Removal Partial denture Design(Part 1)

Dr. Mohamed Ashour Ahmed

Assistant Professor of Removable Prosthodontics, Faculty of Dental Medicine, Al-Azher University

When you realize you've made a

mistake, take immediate steps to

correct it.

Retention

Reciprocation

Support

Indirect Retention

RPD REQUIREMENTS

Bracing and Stabilization

A properly constructed partial denture must

All should be within the physiological limits of the tissues involved

• Forces acting on RPD and factors that influence the magnitude of stresses transmitted to the tissues.

Is the Planning of the form and extent of RPD,

after studying all the factors involved

• Controlling the stresses by RPD

• Design concepts

Removable Partial Denture Design

• Biomechanical aspect of RPD design

* Mechanical ----- related to forces and its application to object----- looseness of teeth ,

bone resorption……etc

Removable Partial Denture Design

• Biomechanical aspect of RPD design

* Bio ------ pertaining to living systems-----inflammation, Caries, bone resorption….etc

FORCES ACTING ON

REMOVABLE PARTIAL

DENTURES

The magnitude and intensity The duration

The direction

The frequency

of these forces

The Ability Of living tissues To Tolerate Forces Is Largely Dependent Upon

Maxfield

Tissues are adapted to receive and absorb forces within their physiological tolerance

Fibers of periodontal ligament are arranged such that their resistance to vertical forces is much greater than that to horizontal forces

Class I Lever: fulcrum: between E and R R X d1= E X d2

Direction of lever arm: E opposite R

Class I Lever:

fulcrum: between E and R

E

R

Free end saddle partial denture without indirect retention

Class II Lever: fulcrum at one end, Resistance R: More than E (force)

Prevent

rotation of

the free

end saddle

around the

fulcrum

line

INDIRECT RETAINERS

Class II Lever

Class III Lever: fulcrum at one endResistance: less than E

Class III Lever: fulcrum at one end

Aker Clasp

a- when force is directed against unsupported end of beam cantilever can act as first class lever Torque on the abutment tooth

F

b- A cantilever design allows excessive vertical movement toward the residual ridge

a

a beam supported only at one end, when force is directed against unsupported end of beam cantilever can act as first class lever

Mesial rest concept for distal extension removable partial dentures

POSSIBLE MOVEMENTS

OF THE PARTIAL

DENTURE

I- Tissue-ward movements

II- Tissue-away movements

III- Horizontal movements: A) Lateral movements

B) Antero-posterior movements.

IV- Rotational movements

Four possible movements of the partial dentures exist

I- Type of movement

II- Causes

III- Function of the partial denture that resist this movement

IV- Components of PD that provide this function

Four possible movements of RPD

Vertical forces acting in gingival direction tending to move the denture towards the tissues

I- Tissue-ward movements

Control direction of force

• Mastication.• Swallowing• Parafunction.

They occur during

• RPD should be designed to resist this movement by providing adequate supporting components

• This function of the partial denture is called

“Support”

• The Resistance to Tissue Ward Movement

Support

• Adequate Distribution of Forces Over the Supporting Sttructure and decrease forces/unit area

• Transferring Occlusal Stresses to the Supporting Oral Structures

Fibers of periodontal ligament are arranged such that their resistance to

vertical forces is much greater

than that to horizontal forces

Tissues are adapted to receive and absorb forces within their physiological

tolerance

Periodontal ligament(0.25mm)

Mucosa(2.0mm)

Different Displacement Between PDL & Mucosa

This Function Is Mainly Provided By:

Properly designed supporting

rests placed in rest seats, which are prepared on the abutment teeth,

Broad accurately fitting

denture bases in distal extension partial dentures.

Rigid major connectors that are neither relieved from the tissues nor placed on inclined planes also provide support

Support is provided by

1. Denture base.

2. Maxillary major connectors.

3. Rests.

4. Rigid portions of clasps placed over the survey line.

Distribute the Forces Over the Supporting Structure

Is the Mandibular major connector play a role in support of RPD ?

Lingual plate ?

II- Tissue - away movements

Vertical forces acting in an occlusal direction tending to displace and lift the denture from

its position

Tissue - away forces occur due to

• This function of the partial denture is called

“Retention”

1. The action of muscles acting along the periphery of the denture

2. Gravity acting on upper dentures.

3. sticky food adhering to the artificial teeth or to the denture base.

Resistance to

movement of the

denture away from

its tissue foundation

(resistance of a

denture to

dislodgment)

Retention

1. Adhesion 2. Cohesion 3. Interfacial

surface tension.

4. At. pressure 5. Gravity

Mechanical

Retention

PhysiologicalPhysical

• The physiologic molding of the tissues around the polished surfaces

• neuromuscular control

• Direct retainers• Indirect R. • Frictional fit • Parts of the

denture engaging tooth and tissue undercuts.

Frictional fit

Mechanical means of Retention

Indirect R.

Direct retainers

Parts of the denture engaging tooth and tissue undercuts.

• Clasps• Attachments

1. Have less surface area.

2. Are bathed in saliva.

3. Lower major connectors are

relieved.

The effect of physical forces is less applicable to lower dentures than upper

because:

contrary to upper major c. that are well adapted and their borders are beaded against the underlying tissues.

4. Strong movements of the tongue

1- Mechanical Direct Retainers, Which Engage Undercuts On Abutment Teeth1.Intracoronal (Precision attachments).2.Extracoronal

AttachmentsIntracoronal

This Function Is Mainly Provided By:

Direct RetainersExtracoronal

2- physiologic forces on polished surfaces of denture bases

3- physical forces on fitting surfaces of denture bases

III - Horizontal movements A) Lateral movements

Horizontal forces developed when the mandible

moves from side to side during function while the teeth are in contact

Lateral movements have a

destructive effect on teeth leading to tilting, breakdown of the periodontal ligament and looseness of abutment teeth.

Masticatory cycle ( frontal View)[Tear Drop appearance]

BracingResistance to Lateral Movement of the Partial Denture

This Function Is Mainly Provided By:

1. Bracing clasp arms placed at or above the survey line of the tooth.

2. Minor connectors in contact with axial (vertical) surfaces of abutment teeth.

3. Proximal plates.

4. Adequate extension of the flanges

BRACINGLateral movement is resisted by:

Maximum extension and coverage of the sides of the residual ridge with the denture base within the physiological limit.

Rigid bracing clasp arms.

Use of a continuous bar resting on the lingual surfaces of natural standing teeth (Kennedy bar).

Rigid major and minor connectors

Reduction of cusp angle inclination of the artificial teeth and balanced occlusion.

Reduced occlusal table.

Horizontal movements B) Antero-posterior movements

There is natural tendency for the upper denture to move forward

and for the lower to move backward.

Horizontal forces which occur during forward and backward movement of the mandible

during function while the teeth are in contact

Forward movement of the upper denture could be resisted by:• Anterior natural teeth.• Palatal slope.• Maxillary tuberosity. • The natural teeth bounding the edentulous space.

The backward movement of the lower denture could be resisted by:

• The slope of the retromolar pad.• The natural teeth bounding the saddle area.• Proximal plates.

Force Denture Resisting structures

AnteriorMaxillary

1. Healthy natural anterior teeth.2. The anterior slope of the hard palate.3. Clasp arms encircle a posterior tooth.4. The tuberosity covered by the saddle.

Mandibular Healthy natural anterior teeth.

Posterior

Maxillary1. Posterior teeth in bounded saddles.2. The labial saddle flange.3. Clasp arms on anterior tooth.

Mandibular1. Posterior teeth in bounded saddles. 2. The steeply sloping retro molar region. 3. Clasp arms on anterior tooth.

Reciprocation It is the resistance to horizontal forces exerted on a tooth by the retentive clasp terminal during insertion and removal of the RPD, this force equal is amount, but in an opposite direction, on the opposite side of the teeth.

Palatal viewProximal view

RECIPROCATIONRECIPROCATION can be achieved by:

Reciprocal clasp arms contacting the tooth prior to or at the same time the retentive tip crosses the survey line of the tooth.

Parts of the major connectors.

Proximal plates.

IV- Rotational movements

1. Rotational movements are due to the variation in compressibility of supporting structures,

2. absence of distal abutment at one end or more ends of denture bases.

3. absence of occlusal rests or clasps beyond the fulcrum line.

Directions of rotational movements

1. Rotation of the extension denture base around transverse fulcrum axis.

2. Rotation of all bases around a longitudinal axis parallel to the crest of the residual ridge.

3. Rotation about an imaginary perpendicular axis

1- Rotation of the extension denture base around transverse fulcrum axis:

A) Rotation of the denture base towards the ridge around the fulcrum

axis joining the two main occlusal rests

B) Rotation of the denture base away from the ridge around the

fulcrum axis joining the two main occlusal rests

1-Rotation of the extension denture base around transverse fulcrum axis:

A) Rotation of the denture base towards the ridge around the fulcrum axis joining

the two main occlusal rests

1-Rotation of the extension denture base around transverse fulcrum axis:

B) Rotation of the denture base away from the ridge around the fulcrum axis

joining the two main occlusal rests

Components of RPD that are used to reduces the tendency the denture to rotate in an occlusal direction about the fulcrum axis

Indirect Retention

2- Rotation of all bases around a longitudinal axis parallel to the crest of the residual ridge

F3-Rotation about an imaginary

perpendicular axis

StabilizationIs the Resistance of Partial Denture to Tipping (Rocking torsional forces)

This movement is counteracted by :1. Providing adequate bracing

2. A rigid major connector.

3. Broad base coverage

4. Balanced contact between upper and lower teeth and reduction of cusp slope.

5. The use of additional rests on teeth other than the abutment tooth.

6. Coverage of the sloping part of the palate ant. (rugea area).

Removal Partial denture Design(Part 2)

Dr. Mohamed Ashour Ahmed

Assistant Professor of Removable Prosthodontics, Faculty of Dental Medicine, Al-Azher University

Don’t wait until it’s

late

The first step in a successful partial denture is to design and plan the case very carefully.

The more time taken with this important step, the more secure

and functional the resulting partial.

The prosthesis must be designed

following the most favorable

biomechanical principles,

as the simple and proper design

helps in reducing the harmful

effects on the supporting

structures

Forces falling on RPD should be properly

Directed vertically on both ridge and abutment

Decreased , within the physiological tolerance

of the tissues

Distributed widely to reduce the force /unit area

Class I RPD

Principles of Class I RPD design

Problems of support associated with free-end saddles RPD is due to:•Lack of posterior abutment Rotation

of the denture and torque to the

abutment.

• Support is derived from both the

residual ridge and abutment teeth.

•Alveolar ridge resorption.

Design Of the Partial Denture framework

Problems of support of the distal

extension bases can be controlled

by reduction of the harmful rotational

movements of the free end bases.

This can be achieved by reducing the

total load transmitted to abutment teeth

and the residual ridge

Periodontal ligament(0.25mm)

Mucosa(2.0mm)

Different Displacement Between PDL & Mucosa

1. Reducing the load

2. Stress equalization (distributing the load between teeth and ridge)

3. Physiologic basing.

Type and accuracy of impression registration (anatomical or functional) (Altered cast technique)

4. Broad stress distribution (dist. the load widely)

Problems of distal extension bases can be reduced by

I. Reducing the load.II. Distributing the load widely.

1.Over more than one abutment tooth.2.Over the maximal area of edentulous ridge.

III. Equitable distribution of the load between teeth and ridgesa. By varying the connection between clasp and saddle:

1.Stress breaking. 2.Combining rigid connection and occlusally approaching clasp.

b. By anterior placement of the occlusal rest: - The RPI and RPA system.

c. Muco-compression impression.IV.Overdenture construction supported by natural teeth or

implant.

Problems of free end saddle can be reduced by

Load on the residual ridge is minimized through

1.Broad tissue coverage and maximum extension of the denture base within the functional limits of muscular movements.

2.Fitness and intimate adaptation of the denture base to the tissue.

3.Functional basing. Mucocompression impression recording of the residual ridges.

4.Use of small and narrow teeth to increase the masticatory efficiency and reduce the mast. Load (canines and premolars instead of premolars and molars)

5.Harmonious occlusion and reducing the cusp angle of art. teeth.

6.Leaving a tooth off the saddle.

7.Placing the artificial teeth on the anterior two-thirds of the base (no 3rd molar).

A, The total occlusal load applied may be reduced by using comparatively smaller posterior teeth represented by the right-hand illustration. B, Less muscular force will be required to penetrate a food bolus with a reduced occlusal table, thereby reducing forces to supporting oral structures.

Factors influencing the effectiveness of tissue support of

a distal extension base 1.Contour and quality of the residual ridge

(nature of the mucoperiosteun )

2.The Extent of area coverage by the denture base

3.Accuracy and fitness of the denture base

4.The accuracy and type of impression registration (anatomical or functional)

5.Total occlusal load applied

1- The Contour and quality of the residual ridge (nature of the mucoperiosteun )

a- Tissue conditioning b- Modify it by surgical intervention

2. The Extent of area coverage by the denture base

3. Accuracy and fitness of the denture base• Materials and techniques that will ensure the greatest

dimensional stability should be selected.

• The better the base fits the denture foundation the less the degree of displacement.

• By using accurate impression technique to obtain maximum tissue coverage for support.

•The broader the area of coverage, the greater the distribution of load, the lesser the displacement; which results in less load per unit area.

4. The accuracy and type of impression registration (anatomical or functional)

Has greater area coverage

More stability under rotating and/or torquing forces

Maintain its occlusal relation with the opposing teeth.

No rapid settelling of the denture base

Distribute the occlusal load equitably and diminish the rotational movement.

A denture base processed to the functional form is generally

5. Total occlusal load applied The more the load applied the higher the

degree of tissue displacement.The magnitude of stresses can be

assessed through:1. General musculature

2. Attrition and wearing of the remaining natural teeth

3. Type of opposing occlusion.

4. Use of fewer, narrower and more effectively shaped artificial teeth.

5. Sex.

6. General health of the patient.

7. Location, length and type of the saddle.

II. Distributing the load widely

1.Over more than one abutment tooth (multiple clasping)

Continuous clasp (Kennedy bar or secondary lingual bar)

extended arm clasp.

lingual plates could be used to splint and distribute the loads over the teeth (especially the lateral loads).

Joining the abutment teeth by crowns help in splinting and wide load distribution.

2. Over the maximal area of edentulous ridge.

a. By varying the connection between clasp and saddle:

Stress breaking:

allows movement between the saddle unit and the retaining unit.

II. Equitable distribution of the load between teeth and ridges

Those having a movable joint. Those having a flexible or semi flexible connection.

Stress breaker

Movable Joint

Flexible connection

1) Stress breakers increase ridge resorption due to the increased load applied to it.

2) It is easily distorted or even fractured during use and repair is difficult.

3) Reduce the effectiveness of indirect retainers.

Disadvantage of stress breaker

Combining ridge connection and occlusally approaching clasp

increase load on the abutment and reduce load falling on the ridge.

They are used in small class II design (mainly in the upper)

Aker Clasp

when force is directed against unsupported end of beam cantilever can act as first class lever Torque on the abutment tooth. A

cantilever design allows also excessive vertical movement toward the residual ridge

Anterior placement of the occlusal rest

Placing the occlusal rest away from the distal extension base achieve mechanical adv. And helps in favorable distribution 0f occ. Load bet. Abut. t. and the ridge

1- By changing the direction of torque on the abutment from the distal to the mesial side of the tooth, the resistance to torque action will be applied to the

neighboring teeth (Buttressing effect )

RPA

RPI

lever arm, represented by distance from rest to denture base, is increased. This

increase in length makes rotational action caused by up-and down

movement of denture base in function more vertical. A vertical force in better tolerated by ridge than is a horizontal

oblique force

1. As rest is moved anteriorly this will increase the area of support (decrease the force /unit area)

2. Wide distribution of the load in an antero-posterior direction. The bone near the abutment will thus share the distal part of the ridge in bearing the occlusal load.

Advantages of Placing the occlusal rest away from the distal

extension base1. By changing the direction of torque on the abutment from the distal to the

mesial side of the tooth, the resistance to torque action will be applied to the neighboring teeth (Buttressing effect ).

2. Achieving mechanical adv. By Changing the stresses acting on the abutment and the saddle from the cantilever action or class I lever to class II lever.

3. Clasp disengagement from the tooth during function provide less stresses on the abutment.

4. The farther the ant. Placement of the rest, the more vertical will be the forces, the less is the horizontal components of force falling on the ridge.

5. As rest is moved anteriorly, will increase the area of support (decrease the force /unit area) and hence less stresses on the ridge and less torque on the abutments.

6. Wide distribution of the load in an antero-posterior direction. The bone near the abutment will thus share the distal part of the ridge in bearing the occlusal load.

The retention of endodontically treated teeth covered by dome shaped copings under partial overdenture could effectively preserve the vertical height of the residual ridge and provide more support to the distal extension base.

The use of endosseous implants replacing the missing distal abutment or supporting an overdenture is one of the concepts used to avoid torquing of the natural tooth abutment and residual ridge resorption.

IV- Overdenture construction

Implant-supported removable partial denture

1.Correct choice of the abut. Tooth with

sufficient alveolar bone support and crown

and root morphology

2.Placement of occlusal rests away from the saddle.

3.Correct choice of direct retainer (flexible clasping).

4.Using stress equalizing design.

Strain on the abutment teeth is minimized through

5. Wide distribution of the load over the teeth:

Strain on the abutment teeth is minimized through

a- By placing additional rests, or

b- by a splinting of one or more teeth, either by fixed partial dentures or by soldering two or more individual restoration together.

6- Using a Kennedy bar to distribute the lateral load on multiple teeth.

7. Preparation and restoration of the abutment

teeth to accommodate the most ideal design of PD

this include

a- Proper form of occ. rest seats

b- Tooth prep. and modification to withstand the functional stresses ( guiding planes, ………..)

8. Providing Posterior Abutments

a- Using an implant at the distal part of the ridge.

b- Salvaging a hopeless badly decayed tooth, an

overdenture abutment

Remember: to solve class I RPD problems

Improve denture support. HOW

Decrease torque by using stress equalization and placement of the rests away from the saddle.

Improve bracing.

Need of indirect retention.

Resin base to accept relining.

?

?

Types of RPD

Tooth Support

Mucosa support

Tooth-mucosa support

Differentiation Between Two MainTypes of Removable Partial Dentures

1. The manner of supported.

2. The method of impression registration and the jaw relation Record required for each type will vary.

3. Indirect retention.

4. Denture base material.

5. Differences in clasp design

A, Kennedy Class I partially edentulous arch. Major support for denture bases must come from residual ridges, tooth support from occlusal rests being effective only at the anterior portion of each base.

B, Kennedy Class III, modification 1 partially edentulous arch provides total tooth support for the prosthesis. A removable partial denture made for this arch is totally supported by rests on properly prepared occlusal rest seats on four abutment teeth.

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