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Principles of rpd design

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Principles of RPD Design, Prosthodontincs, RPD, Removable Partial Denture Designs, McCracken 12th Edition

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Principles of rpd design

  1. 1. Principles of Removable Partial Denture Design Dr. Shujah A Khan MDS Resident, Department of Prosthodontics, DIKIOHS, DUHS
  2. 2. Difference In Prosthesis Support Requirements for movement control are generally functions of whether the prosthesis will be tooth supported or tooth tissue supported For a tooth-supported prosthesis, the movement potential is less because resistance to functional loading is provided by the teeth For a tooth-tissue supported prosthesis, the residual ridge presents with variable potential for support
  3. 3. Differentiation Between Two Main Types of Removable Partial Dentures Certain points of difference are present between Kennedy Class I and Class II types of partial dentures and the Kennedy Class III type of partial dentures Manner in which each is supported: Class I and distal side extension of Class II  Primary Support  Tissues underlying the base and Secondary  Abutment teeth Class III  All support from Abutment Teeth Secondly, the method of impression registration and jaw record required for each type will vary.
  4. 4. Thirdly the need for some kind of indirect retention exists in the distal extension type of partial denture. Fourth, the use of a base material that can be relined to compensate for tissue changes
  5. 5. •Differences in Support The distal extension partial denture derives its major support from the residual ridge with its fibrous connective tissue covering. The length and contour of the residual ridge significantly influence the amount of available support and stability.
  6. 6. The longer the edentulous area covered by the denture base, the greater the potential lever action on the abutment teeth.
  7. 7. Good Support, but Poor Stability Poor Support and Poor to Fair Stability Poor Support and Poor Stability
  8. 8. •Impression Registration Two requirements need to be fulfilled 1. The anatomic form, relationship of remaining teeth and surrounding soft tissues must be recorded accurately. • Impression material that can be easily removed from undercuts without distortion used to fulfil this requirement • Irreversible hydrocolloid, mercaptan rubber base, silicone impression materials and polyethers
  9. 9. 2. The supporting form of the soft tissues underlying the distal extension base of the partial denture should be recorded so firm areas are used as primary stress–bearing areas and readily displaceable tissues are not overloaded - Muco-displacive impression ? • An impression material capable of displacing tissue sufficiently to register the supporting form of the ridge will fulfil this second requirement.
  10. 10. No single impression material can satisfactorily fulfil both of the previously mentioned requirements!!! Hence a combination of materials or same type of material with differing viscosities used?
  11. 11. •Differences in Clasp Design The tooth-supported partial denture, which is totally supported by abutment teeth, is retained and stabilized by a clasp at each end of each edentulous space. Only requirement for such clasps is that they flex sufficiently during placement and removal of the denture to pass over the height of contour of the teeth in approaching or escaping from an undercut area.
  12. 12. While in its terminal position on the tooth, a retentive clasp should be passive and should not flex except when one is engaging the undercut area of the tooth for resisting a vertical dislodging force. Cast retentive arms are generally used for this purpose. Circumferential type Bar type Advantages and Disadvantages
  13. 13. In the combination tooth and tissue supported RPD, the direct retainer must perform still another function in addition to resisting vertical displacement Stress Breaker – Combination Clasp
  14. 14. The amount of stress transferred to the supporting edentulous ridge(s) and the abutment teeth will depend on: 1. Direction and magnitude of the force; 2. Length of the denture base lever arm(s); 3. Quality of resistance (support from the edentulous ridges and remaining natural teeth); and 4. Design characteristics of the partial denture.
  15. 15. The greater the surface area contact of each minor connector to its corresponding guiding plane, the more horizontal the distribution of force.
  16. 16. Maximum contact of the proximal plate minor connector with the guiding plane produces a more horizontal distribution of stress to the abutment teeth.
  17. 17. Minimum contact or disengagement of the minor connector with the guiding plane allows rotation around the fulcrum located on the mesio-occlusal rest, producing a more vertical distribution of stress to the ridge area.
  18. 18. Minor connector contact with the guiding plane from the marginal ridge to the junction of the middle and gingival thirds of the abutment tooth distributes load vertically to the ridge and horizontally to the abutment tooth
  19. 19. •Essentials of Partial Denture Design Design of the partial denture framework should be systematically developed and outlined on an accurate diagnostic cast based on the following concepts: 1. Where the prosthesis is supported? 2. How the support is connected? 3. How the prosthesis is retained? 4. How retention and support are connected? 5. How edentulous base support is connected?
  20. 20. It is first necessary to determine the support; tooth borne or distal extension base? Potential support the abutment can provide, consideration for: 1. Periodontal health 2. Crown and root morphologies 3. Crown to root ratio 4. Bone Index Area (how tooth has responded to previous stress) 5. Location of tooth in arch 6. Relationship of tooth to other support units (edentulous span) 7. Opposing dentition
  21. 21. In evaluation of potential support from the edentulous ridge areas, consideration should be given to: 1. Quality of residual ridge  contour and quality of supporting bone 2. Extent to which the residual ridge will be covered by the denture base 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
  22. 22. The second step in systematic development of the design is to connect the tooth and tissue support units. Major and Minor connectors
  23. 23. The third step is to determine the how the RPD is to be retained  Placement of Direct retainers The key to selecting a successful clasp design for any given situation is to choose one that will 1. Avoid direct transmission of tipping or torquing forces to the abutment 2. Accommodate the basic principles of clasp design by definitive location of component parts correctly positioned on abutment teeth surfaces 3. Provide retention again reasonable dislodgement 4. Be compatible with undercut location, tissue contour and esthetic desires of the patient
  24. 24. The fourth step is to connect the retention units to the support units, direct and indirect retainers must be rigidly attached to the major connector The fifth and last step is to outline and join the edentulous area to the already established design components.
  25. 25. •Components of Partial Denture Design All partial dentures have two things in common: 1. They must be supported by oral structures 2. They must be retained against reasonable dislodging forces
  26. 26. In Class Kennedy III partial dentures three components are necessary Support provided by Rests Connectors (Stabilizing Components) Retainers Kennedy Class I and II  support comes from both the teeth and underlying ridge tissues.
  27. 27. However provision must be made for three other factors: 1. Best possible support from resilient tissues accomplished by Impression technique primarily and secondarily by area covering the denture base area. 2. Method of direct retention • Direct transmission of load to long axis of abutment teeth instead of leverage 3. Distal extension bases  movement of extension base away from tissues is minimum  Indirect retainers
  28. 28. •Tooth Support Support of RPD by the abutment teeth is dependent on the alveolar support, crown and root morphology, rigidity of the framework and design of occlusal rests Through clinical and roentgenographic interpretation the dentist may evaluate the abutment teeth and decide whether they will provide adequate support? The dentist is responsible for preparation and restoration of abutment teeth to accommodate the most ideal design of the partial denture
  29. 29. •Ridge Support Effectiveness of tissue support is dependent upon: 1. Quality of the residual ridge 2. Extent to which it will be covered by the denture base 3. Accuracy and type of impression registration 4. Accuracy of denture bases 5. Design characteristics of the component parts 6. Occlusal load applied
  30. 30. Quality of residual ridge cannot be influenced, except it can be improved by tissue conditioning or modified by surgical intervention Accuracy of impression technique is entirely in the hands of the dentist Accuracy of denture base is influenced by the choice of materials and the exactness of processing techniques
  31. 31. Total occlusal load applied to the residual ridge may be influenced by reducing the area of occlusal contact Fewer or narrower teeth, more effectively shaped teeth? Occlusal forces? Opposing natural teeth vs opposing artificial teeth?
  32. 32. Ridge tissues recorded in their resting or non functioning form are incapable of providing the composite support for distal extension bases Three factors must be considered in the acceptance of an impression: 1. Primary stress bearing areas recorded in their supporting form 2. Tissues other than primary stress bearing areas recorded in their anatomic form 3. Total area covered by the impression should be sufficient to distribute load over large area tolerated by tissues – Snowshoe principle
  33. 33. Denture base processed to the functional form is generally less irregular and provides greater area coverage than does a denture base processed to the anatomic or resting form. A denture base made to anatomic form exhibits less stability under rotating and/or torquing forces than does a denture base processed to functional form and thus fails to maintain its occlusal relation with opposing teeth.
  34. 34. •Major and Minor Connectors A major should be properly located in relation to gingival and moving tissues and should be designed to be rigid  provides proper distribution of forces to and from the supporting components Minor connectors arise from the major connectors and join it with other parts of the denture; serve to connect tooth and tissue support units
  35. 35. •Direct Retainers For Tooth Supported prosthesis: 1. Retain prosthesis against reasonable dislodging forces without damage to abutment teeth 2. Aid in resisting any tendency of the denture to be displaced in a horizontal plane
  36. 36. For Distal Extension Partial Dentures While retaining the prosthesis, it must also be able to flex or disengage when the denture moves tissue-ward under function  Stress Breaker
  37. 37. •Stabilizing Components Stabilizing components of the removable partial denture framework are those rigid components that assist in stabilizing the denture against horizontal movement. All stabilizing components should be able to distribute stresses equally to all supporting teeth without overworking any one tooth. It is necessary that minor connectors have sufficient bulk to be rigid and yet present as little bulk to the tongue as possible
  38. 38. •Guiding Plane Two or more parallel, vertical surfaces of abutment teeth, so shaped to direct a prosthesis during placement and removal. Functions: 1. Provide for one path of placement and removal of the restoration/prosthesis 2. To ensure the intended actions of reciprocal/stabilizing, retentive components 3. Eliminate gross food traps between abutment teeth and components of denture
  39. 39. Proximal guiding plane surfaces should be about one-half the width of the distance between the tips of adjacent buccal and lingual cusps 1/2
  40. 40. Or it should be one-third the buccal lingual width of the tooth 1/3
  41. 41. And should extend vertically about two-thirds the length of the enamel crown portion of the tooth from the marginal ridge cervically. 1/3
  42. 42. A guiding plane should be located on the abutment surface adjacent to an edentulous area. However, excess torquing is inevitable if the guiding planes squarely facing each other on a lone standing abutment adjacent to an extension area are used
  43. 43. •Indirect Retainers An indirect retainer must be placed as far anterior from the fulcrum line as adequate tooth support permits. It must be placed on a rest seat prepared in an abutment tooth that is capable of withstanding the forces placed on it. An indirect retainer cannot function effectively on an inclined tooth surface, nor can a single weak incisor tooth be used for this purpose. Canines/Premolars
  44. 44. •Implant Considerations In Design Kennedy Class III presents less of a challenge to Oral Tissues and patient accommodation than Class I and II. The challenge is chiefly related to prosthesis movement to an extent allowed by tissue displaceability under an applied load. Use of implants can significantly benefit tissue tolerability and reduce any challenge to accommodation presented by prosthesis movement
  45. 45. •Examples of Systemic Approach to Design Class III RPD Entirely tooth supported, may be made to fit the prepared surfaces of the anatomic form of the teeth and surrounding structures. It does not require an impression of the functional form of the ridge tissues, nor does it require indirect retention. Cast clasps of the circumferential variety, the bar type, or the combination clasp may be used.
  46. 46. Unless the need for later relining is anticipated, as in the situation of recently extracted teeth, the denture base may be made of metal, which offers several advantages. The Class III partial denture can frequently be used as a valuable aid in periodontal treatment because of its stabilizing influence on the remaining teeth.
  47. 47. Kennedy Class I – Bilateral Distal Extension RPD Because it derives its principal support from the tissues underlying its base, a Class I partial denture made to anatomic ridge form cannot provide uniform and adequate support. Yet, unfortunately, many Class I mandibular removable partial dentures are made from a single irreversible hydrocolloid impression.  both the abutment teeth and the residual ridges suffer because the occlusal load placed on the remaining teeth is increased by the lack of adequate posterior support
  48. 48. Kennedy Class II RPD Combination of tissue supported and tooth supported restorations Principles of Design for both Class I and II collectively used
  49. 49. Thank You

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