Removable partial dentures(direct retiner) (1)

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Removable partial dentures(direct retiner) (1)

  1. 1. Removable Partial Dentures Direct Retainers Dr.Mohammad Al Sayed 19l4/2008
  2. 2. There are two types of direct retainersIntra-coronal.1Within the contour of the crown e.g. precision attachmentExtra-coronal.2Outside the contour of the crown of the tooth e.g. clasp
  3. 3. Intracoronal retainerconsists of a key and keyway
  4. 4. Extracoronal circumferential direct retainer
  5. 5. Basic RequirementsAll clasps must be designed so that they satisfy the:following six basic requirementsRetention.1Support.2Stability.3Reciprocation.4Encirclement.5Passivity.6
  6. 6. Retention.1Retention is provided by the retentive armwhich prevents the partial denture fromdisplacement away from the tissuestoward the occlusal.While the partialdenture is seated the retentive tip must bepassive. However, it should touch the. tooth in the undercut area
  7. 7. Support.2Support in a clasp is generally provided bythe rest. Thus, while chewing food the restprevents tissueward movement of theclasp assembly, plus directs the forcealong the long axis of the tooth, thus. reduces periodontal tissue damage
  8. 8. Bracing.3.Bracing is the resistance to horizontal forcesThis provided by the rest, minor connector andbracing arm. Actually, the bracing arm mainlyprovides reciprocation which is resistance to. force applied by retentive tip
  9. 9. Encirclement.4There must be more than 180 degreescrown coverage otherwise, the clasp can.slip-off the abutment toothPassivity.5The retentive clasp arm should be passive(no active force) until a dislodging force is.applied
  10. 10. Factors that determine the amount of retentionprovided by a particular clasp arm.The size of the angle of the cervical convergence.1How far into the cervical convergence the clasp terminal is .2.placed.Flexibility of the clasp arm.3:Flexibility of the clasp arm.The length of the clasp arm.1.The diameter of the clasp arm.2.The cross-sectional form.3.The type of metal used.4
  11. 11. :Other factors about clasp designIn a clasp only the tip of the retentive arm should be flexible, the other .1components are rigidSelection of clasp will depend on the survey line (height of contour), .2retention area available, type of partial denture i.e. whether it is tooth-.supported or distal extention removable partial dentureFlexibility of clasp arm can be increased by curving and/or lengthening the .3.armThe thickness of the retentive clasp arm tip should be ½ the thickness at its .4attachment to the body of the clasp armThe greater the diameter of the clasp arm, the lesser will be the flexibility .5.and vice versa.Wrought metal is more flexible than cast metal.6If buccal retention is used on the left side of the arch, buccal retention must .7be used on the right side of the arch
  12. 12. :Classification of extra-coronal retainers Supra bulge clasps (occlusally approaching,.1 ).circumferential or encircling clasps The retentive arm approaches the undercut area from the .suprabulge direction Infrabulge clasps (gingivally approaching, projection or.2 )bar clasps The retentive arm approaches the undercut from the .)infrabulge direction,e.g.Bar clasp arm (I-Bar etc Combination clasps.3 a. ½ clasp is circlet and ½ clasp is Bar type or b. Combination of two types of metals-Chrome-cobalt an wrought wire
  13. 13. Types of circumferential clasps.Circlet clasp.1.Half & half clasp.2.Fish hook or hairpin clasp.3.Reverse circlet clasp.4.Embrasure clasp.5.Ring clasp.6Back action and reverse back action .7.clasp
  14. 14. .Circlet clasp.1Is the most common clasp used for -. removable partial dentures.The least complex in design-It has a rigid reciprocal arm, a rest, a - proximal plate approximating theedentulous area, and a flexible retentive.arm ending in an undercut of 0.01 inchIt may be used on canines, premolars, and -. molars
  15. 15. :Variations of circlet clasp area) Back action clasp: Provides poor bracing due to its length. It is used on premolars (use .02) inch undercutb) Reverse back action: Minor connector is on. buccal, otherwise similar to back actionc) Ring clasp: Variation of back action clasp usually used on upper buccally tilted and. lower lingually tilted molarsd) Other variations of simple circlet clasp “ C ” clasp. or hair-pin clasp
  16. 16. .The half and half clasp: 2The half and half clasp is a modification ofthe circlet clasp with the reciprocal armcoming from one direction and theretentive arm from the other. Two restsare used for this clasp. It is used onmolars and premolars. The retentive tip is.placed into a 0.01 inch undercut
  17. 17. The fish hook or hair-pin clasp. 3The fishhook or hairpin clasp is anothermodification of the circlet clasp. It is mostlyused on teeth with long crowns. It is rarelyused because so much of the teeth iscovered by the retentive arm. The otherdifficulty is lack of flexibility of the retentive.tip because of the bulk of the clasp
  18. 18. :The reverse circlet clasp. 4The reverse circlet clasp is used when theretentive undercut is located on thesurface of the abutment tooth adjacent to.the edentulous spaceIt is indicated in class I & class II distalextension partial dentures where deeptissue undercut precludes the use of.infrabulge clasp
  19. 19. .Embrasure clasp. 5The embrasure clasp is used when there isno modification spaces in the portion ofthe arch that needs retention. The clasp isin fact two circlet clasps back-to-back. Itmay be used on two molars, a molar andpremolar or two premolars. Preparation ofthe double rest and channel going fromthe lingual to the buccal of the teeth needsto be deep enough for strength and not. compromised by the opposing occlusion
  20. 20. .The ring clasp. 6The ring clasp is used on molars. Somedesigners advocate its use for tippedmandibular molars where there is only amesial undercut on the tooth. It has mesialand distal rests and the reciprocal arm is. continuous connecting the two rests

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