Introduction
A retainer is any device fixed to an abutment tooth that holds a removable partial denture in place. Together, the retainers give the denture its anchorage: support, bracing and retention.
How a clasp does its job
- Support: the rest sits in a prepared seat and carries the vertical load along the tooth's axis.
- Retention: only the flexible end of the retentive arm crosses below the survey line into the undercut. It flexes past the bulge on insertion and springs back.
- Reciprocation: the rigid arm on the other side, above the survey line, holds the tooth against the push of the retentive arm.
- Encirclement: the arms together grip more than 180° of the tooth, so it can't slip out.
- Passivity: when the denture is seated, the clasp exerts no force.
- Bracing: the rigid parts resist sideways movement.
Fulcrum line and indirect retention
In a free-end denture, the line joining the two main rests is the fulcrum line. Sticky food can lift the free end, rotating the denture around this line. An indirect retainer, a rest placed as far as possible on the other side of the fulcrum line, stops that rotation.