Removable Partial Dentures

Direct RetainersDr.Mohammad Al Sayed19l4/2008

There are two types of direct retainers

1.Intra-coronalWithin the contour of the crown e.g. precision attachment

2.Extra-coronalOutside the contour of the crown of the tooth e.g. clasp

Intracoronal retainer consists of a key and keyway

Extracoronal circumferential direct retainer

Basic RequirementsAll clasps must be designed so that they satisfy the following six basic requirements:

1.Retention2.Support3.Stability4.Reciprocation5.Encirclement6.Passivity

1.RetentionRetention 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.

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

3.BracingBracing 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.

4.EncirclementThere must be more than 180 degrees crown coverage otherwise, the clasp can slip-off the abutment tooth.

5.PassivityThe retentive clasp arm should be passive (no active force) until a dislodging force is applied.

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.

Other factors about clasp design:1.In a clasp only the tip of the retentive arm should be flexible, the other components are rigid2.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 extention removable partial denture.3.Flexibility of clasp arm can be increased by curving and/or lengthening the arm.4.The thickness of the retentive clasp arm tip should be the thickness at its attachment to the body of the clasp arm5.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

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 claspsa. clasp is circlet and clasp is Bar type orb. Combination of two types of metals-Chrome-cobalt an wrought wire

Types of circumferential clasps1.Circlet clasp.2.Half & half clasp.3.Fish hook or hairpin clasp.4.Reverse circlet clasp.5.Embrasure clasp.6.Ring clasp.7.Back action and reverse back action clasp.

1.Circlet clasp.- Is 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 the edentulous area, and a flexible retentive arm ending in an undercut of 0.01 inch.- It may be used on canines, premolars, and molars.

Variations of circlet clasp are:a) Back action clasp: Provides poor bracing due to its length. It is used on premolars (use .02 inch undercut) b) Reverse back action: Minor connector is on buccal, otherwise similar to back action.c) Ring clasp: Variation of back action clasp usually used on upper buccally tilted and lower lingually tilted molars. d) Other variations of simple circlet clasp C clasp or hair-pin clasp.

The half and half clasp: 2.The half and half clasp is a modification of the circlet clasp with the reciprocal arm coming from one direction and the retentive arm from the other. Two rests are used for this clasp. It is used on molars and premolars. The retentive tip is placed into a 0.01 inch undercut.

3. The fish hook or hair-pin claspThe fishhook or hairpin clasp is another modification of the circlet clasp. It is mostly used on teeth with long crowns. It is rarely used because so much of the teeth is covered by the retentive arm. The other difficulty is lack of flexibility of the retentive tip because of the bulk of the clasp.

4. The reverse circlet clasp:The reverse circlet clasp is used when the retentive undercut is located on the surface of the abutment tooth adjacent to the edentulous space.It is indicated in class I & class II distal extension partial dentures where deep tissue undercut precludes the use of infrabulge clasp.

5. Embrasure clasp.The embrasure clasp is used when there is no modification spaces in the portion of the arch that needs retention. The clasp is in fact two circlet clasps back-to-back. It may be used on two molars, a molar and premolar or two premolars. Preparation of the double rest and channel going from the lingual to the buccal of the teeth needs to be deep enough for strength and not compromised by the opposing occlusion.

6. The ring clasp.The ring clasp is used on molars. Some designers advocate its use for tipped mandibular molars where there is only a mesial undercut on the tooth. It has mesial and distal rests and the reciprocal arm is continuous connecting the two rests.