Direct Retainers (clasps)

By Dr.Swathi Pai on Saturday 17 January 2009 with 0 comments



They counteract dislodging forces at right angles to the occlusal plane.


To be retentive, the terminal end of the retentive clasp must be engaged in areas which are undercut with relation to the path of insertion of the RPD or there will be no resistance to dislodging forces (gravity or sticky foods).


Clasp requirements:

Support- proper rests

Bracing (stability)- minor connectors

Retention- minimum necessary

Reciprocation- reciprocal clasp arm

Encirclement- more than 180 to prevent tooth movement

Passivity- no active force by clasp in place


Types of clasps:

Suprabulge (Aker’s, circumferential) - approaches undercut from above the height of contour.

Infrabulge (Roach or bar) - approaches undercut from under the height of contour.

Combination clasp - circumferential and bar clasp arm or cast clasp arm and wrought wire clasp

Design principles of clasps:

A .010 inch (0.25mm) undercut is the least amount that is measurably effective. Other gauge sizes are .020 and .030 inches. Wrought wire (more flexible) is an .020 undercut.

Any tooth clasped should have a rest!

You must rest next to any edentulous space

Clasp retainers should be at the junction of the middle and gingival 1/3 with the reciprocal arm above this line and the retentive arm below this line!!

The terminal tip of the clasp always points occlusally.

The bracing arm is straight and not tapered.

A minimum of 8mm in length is required for an Aker’s clasp.


The determinants of the resiliency of a clasp:

diameter

length

taper

metal

Category: Prosthodontics Notes

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