Principles of design./ orthodontic seminars


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Principles of design./ orthodontic seminars

  1. 1. Principles of designing removable partial dentures INDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. Contents • Introduction • Principles of design • Philosophies of designing of RPD • Factors influencing design • Systematic approach to desiging of RPD • Essentials of design in Kennedy class I, II, III, IV situations • Surveying and steps involved in surveying • Summary • Conclusion • references
  3. 3. Introduction • Partial or complete loss of natural teeth is the result of disease of the calcified tooth surfaces ( dental caries) or disease of the supporting tissues (periodontal disease). When the individual looses some of his teeth, the remaining teeth and the periodontium, muscles, ligaments, and temperomandibular joint may also be affected.
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  5. 5. • The dentists obligation is to intercept these processes and prevent them from running their full course. Our purpose is to assist the patient to attain and maintain the best physiologic oral health possible. since a high percentage of patients who are partially edentulous require RPD’s, the most reasonable course of action is to plan dental treatment from the perspective of how the design of the prosthesis can best preserve and
  6. 6. • Maintain the residual oral tissues. The dentist must have a logical goal, must plan a systematic approach to achieving the most satisfactory RPD design for the dental conditions individual to that patient, and must develop, in sequence, the methods and means of achieving that design.
  7. 7. Principles of design • Dentists must have a working knowledge of both the mechanical and the biologic factors involved in removable partial denture design and construction. • Any plan of restoration must be based on a complete examination and diagnosis of the individual patient.
  8. 8. • The dentist, not the technician should correlate the pertinent factors and recommend a proper plan of treatment. • A removable partial denture should restore form and function without injury to tissue • A removable partial denture is a form of treatment, not a cure.
  9. 9. Philosophy of design • Stress equalization • Physiologic basing • Broad stress distribution
  10. 10. Principles of partial denture construction • Provide adequate support on the remaining natural teeth to resist vertical stress, and to direct those stresses in the direction of the long axis of the abutment teeth • Provide broad coverage of saddle areas so that the stresses borne by the soft tissue are distributed over a large area.
  11. 11. • Provide adequate tooth borne resistance to lateral stresses • Provide adequate retention • Provide for distribution of stress between the relatively rigid abutment teeth and the relatively resilient saddle areas. • Have rigid bars and connectors
  12. 12. • Avoid covering free gingival margin around the remaining teeth with denture base • Decrease the occlusal table by using narrower artificial teeth • Provide harmony of the occlusal surface of opposing teeth and fewer teeth
  13. 13. Factors influencing design • The arch to be restored with removable partial denture. If both arches are to be restored, following considerations are to be considered. – Orientation of the occlusal plane – Space available for restoration of the missing teeth – Occlusal relationship of the remaining teeth
  14. 14. – Tooth morphology 2. Periodontal condition of the remaining teeth 3. The amount of abutment support remaining 4. Need for splinting 5.If denture is entirely tooth supported or tissue supported.
  15. 15. If one or more distal extension bases are present, the following are to be considered. • Clasp design that will minimize the forces applied to the abutment tooth • Secondary impression to be used • Need for indirect retention • Need for later rebasing, which will decide the type of base material to be used
  16. 16. 6. Need for abutment tooth modification or restoration, which may influence the type of clasp arms to be used and their specific design 7. Type of major connector indicated 8. Material to be used for framework and bases 9. Type of teeth used for replacement
  17. 17. A systematic approach to designing of partial dentures. • Designing of partial denture framework should be systematically developed and outlined on an accurate diagnostic cast. 1. Determine how the partial denture is to be supported 2. Connecting the tooth and tissue supporting units
  18. 18. 3. Retention for the partial denture 4. Connecting the retention units to the supporting units 5. Design the outline and join the edentulous area to the already established design components
  19. 19. Establishing support
  20. 20. • Tooth support – In an entirely tooth supported RPD , rests should be located in such a manner that any load applied to the prosthesis is directed along the long axis of the abutment tooth so that the movement of the prosthesis in an apical direction can be effectively resisted.
  21. 21. • Direct retainers and minor connectors must be designed so that the least possible lateral or torquing stress will be transmitted to the abutment tooth. • Location of placing rests • Occlusal • Cingulum • Incisal surfaces of the abutment tooth
  22. 22. • Rests do act as point of rotation of the prosthesis. The closer the rest is to the edentulous area, the greater is the arc of rotation around the rest.
  23. 23. • In a distal extension RPD, it is advisable to place the rest on the mesial aspect of the primary abutment tooth. The resultant force on the prosthesis will tend to move the tooth mesially and be reciprocated by the dental arch.
  24. 24. Considerations to be considered when evaluating potential support that an abutment tooth can provide • Periodontal health • Crown and root morphology • Crown to root ratio • Length of edentulous span • Opposing dentition
  25. 25. Tissue support • Distal extension RPD derives a great deal of its support from the residual ridge. • Considerations in evaluating the potential of tissue support for RPD’s – Length and contour of the residual ridge – Contour of the edentulous base area – Quality of the supporting bone and overlying mucosa – Forces on supporting tissues – Previous response to stress
  26. 26. Length and contour of residual ridge • Denture base in close approximation to the abutment tooth is primarily supported by the abutment teeth. Proceeding away from the abutment teeth, support for the distal extension base is primarily derived from the underlying tissue.
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  28. 28. Support areas for RPD in mandible • Buccal shelf area( primary) • Retromolar pad area • Slopes of the residual ridge (secondary)
  29. 29. Support areas for an RPD in maxilla • Slopes of the residual ridge (secondary) • Horizontal portion of the hard palate ( primary support) • Crest of the posterior residual ridge ( primary support)
  30. 30. Contour of the edentulous base area • The ideal contour ridge contour is one that has a broad, smooth, rounded ridge crest with nearly vertical buccal and lingual slopes. • Residual ridge crests that are parallel to the opposing ridges or the occlusal plane provide the most advantageous position for distribution of stress.
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  32. 32. • Residual ridges that are flat might offer secondary support but offer little lateral stability • Residual ridges that are sharp and spiny provide the poorest anatomy for both the support and stability of a prosthesis.
  33. 33. Quality of the supporting bone and overlying mucosa. • Absence of cortical bone in specific areas usually indicate poor response of residual bone to stress. • The mucosal covering of the residual ridge plays an important role in support. Tissues covering residual ridges must be recorded in a dynamic form, that is functional form of the supporting areas and anatomic form of the anatomic areas.
  34. 34. Forces acting on supporting tissues • Force activated by Resultant force Counteracted by Sticky foods Vertical lift Retention tongue & muscle forces Vertical lift Adequate denture base coverage Gravity Vertical lift indirect retainers Occlusal load Movt towards Occlusal, cingulum, residual ridge and incisal rests Adequate denture base
  35. 35. Forces around the longitudinal axis Force activated by Resultant force Counteracted by Occlusal force on one Twisitng, tilting Rigid connectors Side of the arch causes Direct retauner design Lifting forces on the Denture base coverage Contrlateral side of Denture tooth placement the arch Contour of the denture base
  36. 36. Forces around the perpendicular axis Force activated by Resultant force Counteracted by Masticatory stress Twisting and spreading Adequate Bracing of the RPD Rigid connectors Denture base coverage Occlusal balance Contour of the denture base
  37. 37. Connection of supporting units
  38. 38. • The connection of the support units is facilitated by major and minor connectors. • A Major and minor connector should be designed meeting the following requirements – They should be rigid and strong enough to withstand masticatory forces
  39. 39. – They should conform to and not interfere with the normal anatomic structures of the mouth – Should avoid food entrapment – They should not interfere with mastication or occlusion.
  40. 40. Design considerations for maxillary major connectors • Should be designed with the comfort of the patient in mind. • To use as much as support from the hard palate as possible. • Free gingival margins and gingival crevices that are traversed by the major connectors should be relieved. • Coverage of the anterior part of the hard palate should be avoided whenever possible
  41. 41. • A bead on the tissue side of the major connector should be prepared along the peripheral outline, so that the major connector will slightly displace the underlying soft tissue to provide a peripheral seal.
  42. 42. Function of beading • Prevent food debris from collecting beneath the major connector • Additional thickness along the bead permits the edges of the polished surface of the connector to be tapered so that a smooth junction with the soft tissues is created • Serves as a finish line for the technician during finishing and polishing of metal framework
  43. 43. Minor connectors • Should be rigid to withstand masticatory stress • Should be positioned in interproximal spaces to avoid tongue interference and should pass vertically from major connector to other components • Should be thicker towards lingual surface and taper towards the contact area. • There should be a minimum of 5 mm space between the vertical minor connectors
  44. 44. • Types of minor connector • Open latticework • Mesh type • Metal base • Finish lines • Internal finish line and external finish line
  45. 45. Providing retention
  46. 46. • Retention achieved by two means • Tissue seal • Mechanical retention • Requirements of a clasp – Support – Bracing (stabilization) – Retention – Reciprocation – Encirclement ( more than 180 degrees) – passivity
  47. 47. General rules concerning clasp retention • Clasp assembly should be designed so that only one retentive arm is used on an abutment teeth. The retentive arm should be opposed by a reciprocal component of the clasp assembly on the opposite side of the abutment tooth
  48. 48. • An abutment tooth that has a definite taper occlusally on which the reciprocal component of the clasp assembly is located should be restored or recontoured to provide for a vertical surface to oppose the retentive arm if adequate bracing for the retentive arm is to be achieved
  49. 49. • The clasp assembly design selected for an abutment tooth should be based on the location and depth of the available undercut area on the abutment tooth. • Complicated clasp assembly designs should be avoided
  50. 50. Guidelines for location of retentive areas • The closer the retentive area to the edentulous area, the greater the potential for retention • Retention areas should be located as widely as possible through the remaing natural teeth in the dental arch to provide eqaulized retention and stability to the rpd
  51. 51. • Whenever possible, retentive areas on one side of the dental arch should be opposed by similar retentive areas on the opposite side of the dental arch.
  52. 52. Factors influencing the effectiveness of indirect retainers • The indirect retainer should be placed at right angles to and as far from the fulcrum line as possible. • An indirect retainer should be always placed on a prepared rest seat on an abutment tooth that is capable of withstanding stresses placed on it, preferably a canine or premolar.
  53. 53. • In distal extension RPD’s, indirect retainers moves the fulcrum line anteriorly to the abutment tooth or teeth contacted by the indirect retainer. This prevents the base from lifting of the soft tissue. • Indirect retainers serve as a third point of reference for the cast RPD framework and thus aid in locating the correct framework position during relining procedures or when altered cast impression technique is used.
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