Retention is the ability of a removable partial denture to resist dislodging forces during function. Bracing: it is the resistance to horizontal components of masticatory force
Types of retention in a removable partial denture
Direct retainer: is a unit of a removable partial denture that engages an abutment tooth in such a manner to resist displacement of the prosthesis away from basal seat tissues.
Suprabulge clasp: a removable partial denture retentive clasp that approaches the retentive undercut from an occlusal or suprabulge direction. Infrabulge clasp: a removable partial denture retentive clasp that approaches the retentive undercut from a cervical or infrabulge direction.
Extracoronal retainers
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.
1- Tooth factor The size of the angle of the cervical convergence. How far is the clasp terminal is placed in angle of cervical convergence
2. 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.
Factors that determine the amount of retention provided by a particular clasp
1. length of clasp arm The length is measured from point of origin to the terminal end. The longer the clasp arm the more flexible it will be. (flexibility directly proportion to the cube of the length)
2.Diameter of clasp arm: The greater average diameter of the clasp arm the less flexibility it will be. (flexibility inversely proportion to the diameter of the clasp)
3. Cross sectional form of the clasp arm: Flexibility may exist in any form ( round, half round and other). The cross sectional form affects flexibility. A round clasp has greater flexibility than a half-round clasp with the same diameter. It also has the ability to flex in all plans, whereas a half round clasp normally flexes in only a single plane.
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Because of the internal structure of wrought wire, it has greater ability to flex than cast alloy. To obtain equal retention, therefore, a greater depth of undercut is required for a wrought wire clasp than for a cast clasp.
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3- stabilization or bracing
4-reciprocal
5- encirclement
6-
In addition to encirclement, other basic principles of clasp design are as follows
1- The occlusal rest must be designed to prevent the movement of the clasp arms toward the cervical 2- Each retentive terminal should be opposed by reciprocal component capable of resisting any transient pressures exerted by the retentive arm during placement and removal. Stabilizing and reciprocal components must be rigidly connected bilaterally (cross-arch) to realize reciprocation of the retentive elements. Reciprocation should be provided on a clasped tooth diametrically opposite the retentive clasp tipStabilizing and reciprocal components must be rigidly connected bilaterally (cross-arch)
3-The amount of retention should always be the minimum necessary to resist reasonable dislodging forces 4. Clasp retainers on abutment teeth adjacent to distal extension bases should be designed so that they will avoid direct transmission of tipping and rotational forces to the abutment. 5. The path of escapement for each retentive clasp terminal must be other than parallel to the path of removal for the prosthesis to require clasp engagement with the resistance to deformation this is retention.6. Unless guiding planes will positively control the path of removal and stabilize abutments against rotational movements, retentive clasps should be bilaterally opposed, i.e., buccal retention on one side of the arch should be opposed by buccal retention on the other, or lingual on one side opposed by lingual on the other
7. Reciprocal elements of the clasp assembly should be located at the junction of the gingival and middle third (or above height of contour ) of the crown of abutment teeth. The terminal end of the retentive arm is optimally placed in the gingival third of the crown.
8. Passivity. A clasp in place should be completely passive. The retentive function is activated only when dialoging forces are applied to the partial denture, One of the main causes of pain or tenderness In an abutment tooth following insertion of a partial denture is incomplete seating of the clasp. If the clasp is not seated, the retentive terminal cannot reach the depth of undercut it was planned to reach and therefore always applies force to the tooth producing pain. 9. A retentive occlusally-approaching clasp should run from the side of the tooth with the least undercut to the side with the greatest undercut 10. Occlusally-approaching retentive clasps should have the terminal third of the retentive arm entering the undercut.