A new design for partial denture circumferential clasp arms.

Partial dentures made from cast cobalt chromium alloys suffer from inadequacies of the retentive spring members. With similar devices made from Type 4 gold alloy, the retentive members are considered satisfactory. Analyses of the combined modulus and strength properties of the alloys show that the lower modulus of elasticity of the gold doubles its overall effectiveness in spring members such as retentive clasp arms. To match this, the strength of cobalt chromium alloys must be doubled. Since this appears unlikely, improvement to the design of the spring members is the only way to rectify this problem. Retentive clasp arms are required to have adequate elasticity to deflect out of the retentive undercut, adequate stiffness to produce retention and adequate strength to resist accidental damage. Current designs have no published rational basis to meet these requirements, and appear far from ideal. An analysis of the forces applying to clasp arms and design for uniform stress along the arm produced a shape which differs substantially from the currently used conical taper. The cross-section dimensions are related to the angular distance from the tip of the clasp. A theoretical comparison of the difference between the theoretical and current designs cannot be made as the current design for tapered clasps is not specific. It is proposed that improved performance over present clasp designs can be obtained by using clasps of the derived shape. However, the manufacture of clasps of the required complex shape and variable size is impractical, and the next best alternative of clasps of constant cross section along their length was examined. Deflection calculations for this form showed that the loss of deflection in using a clasp of constant cross section instead of the ideal was only 23 per cent. It is proposed that clasps of constant cross section along their length, with a thickness chosen according to the size of the tooth used for the undercut, will present a significant improvement in clasp performance. Further, it will provide a systematic method of selecting clasp thickness which is not presently available.