Abstract
Purpose: Non-metal clasp dentures (NMCDs) employing gingiva-colored thermoplastic resin clasps are widely used to enhance the aesthetics of removable partial dentures (RPDs). This study investigated the forces and torsional moments exerted on abutment teeth during removal of thermoplastic resin clasps and compared them with those generated by conventional cobalt-chromium (Co-Cr) alloy clasps.
Methods: Clasps fabricated from Co-Cr alloy and thermoplastic resin with different undercut depths were examined. Fitness accuracy was evaluated, and tensile tests were performed using a miniature six-axis force/torque sensor. Removal forces and torsional moments acting on the abutment tooth were analyzed along the buccolingual (X-axis), mesiodistal (Y-axis), and vertical (Z-axis) directions. Statistical analyses were conducted to compare materials and undercut conditions.
Results: Fitness accuracy ranged from 85 to 130 µm for both clasp types. The thermoplastic resin clasp with a 0.25 mm undercut demonstrated significantly better fitness than the Co-Cr alloy clasp (P < 0.05). Resin clasps with a 0.5 mm proximal undercut generated greater removal forces and torsional moments than metal clasps and resin clasps with smaller undercuts. Increasing the proximal undercut depth in resin clasps resulted in increased retention forces, lateral forces, and torsional moments.
Conclusion: Thermoplastic resin clasps with larger proximal undercuts exert greater lateral forces and torsional moments on abutment teeth during removal. Therefore, appropriate selection of undercut depth is essential to reduce potentially harmful stresses while maintaining adequate retention in NMCDs.
