An investigation of the effectiveness of indirect retainers.

A mandibular bilateral distal-extension base removable partial denture was lifted from a model under standardized conditions. The amount of denture base displacement under various loads and the forces of removal were measured electronically. The framework was constructed to allow repeated changes in the type of clasp, location of indirect retainer, and position of the rest seat on the terminal abutment tooth. The usefulness of an indirect retainer in preventing occlusal displacement of the denture base appears to be limited. The type of clasp had the greatest influence on the amount of denture base movement. Proximal surface guiding planes were also found to be important in preventing lifting of the denture base. Use of mesial instead of distal rests on the terminal abutment teeth did not decrease indirect retention.