Objective: An onlay rest can be used as a retainer for the part of the buccal and lingual margins extending into an undercut area. This retainer is referred to as a ²cap clasp² because its form is like a cap. This study evaluated the retentive force of titanium cap clasps. Methods: Three types of patterns (A: undercut 0.1 mm and width 0.3 mm; B: undercut 0.2 mm and width 0.3 mm; C: undercut 0.1 mm and width 0.5 mm) were made using a stainless steel model. The patterns were cast with CP Ti Grade 2 (T-alloy M, GC, Japan), CP Ti Grade 3 (T-alloy H, GC), and Ti-6Al-7Nb (T-alloy TOUGH, GC) using an argon gas-pressure casting unit (Autocast HC-III, GC). After the titanium cap clasp was sandblasted with 50 µm Al₂O₃ particles and chemically treated with a HNO₃ /HF solution (Chemipolish, Shofu), it was fitted to a screw-driven mechanical testing machine. The retentive forces were measured at a crosshead speed of 40 mm/min up to 1,000 insertion/removal times. The data for these alloys were compared to previous reported data for Type IV gold alloy and Co-Cr alloy (Suzuki, Tsurumi Univ Dent J 14: 141-156, 1988). The results (n=5) were analyzed by ANOVA/Tukey's test (a=0.05). Results: The retentive force of B and C was significantly greater than of A (approximately 1.5 and 2 times, respectively) (p<0.05). As the number of insertion/removal times increased, the retentive force of C decreased severely, but it decreased only moderately for B. The Ti-6Al-7Nb cap clasp exhibited the least decrease among all titanium tested. Conclusions: To maintain appropriate long-term retentive force, the use of Ti-6Al-7Nb is recommended for titanium cap clasps. In addition, a 0.2 mm undercut is better for thicker margins (0.5 mm) if greater retentive force is necessary.

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