrepancy between the central incisors and lateral incisors (P =

.00), and between the central incisors and canines (P = .00).

0

20

40

60

80

100

120

140

160

Pre-treatment model Conventional wax-up model Digital wax-up model

W:H

Rat

io (

%)

Evaluated Models

Central incisors

Lateral incisors

Canines

191

Figure 11-6 Perceived asymmetry percentage of central incisors, lateral incisors and canines of the pre-treatment, conventional wax-up and digital wax-up models.

After comparing the discrepancy between the models for each tooth category, the central

incisors of the digital wax-up models had significantly less discrepancy than the pre-treatment

models (P = .00) and conventional wax-up models (P = .04). However, there has been no

significant difference for the central incisors between the pre-treatment and conventional

wax-up models (P = .09). The lateral incisors and the canines asymmetry percentage did not

differ significantly between the models (P = .46 and P = .32).

11.4.4. Actual symmetry

In relation to the width symmetry, the pre-treatment models had a greater asymmetry

percentage than any wax-up models (Figure 11-7A). For the pre-treatment models, the lateral

incisors were the most asymmetrical. This difference was significant between the central

incisors and lateral incisors (P = .02). However, the difference between the central incisors and

canines, and lateral incisors and canines were insignificant (P = .49 and P = .06). There is no

significant difference between the different teeth category for the conventional wax-up (P =

.08) and digital wax-up (P = .07) models.

After comparing the teeth of the different models, the central incisors asymmetry was

significantly different between the pre-treatment models and conventional wax-up (P = .02)

and digital wax-up (P = .00). Likewise, the lateral incisors asymmetry differed significantly

between the pre-treatment models and conventional wax-up (P = .00) and digital wax-up (P =

.00) models. The central incisors and lateral incisors width asymmetry did not differ

-5

0

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25


Asy

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etr

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192

significantly between the two wax-ups (P = .46 and P = .10). The width asymmetry of the

canines was significantly greater for the pre-treatment than the digital wax-up models (P =

.03). However, there was no significant difference between the pre-treatment and

conventional wax-up models (P = .32) and between the two wax-up models (P = .48).

Similar to the teeth width, the height asymmetry of the pre-treatment models was greater

than the two wax-up models (Figure 11-7B). The lateral incisors of the pre-treatment models

had the greatest height asymmetry. This asymmetry was significantly different between the

central and lateral incisors (P = .00). Otherwise, there was no significant difference between

the canines and central incisors (P = .06) or lateral incisors (P = .34) of the pre-treatment

models. There was no statistical difference between the height asymmetry of the different

teeth category of the conventional wax-up (P = .93) or the digital wax-up (P = .30).

A

-5

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5

10

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25


Asy

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etr

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Canines

193

B

Figure 11-7 Actual asymmetry percentage of the central incisors, lateral incisors and canines for the pre-treatment, conventional wax-up and digital wax-up models. A, The actual width asymmetry. B, The actual height asymmetry.

The height asymmetry of the central incisors and the canines was not significantly different

between the pre-treatment models and conventional wax-up models (P = .15) or digital wax-

up models (P = .66). Likewise, there was no difference between the two wax-up models (P =

.63) for the central incisors. The lateral incisors height asymmetry of the pre-treatment model

was significantly greater than the conventional wax-up models (P = .02), but not the digital

wax-up models (P = .09). However, the lateral incisors height asymmetry of the wax-up models

did not differ significantly (P = .22). For the canines, there was no difference between the pre-

treatment models and conventional wax-up models (P = .37) or digital wax-up models (P = .31).

Likewise, the canine height asymmetry was very similar between the two wax-ups for the

canines (P = .92).

11.5. Discussion

It is clear from this study that the planned prosthodontic treatment can influence the

evaluated objective aesthetic variables. This effect was very clear for the W:H ratio. Therefore,

the hypothesis that the wax-up procedure will idealize the dental aesthetics was accepted. To

a certain extent, there is an overall similarity between the two wax-ups in relation to the PFP,

W:H ratio and symmetry.

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Historically, the GP was proposed to infer that there is a relationship between teeth aesthetics

and mathematical proportion (Levin, 1978). The rationale was that repeated proportion

between the maxillary anterior teeth is associated with greater harmony and aesthetics

(Lombardi, 1973). In addition, the presence of recurrent values may enhance the aesthetic

outcome of the restorative treatment. However, this assumption has not been confirmed by

any other study (Preston, 1993; Hasanreisoglu et al., 2005; Ali Fayyad et al., 2006). Although an

exact match with GP was not observed for any model of this study, relatively close values were

obtained. This is clear for the lateral incisors proportion which was close to what has been

prescribed for GP. However, the canines were larger, which means that the total perceived

maxillary anterior teeth width is greater to what has been assumed by GP. The lack of

coincidence with GP has been confirmed by several earlier studies on natural dentition

(Preston, 1993; Hasanreisoglu et al., 2005; Ali Fayyad et al., 2006). Preston had found that the

PFP in relation to central incisors of the lateral incisors and canines were 66.2% and 55.6%

respectively (Preston, 1993). Similarly, Hasanreisoglu et al. found the lateral incisors and

canines proportions to be 65.9% and 52.3% respectively (Hasanreisoglu et al., 2005). Ali Fayyad

et al. reported that the GP had only existed in 27.1-31.3% of their evaluated population (Ali

Fayyad et al., 2006). The minor dissimilarity between this study and other studies is related to

the inevitable variation between the evaluated population, and the method of measurements.

Nevertheless, the obtained proportion pattern of all of these studies is supportive to the

proportion values of the current study.

This study differs from earlier studies in evaluating the PFP following prosthodontic planning.

Pini et al. had analysed the existence of GP following the treatment of lateral incisors agenesis

(Pini et al., 2012). The treatment was implant placement or orthodontic movement followed

by canines recontouring. They did not confirm the existence of GP for the majority of their

treated cases. Similar to the present study, the GP tended to exist more frequently between

centrals and laterals, and less commonly between laterals and canines. Thus, it could be

speculated that there is no recurrent mathematical proportion, and the canines tend be larger

than what has been proposed by GP (Ward, 2007).

Interestingly, several authors had critiqued the aesthetic value of GP. It appears that the GP

did not coincide with the majority of attractive smiles. Rosenstiel et al. had altered frontal

image by software to incorporate GP. They found that dentists tend to rank frontal images

with GP as less attractive (Rosenstiel et al., 2000). In a follow-up study, they found that lay

persons had minimal preference for images coinciding with GP (Rosenstiel and Rashid, 2002).

Similarly, Mahshid et al. found no relation between GP and what is perceived to be aesthetics

(Mahshid et al., 2004). Basting et al. found that dentists preferred greater proportion than the

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GP (Basting et al., 2006). Further, Ward had confirmed that dentists tend to prefer larger

proportions than GP (Ward, 2007). One of the clear limitations of GP is the appearance of

excessively wide central incisors, which is equivalent to the perceived width of the lateral

incisor and canine (Ward, 2007). Therefore, not only does GP rarely exist naturally, the

utilization of GP is not a reliable method to achieve desirable aesthetics of the anterior

maxillary teeth. Like many aspects in nature, instead of being mathematically determined, the

acceptable proportion for the anterior teeth appears to fit within a range.

The present study adds that PFP of the planned treatment will be dictated by pre-treatment

PFP, even if the tooth dimension is altered. Maintaining the PFP is likely to be due to the teeth

location in the curved arch and their relation to the observer’s view (Ward, 2001;

Hasanreisoglu et al., 2005). For example, from the frontal view, the entire labial surface of the

central incisor is visible, while for the canine, it is primarily the mesial half of the tooth. The

perceived lateral incisor view is dependent on its rotation within the arch. Therefore, it could

be speculated that the interest in establishing a repeatable proportion has been overrated in

the literature. Even if a specific PFP is desirable, a significant alteration in the proportion is not

feasible.

As expected, the teeth W:H ratios of the pre-treatment models were greater than for wax-up

models, which is attributed to the pre-treatment condition of the dentition (Magne et al.,

2003). Since following the treatment, the teeth are likely to be lengthened, the W:H ratio will

decrease. From the aesthetic perspective, lengthening the teeth is beneficial since it will

increase the teeth display, restore anterior teeth relationship and restore natural anatomy

(Abduo and Lyons, 2012). This is well supported by the fact that following the wax-up, the

anterior teeth W:H ratio is similar to natural non-deficient teeth. In a study on natural non-

restored young dentition, Gillen et al. found the W:H ratio of the central incisors, lateral

incisors and canines to be 90.2%, 83.9% and 82.5% respectively (Gillen et al., 1994). Likewise,

Sterret et al. found the ranges to be 85-86%, 76-79% and 77-81% (Sterrett et al., 1999).

Hasanreisoglu et al. had found very similar W:H values (89-91%, 82-83% and 83-87%

respectively) (Hasanreisoglu et al., 2005). Zlateri et al. had found slightly less W:H ratio (82.9%,

78.1% and 81.2%) respectively (Zlataric et al., 2007). Due to the minor variation in the outcome

of these studies, strict adherence to a specific proportion should be avoided. These

measurements are similar to the W:H ration of the wax-up dentition of the present study (83-

88%, 74-79%, and 75-81%). This supports that the restorative treatment is likely to return the

morphology to the baseline natural morphology. A similar outcome was observed by Pini et al,

who evaluated the W:H ratio of the restored anterior dentition that suffered from agenesis of

the lateral incisors (Pini et al., 2013).

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Symmetry has been mentioned as a main determinant for dental aesthetics. After evaluation

of several smiles, Durgekar et al. had found that lay persons tend to prefer symmetrical smiles

(Durgekar et al., 2010). Similarly, Machado et al. had established that minor unilateral vertical

discrepancies can be perceived as unaesthetic (Machado et al., 2013). There is a good

indication from the present study that the symmetrical harmony is enhanced by the wax-up

procedure. This is clear from the reduction of the asymmetry percentage following the

prosthodontic planning. Two forms of symmetrical evaluation were applied. This was

necessary as the perceived frontal symmetry will differ from the actual morphological

symmetry due to the curvature of the arch and angulation of the teeth. For the actual

dimensions symmetry, the greatest vertical and horizontal discrepancy was associated with

the pre-treatment models, and the wax-ups consistently showed improvement in the

symmetry. Since, naturally, there should be relative similarity between contralateral teeth

(Mavroskoufis and Ritchie, 1980), it is clear that the diagnostic wax-ups are useful in idealizing

the aesthetic outcome of the treatment. According to the perceived symmetry, it is clear that

the central incisors were the most symmetrical for all the models, while the canines were the

least symmetrical. This is usually easily observed for the central incisors (Ward, 2001;

Hasanreisoglu et al., 2005). However, for the canines, and to a lesser extent the lateral incisors,

the position and the perceived width will significantly influence the perceived symmetry.

As the central incisors tend to be the most symmetrical in the perceived and the actual

symmetry evaluation, the outcomes of the two wax-ups are likely to be rated as aesthetic. The

importance of central incisors symmetry was emphasized by several investigations. In a web-

based survey, Brunzel et al. had proved that the symmetrical position of the central incisors is

crucial while minor discrepancy in the lateral incisors position can be tolerated (Brunzel et al.,

2006). Yet, in the natural dentition, absolute symmetry of the central incisors is very unlikely

(Mavroskoufis and Ritchie, 1980; Gillen et al., 1994). Mavroskoufis and Ritchie had found that

60% of young individuals had an accumulated central incisors discrepancy of more than 0.2

mm (Mavroskoufis and Ritchie, 1980). Therefore, minimal dimensional discrepancy between

contralateral teeth, as observed after the diagnostic wax-ups in this study, is within the natural

limit. In an aesthetic appraisal by lay persons and orthodontists, it was found that minor

perceived vertical discrepancy of the incisal edges (0.5 mm) within the central incisors was

detected by different investigator groups. However, vertical discrepancy in the lateral incisor

of up to 0.5 mm was acceptable by the orthodontist, while the lay persons had an accepted

discrepancy of up to 1 mm (Machado et al., 2013). On the contrary to the incisors, Pinho et al,

had found that even a 2 mm perceived asymmetry of the canine cusp tip was acceptable by

orthodontists and lay persons (Pinho et al., 2007). This indicates that greater symmetry is more

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critical for teeth closer to the midline, and reasonable deviation from complete symmetry is

not necessary perceived as unaesthetic.

From the aesthetic perspective, this study indicates that the outcome of the conventional and

digital wax-ups is comparable. Digital wax-up appears to exhibit some advantages over

conventional wax-up in relation to the symmetry. However, although the perceived central

incisors symmetry is statistically greater for the digital wax-up models, the actual difference is

minimal (2%), which might not be noticeable clinically. Therefore, the hypothesis that there is

no difference in the aesthetic implications between the two wax-ups was accepted.

Nevertheless, this observation reflects the advantage of using digital technologies in obtaining

dental morphologies. For example, mathematical determination of tooth contour and utilizing

the contralateral mirror image feature will ensure a consistent and symmetrical outcome

(Probst and Mehl, 2008).

It is important to note that for the digital wax-up to be applicable, it should be transferrable to

the clinic. Physical models of the digital wax-up can be produced by 3D printing or milling

(Kasparova et al., 2013). However, it is likely that the actual accumulated discrepancies will

influence the outcomes reported in this study. For example, dimensional error of models

produced by 3D printing can be greater than 100 µm (Inokoshi et al., 2012).

11.6. Conclusions

Within the limitations of this study, it can be concluded that the diagnostic wax-ups had a

positive impact on the aesthetic variables. Despite the aesthetic improvements, the PFP was

minimally affected and no recurrent PFP was observed. The anterior teeth morphology in the

form of the W:H ratio was restored by the wax-ups to natural morphology. There are

indications that the perceived and the actual symmetry had improved following the diagnostic

wax-ups. In terms of tooth dimensions, proportions and symmetry, there is a greater similarity

between the two wax-up protocols. From the aesthetic perspective, digital planning appears to

be very promising. As dental aesthetics are perceived differently, the clinical evaluation of the

aesthetic outcome of each wax-up protocol is yet to be investigated.

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Chapter Twelve

12. General Discussion and Conclusions

Part of this chapter was published in the following article:

Abduo J, Lyons K, Bennamoun M. Trends in computer-aided manufacturing in prosthodontics:

a review of the available streams. International Journal of Dentistry. 2014; Accepted.

(Appendix M)

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The series of conducted research confirm that the diagnostic wax-up alters the pre-treatment

dental models in relation to all the evaluated variables. This observation emphasizes the merit

of conducting diagnostic wax-up. The advantages were observed from the aesthetic and

occlusion perspectives. There is tendency for the planned prosthodontic treatment to return

the dentition to its original and physiological state. Further, the two diagnostic wax-ups were

generally similar in relation to contour, aesthetics and occlusion. Some differences were

observed in relation to accuracy; however, it is very likely to be of subclinical significance.

12.1. Research Methodology

This research utilized the advantages of digital measurements tool. In industry, the digital tools

are frequently utilized to evaluate the precision of workpieces which is mandatory quality

assurance tool (Abduo et al., 2014b). This was feasible in the current research since all the

evaluated models were digital. Over the physical measurements, the advantages of digital

measurements were accuracy, convenience and consistency (Redlich et al., 2008; Dalstra and

Melsen, 2009; Prasad and Al-Kheraif, 2013). Through the software, it is possible to magnify the

models to allow accurate location of the point(s) of interest (Quintero et al., 1999; Kusnoto

and Evans, 2002). This will overcome the problems of measuring the dimensions of very fine

features. Further, the measurements can be repeated which insures the reliability (Delong et

al., 2002; DeLong et al., 2003). In addition, utilizing the digital tool allows accurate

quantification of the distortion, area and dimensions, which are impossible to evaluate

physically (Iwase et al., 2011; Abduo and Bennamoun, 2013).

On the contrary, the digital measurements suffer from some limitations. The accuracy of the

digital quantification is largely dependent on the accuracy of the scanning, model processing

and mesh density. In addition, as the points of interest will be located digitally, positional

inaccuracy on the mesh is inevitable. The issue of accuracy will be discussed in more details in

the accuracy part of the discussion.

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12.2. Tooth Surface Alteration

From the morphological perspective, two surfaces were altered: the axial and the occlusal

surfaces. These surfaces are critical for dental aesthetics, hygiene and function. As one of the

key purposes of prosthodontic treatment is to improve the appearance and function, the

implications of any wax-up on these two surfaces are very important.

12.2.1. Axial surface

It is clear that the diagnostic wax-up had improved the appearance of teeth. However, the

improvement in dental aesthetics is likely to be related to altering the tooth morphology

rather than the arrangement. For example, significant aesthetic improvement was observed

after contour alteration, restoring dental anatomy, symmetry refinement and modifying W:H

ratio. However, the perceived proportion was not affected by the wax-up, which indicates

minimal effect on teeth arrangement. It is not unusual for the tooth anatomy to be modified

through the prosthodontic treatment (Magne and Douglas, 1999a; b; Magne and Belser, 2004).

This was clear after the tooth contour evaluation, especially for the anterior teeth. For

example, it has been observed from this study that the teeth were widened as they emerge

from the gingival tissue. The effect of the wax-up on anterior teeth contour coincides with the

observation of studies that evaluated the effect of prosthodontic treatment on tooth contour

(Meijering et al., 1998; Vasconcelos et al., 2009). This is thought to be advantageous since it

increases the teeth display. In a split-mouth study on four participants, Ehlrich and Hochman

found the participants had preferred the over-contoured crowns by 1 mm over under-

contoured crowns (Ehrlich and Hochman, 1980). In addition, altering the facial tooth

morphology will allow elimination of anterior teeth discrepancies such as anatomical

limitations, asymmetry, and W:H ratio. This confirms the advantages of prosthodontic

treatment on dental aesthetics.

From the hygienic perspective, increasing the tooth contour has been related to gingival

inflammation, periodontal complication and dental caries (Perel, 1971; Sackett and

Gildenhuys, 1976; Sorensen, 1989; Broadbent et al., 2006). On the contrary, more recent

studies consistently indicated that as long as adequate cleanliness is maintained, the

periodontal complications are likely to be prevented, even with over-contoured crowns

(Ehrlich and Hochman, 1980; Sindel et al., 1999; Kohal et al., 2003; Kohal et al., 2004).

Therefore, according to the published clinical studies, it appears that reasonable over-

contouring of up to 1 mm is not likely to be associated with biological complications. Since the

maximal contour recorded in this study, by the two wax-ups was less than 1 mm, the modified

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tooth contours are less likely to induce pathological consequences. Further, the linear increase

of the modified teeth contour coronally means that the contour increase is likely to blend

smoothly with the unaltered tooth surface. This is further supported by the measurements at

the gingival margin being the least (less than 0.2 mm). Therefore, potential implications on oral

health from the two wax-ups are very unlikely.

An additional advantage of widening the teeth is the facilitation of the conservative

preparation of the anterior teeth. When the tooth is prepared for fixed prosthesis, a

considerable amount of tooth surface is reduced to accommodate the prosthesis. This space is

necessary for the durability and the aesthetics of the restoration. After the wax-up, the

preparation can be executed according to the planned restoration surfaces rather than existing

surfaces (Magne and Belser, 2004; Gurel, 2007). This merit was confirmed by the current

research for the anterior teeth. Since the anterior teeth tend to have less tooth substance than

posterior teeth, they will benefit significantly from any conservative measure. This will reduce

the risk of pulpal and mechanical complications (Goodacre et al., 2001). Further, since the

anterior teeth have the advantage of being accessible for cleaning, it is less likely for the oral

health to be affected by over-contouring.

On the other hand, an interesting observation of this research is that the contour of the molar

teeth was minimally affected. This is likely to be advantageous as it will maintain the ease of

cleaning of the less accessible areas (Becker and Kaldahl, 1981). In addition, it is less likely for

the appearance of posterior teeth to benefit from over-contouring as they are not in the

aesthetic zone.

The quantitative evaluation of this study had revealed that the prosthodontic treatment had

restored the W:H ratio of the maxillary anterior teeth to the natural dimensions. Before the

wax-up, the W:H ratio was greater, which might be due to the pre-treatment condition of the

dentition (Magne et al., 2003). While after the treatment, the teeth are likely to be lengthened

which might reduce the W:H ratio. This is associated with greater tooth display and aesthetic

appearance (Abduo and Lyons, 2012). The studies that evaluated the W:H ratio for the central

incisors, lateral incisors and canines of the natural dentition revealed similar dimensions to

what has been observed in this research after the wax-up (Gillen et al., 1994; Sterrett et al.,

1999; Hasanreisoglu et al., 2005; Zlataric et al., 2007). Therefore, after the wax-up, the anterior

teeth W:H ratio will be similar to the W:H ratio of natural non-deficient teeth. Similarly, our

research confirmed the observation of Pini et al. after the management of lateral incisors

agenesis (Pini et al., 2013).

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In terms of anterior teeth symmetry, the present research indicated that the symmetrical

harmony was enhanced after the wax-up. This was observed for the actual and the perceived

symmetry, and the vertical and horizontal tooth symmetry. Since, naturally, there should be

relative similarity between contralateral teeth (Mavroskoufis and Ritchie, 1980), it is clear that

the diagnostic wax-ups are useful in idealizing the aesthetic outcome of the treatment.

Dentition with greater symmetry is perceived as more aesthetic (Durgekar et al., 2010;

Machado et al., 2013)

A new observation of the research is that the wax-up or had minimal influence on the

positional variable such as the PFP, and the PFP of the pre-treatment dentition tend to

determine the post-treatment PFP. Similar outcome was observed by Pini et al. who analysed

the PFP following the lateral incisors agenesis (Pini et al., 2012). Maintaining the PFP is likely to

be due to the teeth location in the curved arch and their relation to the observer’s view (Ward,

2001; Hasanreisoglu et al., 2005). For example, from the frontal view, the entire labial surface

of the central incisor is visible, while for the canine, it is primarily the mesial half of the tooth.

The perceived lateral incisor view is dependent on its rotation within the arch. Therefore, it

could be speculated that the interest in establishing a repeatable proportion has been

overrated in the literature. Even if a specific PFP is desirable, a significant alteration in the

proportion is not possible by prosthodontic treatment.

After comparing the two wax-ups, there is an overall similarity. This applies to the contour,

W:H ratio, PFP and symmetry. The contour of the digital wax-up tends to be greater than the

conventional wax-up. On the other hand, the contour around the conventional wax-up teeth

tends to be more consistent than the digital wax-up. This difference between the two wax-ups

was statistically significant; however, the actual difference is minimal. Therefore, the clinical

impact of this difference is most likely minimal. Therefore, the hypothesis that the

conventional and digital wax-ups cause similar contour alterations is accepted. The tendency

for the digital wax-up to cause greater labial contour could be due to the difficulty in locating

the gingival margin digitally on the scanned pre-treatment model (Tan et al., 2008; Abduo et

al., 2010; Han et al., 2011). This is more difficult than the actual and physical location of the

gingival margin as per the conventional wax-up.

In relation to symmetry, the digital wax-up was associated with greater symmetry than the

conventional wax-up. Similar to the contour, the actual difference in the symmetry is minimal,

which might not be noticeable clinically. Thus the hypothesis that the aesthetic effects of the

conventional and digital wax-ups are similar was accepted. Nevertheless, this observation

reflects the advantage of using digital technologies in obtaining consistent and symmetric

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dental morphologies. This might be attributed to the mathematical determination of tooth

contour and utilizing the contralateral mirror image feature (Probst and Mehl, 2008). An

additional advantage of the digital wax-up was the better overall contour and sharper

appearance. This is likely to be related to the utilization of physiological teeth moulds which

were rated as highly aesthetic and normally used for definitive prosthesis. Other researchers

found that the digitally modified teeth tend to exhibit exaggerated anatomical features (line

angles, cusp tips and marginal ridges) (Paulus et al., 1999; Mehl et al., 2005a). The advantage

of the high anatomical definition is the digital wax-up might be perceived as more natural and

aesthetic (Figure 12-1). On the contrary, the conventional wax-up is based on manual wax

handling which might hinder development of optimal aesthetic appearance. Nevertheless, in

clinical practice, this effect is not very critical as the definitive prosthesis is provided at greater

definition and aesthetics.

A B

Figure 12-1 Frontal virtual images of (A) conventional wax-up model and (B) digital wax-up model. It is clear that the teeth of the digital wax-up model exhibited more defined features which might enhance the overall aesthetics.

12.2.2. Occlusal Surface

In relation to the occlusal surface, the two wax-ups had caused prominent occlusal alterations,

and returned the teeth to more to what is thought to be natural morphology. This was true for

the static and dynamic occlusion. In relation to the static occlusion, the occlusion quality was

evaluated from quantifying the contact number and area. Regardless of the wax-up procedure,

the contact number, contact area had significantly increased after the wax-up, which supports

that the planned prosthodontic treatment positively influences the number and area of

occlusal contacts. The lower occlusal contact number and area for the pre-treatment models

could be attributed to the pre-treatment dentition’s tendency to suffer from dental problems

that influence the morphology, such as large restoration, tooth wear and chipping. Such

abnormalities can affect the quality of the occlusal contact and contact area.

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Overall, the studies on natural and intact dentition revealed that the intercuspal occlusal

contact number was similar to the contact number obtained following the wax-up treatment

of this study (McNamara and Henry, 1974; Korioth, 1990b; Ciancaglini et al., 2002; Delong et

al., 2002). Thus, it might be reasonable to assume that the planned prosthodontic treatment

will restore the occlusal anatomy to a more natural anatomy and the occlusal relationship

might return to the baseline relationship. Such an observation is attributed to the more natural

dental anatomy that can be generated following the wax-up. This is in accordance with the

clinicians’ recommendations on idealising static occlusal contacts with prosthodontic

treatment (Wiskott and Belser, 1995; Koyano et al., 2012). The envisioned improvement of

occlusal contacts will potentially contribute to a more stable and functional occlusion (Owens

et al., 2002). Therefore, in addition to aesthetic improvement following the prosthodontic

treatment, the oral function is more likely to improve as well.

In relation to the occlusal contact area, there was a marked variation from previous studies.

Studies on physical measurements of the occlusal area found less area than what has been

reported in this study (Hidaka et al., 1999; Delong et al., 2002; Alkan et al., 2006). Conversely,

studies that measured the area digitally recorded a relatively large contact area (Iwase et al.,

2011) which was similar to the wax-up models of this study. The variation in the outcome of

the studies could be related to the method of area quantification (Owens et al., 2002). It has

been acknowledged that a slight vertical discrepancy of the maxilla-mandibular tooth

relationship will cause an exponential reduction of the recorded area (Wilding et al., 1992;

Hidaka et al., 1999). Several of the earlier studies have applied occlusal medium to quantify the

area, in such cases, vertical displacement of the jaw will likely have occurred, resulting in an

underestimation of the contact area. On the other hand, digital evaluation of the area is likely

to overestimate the contact area due to the risk of the models overlapping (Iwase et al., 2011).

Nevertheless, it is clear that the wax-up process increases the contact area, which is indicative

that the planned prosthodontic treatment will increase the contact area at the occlusal phase

of chewing, hence resulting in more efficient chewing.

A significant determining factor of the contact number and area was the location of the tooth

in the arch. There was a dominance of contact number and area on the posterior teeth (two

times the anterior teeth), which corroborates several earlier investigations on natural

dentition (Ehrlich and Taicher, 1981; McDevitt and Warreth, 1997) and restored dentition (Yi

et al., 1996). Likewise, similar relationship was observed for the contact area of the natural

dentition (Yurkstas and Manly, 1949; Owens et al., 2002). The more profound contacts on the

posterior teeth are due to greater area, cuspal morphology and interdigitation of the opposing

teeth. The anterior teeth, on the other hand, have more confined surfaces and incisal edges.

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Further, this finding fits with the mutually-protected occlusion concept, where the posterior

teeth prevent excessive contact of the anterior teeth at maximum intercuspation (The Glossary

of Prosthodontic Terms, 2005). Although this finding is correct for the pre-treatment and the

wax-up models, the difference between the anterior and posterior teeth is greater following

the prosthodontic treatment, which indicates the idealisation of the occlusion scheme

following prosthodontic planning. Thus, it could be speculated that the posterior teeth receive

greater benefit in terms of contact number and area following the prosthodontic treatment

than the anterior teeth. This is advantageous from the functional perspective, as the posterior

teeth are responsible for food chewing and grinding.

In relation to the effect on the lateral occlusion scheme, the two wax-ups had influenced the

lateral occlusion in relation to the prevalence of each scheme at different position. Further,

there has been an effect on the contacting teeth type and quantity. Wide range of eccentric

movements was selected in this study (Ogawa et al., 1998) as this will cover full functional and

parafunctional movements such as chewing, grinding and bruxism (Suit et al., 1976; Woda et

al., 1979; Ogawa et al., 1996; 1997).

In terms of the prevalence of each lateral occlusion scheme, it is clear that with greater

excursion, the prevalence of CGO tends to increase. This was in accordance with the studies

that evaluated the prevalence of each lateral occlusion scheme at different positions for the

natural dentitions of young participants (Yaffe and Ehrlich, 1987; Al-Nimri et al., 2010).

Although the present study had evaluated the lateral occlusion scheme of restored dentitions,

overall, the figures were supporting to the outcome of the earlier studies. The differences in

documenting the prevalence of each occlusion scheme could be related to the different

classification and recording methods of the lateral occlusion scheme (Ogawa et al., 1998;

Abduo et al., 2013). In addition, it is important to emphasize that a single lateral occlusion

scheme did not exist for any model for the entire excursive path. Therefore, in accordance

with all the published clinical studies, true CGO or GFO seldom exists clinically (Woda et al.,

1979; Becker and Kaiser, 1993; Turp et al., 2008; Abduo et al., 2013), even after restoring the

dentition.

The dynamic nature of the lateral occlusion scheme at the different arch positions is attributed

to teeth morphological factors. In the initial phase of excursion, the cusps are articulated

against wider fossa surfaces (Schuyler, 1963). As excursion progresses the total contact area

reduces, thus more teeth will be discluded. The pattern of lateral occlusion scheme alteration

with different excursion was observed for the pre-treatment and wax-up models. However,

the wax-up models had more consistent gradual occlusion scheme changes. This might be due

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to the pre-treatment dentitions suffering from morphological abnormalities, such as tooth

wear or failed restorations. Subsequently, a wider contact area between worn down teeth

might be evident and will maintain a greater number of contacts, even after maximal excursion

(Beyron, 1954). Whereas the dentitions of the wax-up models had restored natural dental

morphology and less prominent wear facets, which can produce steeper articulation and cause

a consistent reduction of the total number of tooth contacts during excursion. This was

evident in the initial phases of excursion (0.5-1.0 mm) where the contacts quantity was

significantly greater for the two wax-up models than at the later stages of excursion. In

addition, it was clear that the proportion of CGO at later stages of excursion was greater for

the two wax-ups than the pre-treatment models. This was also likely to be related to restoring

the cuspal morphology of the canines to the original canine morphologies. The studies that

evaluated the implication of individual age on excursion scheme confirmed that the younger

the individual, the greater the prevalence of CGO (Panek et al., 2008; Abduo et al., 2013). This

was attributed to the prominent dental morphology and less cuspal wear of the young

dentitions (Panek et al., 2008; Abduo et al., 2013). Following the prosthodontic planning, it was

clear that the frequency of the canine contact was increased, followed primarily by the first

premolars. This was attributed to restoring the original canine cuspal morphology, while the

canines of pre-treatment models suffered from greater tooth wear (Beyron, 1954). Therefore,

it could be speculated that the planned prosthodontic treatment can return the dentition

occlusion scheme closer to what is expected to be the original state.

After comparing the two wax-ups there was an overall similarity in the static and lateral

occlusion schemes. Therefore, the hypotheses that there is no difference between the

conventional and digital wax-up in relation to static and dynamic occlusal relationships were

accepted. For the static occlusion, there is tendency for the digital wax-up to exhibit greater

contact number than the conventional wax-ups, while for the contact area, the conventional

wax-up was associated with greater area. However, the difference was not statistically

significant. Likewise, the two wax-ups induced similar prevalence of the lateral occlusion

scheme and the pattern of occlusal contacts frequency. However, the digital wax-up appeared

to have an exaggerated role of the canine teeth from the greater frequency of canine contacts.

Yet, this difference was not to a statistical significant level. The observed slight difference is

most likely related to the differences between the occlusal morphology generated by each

wax-up. The dental morphologies that can be attained digitally by the software tend to be well

defined (Mehl et al., 2005a). As the digital wax-up utilizes an average tooth form, the final

tooth morphology tends to exhibit more defined and steeper anatomical features (Ender et al.,

2011). This means greater cuspal angle, more pointy cusps and deeper grooves and fossae

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(Figure 12-2). The more defined occlusal anatomy could explain the greater contact number

and the lower area of the posterior teeth for the digital wax-up models that was observed in

the current study. Further, the greater prevalence of CGO tends to develop with more defined

canine anatomy. Therefore, in terms of occlusion rehabilitation, it is reasonable to state that

the two wax-ups generated a similar outcome.

A B

Figure 12-2 Examples of digital (A) maxillary and (B) mandibular posterior teeth that illustrate the well-defined occlusal anatomy.

Despite the lack of a significant statistical difference between the two wax-up protocols

applied in this study, it is important to reinforce that the digital wax-up is still in its early phase

of application and requires additional investigation. For example, this study did not consider

non-working side contacts, which could influence the observed lateral occlusion scheme. The

omission of observing the non-working side contacts was necessary, as virtual modelling of the

condylar movement is very imprecise (Schierz et al., 2014). Several authors had acknowledged

that one of the limitations of the available CAD/CAM systems is lack of a reliable virtual

articulation system that can simulate dynamic motion (Kordass et al., 2002; Rohrle et al., 2009;

Koralakunte and Aljanakh, 2014). To overcome this problem, few reports on virtual articulators

were published with promising outcome (Kordass et al., 2002; Fang and Kuo, 2008;

Solaberrieta et al., 2013; Solaberrieta et al., 2014b). Yet, after comparing the virtual

articulation with the conventional articulation, greater deviation in the dynamic motion was

observed with the virtual articulation (Solaberrieta et al., 2014a). As a result the authors had

concluded that the level of accuracy of the virtual articulator in simulating dynamic motion

might be acceptable for orthodontic treatment, but not for prosthodontic treatment

(Solaberrieta et al., 2014a).

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12.3. Accuracy

The precision of digital dentistry has been an interest since the introduction of this technology.

For it to be accepted, it has to exhibit similar accuracy to conventional protocol. Digital

dentistry is thought to have the advantage of omitting several steps, which will reduce the

inaccuracy contributed from each step. In this research project, the accuracy was evaluated at

two levels: gingival and occlusal levels. Both of these levels dictate the treatment outcome,

usability of the wax-up and transferring the information intra-orally. Good wax-up will ensure

accurate evaluation and minimal adjustments.

12.3.1. Gingival accuracy

The gingival accuracy was evaluated in this research as it represents the effect of wax-up on

the unaltered surface. The digital protocol is more accurate than the conventional protocol

(Abduo et al., 2014a). This difference is most likely mathematical and related to the method of

accuracy evaluation. The conventional wax-up required additional steps such as model

duplication and subsequent scanning. The additional material and steps will inevitably add

errors in the form of dimensional changes (DeLong et al., 2003). On the contrary, the digital

wax-up was conducted directly on the virtual pre-treatment model, which will reduce the

chance of inaccuracies. Nevertheless, despite the statistical difference, the actual difference is

minimal which might not have a clinical implication. As a result, the hypothesis that the

precision of the two wax-ups is similar can be accepted in relation to the gingival tissues.

On the other hand, the digital wax-up appears to be more deficient in locating the wax-up

margin than the conventional wax-up. This was observed from the contour study, where the

contour increase was greater on the gingival margin for the digital wax-up than for the

conventional wax-up. Locating the restoration margin on virtual mesh was reported to

invariably lead to a slight deviation from the exact gingival margin (Abduo et al., 2010). This

might contribute to greater inconsistency of the contour alteration of the digitally modified

teeth. Some investigators reported that a possible consequence of this limitation is the

marginal discrepancy of the digitally produced crowns in comparison to the conventionally

produced crowns (Tan et al., 2008; Han et al., 2011). On the contrary, the conventional wax-up

is based on actual and tactile feeling of the gingival margin which could lead to a more

accurate outcome.

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12.3.2. Occlusion accuracy

From the occlusal perspective, the digital wax-up had greater inaccuracy than the conventional

wax-up. Therefore, the hypothesis that there is no difference between conventional and digital

wax-ups in relation to occlusion precision is rejected. The greater discrepancy of the digital

wax-up is very likely to be related to the mathematical determination of the dental anatomy.

The external surface of the digital teeth is formed of vertices connected by accumulating

triangles of polygonal mesh. This polygonal mesh is composed of flat triangles which

approximate the curved surfaces of the dental restoration. Although the polygonal mesh is

necessary to virtually modify the dental anatomy, it could introduce some discrepancies in the

approximation of curves. The effect of the polygonal mesh is further accentuated by the

presence of prominent curvature of the teeth which is evident with the digital wax-up (Pfeiffer,

1999; Luthardt et al., 2002). Further, it is likely for the opposing meshes to overlap during the

digital modelling as opposed to the actual determination of the occlusal contacts on

conventional wax-up. Many researchers have developed computer algorithmic systems for

tooth surface design (Paulus et al., 1999; Mehl et al., 2005a; b; Ender et al., 2011). In

accordance with the outcome of this research, regardless of the tooth modification technique,

virtual reconstruction generally was found to cause up to 0.5 mm vertical discrepancies of the

completed restoration and in many cases, the operator is expected to manually adjust the

occlusal contacts (Ender et al., 2011). Therefore, although the digital wax-up anatomy is very

natural and well-defined, it might be responsible for greater occlusal discrepancies. In

addition, as the digital wax-ups are yet to be produced by CAM processes, the accumulated

final inaccuracy will be greater to what has been observed in this study. The problem of further

processing of the digital wax-up will be discussed in the following section. Nevertheless, since

in clinical practice the wax-ups are used primarily for provisional restorations, minor occlusal

discrepancies can be modified easily in the clinic, with no major consequences on the

definitive prostheses (Magne and Douglas, 1999a; b; Magne and Belser, 2004).

12.3.3. Digital processing precision

The rationale of digital protocol in reducing the inaccuracy by omitting manufacturing steps

and reducing materials is logical, however, it is strongly dependent on the involved steps. The

adequacy of the digital model is largely dependent on the reliability of the scanning procedure.

The scanning procedure and the surface generation are associated with some discrepancies

(Persson et al., 2006). The virtual model will initially be represented as a point cloud that is

used to form the model boundary representation. The reconstruction procedure commences

as the adjacent points are connected with straight lines using fitting algorithms. Eventually, a

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continuous surface is obtained in the form of triangulated mesh or polygonal 3D model. Still, at

this stage, the surface suffers from noise that requires elimination, which might further

influence the surface accuracy. Curved surfaces are represented with numerous flat surfaces.

True mathematical sphere can then be obtained by extrapolating the points according to the

algorithmic principle of curve of best fit (Willer et al., 1998; Pfeiffer, 1999; Luthardt et al.,

2002) to form the surface boundary. Therefore, in principles, the greater the vertices recorded

for the point cloud, the closer the curve of best fit represents the overall morphology (Rudolph

et al., 2007). Further, exaggerated curvature is prone to greater loss of details.

The initially scanned image is very complex, detailed and with an enormous size, which makes

its manipulation very difficult. To enhance the software performance, the virtual model can be

simplified by a process of decimation (Willer et al., 1998; Persson et al., 2006), whereby the

number of points is selectively reduced to minimise the overall size of the 3D polygonal image.

The effect of the decimation is more prominent on flat surfaces where less number of points is

needed compared to curved surfaces. Therefore, while reducing the image size, the overall

geometry of the polygonal 3D model is not affected (Figure 12-3A, 3B, 3C and 3D). The

drawback of excessive decimation is the possibility of virtual rounding the sharp edges and the

actual surface irregularities which will cause inaccuracy of the virtual model (Figure 12-3E)

(Willer et al., 1998; Persson et al., 2006).

A

B

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C

D

E

Figure 12-3 An example of the effect of dental model simplification. (A) The originally scanned model is composed of dense points. (B) The model after 50% decimation. (C), 25% decimation. (D) 12.5% decimation. (E) 6.75% decimation model. The decimation primarily affects flat surfaces. Excessive decimation causes greater the loss in the resolution.

Even after the completion of the digital wax-up by the software, it has to be produced into a

physical model. Once the digital wax-up model is completed by the software, the data is

transferred to a CAM software that controls the production unit. The aim of CAM process is to

produce an accurate model as represented by the software. Actual model can be produced by

subtractive or additive manufacturing.

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Subtractive manufacturing is based on milling the workpiece from a larger blank by a computer

numeric controlled (CNC) machine. The CAM software automatically translates the CAD model

into tool path for the CNC machine. This involves computation of the commands series that

dictate the CNC milling, including sequencing, milling tools, and tool motion direction and

magnitude (Rekow et al., 1991). Due to the unevenness of the features of dental morphology,

the milling machines combine burs with different sizes. The accuracy of tool positioning has

been reported to be within 10 µm (Rekow et al., 1991). The CAM software also incorporates

compensation steps for the cutter tool diameter which ensures that the milling bur reaches

the desired surface without sacrificing necessary segment of the workpiece (Rekow et al.,

1991; Ortorp et al., 2011). The dental CNC machines are composed of multiaxis milling devices

to facilitate the 3D milling of dental workpieces.

A key advantage of milling is ensuring the durability of the workpiece since it is milled from an

industrial grade blank. Milling can reduce fabrication flaws in dental prostheses, by relying

more on the tighter quality control processing of the material manufacturer rather than

commercial laboratory (Rekow and Thompson, 2005) so that manufacturing deficiencies, such

as porosities and inhomogeneous consistency, are reduced (Sadan et al., 2005; Denry and

Kelly, 2008; Abduo and Lyons, 2013). On the other hand, milling was found to be associated

with surface damage in the form of surface micro-fractures, chipping defects, and altered

surface quality (Sindel et al., 1998; Ezugwu et al., 2003; Abele and Frohlich, 2008; Rekow et al.,

2011) and could constitute a point for crack propagation within the restoration under occlusal

forces (Sindel et al., 1998). The cutting conditions also cause excessive vibrations, especially in

thin edges (Terminasov and Yokhontov, 1959). The extent of the damage is dependent on the

material of the workpiece (Xu et al., 1998) and ranges from 15 to 60 µm (Sindel et al., 1999;

Luthardt et al., 2004; Kim et al., 2010).

The accuracy of digital dentistry has been heavily evaluated by quantifying the fit of digitally

produced dental restorations in comparison with conventional restoration. There is an overall

tendency for the restorations produced by conventional methods to exhibit better fit than

milled restorations. This applies to milled metal (Tan et al., 2008; Han et al., 2011) and ceramic

restorations (Abduo et al., 2010; Aboushelib et al., 2012). The production of the fine details by

milling is largely dependent on the diameter of the smallest milling bur which is normally

about 1 mm (Rekow et al., 1991; Beuer et al., 2008; Ortorp et al., 2011); however, smaller

diameter milling burs do not appear to produce fine detail for accuracy (Bornemann et al.,

2002; Tinschert et al., 2004). Ortorp et al. reported that, in order to mill the angle with a

diameter less than the diameter of the smallest fitting bur, a drill compensation feature has to

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be incorporated within the software to provide room for the bur movement (Figure 12-4)

(Ortorp et al., 2011).

A B E

Figure 12-4 The effect of bur diameter in line angle production. (A) Sharp virtual line angle cannot be produced by rounded bur. Therefore, surface inaccuracy will occur on the milled restoration in the form of (B) negative error after over-milling of the sharp corner, or (C) positive error after under-milling of the sharp corner.

Additive manufacturing systems have recently been introduced as a method to construct

dental models and defined as the process of joining materials to make objects from 3D model

data, usually layer upon layer (Webb, 2000; Davis, 2010; van Noort, 2012). Once the CAD

design is finalized, it is segmented into multislice images. For each millimetre of material, there

are 5–20 layers, which the machine lays down as successive layers of liquid or powder material

that are fused to create the final shape. This is followed by workpiece refinement to remove

the excess materials and supporting arms. Similar to the subtractive systems, a form of CNC

machine is used with a processing head that moves in two axes (𝑥- and 𝑧-axes) and the

specimen platform or the processing head moves in the vertical axis (𝑦-axis) (Choi and Chan,

2004; Choi and Cheung, 2005).

Originally, the additive manufacturing methods were implemented to fabricate prototype

models and patterns with reliable accuracy and repeatability that could be produced in a short

time. In prosthodontics, additive manufacturing can fabricate a preproduction pattern (wax or

plastic) that can be transformed to definitive workpieces in metals, resins, or ceramics (Webb,

2000; Davis, 2010; van Noort, 2012). The application of additive manufacturing in dentistry is

useful due to its ability to produce a variety of shapes that conform to any biological shape,

such as teeth. The additive systems used in dentistry are stereolithography, selective laser

sintering or melting, and 3D printing. Regardless of the method, all share the following

features that distinguish them from subtractive manufacturing:

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Incremental vertical object build-up.

No material wastage.

Large objects production.

Passive production (i.e., no force application).

Fine details production.

These features make them ideal methods for dental models fabrication. The dental models can

be produced by stereolithography or 3D printing. Stereolithography produces the solid layers

using a concentrated ultraviolet light beam that moves on a curable liquid polymer pool. As

the first layer is polymerized, a platform is lowered a few microns and the next layer is cured.

This process is repeated until the whole solid object is completed. The object is then rinsed

with a solvent and placed in an ultraviolet oven to thoroughly cure the resin. In dentistry,

stereolithography is routinely used to produce resin objects such as surgical templates, facial

prosthesis patterns, occlusal splints, burnout resin patterns, and investing flasks (Chang et al.,

2006; Mandelaris and Rosenfeld, 2008; Bi et al., 2013; Salmi et al., 2013).

3D printing extrudes material from a nozzle that solidifies as soon as it is deposited on the

manufacturing platform. The layer pattern is achieved through horizontal nozzle movement

and interrupted material flow. This is followed by vertical movement for the sequential layer

deposition. There are a range of materials that can be used for 3D printing. This includes

thermoplastic materials, such as waxes, resins, or fused filament, which pass through a heated

nozzle and solidifies immediately after extrusion. Alternatively, liquid ceramic or resin

materials with a binder can be printed (Sun et al., 2009; Inokoshi et al., 2012), which, following

deposition, solidifies immediately (Ebert et al., 2009; Silva et al., 2011). Some systems also

allow for multicolour production (Inokoshi et al., 2012). This approach is used in dentistry to

fabricate dental models, facial prosthesis patterns, acrylic prostheses, investing flasks, and

castable or ceramic frameworks (Sun et al., 2009; Silva et al., 2011; Inokoshi et al., 2012).

In comparison to subtractive processing, this method is more economical since it does not

result in any material wastage, and any unused material is completely reusable for future

processing (Webb, 2000; Davis, 2010). In addition, there is minimal restriction on the ability to

fabricate large workpieces (such as whole arch model), which is not the case with subtractive

methods that are more suitable for smaller workpieces (Chen et al., 1997; Runte et al., 2002;

Feng et al., 2010a). Additive manufacturing also allows the fabrication of workpieces with

different consistencies and material properties (Ebert et al., 2009).

Among the advantages of additive manufacturing is the ability to produce customized models

that represent patient hard and/or soft tissues (Webb, 2000; Davis, 2010). The workpieces can

215

include detailed morphology, sharp corners, undercuts, or voids. Because no drilling tool is

involved, no compensation feature is required as is necessary for the subtractive

manufacturing. Further, the whole production process is passive and involves no force

application. However, due to the production procedure, which involves sequential layering,

the external surface tends to have stepped and coarse morphology representing each

fabrication layer along the construction direction (Choi and Chan, 2004). Such stepping

adversely affects the surface texture and the overall dimensional accuracy of the workpiece

(Choi and Chan, 2004), which might influence the surface quality (Williams et al., 2004;

Williams et al., 2006) (Figure 12-5). The vertical walls were minimally affected by stepping

while the corrugated or sloping surfaces are more prominently influenced (Vandenbroucke

and Kruth, 2007). Therefore, concerns were raised regarding the accuracy of the occlusal

surface of prostheses produced using this technique (Silva et al., 2011). The accuracy of

additive technique is dependent on layer thickness and the width of curing beam. The thinner

the layers and the narrower the curing beam, the more accurate the final product; however,

increasing the number of layers and reducing the diameter of the beam will exponentially

increase the fabrication time (Kathuria, 1999; Khaing et al., 2001; Choi and Chan, 2004).

A B C D

Figure 12-5 The effect of layered production on the surface accuracy. (A) Smooth surface is ideal for dental models. (B) Thick layers will increase the prominence of surface stepping. (C and D) As the layers thickness is reduced the surface accuracy will increase. The corrugated surface (occlusal surface) is more affected by the steps than the vertical surfaces.

In the dental literature, there are a limited number of studies that have evaluated the accuracy

of prostheses fabricated by additive manufacturing. Salmi et al. reported the dimensional

accuracy of occlusal splits fabricated by stereolithography to be 0.3 mm (Salmi et al., 2013). In

relation to 3D printing, Ebert et al. reported that this method allows the fabrication of very

accurate ceramic workpieces (Ebert et al., 2009), and the production of sections of 100 µm is

feasible. Silva et al. reported that the tolerance of the fabricated workpiece is less than 25 µm

(Silva et al., 2011). In comparison, an evaluation of the dimensional errors of printed dentures

found a mean deviation of 5 µm, but dimensional distortions of up to several 100 µm were

detected (Inokoshi et al., 2012). With regular advancement and development in these

216

technologies, it is very likely that it is very likely that significant quality improvement will occur

in the future making this technology very competitive with the existing fabrication methods

(Silva et al., 2011) (Figure 12-6).

A B

C D

Figure 12-6 (A) An example of a maxillary model produced from the conventional method. (B) A virtual image of the same model. (C) The same model after production by 3D printing. (D) A magnified image of the buccal surface of the 3D printed model illustrating the model layers that may influence the surface quality.

Therefore, although the digital tools are very attractive to be applied in routine dentistry, they

suffer from inherent limitations. Nevertheless, the field is continuously improving to a

competitive level that supports its application more frequently in prosthodontics.

12.4. Future Research

Overall, this research illustrates a promising outcome of digital diagnostic wax-up in being

comparable to conventional wax-up in relation to all the evaluated parameters. Due to the

advantages of the digital wax-up, it is likely that greater refinement will be observed in the

217

future. While the outcome of digital wax-up is promising, recommendations to alter the

current protocol should be based on additional evidence. The research field will benefit from

further experimentation in three aspects: clinical appraisal, protocol validation and innovative

improvements.

Clinically, the performance and the advantages of the digital wax-up should be evaluated. This

will clinically validate the pros and cons of the digital wax-up that were observed in this study,

and will determine the clinical significance of the differences detected between the two wax-

up protocols. This series of experiments provided an outcome of objective analyses of

accuracy, occlusion and aesthetics. However, as discussed earlier, individuals vary in their

perception of their dentitions in terms of comfort and appearance. Thus, future subjective

investigations are necessary to evaluate the actual impact of digital planning and treatment on

the masticatory system and adaptation. Further, it is of necessity to analyse the subjective

patient appraisal of the aesthetic. This is best evaluated by a patient preference survey on

dental aesthetics following each wax-up.

Protocols involving digital dentistry should be evaluated for practicality and cost-effectiveness.

While it is tempting to utilize digital technology, it is known that the involved equipment

requires a significant initial investment. Alternatively, clinicians may opt to outsourcing some

of the digital protocol steps and the computer-aided production using commercially available

systems. Nevertheless, the question remains, will digital wax-up be more convenient and

efficient than conventional wax-up? Factors that can influence the practicality and cost-

effectiveness of the digital wax-up are the complexity of the case, the number of involved

teeth and access to a digital laboratory.

Recently, more innovative applications of digital dentistry have been suggested. For example,

digital modelling can be further exploited by the incorporation of digital colouring and digital

tooth shade determination (van Noort, 2012). This will provide additional simplification and

predictability of the clinical protocol. In addition, with the aid of the software, mathematical

modelling can be employed to select the best prosthesis material and design. At the industrial

level, digital manufacturing systems are now available to produce a workpiece from different

materials. It is not unlikely for such technologies to be utilized to produce dental prostheses

from multiple materials in the near future.

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12.5. Conclusions

In conclusion, this research confirms the implications of diagnostic wax-up on teeth contours,

aesthetics and occlusion. Following the wax-up procedure, the dentition is restored to a state

that is close to the benchmark condition. After comparing the digital wax-up to the

conventional wax-up, the following points can be concluded:

Overall, the two wax-ups are very similar in relation to precision, contour, static occlusion,

lateral occlusion and aesthetics.

At the virtual level, the digital wax-up is slightly more accurate than the conventional wax-

up around the gingival tissues. This observation is yet to be confirmed after physical

production of the digital wax-up model. However, for the occlusion, the digital modelling

appears to be slightly more deficient in obtaining accurate occlusal contacts than the

conventional wax-up.

The digital wax-up had caused more axial contour increase than the conventional wax-up.

Yet, the magnitude of this difference is minimal.

In relation to the intercuspal occlusal contact number and area, the two wax-ups had

yielded similar outcome. Similarly, the lateral occlusal relationship was similarly affected

by the two wax-ups.

In relation to the evaluated aesthetic variables, the digital wax-up had the advantage of

the ease of obtaining natural and symmetrical appearance.

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Appendix

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Appendix A

evaluation of virtual planning as a tool for prosthodontic treatment · evaluation of virtual...

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