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Design considerations for a distal extension rpd/prosthodontic courses

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Design considerations for a distal extension rpd/prosthodontic courses

  1. 1. Design considerations for aDesign considerations for a distal extension RPDdistal extension RPD INDIAN DENTAL ACADEMY Leader in continuing Dental Education www.indiandentalacademy.comwww.indiandentalacademy.com
  2. 2. IntroductionIntroduction • ““Plans may be useless but planning isPlans may be useless but planning is indispensable.”indispensable.” • RPDs are intended to be placed in theRPDs are intended to be placed in the mouth and removed. Hence, they cannotmouth and removed. Hence, they cannot be rigidly connected to the teeth orbe rigidly connected to the teeth or tissue. This makes them subject totissue. This makes them subject to movement within the oral cavity.movement within the oral cavity. • Movements induce stress.Movements induce stress. • It is important for the clinicians toIt is important for the clinicians to understand the possible movements andunderstand the possible movements and be able to logically design thebe able to logically design the component parts of the RPD to helpcomponent parts of the RPD to help control these movements.control these movements. www.indiandentalacademy.comwww.indiandentalacademy.com
  3. 3. CLASSIFICATIONCLASSIFICATION: by: by Beckett(1953), Craddock(1956)Beckett(1953), Craddock(1956) and Osborne and Lammie (1974)and Osborne and Lammie (1974) • Class I – denture supported by mucosaClass I – denture supported by mucosa and underlying boneand underlying bone • Class II – denture supported by teeth.Class II – denture supported by teeth. • Class III- denture supported by aClass III- denture supported by a combination of mucosa and tooth bornecombination of mucosa and tooth borne meansmeans • Class IV- denture supported by implants.Class IV- denture supported by implants. www.indiandentalacademy.comwww.indiandentalacademy.com
  4. 4. Differentiation between the twoDifferentiation between the two main types of removable partialmain types of removable partial denturesdentures • Manner of supportManner of support • Method of impression registrationMethod of impression registration • Need for some kind of indirect retentionNeed for some kind of indirect retention • Denture base materialDenture base material • Differences in clasp design.Differences in clasp design. www.indiandentalacademy.comwww.indiandentalacademy.com
  5. 5. Biomechanical considerations.Biomechanical considerations. • A better understanding of the methods ofA better understanding of the methods of controlling forces on RPD may be achieved by acontrolling forces on RPD may be achieved by a brief review of the development of forces. Thisbrief review of the development of forces. This considers the application of mathematicalconsiders the application of mathematical formulas.formulas. • As Tylman states, “ great caution and reserveAs Tylman states, “ great caution and reserve are essential whenever an attempt is made toare essential whenever an attempt is made to interpret biological phenomenon entirely byinterpret biological phenomenon entirely by mathematical computation.”mathematical computation.” www.indiandentalacademy.comwww.indiandentalacademy.com
  6. 6. RPD as a simple machineRPD as a simple machine • LeverLever • WedgeWedge • Inclined planeInclined plane • ScrewScrew • Wheel and axleWheel and axle • pulleypulley www.indiandentalacademy.comwww.indiandentalacademy.com
  7. 7. LeverLever • In its simplest form, a lever is a rigid barIn its simplest form, a lever is a rigid bar supported somewhere along its length. Itsupported somewhere along its length. It may rest on the support or may bemay rest on the support or may be supported from above.supported from above. • There are three types of levers;There are three types of levers; www.indiandentalacademy.comwww.indiandentalacademy.com
  8. 8. First class leverFirst class lever www.indiandentalacademy.comwww.indiandentalacademy.com
  9. 9. Second class leverSecond class lever www.indiandentalacademy.comwww.indiandentalacademy.com
  10. 10. Third class leverThird class lever www.indiandentalacademy.comwww.indiandentalacademy.com
  11. 11. Inclined planeInclined plane • The inclined plane is a plane surface setThe inclined plane is a plane surface set at an angle, other than a right angle,at an angle, other than a right angle, against a horizontal surface . The inclinedagainst a horizontal surface . The inclined plane permits one to overcome a largeplane permits one to overcome a large resistance by applying a relatively smallresistance by applying a relatively small force through a longer distance than theforce through a longer distance than the load is to be raised.load is to be raised. www.indiandentalacademy.comwww.indiandentalacademy.com
  12. 12. www.indiandentalacademy.comwww.indiandentalacademy.com
  13. 13. WedgeWedge • A wedge by definition is a solid object withA wedge by definition is a solid object with a broad base and its two sides arising toa broad base and its two sides arising to intersect each other forming an acuteintersect each other forming an acute angle opposite the base.angle opposite the base. www.indiandentalacademy.comwww.indiandentalacademy.com
  14. 14. Movement of the denture aroundMovement of the denture around an axisan axis • The imaginary line passing through the teethThe imaginary line passing through the teeth and direct retainers, around which the dentureand direct retainers, around which the denture rotates slightly when subjected to various forcesrotates slightly when subjected to various forces directed toward or away from the residual ridgedirected toward or away from the residual ridge is called a fulcrum line.is called a fulcrum line. • There can be three fulcrums around which aThere can be three fulcrums around which a RPD may rotate. Most of these movementsRPD may rotate. Most of these movements occur simultaneously.occur simultaneously. www.indiandentalacademy.comwww.indiandentalacademy.com
  15. 15. Denture rotation along the fulcrum lineswww.indiandentalacademy.comwww.indiandentalacademy.com
  16. 16. One fulcrum ( the most commonlyOne fulcrum ( the most commonly identified).identified). www.indiandentalacademy.comwww.indiandentalacademy.com
  17. 17. Second fulcrum lineSecond fulcrum line www.indiandentalacademy.comwww.indiandentalacademy.com
  18. 18. Third fulcrum lineThird fulcrum line www.indiandentalacademy.comwww.indiandentalacademy.com
  19. 19. The second and third fulcrums induce horizontalThe second and third fulcrums induce horizontal stresses. These stresses are better toleratedstresses. These stresses are better tolerated when they occur close to the horizontal axis ofwhen they occur close to the horizontal axis of rotation of the toothrotation of the tooth.. www.indiandentalacademy.comwww.indiandentalacademy.com
  20. 20. Factors influencing magnitude ofFactors influencing magnitude of stresses transmitted to thestresses transmitted to the abutment teeth.abutment teeth. • 1.Length of the span.1.Length of the span. • The potential of theThe potential of the lever system tolever system to relatively magnify arelatively magnify a force is illustrated as-force is illustrated as- mechanicalmechanical advantage= effortadvantage= effort arm/ resistance armarm/ resistance arm www.indiandentalacademy.comwww.indiandentalacademy.com
  21. 21. • 2. Quality of support2. Quality of support of ridgeof ridge www.indiandentalacademy.comwww.indiandentalacademy.com
  22. 22. • 3.Qualities of the3.Qualities of the claspclasp • Clasp designClasp design • Length of theLength of the claspclasp • Material used inMaterial used in clasp constructionclasp construction www.indiandentalacademy.comwww.indiandentalacademy.com
  23. 23. • 4. Abutment tooth surface4. Abutment tooth surface • The surface of a gold crown or restorationThe surface of a gold crown or restoration offers more frictional resistance to claspoffers more frictional resistance to clasp arm movement than does the enamelarm movement than does the enamel surface of tooth. Therefore, greater stresssurface of tooth. Therefore, greater stress is exerted on a tooth restored with goldis exerted on a tooth restored with gold than on tooth with intact enamel.than on tooth with intact enamel. www.indiandentalacademy.comwww.indiandentalacademy.com
  24. 24. • 5. Occlusal harmony5. Occlusal harmony • Occlusion with deflective occlusal contactsOcclusion with deflective occlusal contacts between opposing teeth generatesbetween opposing teeth generates horizontal forces which magnify by thehorizontal forces which magnify by the factor of leverage and transmit destructivefactor of leverage and transmit destructive forces to the abutment teeth and residualforces to the abutment teeth and residual ridges.ridges. • Opposing occlusion influences stress.Opposing occlusion influences stress. www.indiandentalacademy.comwww.indiandentalacademy.com
  25. 25. • Area of the denture base against whichArea of the denture base against which the force is applied.the force is applied. • Artificial teeth should be arranged so thatArtificial teeth should be arranged so that the bulk of the masticating force is appliedthe bulk of the masticating force is applied to the center of the denture bearing area,to the center of the denture bearing area, both anteroposteriorly and buccolingually.both anteroposteriorly and buccolingually. www.indiandentalacademy.comwww.indiandentalacademy.com
  26. 26. Amount of stress transferred toAmount of stress transferred to ridges and abutment teeth-ridges and abutment teeth- • The direction and magnitude of forceThe direction and magnitude of force • The length of the lever armsThe length of the lever arms • The quality of resistance ( support fromThe quality of resistance ( support from the edentulous ridges and remainingthe edentulous ridges and remaining natural teeth).natural teeth). • The design characteristics of the partialThe design characteristics of the partial denture.denture. www.indiandentalacademy.comwww.indiandentalacademy.com
  27. 27. CONTROLLING STRESS BYCONTROLLING STRESS BY DESIGN CONSIDERATIONSDESIGN CONSIDERATIONS www.indiandentalacademy.comwww.indiandentalacademy.com
  28. 28. DIRECT RETENTION;DIRECT RETENTION; • Retentive clasp armRetentive clasp arm • Forces of adhesion and cohesionForces of adhesion and cohesion • Frictional controlFrictional control • Neuromuscular controlNeuromuscular control • Clasp positionClasp position • Clasp design –Clasp design – • 1. reverse circumferential cast clasp1. reverse circumferential cast clasp • 2. bar clasp2. bar clasp • 3.Combination clasp3.Combination clasp • Splinting of abutment teethSplinting of abutment teeth www.indiandentalacademy.comwww.indiandentalacademy.com
  29. 29. INDIRECT RETENTIONINDIRECT RETENTION • It assists in direct retention.It assists in direct retention. • Moves the fulcrum away from the force.Moves the fulcrum away from the force. • The necessity and design of the indirect retainerThe necessity and design of the indirect retainer varies with each type of prosthesis.varies with each type of prosthesis. • As the rotation axis is moved anteriorly or awayAs the rotation axis is moved anteriorly or away from the edentulous area, the arc of rotationfrom the edentulous area, the arc of rotation becomes greater and more of the denture basebecomes greater and more of the denture base force is delivered in a vertical direction.force is delivered in a vertical direction. www.indiandentalacademy.comwww.indiandentalacademy.com
  30. 30. Increases the length of the resistance arm www.indiandentalacademy.comwww.indiandentalacademy.com
  31. 31. OCCLUSIONOCCLUSION • The occlusion provided should beThe occlusion provided should be harmonious with movements of the TMJharmonious with movements of the TMJ and the neuromusculature.and the neuromusculature. • Natural teeth should guide theNatural teeth should guide the movements.movements. • The number and buccolingual width ofThe number and buccolingual width of teeth may be reducedteeth may be reduced • Steep cuspal inclines should be avoidedSteep cuspal inclines should be avoided.. www.indiandentalacademy.comwww.indiandentalacademy.com
  32. 32. DENTURE BASE:DENTURE BASE: • Should cover as much of the areaShould cover as much of the area possible within physiologic limitspossible within physiologic limits • Flanges for horizontal stabilizationFlanges for horizontal stabilization • Accurate adaptation and relineableAccurate adaptation and relineable • Retromolar pad and TuberositiesRetromolar pad and Tuberosities coverage in addition to providing supportcoverage in addition to providing support can contribute to indirect retention.can contribute to indirect retention. • Polished surface should assist retention.Polished surface should assist retention. www.indiandentalacademy.comwww.indiandentalacademy.com
  33. 33. MAJOR CONNECTOR.MAJOR CONNECTOR. • Should contribute to load distributionShould contribute to load distribution whenever called for.whenever called for. • A lingual plate major connector supportedA lingual plate major connector supported by rests is the one preferred in theby rests is the one preferred in the mandibular arch and the full palatal platemandibular arch and the full palatal plate major connector for the maxillary arch.major connector for the maxillary arch. • Basic principles for design is valid.Basic principles for design is valid. www.indiandentalacademy.comwww.indiandentalacademy.com
  34. 34. MINOR CONNECTORSMINOR CONNECTORS • With guiding planes offers horizontalWith guiding planes offers horizontal stability to the partial denture.stability to the partial denture. • The abutment tooth receives stabilizationThe abutment tooth receives stabilization against lateral stresses.against lateral stresses. • When the number of teeth contactedWhen the number of teeth contacted increases, the force resisted by each toothincreases, the force resisted by each tooth is minimized.is minimized. www.indiandentalacademy.comwww.indiandentalacademy.com
  35. 35. RESTSRESTS • Should direct forces along the long axis of theShould direct forces along the long axis of the toothtooth • Should not act as an inclined plane or wedgeShould not act as an inclined plane or wedge • Should provide free ball and socket movement.Should provide free ball and socket movement. • Moving the rest position from a distal to a mesialMoving the rest position from a distal to a mesial location on one tooth can change the direction oflocation on one tooth can change the direction of movement of the retainer 180 degrees.movement of the retainer 180 degrees. www.indiandentalacademy.comwww.indiandentalacademy.com
  36. 36. • Retainer position must always beRetainer position must always be coordinated with the rest position so thatcoordinated with the rest position so that the retainer disengages from tooth contactthe retainer disengages from tooth contact during movement of the prosthesis.during movement of the prosthesis.www.indiandentalacademy.comwww.indiandentalacademy.com
  37. 37. Guide planesGuide planes Maximum contact minimum contact optimum contact www.indiandentalacademy.comwww.indiandentalacademy.com
  38. 38. STRESS BREAKERS( STRESSSTRESS BREAKERS( STRESS EQUALIZERS)EQUALIZERS) • It separates the action of the retaining elementsIt separates the action of the retaining elements from the movement of the denture base byfrom the movement of the denture base by allowing independent movement of the dentureallowing independent movement of the denture base.base. • They effectively dissipate vertical stresses but atThey effectively dissipate vertical stresses but at the cost of horizontal stability.the cost of horizontal stability. • Disadvantages:Disadvantages: • 1. excessive ridge resorption1. excessive ridge resorption • 2. tissue impingement2. tissue impingement • 3. inefficient mastication3. inefficient mastication www.indiandentalacademy.comwww.indiandentalacademy.com
  39. 39. • Examples:Examples: • Split bar major connector, TiconiumSplit bar major connector, Ticonium Hidden- lock RPD, hinge type of stressHidden- lock RPD, hinge type of stress breaker, Dalbo extra coronal retainer.breaker, Dalbo extra coronal retainer. www.indiandentalacademy.comwww.indiandentalacademy.com
  40. 40. OTHER DESIGNOTHER DESIGN CONSIDERATIONSCONSIDERATIONS • OVERLAY ABUTMENT: salvaging of anOVERLAY ABUTMENT: salvaging of an existant tooth using endodontic,existant tooth using endodontic, periodontal or a combination of treatmentsperiodontal or a combination of treatments should be considered. The tooth shapedshould be considered. The tooth shaped into a dome can provide better supportinto a dome can provide better support and decrease resorption of the ridges.and decrease resorption of the ridges. www.indiandentalacademy.comwww.indiandentalacademy.com
  41. 41. Essentials of partial dentureEssentials of partial denture design.design. www.indiandentalacademy.comwww.indiandentalacademy.com
  42. 42. First step- determine supportFirst step- determine support for the partial denturefor the partial denture • ABUTMENT TOOTHABUTMENT TOOTH • periodontal healthperiodontal health • crown and root morphologiescrown and root morphologies • crown to root ratioscrown to root ratios • bone index areasbone index areas • location of the tooth in the archlocation of the tooth in the arch • relationship to other support unitsrelationship to other support units • the opposing dentitionthe opposing dentition www.indiandentalacademy.comwww.indiandentalacademy.com
  43. 43. • RESIDUAL RIDGESRESIDUAL RIDGES • quality of the residual ridgequality of the residual ridge • Extent of residual ridge coverage by the dentureExtent of residual ridge coverage by the denture basebase • Type of impression registrationType of impression registration • Accuracy of the denture baseAccuracy of the denture base • Design of the partial denture frameworkDesign of the partial denture framework • Total occlusal load applied.Total occlusal load applied. www.indiandentalacademy.comwww.indiandentalacademy.com
  44. 44. Second step- to connect the toothSecond step- to connect the tooth and tissue support unitsand tissue support units • Involves the designing of major and minorInvolves the designing of major and minor connectors in compliance with basicconnectors in compliance with basic principles and conceptsprinciples and concepts www.indiandentalacademy.comwww.indiandentalacademy.com
  45. 45. Third step- to determine how theThird step- to determine how the partial denture is to be retainedpartial denture is to be retained • DIRECT RETENTION:The key toDIRECT RETENTION:The key to selecting a successful clasp design is toselecting a successful clasp design is to choose a clasp that will-choose a clasp that will- • 1. avoid direct transmission of tipping or1. avoid direct transmission of tipping or torquing forces to the abutment.torquing forces to the abutment. • 2. accommodate the basic principles of2. accommodate the basic principles of clasp designclasp design • www.indiandentalacademy.comwww.indiandentalacademy.com
  46. 46. • 3. provide retention against reasonable3. provide retention against reasonable dislodging forcesdislodging forces • 4. be compatible with undercut location,4. be compatible with undercut location, tissue contour and esthetic desires of thetissue contour and esthetic desires of the patient.patient. www.indiandentalacademy.comwww.indiandentalacademy.com
  47. 47. INDIRECT RETENTIONINDIRECT RETENTION • To counteract any lifting of the distalTo counteract any lifting of the distal extension base away from the supportingextension base away from the supporting tissues.tissues. • Resistance to anteroposteriorResistance to anteroposterior displacement by-displacement by- • 1. clasps encircling M-D convexity1. clasps encircling M-D convexity • 2. continuous clasps2. continuous clasps • 3. onlays3. onlays • 4. embrasure hooks4. embrasure hookswww.indiandentalacademy.comwww.indiandentalacademy.com
  48. 48. FOURTH STEP- to connect theFOURTH STEP- to connect the retention units to the support unitsretention units to the support units • The direct and indirect retainers are rigidlyThe direct and indirect retainers are rigidly attached to the major connector.attached to the major connector. • The components designed to contact theThe components designed to contact the tooth surfaces may be prepared to act astooth surfaces may be prepared to act as guide planes.guide planes. www.indiandentalacademy.comwww.indiandentalacademy.com
  49. 49. FIFTH STEP- to outline and join theFIFTH STEP- to outline and join the edentulous area to the alreadyedentulous area to the already established design componentsestablished design components • Support is the primary concern in selectingSupport is the primary concern in selecting designing and fabricating a distaldesigning and fabricating a distal extension denture base.extension denture base. • Of secondary importance are- esthetics,Of secondary importance are- esthetics, stimulation of the underlying tissues andstimulation of the underlying tissues and oral cleanliness.oral cleanliness. www.indiandentalacademy.comwww.indiandentalacademy.com
  50. 50. Conclusion:Conclusion: Adequate time must be spent in properlyAdequate time must be spent in properly diagnosing the particular case and devising adiagnosing the particular case and devising a workable treatment plan. Once this is doneworkable treatment plan. Once this is done the mechanical task of laying out the “nutsthe mechanical task of laying out the “nuts and bolts” of removable partial dentureand bolts” of removable partial denture design can be carried out to ensure that ourdesign can be carried out to ensure that our patients receive full benefits of our knowledgepatients receive full benefits of our knowledge and skillsand skills.. Any plan conceived in moderation mustAny plan conceived in moderation must fail when the circumstnaces are set infail when the circumstnaces are set in extremes.extremes. www.indiandentalacademy.comwww.indiandentalacademy.com
  51. 51. REFERENCESREFERENCES • Removable partial prosthodontics- McRemovable partial prosthodontics- Mc Crackens 11Crackens 11thth eded • Clinical removable partial prosthodontics-Clinical removable partial prosthodontics- Stewart 2Stewart 2ndnd eded • Partial dentures- John Osborne 4Partial dentures- John Osborne 4thth eded • Textbook of removable partial dentures-Textbook of removable partial dentures- KratochvilKratochvil • Removable partial prosthodontics-Removable partial prosthodontics- Grosso- Miller 3Grosso- Miller 3rdrd eded • Combinations in fixed- removableCombinations in fixed- removable prostheses- JPD1971,26prostheses- JPD1971,26www.indiandentalacademy.comwww.indiandentalacademy.com
  52. 52. www.indiandentalacademy.comwww.indiandentalacademy.com

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