Clasp Retention: The Effects of Five Variables

An attempt was made to determine the relative retention of each of the four most commonly used clasp assemblies. They are the rest, proximal plate, 1 bar (RPI); rest, proximal plate, facial circumferential arm (RPA); modified T; and Akers (facial and lingual circumferential arms). A mandibular bilateral distal extension base removable partial denture design was selected because it is most commonly used. A test model was made by removing all mandibular molars from a Columbia Dentoform and replacing the ivorine second bicuspids with natural teeth. These, in turn, were later replaced with gold crowns that were identical in contour to the natural teeth. Eight removable partial dentures were constructed using preformed plastic lingual bars, minor connectors, denture base retention loops, and clasps. Artificial teeth and the denture bases were processed to each framework as for a clinical patient. Each removable partial denture was placed in the Instron testing machine and pulled from the test model. The investigation examined the effectiveness of each of four types of clasps in resisting displacing forces, in both vertical (occlusal) and mesio‐occlusal directions. Each clasp was tested using natural teeth and gold crowns, in dry and wet environments, both with and without indirect retainers. Results indicate that, (1) The RPA clasp was the most retentive of all on natural abutments, against both directions of pull, but only slightly so over the RPI clasp with a vertical pull. (2) The Akers clasp was most retentive against a mesio‐occlusal pull on the gold abutments. (3) Natural tooth structure gives slightly more frictional resistance than a gold surface. (4) The presence of artificial saliva caused little change in forces required for dislodgment. (5) The absence of indirect retainers greatly reduced the retentive properties of the RPD frameworks.