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Presented by Dr.Abbasi Begum .MPG dept. of Prosthodontics

Basic principles of Removable Partial Denture Design

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1. Introduction2. Biomechanics of RPD.3. Biomechanical considerations.4. Possible type of movement taking place.5. Differences in design between tooth supported and

tissue supported prosthesis.6. Factors contributing to amount of stress transmitted

to the abutment.7. Design considerations of prosthesis to control stress8. conclusion 9. Reference.

Contents

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Introduction

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1. A thorough, general examination of the mouth with roentgenograms.

2. A thorough prophylaxis, and the treatment of, and removal of all questionable teeth.

3. The repair of all carious lesions.4. Obtaining a preliminary impression and

study casts and articulating them.5. Drawing a tentative design on study casts,

preparing occlusal rests and teeth by grinding.

Good removable partial dentures are the results of:

Planning and designing removable partial dentures-Colonel Arthur H. Schmidt -JPD-November 1953Volume 3, Issue 6, Pages 783–806

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6. Obtaining an accurate impression after all teeth are prepared.

7. Remembering that the basis for all successful partial dentures is summed up in the words 

Plan—Design—Construct.

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The principles for removable partial denture design were first expounded by A.H. Schmidt in 1956

1. The dentist must have a thorough knowledge of both the mechanical and biologic factors involved in removable partial denture design

2. The treatment plan must be based on a complete examination and diagnosis of the individual patient

PRINCIPLES OF REMOVABLE PARTIAL DENTURE DESIGN

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3.The dentist must correlate the pertinent factors and determine a proper plan of treatment.

4.A removable partial denture should restore form and function without injury to the remaining oral structure.

For example in restoring occlusion, the prosthesis should also restore a normal or desirable facial contour and not impede the normal movement of the tongue and other tissues.

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Forces due to a removable restoration can be widely distributed, directed and minimized by

the selection Design The location of components of removable

partial denture and By developing a harmonious occlusion

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Why is it important to understand the biomechanics?????????

Biomechanics of RPD

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Removable partial denture forces in oral cavity.

causes movement of various component of the RPD.

So its important to understand the movements taking place on these components and logically help design them in order to control the movements taking place in them.

what are the types of movements taking place in the oral cavity????

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Sagittal plane

Horizontal plane

Frontal plane.

Stewart-clinical removable partial prosthodontics-3rd edition

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Designing of an rpd is based on both biological and mechanical considerations.

Most of us dentist consider the mechanical aspects but it is also important to understand the biological aspect.

Biological aspects :-1. whether tooth used for support can bear the

loads falling on it.2. The type of underlying mucosa.

The resistance to load from a tooth is based on the amount of force falling on it, the duration of force and direction of force applied.

Biomechanical considerations

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1. A RPD lever, mainly distal extension.Based on the type of lever the forces applied

on the abutment teeth varies.The lever has the potentiality to increase

the forces falling on the tooth.2. Cantilever type design rpd should be avoided.3. Tooth tends to withstand vertical forces

than non vertical forces because of the number of pdl fibres involved.

4. An abutment tooth will withstand non vertical forces if the forces are applied as close to the horizontal axis of rotation.

Mechanical aspects:-

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Lever and Inclined Plane ActionsThe Inclined Plane

In this system, two objects share an interface that forms an acute angle with the horizontal plane. Application of vertical power (P) causes the objects to move in opposite directions

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consist of a fulcrum (F) about which the lever rotates, a power source (P) that mobilizes the lever to do work, and a resistance (R) to which the lever does work.

The lever

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Fulcrum is

Fulcrum is placed between effort and load

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The most efficient lever The most inefficient lever is a third-class lever

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When externally loaded, a distal extension removable partial denture may function as a first-class lever.

A fulcrum (F) exists near the rest seat. The power that activates the lever results from occlusal loading (L) of the extension base.

As the lever functions, aspects of the prosthesis anterior to the fulcrum will move in a superior direction

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Forces arising from three principal fulcrums

principal fulcrum line- A horizontal fulcrum line Rotational movement around this

fulcrum line is the greatest in magnitude but , not necessarily the most damaging.Resultant force on the abutment teeth -

mesio-apical or disco-apical

Forces Acting on the RemovablePartial Denture

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A second fulcrum line - in the sagittal plane and extends through the occlusal rest on the terminal abutment and along the crest of the residual ridge on one side of the arch

This fulcrum line controls the rotational movements of the denture in the frontal plane

easier to control

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The third fulcrum- located in the vicinity of the midline, just lingual to the anterior teeth

Oriented vertically and controls rotational movement in the horizontal plane

Forces can be extremely damaging

Significant attention during the design process.

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Difference in design between tooth supported and tissue supported prosthesis

Tooth supportedClass III,class IVDerive support from

teethThe movement

potential is less because the teeth provide resistance to functional loading.

single universal design can be followed for teeth supported prosthesis

The denture base is made up of metal bases.

Tissue supportedSeen in classI and

classIIcases.Tissue maximum amount

of support (tissue provides primary support and teeth provide secondary support).

Too much tissue movement because of the dynamic state of the tissue.

Multiple complex design have to followed.

Denture base acrylic resin

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1) Length of edentulous span:- 2) Quality of ridge support3) Clasp flexibility4) Clasp design5) Length of the clasp6) Material used for clasp construction7) Surface characteristics of an alloy8) Occlusal harmony:-

Stewart-clinical removable partial prosthodontics-3rd edition

Factors contributing to the amount of stress transmitted to the abutment

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1. Length of edentulous span

Longer edentulous span

longer denture base

greater force transmitted to abutment teeth

Retain a Posterior abutment to serve as vertical support, even as an overdenture abutment Improved patient service

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Broad ridges with parallel sides

longer flanges

stabilize the denture against lateral forces.

2.Quality of ridge supportA) Form of residual ridge Better support by ridge less stress on abutment

teeth

Large well formed ridges less stress on abutment

Small thin, knife like ridges are very poor.

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B) Type of mucosa Influences magnitude of stresses transmitted

to abutment teeth. Healthy mucosa capable of bearing greater

functional loads

Soft, flabby,

displaceable mucosa little vertical support of

denture allows excessive

movement of denture

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3. clasp as a factor

CLASP

Fexibility

Design

Length

Material used

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More flexible the retentive arm of clasp less stress to abutment tooth

Wrought-wire retentive clasp Class I and Class II applicationsIt has tendency to produce very high amount of

lateral stress.But not indicated in cases with poor ridges as

it cannot with stand lateral stresses leading to heavy stresses on the ridges.

Clasp flexibility

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Clasp design

Clasp should be passive once framework seated completelyIt should be such that it doesn't apply force on the teeth while it is seated on the teeth.

So complete seating of prosthesis is mandatory.

Reciprocal arm should designed that it lies above the height of contour.

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Length of claspIncrease in length

Increased flexibility

•Doubling the length of a clasp will increase its flexibility fivefold

•Clasp length may be increased by using a curved, rather than straight, course on anabutment tooth

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Material

• Co cr alloys based clasp apply more force on the abutment teeth than gold based alloy.

• So thinner diameter co cr clasp can be used to reduce the amount of force applied.Surface characteristics of

an alloyGold crowns more resistance to clasp than enamel.

Abutment restored with gold experiences more forces than intact enamel.

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8.Occlusal harmony:-

It plays an important role.

Deflective contacts should be avoided.

Prosthesis opposing natural dentition face more forces than from a natural dentition.

Occlusal force should be directed to middle of residual alveolar ridge closer to the abutment.

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Ideally, the occlusal load should be applied in the center of the denture-bearing area, both anteropostenorly and faciolingually.

The second premolar and first molar regions represent the best areas for the application of the masticatory loads.

Artificial teeth should be arranged so that the bulk of the masticatory forces are applied in these areas

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Direct retention

Retentive clasp arm -Responsible for transmittingmost of the destructive forces to the abutments

So retentive clasp arm should provide adequate retention with least forces.

This can be done by providing retention from other components of denture.

the support and stability of the prosthesis also may be improved.

Other components that provide additional retention are:- Adhesion cohesion. Frictional grip. Neuromuscular control.

Design Considerations: Controlling Stress

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Clasp position :-Often, the spatial distribution of

retentive clasps is more important to retention than the number of clasps.

The following configurations can be followed while determining clasp position.

1.Quadrilateral configuration:- it is used in class 3 situation with modification space.

2.Tripodal configuration:- class 2 modification 1

3.Bilateral configuration

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1. Quadrilateral configuration:- Class 3 with modification space. Clasp assembly on both abutment teeth

adjacent to edentulous space. In case of absence of modification space

clasp assembly anteriorly and posteriorly are given on the dentulous opposite arch.

2. Tripodal configuration:- Class 2 with modification space. Clasp present adjustment to the edentulous

space. On the modification space side clasp on

both the abutment teeth. If modification is absent clasp placed as far

anteriorly and posteriorly on the teeth. Not as effective as quadrilateral

configuration but better in class 2 cases.3. bilateral configuration:-

Class 1 situations. Provides least stress reduction.

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CLASP DESIGN:-

Cast circumferential clasp:-• Class 2 and class 1 cases clasp assembly involving disto occlusal rest and retentive tip involving mesiofacial undercut is prevented.• Terminal end of such clasp tipping forces on the abutment teeth.• A clasp that originates from the mesioocclusal rest and engages the distofacial undercut or a reverse circulate clasp should be used.

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Vertical projection clasp:-

• T clasp or modified t clasp can be used on an abutment adjacent to the distal extension space.• It is used when the a distofacial undercut

is seen on the abutment.• It is not indicated in case of mesiofacial

undercut.• I clasp is better used involving

mesiofacial undercut and mesioocclusal rest seat.• It doesn’t apply any stress on teeth.

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Combination clasp:-Used when a distal extension is presented with a

mesiofacial undercut.

Flex more and in multiple spatial planes..

Combination of both cast and wrought wire is used.

Wrought wire is used as the retentive arm and cast metal is used as the reciprocal arm

More flexible and produces less stress on the abutment.

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Splinting two or more teeth increases the periodontal ligament area and distributes applied loads more effectively

Splinting of abutment teeth

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Loss of periodontal attachment has occurredAbutment tooth - tapered root or short roots

to the extent that there is not an acceptable amount of periodontal ligament attachment present

The joining of two such teeth by crowns will produce an acceptable multirooted abutment tooth

Indications

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Splinting removable prosthesis

Not done if fixed splinting is possible.

The splinting consists of clasping more than one tooth on each side of the arch and using additional rests for increased support.

All clasps need not to be retentive.

Prepared guiding planes may provide additional horizontal stability.

Results in decreased mobility.Cross arch stabilization

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Swing-Lock partial denture, can be used to splint teeth effectively

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Helps resist rotation and/or displacement of a removable partial denture

The indirect retainer is essential in the design of Class I and Class II removable partial dentures

The indirect retainer or retainers must bepositioned as far anterior to the fulcrum line as

possible

Indirect retention

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not as critical in a Class I archRequired- modification space on the tooth-

supported side of the arch, abutment teeth on both sides of the space should be rested .

CLASS II

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Class III situation usually doesn’t need a indirect retainer as there is no lever effect.

Class IV situation it just opposite to the class I situation with indirect retainer present as far posteriorly as possible.

Auxiliary rests

Mesio-occlusal rests on the firstpremolars serve to support this long lingual plate major connector

Auxiliary rests

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Smoothly functioning occlusion in harmony with TMJ & neuromusculature-minimize the load transferred to the teeth and soft tissues. The contacts of the remaining natural teeth

should be the same whether the removable partial denture is in mouth or not.

The number of teeth replacing the natural teeth should be reduced to decrease the amount of force falling on the ridge.

Artificial posterior teeth should have sharp cusp with low incline plane in order to increase the cutting efficiency and prevent horizontal interferences force.

Occlusion

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The denture base should extend as much denture bearing area as possible in order to dissipate the forces falling on the prosthesis.

Denture flanges should be as long as possible. Maxillary denture base extend maxillary tuberosity. Mandibular denture base retromolar pad. Overextension of denture base should be avoided. Accurate adaptation of denture base is necessary for

proper retention of denture base. The external polished surface of the denture should be

contoured properly in order to aid in retention of the dentures.

Denture bases

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Major connectors:-

A major connector maximum coverage area dissipate the occlusal forces falling on it.

In maxilla a palatal full coverage major connector is more preferred as it contacts all the remaining teeth with a lingual plate thus helps in dissipating the forces.

In mandible the lingual plate major connector Is most preferred because it covers the whole lingual surface of anterior teeth.

Thus distributing the forces to all the teeth. It is particularly helpful in splinting

periodontally weakened teeth. It also provides rigidity and cross arch

stabilization.

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Minor connector The minor connector connecting the guide

plane to the major connector plays a very important role in dissipating functional stresses. Because of its close adaptation to the abutment teeth.

This minor connector helps in two major functions. It provides a single path of insertion. Improved stability by providing increased

resistance to horizontally directed forces Additional guide planes can be

incorporated on other teeth to help dissipate the lateral stresses falling on the single tooth.

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Rest and rest seats:- Rest seats are essential as they transmit

force vertically along the long axis of the teeth.

rest seats prevent formation of any lateral stresses.

Rest seats should be designed in such a way that they are less than 90 degrees to the path of insertion.

Thus helping the rest seat to grab the tooth securely and prevent its migration.

Occlusal rest seats should be rounded and some amount of space should be present between the rest and rest seat to allow free movement as a movement of ball and socket joint.

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Conclusion

The principles of surveying, the significance of the survey line, the relation of the clasp to the survey line, and the opportunity presented by tilting the cast to control the location of undercuts are basic factors which will enable the prosthodontist to solve any removable partial denture problem

Thorough understanding and application of these these basic principles will lead to a successful treatment outcome .

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Mc crackens removal partial denture. 11th edition.

Stewarts clinical removable prosthodontics. 3rd edition

Beumer J, Curtis TA, Firtell DN. Maxillofacial rehabilitation, prosthodotics and surgical considerations.

Planning and designing removable partial dentures-Colonel Arthur H. Schmidt -JPD-November 1953Volume 3, Issue 6, Pages 783–806

References

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Thank you