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DIRECT RETAINERs By Dr hisham mously

Part 2 . DIRECT RETAINERs By Dr hisham mously. TYPES OF CLASP ASSEMBLIES : Clasps Designed to Accommodate Functional Movement 1. RPI, RPA and Bar clasp 2. Combination clasp .

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DIRECT RETAINERs By Dr hisham mously

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  1. Part 2 DIRECT RETAINERsBy Drhishammously

  2. TYPES OF CLASP ASSEMBLIES: Clasps Designed to AccommodateFunctional Movement 1. RPI, RPA and Bar clasp 2. Combination clasp

  3. Distal extension acts as a long “effort arm” across the distal rest” fulcrum” results in harmful tipping or torquing of the tooth. Two strategies may be adopted to change the fulcrum location (mesial rest concept clasp assemblies), or use of a flexible arm (wrought-wire retentive arm). Mesial rest includes the RPI and RPA mesio-occlusal rest with distal guiding plane. Proximal plate, in conjunction with the minor connector supporting the rest, provides the stabilizing and reciprocal aspects of the clasp assembly. The I-bar gingival third of the buccal or labial surface of 0.01-inch, no more than 2 mm of its tip contacting. RPI, RPA, and Bar Clasp

  4. Three basic approaches RPI : location of the rest, the design of minor connector (proximal plate) and location of the retentive arm. Entire length of the proximal tooth surface, from the marginal ridge to the junction of the middle and gingival thirds. Contraindications for a bar-type (exaggerated buccal or lingual tilts, severe tissue undercut, or a shallow buccal vestibule) modification should be considered for the RPI system (the RPA clasp). RPI, RPA, and Bar Clasp

  5. The term bar clasp is generally preferred over the less descriptive term Roach clasp arm. The bar clasp arm has been classified by the shape of the retentive terminal, T, modified T, I, or Y. If a tissue undercut prevents the use of a bar clasp arm, a mesially originating ring clasp, a cast, or a wrought-wire clasp or reverse-action clasp may be used. Indications for use of a bar clasp arm (1) a small degree of undercut (0.01 inch) in the cervical third. (2) tooth-supported partial dentures. (3) distal extension base situations. and (4) in situations in which esthetic considerations. RPI, RPA, and Bar Clasp

  6. To reduce the effect of the Class I lever in distal extension. The combination clasp consists of a wrought-wire retentive clasparm and a cast reciprocal clasp arm. Retentive arm is almost circumferential, but may be used in the manner of a bar. Advantages of the combination clasp include Flexibility (abutment tooth adjacent to a distal extension base or weak abutment when a bar-type direct retainer is contraindicated) and adjustability and the appearance. Combination Clasp

  7. The most common use of the combination clasp is on an abutment tooth adjacent tooth adjacent to a distal extension base where only a mesial undercut exists on the abutment or where a large tissue undercut contraindicates a bar-type retainer. Disadvantages: (1) extra steps in fabrication. (2) distorted by careless handling; (3) less accurately adapted therefore may provide less stabilization. The disadvantages listed previously should not be prevent its use, problems are minimized by selecting the best wrought wire. Combination Clasp

  8. TYPES OF CLASP ASSEMBLIES: Clasps Designed Without Movement Accommodation 1. Circumferential clasp 2. Ring clasp 3. Embrasure clasp 4. Back action clasp 5. Multiple clasp 6. Half-and-half clasp 7. Reverse- action clasp

  9. Most logical clasp to use with all tooth-supported partial dentures because of its retentive and stabilizing ability. Disadvantages: 1. More tooth surface is covered. 2. Occlusal approach may increase the width of the occlusal surface of the tooth. 3. Half-round form prevents adjustment to increase or decrease retention. Circumferential Clasp ( Aker clasp)

  10. The basic form of the circumferential clasp is a buccal and lingual arm used ( one retentive clasp arm, opposed by a nonretentive reciprocal arm). Circumferential Clasp

  11. Circumferential type, which encircles nearly all of a tooth from its point of origin used when a proximal undercut cannot be approached by other means. Supporting strut on the nonretentive side, with or without an auxiliary occlusal rest on the opposite marginal ridge. The advantage of an auxiliary rest is that prevent further movement. Ring Clasp

  12. Unmodified Class II or Class III partial denture, no edentulous spaces are available on the opposite side. Sufficient space must be provided between the abutment teeth, yet the contact area should not be eliminated entirely, based on the patient’s age, caries index, oral hygiene, tooth contours, used with double occlusal rests, to avoid interproximal wedging by the prosthesis, always be used whenever food impaction is possible, two retentive clasp arms and two reciprocal clasp arms. Embrasure Clasp

  13. Back-action Clasp: Modification of the ring clasp; difficult to justify. Multiple Clasp: Two opposing circumferential clasps joined at the terminal end of the two reciprocal arms. When additional retention and stabilization are needed.

  14. Half-and-half Clasp: Consists of a circumferential retentive arm arising from one direction and a reciprocal arm arising from another. The second arm must arise from a second minor connector, with or without an auxiliary occlusal rest. Reverse-action Clasp Hairpin; to allow a proximal undercut to be engaged from an occlusal approach same use of a ring clasp.

  15. The teeth that are engaged by a removable partial denture must be contoured to support, stabilize, and retain the functioning prosthesis. Critical areas of an abutment that provide for retention and stabilization (reciprocation) can only be identified with the use of a dental cast surveyor. Relationship of the vertical arm to the cast represents the path of placement ANALYSIS OF TOOTH CONTOURS FOR RETENTIVE CLASPS

  16. Identifies the location on the clinical crown where the greatest convexity exists. This line, called the height of contour (specific to the surveyor-defined path) This surveyor-defined path and the subsequent tooth height of contour will indicate the areas available for retention an those available for support, and the existence of tooth and other tissue interference to the path of Placement. ANALYSIS OF TOOTH CONTOURS FOR RETENTIVE CLASPS

  17. The importance of angle of cervical convergence lies in its relationship to the amount of retention ANALYSIS OF TOOTH CONTOURS FOR RETENTIVE CLASPS

  18. Clasp retention is based on the resistance to deformation of the metal. For a clasp to be retentive, it must be placed in an undercut area of the tooth. Dependent on several factors: Tooth factors are the size of the angle of cervical convergence (depth of undercut) and how far the clasp terminal is placed into the angle of cervical convergence. Prosthesis factors include the flexibility of the clasp arm ( length, diameter , cross-sectional form or shape, and the material used in making the clasp). AMOUNT OF RETENTION

  19. To be retentive, a tooth must have an angle of convergence cervical to the height of contour. This line, which Kennedy called the height of contour, is its greatest convexity. DeVan added the terms suprabulge, denoting the surfaces sloping superiorly, and infrabulge, denoting the surfaces sloping inferiorly. Any areas cervical to the height of contour may be used for the placement of retentive clasp components, whereas areas occlusal to the height of contour may be used for the placement of nonretentive, stabilizing, or reciprocating components. The location and depth of a tooth undercut available for retention are therefore only relative to the path of placement and removal of the partial denture. Size of and Distance Into the Angle of Cervical Convergence

  20. The most suitable path of placement is generally considered to be the path of placement that will require the least amount of mouth preparation necessary to place the components of the partial denture in their ideal position on the tooth surfaces and in relation to the soft tissue. The path of placement also must take into consideration the presence of tissue undercuts that would interfere with the placement of major connectors, the location of vertical minor connectors, the origin of bar clasp arms, and the denture bases. Guiding planes control the path of placement and removal, they can also provide additional retention. If some degree of parallelism does not exist during placement and removal, trauma to the teeth and supporting structures. Size of and Distance Into the Angle of Cervical Convergence

  21. The longer the clasp arm the more flexible. Measured from the point at which a uniform taper begins. Bar clasp arm will usually be longer than a circumferential clasp arm. Diameter of Clasp Arm The greater the average diameter of a clasp arm the less flexible. Diameter will be at a point midway between its origin and its terminal end. Length of Clasp Arm

  22. The only universally flexible form is the round form, which is practically impossible to obtain by casting and polishing. Cast retentive clasp arms are more acceptable in tooth-supported partial dentures in which they are called on to flex only during placement and removal of the prosthesis. A round clasp is the only circumferential clasp form that may be safely used to engage a tooth undercut on the side of an abutment tooth away from the distal extension base. Cross-sectional Form of the Clasp Arm

  23. Cast gold is that its bulk must be increased to obtain the needed rigidity. Greater rigidity with less bulk is possible through the use of chromium-cobalt alloys. Wrought-wire clasp arm has toughness exceeding that of a cast clasp arm. The tensile strength of a wrought structure is at least 25% greater than that of the cast alloy from which it was made. It may therefore be used in smaller diameters to provide greater flexibility without fatigue. Material Used for the Clasp Arm

  24. Depend on the location of the retentive part of the clasp arm, not in relation to the height of contour, but in relation to the angle of cervical convergence. The retention on all principal abutments should be as equal as possible. Relative Uniformity of Retention

  25. When the direct retainer comes into contact with the tooth, the framework must be stabilized against horizontal movement for the required clasp deformation to occur. Derived from either cross-arch framework contacts or a stabilizing or reciprocal clasp in the same clasp assembly. Must be in contact during the entire period of retentive clasp deformation. Diameter must be greater than the average diameter of the opposing retentive arm to increase desired rigidity. Stabilizing-reciprocal Cast Clasp Arm

  26. Minor Connectors • Connects components to the major connector • Direct retainer • Indirect retainer • Denture base

  27. Minor Connectors

  28. Functions of Minor Connectors • Unification and rigidity • Stress distribution • Bracing through contact with guiding planes • Maintain a path of insertion

  29. Types of Minor Connectors • Embrasure Minor Connectors • Between two adjacent teeth

  30. Types of Minor Connectors • Embrasure Minor Connectors • Triangular shaped in cross section • Joins major connector at right angles • Relief placed so connector not directly on soft tissue

  31. Gridwork Minor Connectors • Connect the denture base and teeth to the major connector

  32. Gridwork Minor Connectors • Adjacent edentulous spaces • Usually connect major connector to direct retainers • Open lattice work or mesh types

  33. Gridwork Minor Connectors • Mesh type • Flatter • Potentially more rigid • Less retention for acrylic if openings are small

  34. Gridwork Minor Connectors • Lattice Type • Potentially superior retention • Interferes with setting of teeth, if struts are too thick • Both types are acceptable if correctly designed

  35. Gridwork Relief • Mechanical retention of denture base resin • Allows the acrylic resin to flow under the gridwork

  36. Gridwork Relief • Relief wax is placed in the edentulous areas • 1 mm of relief

  37. Junction With Major Connector • Butt joint with slight undercut in metal • Maximum bulk of the acrylic resin • Prevents thin, weak edges fracturing

  38. Mandibular Gridwork Design • Extend 2/3 of the way from abutment tooth to retromolar pad • Never on the ascending portion of the ridge Stewart's, Fig. 2-55

  39. Maxillary Gridwork Design • Gridwork • 2/3 of the length of from abutment to the hamular notch • Major connector • extends fully to the hamular notch

  40. Gridwork Design Facially just over the crest of the residual ridge

  41. Mandibular Tissue Stops • Contact of metal with cast at posterior of distal extension gridwork • Prevents distortion at free end during hydraulic pressure of processing

  42. No Tissue Stops In Maxilla • Maxillary major connector acts as a tissue stop (no relief)

  43. Proximal Plates • Minor connectors originating from the gridwork in an edentulous area • Broad contact with guiding planes • May or may not terminate in an occlusal rest

  44. Proximal Plates

  45. Proximal Plates • Shifted slightly lingually • Increases rigidity • Enhances reciprocation • Improves esthetics • Often a triangular space below the guiding plane (an undercut)

  46. Proximal Plates • Rigid, cannot be placed in undercut • Block-out placed in undercuts prior to waxing and casting the framework

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