Designing in removable partial dentures /certified fixed orthodontic courses by Indian dental academy

Designing in Removable partial dentures

INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

www.indiandentalacademy.com

Introduction Authorities in the field of removable

partial denture design may differ on their approach in developing the design of each individual prosthesis.

There is however, complete agreement that the correct design incorporates proper use and application of mechanical and biologic principles.


Simple mechanical principles have to understood and applied in designing of the removable partial denture.

Machines are classified into 2 categories as simple and complex.

Complex machines are combinations of many simple machines.


Simple machines Lever, wedge,

screw, wheel and axle, pulley and inclined plane.

A lever is a rigid bar supported somewhere along its length.

Support point of the lever is called the fulcrum.


Of all simple machines lever and inclined plane principle is involved in partial denture design.


Types of lever.


First class lever.


Location of stabilizing and retentive components in relation to the horizontal axis of rotation

An abutment tooth will better tolerate off vertical forces if these forces occur as near as possible to the horizontal axis of rotation of the abutment.


Tooth Vs tooth- tissue supported. They differ in Manner in which each is supported. Method of impression registration. Need for some indirect retention. Denture base material.

Acrylic/metal. Difference in clasp design.


Forces acting on partial denture The all tooth supported RPD is rarely

subjected to induced stresses ,because leverage-type forces are not involved and there are no fulcrums around which the partial denture may rotate.

The distal extension partial denture is subjected to rotation around 3 principal fulcrums.


During the formulation of design these fulcrums and the movement that may take place around them must be kept in mind and components positioned to counteract the movement.


Horizontal fulcrum line.

It is in the horizontal plane extending through two principal abutments.

Controls rotational movement of denture in sagittal plane (towards or away from the ridge.)


Magnitude of rotational movement is greatest around this fulcrum but not the most damaging.

Difficult to control movement around this fulcrum line

Resultant forces are in the apical direction.


Fulcrum on sagittal plane Extends through the

occlusal rest on the terminal abutment and along the crest of the residual ridge on one side of the arch.

Controls the rotational movement in vertical plane(rocking,or side to side movements).


Movements are easier to control around this fulcrum.

They are of lesser magnitude.

More damaging forces as direction is horizontal.


Vertical fulcrum line Located in

midline ,lingual to anterior teeth.

Controls movement of the denture in horizontal plane (flat circular movements of the denture).www.indiandentalacademy.co

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Principles by A.H. Schmidt (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.


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.

5. A removable partial denture is a form of treatment and not a cure.


Philosophy of design Of the various schools of thought , none

are backed by scientific research or statistics.

They are ideas of dentists who by extensive clinical experience have formulated rules by which they produce a design .

The challenge in design lies primarily in class 1 and 2 arches and to some extent in the class 4 arches.


There are 3 basic , underlying approaches to distributing the forces acting on partial denture between the soft tissues and teeth.

Stress equalization Physiologic basing Broad stress distribution.


Stress equalization

Resiliency of the tooth secured by the periodontal ligament in an apical direction is not comparable to the greater resiliency and displaceability of the mucosa covering the edentulous ridge.


There fore , it is

believed that a a type of stress equalizer is needed to replace the rigid connection between denture base and direct retainer.


Most common type is a hinge device which permits vertical movement of the denture base. it can be adjusted to control the amount of vertical movement.


Advantages.1. Minimal direct retention is

required- as denture base acts more independently.

2. Has the massaging or stimulating effect on the underlying bone and soft tissue. Which minimizes tissue change and resulting Rebasing procedures.


Disadvantages.1. Construction of stress director is

complex and costly.2. Constant maintenance required.3. Difficult or impossible to repair.4. Lateral movements of base can

lead to rapid resorption of the ridges.

This school of thought had got fewer advocates.


Physiologic basing

This school of thought too believes that there is relative lack of movement in abutment teeth in an apical direction.

But it believes that stress equalization can be best achieved by either displacing or depressing the ridge mucosa

during the impression making procedure or by relining the denture base after it has

been constructed


The tissue surface is recorded in functional form and not anatomic form.

Rpd constructed from tissue displacing impression will be above the plane of occlusion when the denture is not in function.


To permit vertical movement from rest position to functional position the retentive clasps have to have minimum retention and also their number has to be less.


Advantages.1. Intermittent base

movement has a physiologically stimulating effect on the underlying bone and soft tissue.

2. Less need for relining and Rebasing.

3. Simplicity of design and construction because of minimal retention requirements.


4. Light weight prosthesis with minimal maintenance and repair.

5. The looseness of the clasp on the abutment tooth reduces the functional forces transmitted to the tooth.


Disadvantages.1. Denture is not well stabilized

against lateral forces.2. There will be always premature

contact when mouth is closed .3. It may be uncomfortable

sensation to the patient.4. It is difficult to produce effective

indirect retention.


Broad stress distribution

Advocates of this school of thought believe that excessive trauma to the remaining teeth and residual ridge can be prevented by distributing the forces of occlusion over as many teeth and as much of the available soft tissue area as possible.

Achieved by means of additional rests,indirect retainers,clasps and broad coverage denture bases.


advantages1. Teeth can be splinted .

2. Prosthesis are easier and less expensive to construct.

3. No flexible or moving parts so less danger of distorting the denture.


4. Indirect retainers and other rigid components provides excellent horizontal stabilization.

5. Less relining required.


disadvantages

1. Greater bulk may cause prosthesis to be less comfortable.

2. Increased amount of tooth coverage can lead to dental caries


Length of span Factors

influencing magnitude of stresses transmitted to abutment teeth

1. Length of span.

2. Quality of support of ridge.

3. Clasp.1. Qualities 2. Design.3. Length4. Material.

4. Abutment tooth surface

5. Occlusal harmony.

The longer the edentulous span ,the longer will be the denture base and the greater will be the force transmitted to the abutment teeth.


Quality of support of ridge

Factors influencing magnitude of stresses transmitted to abutment teeth

1. Length of span.





Large well formed ridges are capable of absorbing greater amounts of stress than are small,thin,or knife-edged ridges

broad ridges with parallel sides permit the use of longer flanges which help in stabilizing the denture against the lateral forces.

Type of mucoperiosteum also influences the magnitude of stresses transmitted to abutment teeth. www.indiandentalacademy.co

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•Sharp spiny ridge will provide poor support,poor to fair stability.

•Soft ,flabby displaceable tissue-poor support, poor stability- leads to vertical and lateral instability and transmission of stress to the adjacent abutment tooth.

•Flat ridge will provide good support,poor stability


Qualities of clasp Factors


1. Length of span.





More flexible the clasp less stress is transmitted to the abutment tooth.

But at the same time it contributes less resistance to the lateral and vertical stresses transmitted to the residual ridges.


Design of clasp Factors


1. Length of span.





A clasp that is designed so that it is passive when it is completely seated on the abutment tooth will exert less stress on the tooth than one that is not passive.

A clasp should be designed so that during insertion or removal of the prosthesis the reciprocal arm contacts the tooth before the retentive tip passes over the greatest bulge of the abutment tooth.


Length of clasp Factors


1. Length of span.





More flexible the clasp less stress it will exert on the abutment tooth.

Flexibility can be increases by lengthening the clasp.


Clasp length may be increases by using a curved rather than a straight course on an abutment tooth


Material used for clasp fabrication

Factors influencing magnitude of stresses transmitted to abutment teeth

1. Length of span.





Crome alloy being more rigid will exert greater stress on the abutment tooth.


Clasp arm of chrome alloys are constructed with a smaller diameter than a gold clasp


Amount of clasp surface in contact with tooth


Abutment tooth surface Factors


1. Length of span.





Surface of gold crown or restoration offers more frictional resistance to clasp arm movement than does the enamel surface of the tooth.

Greater stress is exerted on a tooth restored with gold than on a tooth with intact enamel.


Occlusal harmony. Factors influencing magnitude of stresses transmitted to abutment teeth

1. Length of span.





When deflective occlusal contacts are present between opposing teeth destructive horizontal forces which are magnified by leverage are transmitted to the abutment and ridge.


Partial denture constructed opposing a complete denture will be subjected to a much less occlusal stress than one opposed by natural dentition. Force exerted by natural teeth –300

pounds per square inch. Complete denture – 30 pounds per

square inch. Occlusal load applied to the distal

end of denture base will result into more stress transmitted to the abutment teeth.


Ideally,the occlusal load should be applied in the center of the denture –bearing area, both antero-posteriorly and bucco-lingually


Design considerations for stress control

1. Direct retention • Adhesion ,cohesio

n• Frictional • neuromuscular

2. Clasp position Quadrilateral Tripod bilateral

3. Clasp design• Circumferential

clasp• Bar clasp.• Combination clasp

4. Splinting of abutments

5. Indirect retention6. Occlusion7. Denture base 8. Major connector 9. minor connector 10. Rests

At present there is no way that all forces can be totally negated or countered.

Long term clinical observations have proved that a design philosophy that strives to control these forces within the physiologic tolerance of the teeth and supporting structures can be successful.


Direct retention


1. Direct retention • Adhesion ,c

ohesion• Frictional • neuromusc

ular2. Clasp position

Quadrilateral

Tripod bilateral

3. Clasp design• Circumfere

ntial clasp• Bar clasp.• Combinatio

n clasp 4. Splinting of

abutments 5. Indirect

retention6. Occlusion7. Denture base 8. Major connector 9. minor

connector 10. Rests

The retentive clasp arm is responsible for transmitting most of the destructive forces to the abutment teeth.

Clasp retention should be kept at the minimum yet provide adequate retention to prevent dislodgement of the denture.

Other components should be used to contribute for the retention so that amount of retention provided by clasp can be reduces www.indiandentalacademy.co

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Adhesion and Cohesion





Quadrilateral

Tripod bilateral






connector 10. Rests

For adhesion and cohesion to work Maximum area of available support

should be used. Denture base should be accurately

adapted to the underlying mucosa. Though peripheral seal cannot be

developed due to presence of teeth .Atmospheric pressure helps in retention of the maxillary partial denture when metal casting is accurate and margins of connector are beaded.


Frictional control Design

considerations for stress control




Quadrilateral

Tripod bilateral






connector 10. Rests

The partial dentures should be designed so that guide planes are created on as many teeth as possible.

These planes can be on enamel surfaces of the teeth or in restorations placed on the teeth.


The frictional contact of the prosthesis against these parallel surfaces can contribute significantly to the retention of the denture.


Function of guide planes1. To provide for one path of

placement and removal of the restoration.

2. To ensure the intended actions of reciprocal,stabilizing, and retentive components.

3. To eliminate gross food traps between abutment teeth and components of the denture.


Proximal guiding plane surfaces should be about 2/3rd as wide as the distance between the tips of adjacent buccal and lingual cusps or

about 1/3rd of the buccal lingual width of the tooth.

Vertically it should extend 2/3rd of the length of the enamel crown portion of the tooth from the marginal ridge cervically.

Care must be taken to avoid creating buccal or lingual line angles.


Contact of guide planes


Neuromuscular control





Quadrilateral

Tripod bilateral





retention6. Occlusion7. Denture base 8. Major

connector 9. minor

connector 10. Rests

The innate ability of the patient to control the action of the lips, cheeks, tongue can be a major factor in the retention of a denture.

A properly contoured denture base, however, can aid the patient’s neuromuscular control of the prosthesis.


Clasp position





Quadrilateral

Tripod bilateral






connector 9. minor

connector 10. Rests

The position of retentive clasp is more important than the number of retentive clasp used in any design.

The number of clasps used is determined by classification.


Quadrilateral configuration





Quadrilateral

Tripod bilateral






connector 10. Rests

Is indicated in class 3 arches particularly when modification space exists on the opposite side.


A retentive clasp is positioned on each abutment tooth adjacent to the edentulous spaces.

In this design leverage is effectively neutralized.


When no modification space exists the goal should be to place one clasp as far posterior on the Dentulous side as possible and one as far anterior as space and esthetics permit.


Tripod configuration Design





Quadrilateral

Tripod bilateral






connector 10. Rests

Used primarily for class 2 arches.

If there is a modification space on the Dentulous side .


If there is no modification space present .

One clasp on the Dentulous side of the arch should be positioned as far posterior, and the other, as far anterior as factors such as interocclusal space, retentive undercut, and esthetics considerations will permit.


Bilateral configuration Design





Quadrilateral

Tripod bilateral






connector 10. Rests

Used in class 1 cases. In this configuration the

clasps exert little neutralizing effect on the leverage induced stresses generated be the denture base. These stresses must be controlled by other means.


The terminal abutment tooth on the each side of the arch must be clasped regardless of where it is positioned.


Clasp design





Quadrilateral

Tripod bilateral





retention6. Occlusion7. Denture base 8. Major connector 9. minor connector 10. Rests www.indiandentalacademy.co

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Circumferential clasp


1. Direct retention • Adhesion ,

cohesion• Frictional • neuromus

cular2. Clasp position

Quadrilateral

Tripod bilateral


ntial clasp• Bar clasp.• Combinati

on clasp 4. Splinting of



connector 9. minor

connector 10. Rests

Conventional circumferential clasp originating from distal rest and engaging mesiobuccal retentive undercut should be avoided at all cost in distal extension removable partial denture.


As denture base moves towards the tissue the clasp puts distal tipping force on the abutment tooth.


Alternatives Reverse circlet clap

Approaches a distobuccal undercut from the mesial surfaces of a terminal abutment tooth.

As occlusal load is applied, retentive terminal moves gingivally and loses contact with the tooth surface and no stresses are transmitted.


Disadvantage

It may produce wedging force between 2 teeth's- can be countered by making rest on the approximating surface too.


Vertical projection or Bar clasp.





Quadrilateral

Tripod bilateral






connector 10. Rests

It is used in distal extension partial denture when retentive undercut is located on the distobuccal surface.

Never when tooth has a mesiobuccal undercut.


Functions similar to the reverse circumferential clasp with the advantage of not producing any wedging forces .


One school of thought advocated omitting of the distal rest in favour of a mesial rest for the following reasons. As the fulcrum line is still distal to the

clasp terminal when distal rest is used . With use of mesial rest the lever arm is

increases and forces are directed to the ridge in more vertical direction which are better tolerated by the ridge.


Disadvantage a space is

created between framework and tooth surface leading to food trapment


What is the consensus? Least unfavorable torque is

when…..

…T clasp with distal -occlusal rest and a rigid circumferential reciprocating clasp.


Combination clasp





Quadrilateral

Tripod bilateral






connector 10. Rests

It is used when mesiobuccal undercut exists on an abutment tooth adjacent to a distal extension edentulous ridge.

Only the retentive arm is wrought metal.

Reciprocation and stabilization against lateral movement must be obtained through the use of the rigid cast elements that make up the remainder of the clasp.


Wrought wire can flex in any spatial plane and can absorb torosional stress in both the vertical ad horizontal planes.

A cast clasp flexes in the horizontal plane only.

A short wrought wire arm can be destructive element because of its reduced ability to flex compared with a longer wrought –wire arm.


Advantages. Flexibility. Adjustability. Minimum tooth contact. Better esthetics.


Splinting of abutments





Quadrilateral

Tripod bilateral






connector 10. Rests

Rationale It increases the periodontal

ligament attachment area and distributes the stress over a larger area of support.


Indications by crowns.

1. Loss of periodontal attachment by disease or therapy.

2. Abutment has tapered or short roots

3. Second premolar as abutment with edentulous space anterior to it- splinted with canine by FPD.


Splinting by crowns stabilizes the teeth in mesiodistal direction.

Splint should include canine to achieve the stabilization in buccolingual direction as well.


An extremely week teeth should not be splinted with a strong teeth.


Splinting by clasps Should not be done if

fixed splinting is possible. More tan one teeth are

clasped on each side of the arch ,using a number of rests for additional support and stabilization of the teeth and prosthesis.

Most of the clasp arm will not be retentive.


Indirect retention





Quadrilateral

Tripod bilateral






connector 10. Rests

Indirect retainer. The component of removable partial

denture that assists the direct retainer in preventing displacement of the distal extension denture base by functioning through lever action on the opposite side of the fulcrum line when the denture base moves away from the tissues in pure rotation around the fulcrum line. (GPT-7)

Indirect retainer also contributes to a lesser degree, to the support and stability of the denture.


Design considerations

Should never be on an inclined tooth or on a single weak incisor.


Class 1.

It must always be used and positioned as far anteriorly as possible.


Class 2

its use is not as critical as in class 1.

If no modification space exists .

An abutment tooth with suitable contours for clasping should be selected as far anterior on the tooth-supported side as possible.

This rest and clasp assembly, may serve as the indirect retainer if it is located far enough anterior to the fulcrum line.www.indiandentalacademy.co

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If modification space exists.

The most anterior abutment on the tooth supported side, with its rest and clasp assembly, may be located far enough anterior to the fulcrum line to serve as the indirect retainer.

A definite rest seat positioned even farther anterior,if possible,may increase the effectiveness of the indirect retention.


Class 3 Indirect retention is

not ordinarily used. Auxillary rests must

for lingual plate major connector.

Auxillary rests may be needed to provide additional vertical support for a long lingual bar major connector or an extensive palatal major connector.


If the contours of the posterior abutment teeth in class 2 or 3 partial denture are not suitable for retention

In such case non retentive stabilizing clasp are designed for posterior teeth and anterior indirect retention is a must.


Class 4 The

indirect retainer must be located as far posterior as possible.


Occlusion





Quadrilateral

Tripod bilateral






connector 10. Rests

Occlusion which is in harmony with movements of TMJ and neuromusculature will minimize the stress transferred to the abutment teeth and residual ridge.


Design considerations The initial occlusal contact should

always be in the remaining natural teeth.

Mandible should not be guided into protrusive or lateral movements by the metal or artificial teeth.

Reduced buccolingual width of replaced teeth reduces the stress transmitted.


if number of teeth replaced is reduced stress transmitted will be less

Sharp cutting surfaces and sluiceways can help relive some unnecessary force during mastication.

Steep cuspal inclines on the artificial teeth should be avoided because they tend to introduce horizontal forces .


Denture base





Quadrilateral

Tripod bilateral






connector 10. Rests

….to reduce the stress to the abutment teeth? denture base should cover

maximum area of the supporting tissue as possible.

Denture base flanges should be as long as possible-to help stabilize against horizontal movements.


Design considerations…. Distal extension denture base should cover

the retro molar area and tuberosity of maxilla as these structures better absorb stress.

Overextension should be avoided as interference with functional movements of surrounding tissues will transmit stresses to the remaining teeth.

Accurate adaptation of denture base leads to less tendency for movement during function.

Contour of the polished surfaces also helps in reducing the stress transmitted.


Major connector





Quadrilateral

Tripod bilateral






connector 10. Rests


Design considerationsMandibular arch. Lingual plate major connector can aid in

distribution of functional stress and so is advised if anterior teeth are periodontally weakened.

Also indicted in class 1 arches when the need for additional resistance to horizontal rotation of the denture is required because of excessively resorbed residual ridges.

Another indication is in shallow floor of mouth.


Added rigidity provided by lingual plate also helps in distributing stress created on one side of the arch to the other side (CROSS ARCH STABILIZATION).

A lingual bar should be tapered superiorly with a half pear shape in cross section and should be relived sufficiently.


Maxillary arch. Broad palatal major connector that

connects several of the remaining natural teeth through lingual plating can distribute stress over a large area.

Major connector covering hard palate contributes to support, stability, and retention of the prosthesis.and reduces the stress that is transmitted to the abutment teeth.


Minor connector



ohesion• Frictional • neuromuscu

lar2. Clasp position

Quadrilateral

Tripod bilateral

3. Clasp design• Circumferen

tial clasp• Bar clasp.• Combinatio


abutments 5. Indirect retention6. Occlusion7. Denture base 8. Major connector

9. Minor connector

10. Rests

The intimate tooth to partial denture contact is brought by minor connector

It serves too purposes . Provides horizontal stability

to the partial denture against lateral forces on the prosthesis.

Through this contact, the tooth receives stabilization against lateral stresses.


If more guiding planes are incorporated

in design the force transmitted to each teeth can be minimized.

When crown restorations are used, a lingual reciprocal clasp arm may be inset into the tooth contour by providing a ledge on the crown on which the clasp arm may rest.


design modification of minor connector.

Places the minor connector in the center of the lingual surface of the abutment tooth.

Advantage: it reduces the amount of

gingival tissue coverage.

Provides enhanced bracing and guidance during placement.


Disadvantages. Increases encroachment of tongue

space More obvious borders. potentially greater space between

the connector and abutment tooth.


Rests





Quadrilateral

Tripod bilateral






connector 10. Rests

Control stress by directing stress along the long axis of abutment teeth.

Periodontal ligament is better suited to withstand vertical than horizontal forces.


Design considerations

Floor of rest seat preparation must be less than 90 degrees with long axis of tooth as this design grasps the tooth to prevent its migration.

When an angle is more than 90 degrees inclined plane effect is set up and stress on abutment is magnified.

in class 1 and 2 Preparation should be saucer shaped without

any sharp angles and ledges.

www.indiandentalacademy.co

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Rest should be free to move within the rest seat to release the stresses which would otherwise transmit to the tooth.

More the no of teeth that bear rest seats, the less will be the stress places on each individual tooth.


Essentials of partial denture design. It should be systemically

developed and outlined on an accurate diagnostic casts.


First step Decide how the partial denture

has to be supported. If Tooth supported.

Evaluate1. Periodontal health2. Crown and root morphologies3. Crown to root ratio.4. Bone index area.5. Location of tooth in arch.6. Length of edentulous span.7. Opposing dentition.


If tooth and tissue supported. Also Consider

1. Quality and contour of supporting bone and mucosa

2. Extend to planned coverage of ridge.3. Type and accuracy of impression

registration.4. Accuracy of denture base.5. Design characteristics of the

component parts of framework.6. Anticipated occlusal load.


Denture base areas adjacent to abutment teeth are primarily tooth supported.

As we proceed away from abutment teeth, they become more tissue supported.


Second step Connect the tooth and tissue

support units.

These connection is facilitated by designing and locating major and minor connectors in compliance with the basic principles and concepts.


Third step. Determine how the partial denture is to

be retained. Selecta clasp design that will

1. Avoid direct transmission of tipping for torquing forces to the abutment

2. Accommodate the basic principles of clasp design by definitive location of components parts correctly positioned on abutment tooth surfaces.

3. Provide retention against reasonable dislodging forces.

4. Be compatible with undercut locations,tissue contour,and esthetic desires of the patient.


Fourth step. Connect the retention units to the

support units


Fifth step. Outline and join the edentulous

area tote already established design components.


To be continued …… Designing of major connectors . Designing of minor connectors Miscellaneous factors in designing.


Thank you

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Designing in removable partial dentures /certified fixed orthodontic courses by Indian dental academy