24 clinical presentations of tress … · peripheral rim of enamel transferring occlusal load...

Publication of one recent article that demonstrates stressdistribution within tooth structure has improved clini-

cians’ understanding of subtle compression and stressfracture presentations in teeth.1 Until the publication ofthis benchmark article, numerous fracture presentationsobserved clinically have been difficult to explain. Straindistribution within the tooth is related to its structure.Enamel acts as a stress distributor, transferring the loadvertically to the root, and horizontally via the dentino-enamel junction (DEJ) to the dentin of the crown. A thickzone (~200 µm) in the dentin at the DEJ undergoes greaterstress than the central coronal dentin.

Recently discovered structures within the occlusalsurface of molars2,3 indicate that conventional cavitydesigns are disharmonious with the tooth’s natural

mechanical stress distribution system.4-7 This understand-ing has resulted in the development of a discipline termedmicrodentistry. This philosophy urges the use of modernmethods of caries detection for early accurate minimalintervention in the caries process to preserve internalmechanical structures within the tooth that are vital to itslong-term mechanical viability.

Moiré FringesTo understand the various presentations of tooth fracturecaused by the disruption of the natural stress distributionmechanism within the tooth, the significance of the Moiréfringes must be considered. To date, stress studies thatutilize polarized light have generally been conductedwith plastics to show stresses that occur when loads areapplied. This technique is not effective in natural denti-tion due to their inability to transmit light, so these studies

CLINICAL PRESENTATIONS OF

STRESS DISTRIBUTION IN TEETH AND

THE SIGNIFICANCE IN OPERATIVE DENTISTRYGraeme Milicich, BDS*

J. Tim Rainey, DDS†

Pract Periodont Aesthet Dent 2000;12(7):695-700

Figure 1. The direction of the Moiré fringes indicatesthe direction of the stress. Thicker zones in the fringesrepresent increased stress and tighter fringes indicategreater stress.

*Private practice, Hamilton, New Zealand.

†Adjunct Associate Professor, University of Texas Health ScienceCenter School of Dentistry, San Antonio, Texas.

Graeme Milicich, BDSP.O. Box 378Hamilton, New Zealand

Tel: (011) 64-7-858-0750Fax: (011) 64-7-849-5632E-mail: [email protected]

Stress distribution in human tooth structure can be visu-alized through the use of Moiré fringes, which hasimproved the clinical understanding of recently identifiedanatomical structures in molar occlusal surfaces. This arti-cle discusses the concept of a “peripheral rim of enamel”and describes the manifestation of compressive and ten-sile fractures within the peripheral rim of enamel anddentin. It also emphasizes the benefits of microdentistrytechniques and minimally invasive preparation designsin the long-term preservation of the natural tooth structure.

Key Words: Moiré fringes, stress, microdentistry, caries,

peripheral, enamel

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have most often focused on stress distribution associ-ated with intracoronal pins and posts.

The Moiré fringes study allows the visualization ofstress distribution within natural tooth structure (Figure 1).Since it has been postulated that the source of abfrac-tion lesions is stress concentration at the cervical DE J,the relationship demonstrated in the Moiré fringes studyconfirms this hypothesis. The Moiré fringes show theperipheral rim of enamel transferring occlusal load directlyto the root of the tooth. Compression load in the enameltransfers via the DE J into horizontal load in the dentin ofthe crown. In this transfer, a stress concentration occursat the DE J as it converts the vertical load in the enamelinto the horizontal load in the dentin.

The DE J is a zone approximately 200 µm thick wherecollagen density and mineral content are each approxi-mately 50%, compared to a 30% collagen/70% min-eral ratio in the body of the dentin. Since the DEJ is moreelastic due to its greater collagen content, it allows micro-compression to occur between the enamel and the dentin,which enables the enamel rim — with its high elastic mod-ulus — to transfer a vertical load directly to the root struc-ture. Once this initial load transfer occurs, the sheer loadcreated within the DE J transfers into a horizontal load inthe body of the crown dentin, with the stress decreasingtoward the center of the tooth. This increase in load inthe DE J and decrease of stress in the central body of thedentin is evidenced in the Moiré fringes study.

Peripheral Rim TheoryOnce this load distribution system is understood, the func-tion of the enamel rim assumes greater significance. Fromthe perspective of stress distribution, the occlusal enamel

Figure 2. Delamination of the peripheral rim and occlusalenamel is apparent as a white line fracture. Note theocclusal abfraction and recurrent caries associated withstress concentration.

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Practical Periodontics & AESTHETIC DENTISTRY

is a separate entity from the peripheral rim enamel.In simplified terms, the function of the peripheral rim canbe visualized through a comparison to an invertedteacup. Several years ago, an automobile company suc-cessfully balanced a 2-ton car on four inverted chinateacups. When loaded correctly, the fragile teacups wereable to support significant loads and successfully trans-fer the load to the floor. In the same manner, the enamel

Figure 4. A subocclusal transverse oblique ridgethat extends from the distolingual to mesiobuccalaspects of mandibular molars. A supporting webof enamel is connected to this structure.

Figure 5. A suspensory web of enamel in thearea formerly referred to as the mesial fossae ofmaxillary molars. This web is also foundmesially in the adjacent region formerly referredto as the marginal ridge fossae.

Figure 3. Occlusal abfraction guttering occursbetween the peripheral rim and the cavity mar-gin due to stress concentration. The compressivefacets are marked in blue.

Subocclusal trans-verse oblique ridge

Suspensory webof enamel


still support reasonable vertical compressive loads. Whenlateral compressive loads are applied to the rim, how-ever, it distorts easily. Squeezed between two forces onopposite sides of the rim, an opened can becomes ovatewith the apex of the deformation occurring 90 degreesaround the rim from the compression forces. This is a sim-plified image, but the effects can be observed clinicallyin teeth that are failing due to the presence of restora-tions. An occlusal cavity in a posterior tooth can cause itto flex under compressive loads on external cusp planesto an extent that the distortion in the tooth results in struc-tural failure of the peripheral rim.8-10

Considering tooth structure in this manner, the mar-ginal ridge becomes a part of the peripheral rim ratherthan a separate morphological identity. Its significanceto the overall stress distribution system increases whencavity designs are considered for the treatment of pri-mary interproximal caries. The peripheral rim can be con-sidered a tension ring. When a minimal MOD-type cavityis prepared in a sound tooth, the cusps spring apartapproximately 10 µm, which indicates that the tooth struc-ture is under tension.11 When the marginal ridge is simplya part of the peripheral rim of enamel, its removal totreat Class II interproximal caries has long-term signifi-cance for the structural integrity of the tooth.

Clinical Evidence and SignificanceIf the peripheral rim of enamel functions almost indepen-dently of the occlusal enamel and is able to absorb com-pressive loads without fracturing, a transfer of this load viathe DE J to a horizontal load in the dentin should occur. Ifthis redistribution of the compressive load cannot be distrib-uted through the body of the dentin due to the blocking

Figure 9. Radiograph of conventional contact point caries.

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transfers load to the root. The theory that the peripheralrim of enamel is able to support significant vertical loadsbecomes evident in a clinical setting. Clinicians canobserve how the teeth fail in function when their stressdistribution system is disrupted by cavity designs.

A simple tin can is also valuable in this discussion.Untouched, it can support significant vertical and lateralcompressive loads. Even with the lid removed, it can

Figure 7. Occlusal view of a peripheral rimenamel fracture and associated occlusal effectcaries in a mandibular molar.

Figure 8A. Peripheral rim fractures are a resultof compressive loads on the buccal cusps.8B. The development of caries occurs due to theperipheral rim fracture.

Figure 6. Caries is established through an enamelfracture. Minimal decalcification is evident andinverted. Red line indicates areas of normal inter-proximal contact point caries decalcification.

A B


effect of a cavity, then one would expect to find clinicalevidence of stress concentration between the peripheralrim enamel and the cavity wall. Differences in elastic mod-ulus between the enamel and the underlying dentin couldcause vertical delamination and fracturing along theDE J.12,13 The failure of the stresses to be distributed throughthe dentin, due to the blocking effect of the cavity, shouldcause stress concentration between the peripheral rim andthe cavity margin. This would have the same effect asstress concentration at the cervical enamel margin has incausing abfraction lesions. The result would be an occlusalabfraction lesion. Once the theory is understood, the clin-ical evidence becomes clear (Figures 2 and 3).

The preparation of cavities in tooth structure disruptsthe natural load distribution and creates zones of stressconcentration. The clinical presentation of this stress con-centration depends primarily on two factors: the type ofcavity prepared, and whether the applied load is com-pressive or tensile.13-15

Conventional Extension-For-Prevention TypeOcclusal CavityDentin has a moderate elastic modulus, and enamelhas a high elastic modulus. Between them lies the DE J,which, due to its high collagen content, has a low elas-tic modulus in comparison to both dentin and enamel.While dentin can deform elastically under load (due toits moderate elastic modulus), enamel will fracture ratherthan deform.16 Once the occlusal surface of a mandibularmolar has been removed for amalgam placement, thetooth begins to behave like the tin can with the topremoved. First, the conventional extension-for-prevention

cavity design required by amalgam removes an impor-tant occlusal cross-bracing structure — the subocclusaloblique transverse ridge in mandibular molars (Figure 4),and the maxillary molar mesial subocclusal enamel web(Figure 5). Second, amalgam provides little mechanicalsupport when the tooth is under load.5-7,10,11

One of the most common loads on mandibular molarsis a compressive load on the outer face of the buccalcusps. As the unsupported dentin deforms under load, a

698 Vol. 12, No. 7

Figure 12. Caries associated with peripheral rimfracture and the causative compression facets.Note the inverted enamel decalcification.

Figure 13. Radiograph demonstrates occlusaleffect caries depicted in figures 11 and 12.

Figure 11. Occlusal view of posterior tooth withperipheral rim fracture.


Figure 10. Radiograph of occlusal effect caries associatedwith the conventional extension of the cavity in figures 8and 9.


compressive distortion in the cusps is created.8,13 The toothbecomes ovate, which places the peripheral rim enamelin the contact point areas under tension. Eventually, ver-tical fracturing of the enamel may occur in the marginalridge zone of the peripheral rim. These enamel frac-tures are not to be confused with naturally occurring stressrelief lamellae in virgin teeth. Continual flexure in thisfracture can eventually result in caries propagation through

the enamel crack (Figure 6). This caries presentation canbest be described as “occlusal effect caries.” The inter-proximal caries is developing only because an occlusalcavity has been prepared in the tooth.

The proposition that preparation of an occlusal cav-ity in a tooth creates the potential for interproximal carieshas serious significance for the development of minimalintervention microdentistry. Radiographically, this type ofcaries presentation is difficult to diagnose.17 Apart froma faint and diffuse graying of the contact point enamel,little enamel decalcification can be detected. Due tothe lack of a conventional interproximal enamel cariesmarker, the dentin caries is often overlooked unless frac-ture has been noted at clinical examination, and the radi-ographs are viewed with that in mind (Figures 7 through13). While these examples are exemplified in mandibu-lar molars, this effect can occur in any tooth that has acompressive force applied to the cusp where a moder-ate-sized restoration has been placed.

Tension Fracturing of Cusps Vertical fracturing in the contact point area of the periph-eral rim and associated caries can occur when cuspsare placed under tension loads. Although the interprox-imal caries presentation remains the same as with com-pression fractures, there is generally an associated dentinfracture under the tension facet cusp. This is due to theinability of dentin to absorb tensile forces once it hasbeen separated from the surrounding tooth structure bya cavity preparation that removed the occlusal enamelcross-bracing structures.9 -11 Tooth structure is designedto absorb compressive and tension loads as a total

Figure 16. Occlusal effect fracture caries andthe dentin fracture associated with the tensioncusp. The most common fracture presentationobserved clinically is tension cusp fractures.

Figure 14. A tension facet and peripheral rimfracture are evident at the distal contact point.

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Figure 17A. Dentin fractures occur beneath the tensionfacet cusp. 17B. Cavity preparations that remove theocclusal enamel cross-bracing structures often result intension cusp fractures.

Figure 15. Radiograph of caries development inthe peripheral rim fracture. Note the minimaldepth of the extension-for-prevention amalgamcompared to areas where decay was removed.

A B


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biomechanical entity and does not adapt well whenthis force distribution system is disrupted to any extentby conventional cavity designs (Figures 14 through 17).

ConclusionsIf the preparation of an extension-for-prevention occlusalcavity can cause long-term failure of the tooth, then cavitydesigns must be addressed to accommodate the naturalstress distribution mechanisms within the tooth. It couldbe necessary to conserve the occlusal cross-bracing struc-tures of the subocclusal oblique transverse ridge andmaxillary molar mesial subocclusal enamel web, and —if at all possible — to avoid the cutting of the peripheralrim of enamel. To conserve these structures, contempo-rary caries diagnostic techniques must be understoodand utilized so that caries can be treated in its prelimi-nary stages without unnecessary destruction of these vitalanatomical entities. The utilization of magnification, cariesdetection dye,18 laser caries diagnosis,19 and minimallyinvasive cavity designs utilizing air abrasion to avoid themicrofracturing that occurs in enamel when cavities areprepared with high-speed rotary instrumentation are allpromising new techniques.20-23 Due to the way it selec-tively cuts compromised tooth structure, air abrasion allowsconservation of sound tooth structure. It was this tech-nology that allowed anatomical structures such as thesubocclusal oblique transverse ridge and maxillary molarmesial subocclusal enamel web to be observed clini-cally. In contrast, high-speed rotary burs indiscriminatelycut both sound and unsound tooth with equal ease.

In addition to the need for minimally invasive den-tistry, the techniques and materials utilized in restoringminimal cavities should be considered. To conserve theintegrity of the peripheral rim and occlusal cross-bracingstructures, tunnel preparations become an appropriatetreatment option when treating primary interproximalcaries in posterior teeth. This approach will prevent com-pressive fracturing of the peripheral rim and tension frac-turing of cusps that occur with conventional MOD-typecavity preparations. As a consequence, the use ofautopolymerizing glass-ionomer cement has to be con-sidered when restoring this form of cavity design.24-27

Current cavity designs and treatment concepts haveevolved from a tradition based on the mechanical require-ments of amalgam, often to the exclusion of the biome-chanical requirements of the tooth. Only by reassessingtheir restorative criteria and modalities can cliniciansavoid the perpetuation of the treatment cycle to whichpatients are currently subjected.28-30

References1. Wang RZ, Weiner S. Strain structure in human teeth using Moiré

fringes. J Biomech 1998;31(2):135-141.2. Rainey JT. A subocclusal transverse ridge: Identification of a pre-

viously unreported tooth structure: The Rainey ridge. J Clin PediatrDent 1996;21(1):9-13.

3. Rainey JT. The maxillary molar mesial subocclusal enamel web:Identification of a previously unreported tooth structure: The maxil-lary Rainey web. J Clin Pediatr Dent 1998;22(3):195-198.

4. Gale EN, Osborne JW. The effects of alloy, cavity width and toothposition on marginal failure of Class II amalgams. J Dent Res1980;59(special issue A):293:Abstract 101.

5. Gelb MN, Barouch E, Simonsen RJ. Resistance to cusp fracturein Class II prepared and restored premolars. J Prosthet Dent1986;55:184-185.

6. Larson TD, Douglas WH, Geistfield RE. Effect of prepared cavi-ties on the strength of teeth. Oper Dent 1981;6:2.

7. Mondelli J, Steagall L, Ishikiriama A, et al. Fracture strength ofhuman teeth with cavity preparations. J Prosthet Dent 1981;43(4):419-422.

8. Grimaldi J. Measurement of the Lateral Deformation of the ToothCrown Under Compressive Cuspal Loading. [thesis]. University ofOtago, Dunedin, New Zealand, 1971.

9. Panitvisai P, Messer HH. Cuspal deflection in molars in relationto endodontic and restorative procedures. J Endod 1995;21(2):57-61.

10. Re GJ, Norling BK. Forces required to crack unfilled and filled molarteeth. J Dent Res 1980;59(special issue A):351:Abstract 334.

11. Larson TD, Douglas WH, Geistfield RE. Effect of prepared cav-ities on the strength of teeth. Oper Dent 1981;6(1):2-5.

12. Khers SC, Carpenter CW, Vetter JD, Staley RN. Anatomy ofcusps of posterior teeth and their fracture potential. J Prosthet Dent1990;64(2):139-47.

13. Rasmussen ST. Fracture properties of human teeth in proximity tothe dentinoenamel junction. J Dent Res 1984;63(11):1279-1283.

14. Cavel VT, Kelsey WP, Blankenau RJ. An in vivo study of cuspalfracture. J Prosthet Dent 1985;53:38-42.

15. Sano H, Ciucchi B, Mathews WG, Pashley DH. Tensile proper-ties of mineralized and demineralized human and bovine dentin.J Dent Res 1994;73(6):1205-1211.

16. Renson CE, Braden M. Experimental determination of the rigid-ity modulus, Poisson’s ratio and elastic limit in shear of humandentin. Arch Oral Biol 1975;20(1):43-47.

17. Christensen GJ. Dental radiographs and dental caries: A chal-lenge. J Am Dent Assoc 1996;127(6):792-793.

18. Al-Sehaaibny F, White G, Rainey T. The use of caries detector dyein diagnosis of occlusal carious lesions. Pediatr Dent 1996;20(4):293-298.

19. Ross G. Caries diagnosis with the DIAGNOdent Laser: A user’sproduct evaluation. Ont Dent 1999;76(2):21-24.

20. Xu HH, Kelly JR, Jahanmir S, et al. Enamel subsurface damage dueto tooth preparation with diamonds. J Dent Res 1997;76(10):1698-1706.

21. Summitt JB, Osborne JW. Initial preparations for amalgam restora-tions: Extending the longevity of the tooth-restoration unit. J AmDent Assoc 1992;123(11):67-73.

22. Ferianakis K, White GE. Newer Class I cavity preparation forpermanent teeth using air abrasion and composite restoration.J Clin Pediatr Dent 1999;23(3):210-216.

23. Walls AW, Murray J J , McCabe JF. The management of occlusalcaries in permanent molars. A clinical trial comparing a mini-mal composite restoration with an occlusal amalgam restoration.Br Dent J 1988;164(9):288-292.

24. Hunt PR. A modified Class II cavity preparation for glass-ionomerrestorative materials. Quint Int 1984;15 (1011):101-108.

25. Knight GM. The tunnel restoration — Nine years of clinical experi-ence using capsulated glass ionomer cements. Case report. AustDent J 1992;37(4):245-251.

26. Ten Cate JM, Van Duinen RN. Hypermineralization of dentinallesions adjacent to glass-ionomer cement restorations. J Dent Res1995;74(6):1266 -1271.

27. Brantley CF, Bader JD, Shugars DA, Nesbit SP. Does the cycle ofre-restoration lead to larger restorations? J Am Dent Assoc 1995;126(10):1407-1413.

28. Siurjons, H. Extension for prevention: Historical development andcurrent status of GV Black’s concept. Oper Dent 1983;8(2):57-63.

29. Hodges DJ, Mangum FI, Ward MT. Relationship between gapwidth and recurrent dental caries beneath occlusal margins ofamalgam restorations. Commun Dent Oral Epidemiol 1995;23:200-204.

30. Anusavice K J, ed. Quality Evaluation of Dental Restorations.Carol Stream, I L: Quintessence Publishing; 1989:61-68.


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1. What is the significant conclusion from the Moiré Fringesstudy?a. Enamel has no significance in stress transfer.b. Dentin fails under occlusal load.c. Stress is evenly distributed throughout tooth structure.d. A tooth is a complex structure designed to effectively

distribute and absorb stress.

2. What recently reported anatomical structure is significantin occlusal stress distribution?a. The marginal ridge.b. The oblique transverse ridge.c. The subocclusal oblique transverse ridge and maxillary

molar mesial subocclusal enamel web.d. The dentinoenamel junction.

3. What is the biomechanical significance of the DEJ?a. It allows caries to spread laterally.b. It acts to transfer vertical load in the enamel into a

horizontal load in the dentin.c. It causes fracturing between enamel and dentin.d. It has no functional significance.

4. What is the significance of the peripheral rim of enamel?a. It is very stable in vertical compressive load.b. The marginal ridge is a part of the peripheral rim.c. Cutting an occlusal cavity disrupts the ability of the

peripheral rim to distribute occlusal loads.d. All of the above.

5. What is the potential effect of cutting a conventionalG.V. Black extension for prevention of occlusal cavity?a. It has no effect.b. It unnecessarily cuts sound tooth structure.c. It has the potential to cause recurrent caries.d. It allows compressive flexing of the tooth resulting in

peripheral rim enamel fracture and the potential forinterproximal caries.

6. What is occlusal effect caries?a. Recurrent caries that occurs due to underextension of

the cavity.b. Compressive distortion of the peripheral rim due to the

presence of an occlusal cavity causing vertical fractureof the enamel allowing caries to establish.

c. Failure of the occlusal restoration due to compressiveloads leading to recurrent caries.

d. Fracturing of the occlusal enamel under compressive loadleading to recurrent caries.

7. What causes occlusal abfraction and peripheral rim guttering?a. The presence of an existing large cavity.b. Stress concentration between the peripheral rim and

the restoration.c. Fracturing between the peripheral rim and underlying

dentin along the DEJ.d. All of the above.

8. What is the difference in fracture presentation betweencompressive loads and tensile loads on cusps?a. There is no difference.b. Tensile loads cause horizontal fractures through dentin.c. Compressive loads cause cusps to fall off.d. Tensile loads cause occlusal abfractions.

9. Why is occlusal effect caries difficult to diagnose radio-graphically?a. There is most often minimal enamel decalcification in

the fracture so it will not appear on the radiograph.b. X-rays will not penetrate the area.c. Occlusal effect caries does not exist.d. Fractures can easily be seen on radiographs.

10. What is required to prevent occlusal effect caries andperipheral rim fracturing?a. Accurate caries diagnosis.b. Conservation of the important anatomical structures of

the peripheral rim, subocclusal oblique transverse ridge,and maxillary molar mesial subocclusal enamel web.

c. Minimally invasive tunnel preparations to treat inter-proximal caries to conserve the peripheral rim.

d. All of the above.

To submit your CE Exercise answers, please use the answer sheet found within the CE Editorial Section of this issue and complete as follows:1) Identify the article; 2) Place an X in the appropriate box for each question of each exercise; 3) Clip answer sheet from the page and mailit to the CE Department at Montage Media Corporation. For further instructions, please refer to the CE Editorial Section.

The 10 multiple-choice questions for this Continuing Education (CE) exercise are based on the article “Clinical presentations of stress distrib-ution in teeth and the significance in operative dentistry” by Graeme Milicich, BDS, and J. Tim Rainey, DDS. This article is on Pages 695-700.

Learning Objectives:This article discusses the concept of a “peripheral rim of enamel” and describes the compressive and tensile fractures within this structureand dentin. Upon reading this article and completing this exercise the reader should:

• Understand the benefits of microdentistry techniques and minimally invasive preparation designs for the preservation ofnatural tooth structure.

• Identify various tooth structures involved with stress distribution during cavity preparation.

CONTINUING EDUCATION

(CE) EXERCISE NO. 24CE

CONTINUING EDUCATION

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24 clinical presentations of tress … · peripheral rim of enamel transferring occlusal load...

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