DESIGNING OF RPD

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Presentation Transcript

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Assalamaa aleh kum

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CONCEPTS OF DESIGNING PARTIAL DENTURE Prof. Dr. S. Venugopal. MDS.

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Great controversy continues to exist as to what constitutes correct design and adequate support for the free-end or distal extension removable partial dentures. The method for using and equalizing support from the edentulous ridge and remaining teeth remains the main issue. The different methods used have given rise to various design philosophies.

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There are 3 basic underlying approaches to distributing the forces acting on a partial denture, between the soft tissue and the teeth. Stress equalization Physiologic basing Broad stress distribution

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STRESS EQUALIZATION Emphasizes that the 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. Due to this, forces are transmitted to the abutment teeth as the denture bases are displaced in function.

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It is the belief of this school, that a rigid connection between the denture bases and the direct retainer on the abutment tooth is damaging and that some type of stress director or stress equalizer (a flexible or movable joint between teeth and metal frame work so that the base can move independently of the clasp) is essentially to protect the vulnerable abutment teeth.

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Advantages: This design usually calls for minimum direct retention because the denture base operates more independently than a conventional denture. Internal attachments for retention of a stress broken prosthesis are widely used. Theoretically at least the stress director eliminates the tipping on the tooth, thus preventing bone resorption about the tooth.

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Intermittent pressure against the mucosa caused by the movement of the denture base has a massaging effect on the underlying bone and soft tissue. Thus it is claimed, minimizes tissue change and reduces the necessity of relining and rebasing the denture to compensate for tissue changes as is required for most distal extension partial dentures.

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Disadvantages The stress director is comparatively fragile. Its lack of ability to prevent damaging lateral stresses from occurring on the edentulous ridge can result in rapid resorption of bone and settling of the denture. If sufficient thickness of metal is used in the hinge to prevent lateral movement, the prosthesis becomes heavy, bulky and annoying to the patient.

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Its construction is complex and costly. It requires constant maintenance and may be difficult or impossible to repair Of the 3 schools of thought the stress breaker school has the least advocates, appealing mostly to the more sophisticated or experienced practitioner.

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PHYSIOLOGIC BASING This philosophy of design believes that equalization can best and most simply be accomplished by some form of physiologic basing or lining of the denture base. This is produced either by depressing or displacing the ridge mucosa during the impression making procedure or by relining the denture base after it has been constructed.

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The reason for displacing the mucosa during the impression procedure is to record the soft tissues in their functioning form and not anatomic form so that the denture base formed over displaced tissue will adapt more readily to the depressed tissue and will be better able to withstand the force generated. Hence the artificial teeth of the removable partial denture constructed from a tissue displacing impression will be positioned above the plane of occlusion when the denture is in the

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mouth and not in function. To permit the vertical movement of the partial denture from the rest position to the functional position, the direct retainers must be designed with minimal retention and their number should be limited.

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ADVANTAGES: The intermittent base movement that occurs when occlusal loads are applied and removed has a stimulating effect on the underlying bone and soft tissue, thus reducing the frequency to reline or rebase the prosthesis. Simplicity of design and construction due to minimal retention requirements. The looseness of the clasp on the abutment tooth reduces functional forces transmitted on the tooth and hence aids in their preservation.

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Disadvantages The denture is not well stabilized against lateral forces due to minimum number and flexibility of the direct retainers. The residual ridge receives a greater proportional amount of force than the abutment teeth. Many critics believe that the tooth with its periodontal ligament is better able to withstand lateral forces than the residual ridge. The load of supporting the denture is limited to a few teeth instead of being shared by a number of teeth as in other philosophies.

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As artificial teeth are always slightly above the occlusal plane when the denture is not in function, there will always be slight premature contacts between the opposing teeth and the denture teeth when the mouth is closed, resulting in an uncomfortable sensation for many patients and may result in a sense of insecurity. It is difficult to produce effective indirect retention because of the vertical movement of the denture and the minimal retention of the direct retainer. By the time the indirect retainer engages a rest seat to prevent the denture base from being dislodged, the direct retainer would have lost contact with the abutment tooth.

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Broad stress distribution: Advocates of this school 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 available soft tissue areas as possible. This is accomplished by the use of additional rests, indirect retainers, clasps and broad coverage denture bases.

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Advantages: The forces of occlusion are reduced on any one tooth or area of the ridge and hence will never exceed the physiological limit of the teeth with no resultant resorption of the alveolar bone. Multiple tooth contacts by direct retainers, additional rests and minor connectors can be planned so that lateral forces can be distributed over as many teeth as possible. Clasps used constitute a form of removable splinting especially when fixed splinting is not indicated due to a guarded prognosis for economic reasons.

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Proponents believe prosthesis is easier and less expensive. No flexible components so less danger of distorting the denture and less subject to breakage. Due to increased stability, this partial denture doesn’t require relining as frequently as other types because residual ridge doesn’t bear as much of the horizontal load.

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Disadvantages Greater amount of tooth and soft tissue coverage and increased bulk may cause the prosthesis to be less comfortable and less accepted by patients compared to other simpler designs. Due to increased amount of tooth surface, mouth must be kept meticulously clean. With today’s increased awareness and currently available aids, this factor may not be as critical as it might once have been thought.

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If certain principles are applicable for the solution of given conditions, the same principles will be operative in another instance showing repetition of these conditions. If this is found not to be true, it indicates that something less than basic principles are being applied. The first concept of design should follow or accompany the preliminary survey when oral changes are being planned. Final design should be determined at the time of final survey, when it is possible to measure the mouth preparation changes that have actually been accomplished. The design should be decided by the prosthodontist who planned/executed the mouth preparation for only he can know the oral conditions.

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Tooth-tissue supported removable partial dentures Class I: In class I, the abutment teeth are situated anterior to edentulous areas. This type receives support from soft tissues and the remaining teeth. The different nature of the available support requires careful consideration when designing the removable partial denture.

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Principles for Class I Removable Partial Dentures (Bilateral Distal Extension) For Class I Clasps 2 clasps on each terminal tooth are sufficient. For distobuccal undercut a vertical projection clasp is indicated For mesiobuccal undercut wrought wire clasp. Reciprocal arm should be rigid. Can also be replaced with lingual plate.

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For Class II Clasps Usually 3 clasps are designed. The distal extension side can be designed similar to Class I The tooth supported, or modification side should have two retentive clasps:one as far posterior and one as far anterior as esthetics permit. If modification space is present it is usually most convenient to clasp either side abutment. Reciprocal arm should be rigid. Can also be replaced with lingual plate.

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For Class I and II 2. Rests For maximum possible support. Designed for direction of stress in a direction along the long axis of the tooth. Should always be placed on the tooth adjacent to the edentulous area.

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For Class I and II 3. Indirect Retention Location: as far anterior to the fulcrum line 2 indirect retainers for Class I situation, and 1 on the on the opposite side as the edentulous area in Class II is sufficient. Lingual plate with rest seats is an efficient retainer.

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For Class I and II 4. Major Connector Must be rigid and not impinge on soft tissue. Maxillary: support from hard palate whenever possible. Mandibular: lingual plate for increased rigidity, indirect retention and elimination of areas of food impaction.

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For Class I and II 5. Minor Connector Rigid Positoned to enhance comfort, cleanliness, and placement of artificial teeth.

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For Class I and II 5. Minor Connector Rigid Positoned to enhance comfort, cleanliness, and placement of artificial teeth.

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For Class I and II 6. Occlusion Centric relation and occlusion should coincide. Artificial teeth should be positioned to reduce stress. Positoned to enhance comfort, direction of stress along long axis, and efficiency of mastication.

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For Class I and II 7. Denture Base Broad coverage for equal stress distribution. Borders should not interfere with the functional movements of the tissues. Selective pressure technique is indicated for edentulous space.

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b. For Class III Clasps Quadrilateral positioning is ideal. Type of clasp is not critical. Selection depends on considerations for tooth, esthetics. Bracing arm should be rigid.

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b. For Class III 2. Rests Should be placed on the adjacent abutments. Should support the major connector 3. Indirect Retainers Usually not required

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For Class III 4. Major Connector and Minor Connector Requirements same as Class I 5. Occlusion Requirements same as Class I.

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For Class III 6. Denture Base Functional impression is not neccesary Extent of coverage should be determined by appearance, comfort and avoidance of food impaction.

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c. For Class IV Clasps Quadrilateral positioning is ideal. Location should be as anterior as possible adjacent to edentulous area and as far posterior to the fulcrum line as possible. Type of clasp is not critical. Selection depends on considerations for tooth, esthetics.

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c. For Class IV 2. Major Connector Rigid and broad coverage. E.g. maxillary palatal coverage. 3. Indirect Retainer Should be located as far posterior to the fulcrum line as possible.

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PROBLEMS OF DESIGN Some problems of design are recurrent in more than one group of partial denture situations. Number and location of the modification space will determine how much trouble may result: If the modification space is in anterior part of the arch, the path of insertion from the right or left angulations is impossible. An additional edentulous space bounded by teeth will mean more opposing proximal surfaces with the increased possibility of interference to the insertion of the finished restoration.

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When host teeth are not replaced, it is likely that those remaining will have migrated out of normal alignment. Hence the modification space becomes either larger or smaller than the original tooth. This makes the placing of a substitute quite difficult. It also results in frequent fracturing of the substitute from modification of the frame work

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THANK YOU My sir… with your preaching I am here and sharing the knowledge with the world of new dental doctors…