Introduction
The clasp is the most common direct retainer of a partial denture. It partly encircles an abutment tooth and holds the denture with the flexible tip of its arm in the undercut.
Parts of a clasp
Each clasp should be supported by a rest, the arms together should encircle more than 180° of the tooth, and molar ring clasps need rests both mesially and distally.
Wrought wire clasps
Made from drawn wire of round cross-section (stainless steel or other alloys), usually 0.8–1.0 mm thick, bent with pliers on the cast. They are cheap, soft, easy to shape, adjust and repair, and are used in acrylic resin partial dentures and in combination clasps on cast frameworks.
Preparing the cast
Survey the cast and draw the tooth's equator (survey line) with graphite, marking the zones above and below it. The clasp arm must pass through three of these zones.
Common wire clasps
Acrylic resin clasps
Tooth- or gum-coloured clasps made of resin, for example Kemeny dentoalveolar clasps, which rest on the tooth and the alveolar mucosa. Modern flexible dentures use nylon or other thermoplastic clasps.