Removable Partial Dentures Direct Retainers

1. Removable Partial DenturesDirect Retainers

2. Direct Retainer • It is that component of removable partial denture that is used to retain and prevent dislodgement consisting of a clasp assembly or precision attachment

3. There are two types of direct retainers 1.Intra-coronal Within the contour of the crown e.g. precision attachment 2.Extra-coronal Outside the contour of the crown of the tooth e.g. clasp

4. Intracoronal retainer consists of a key and keyway

5. Extracoronal circumferential direct retainer

6. Basic Requirements • All clasps must be designed so that they satisfy the following six basic requirements: • 1.Retention • 2.Support • 3.Stability • 4.Reciprocation • 5.Encirclement • 6.Passivity

7. 1.Retention • Retention is provided by the retentive arm which prevents the partial denture from displacement away from the tissues toward the occlusal. • While the partial denture is seated the retentive tip must be passive. • However, it should touch the tooth in the undercut area.

8. 2.Support • Support in a clasp is generally provided by the rest. • Thus, while chewing food the rest prevents tissue ward movement of the clasp assembly, plus directs the force along the long axis of the tooth, thus reduces periodontal tissue damage.

9. 3.Bracing • Bracing is the resistance to horizontal forces. • This provided by the rest, minor connector and bracing arm. • Actually, the bracing arm mainly provides reciprocation which is resistance to force applied by retentive tip.

10. 4.Encirclement There must be more than 180 degrees crown coverage otherwise, the clasp can slip-off the abutment tooth. 5.Passivity The retentive clasp arm should be passive (no active force) until a dislodging force is applied.

12. Factors that determine the amount of retention provided by a particular clasp arm 1.The size of the angle of the cervical convergence. 2.How far into the cervical convergence the clasp terminal is placed. 3.Flexibility of the clasp arm. Flexibility of the clasp arm: 1.The length of the clasp arm. 2.The diameter of the clasp arm. 3.The cross-sectional form. 4.The type of metal used.

13. Other factors about clasp design: 1.In a clasp only the tip of the retentive arm should be flexible, the other components are rigid 2.Selection of clasp will depend on the survey line (height of contour), retention area available, type of partial denture i.e. whether it is tooth-supported or distal extension removable partial denture. 3.Flexibility of clasp arm can be increased by curving and/or lengthening the arm.

14. 4.The thickness of the retentive clasp arm tip should be ½ the thickness at its attachment to the body of the clasp arm 5.The greater the diameter of the clasp arm, the lesser will be the flexibility and vice versa. 6.Wrought metal is more flexible than cast metal. 7.If buccal retention is used on the left side of the arch, buccal retention must be used on the right side of the arch

16. Classification of extra-coronal retainers: 1.Supra bulge clasps (occlusally approaching, circumferential or encircling clasps). The retentive arm approaches the undercut area from the suprabulge direction. 2.Infrabulge clasps (gingivally approaching, projection or bar clasps) The retentive arm approaches the undercut from the infrabulge direction,e.g.Bar clasp arm (I-Bar etc.) 3.Combination clasps a. ½ clasp is circlet and ½ clasp is Bar type or b. Combination of two types of metals-Chrome-cobalt an wrought wire

17. Thank you…..