Methods: Variable designed thermoplastic resin clasps (undercut: 0.25 mm, 0.5 mm, and 0.75 mm, thickness: 0.5, 1.0, 1.5, and 2.0 mm, width: 5.0 mm) were fabricated with two thermoplastic resins: Polyamide [Valplast (VAL), Unival, Japan] and Polycarbonate [Reigning (REI), Tousinyoukou, Japan]. Using an 18-8 stainless tooth die simulating the first molar, the clasp patterns were waxed up. According to the manufacture’s instruction, each thermoplastic resin was injected using an air-pressure injection system (MIS-II, i-CAST) under 1.0 MPa. The clasps were repeated the insertion and removal motion up to 10,000 cycles in distilled water at 37±2°C. The retentive force (N) of each resin clasp was measured with a tensile test apparatus (EZ-S, Shimizu, Kyoto, Japan) at a crosshead speed of 50 mm/min. The data were analyzed by one-way ANOVA and Tukey’s test (α= 0.05).
Results: The retentive forces of both thermoplastic resins were proportionally increased as undercuts were increased. REI showed the significantly greater initial retentive forces than VAL (p<0.05). REI clasps with 0.75 mm undercut failed from 3,000 to 6,000 insertion and removal load cycles though no failures were observed in VAL clasps up to 10,000 cycles.
Conclusions: The retentive forces of both thermoplastic resin clasps depended on elastic modulus of each resin, undercuts, thickness and widths of clasp as well as a conventional metal clasp. Although REI failed by insertion/removal motion, VAL clasps kept the constant retentive forces or less than 50 % decrease without failure.