The load exerted by removable partial denture clasps conforming to average tooth curvatures, in clinically encountered undercuts

Purpose: The aim of this study was to establish clinically relevant guidelines for the selection of clasps for removable partial dentures (RPDs) by adapting RPD clasp arms of differing materials and diameter to a three-dimensional model based on the average curvature of premolars and molars. Method: Randomly selected discarded casts were collected that had intact first premolars and first molars. The normal clasp position for the buccal surfaces of these teeth was drawn on the cast and the teeth were then sectioned to this line, and scanned using a flatbed scanner. The average curvature and length for each group was determined using a graphics-drawing programme and a 3D model was printed using these data. Clasp materials were then be adapted to these models using the wrought wires: Leowire (Leone, Fiorentino, Italy) (stainless steel); Remanium Hard (Dentaurum, Pforzheim, Germany) (stellite alloy of chromium and cobalt); Noninium (Dentaurum, Pforzheim, Germany) (stainless steel, nickel free); and cast clasps cast in the stellite alloy Vitallium (Dentsply, Ontario, Canada). Two diameters of the wrought wire clasp groups were used (0.9mm and 1.0mm). Ten samples for each diameter and material were adapted to the 3D models as they would in a clinical case. Each clasp was then randomly deformed beyond its proportional limit in a tensile testing machine (Instron Corporation, High Wycombe, United Kingdom) and the forces exerted at that limit and at deflections of 0.25 mm, 0.5 mm, and 0.75 mm were measured, as these are the clinically encountered undercuts. Statistical analysis was carried out to determine if the forces exerted at these deflections were within the proportional limit of the clasp, but also significantly within the realistic limit (defined as two standard deviations from the proportional limit) to allow for variations in manufacture and application of the clasp arms. Results: The results confirmed the effect of material, length, and diameter on flexibility for the wrought wires. A table was produced with guidelines for those clasps which could be used safely within their realistic limits and therefore should provide longevity of service. The greatest force exerted on premolar clasp length arms was provided by Leowire at 0.25mm undercut (606g); although it was 2% greater than its realistic limit, this was considered sufficiently within the safety factor to iii recommend its clinical use. The greatest force exerted on molar clasp length arms was provided by Remanium Hard at a deflection of 0.5 mm (417g). Cast clasps for premolars should not be prescribed as they were all well above their realistic limits. Cast clasps should only be selected for molars if the clasp arm is longer than 14.5mm. At a deflection of 0.25 mm the cast arm exerted a force of 773g but whilst this was below its proportional limit, it was higher than its realistic limit. Conclusions: The results of this study provide valid guidelines for the clinical application of clasp arms in removable partial dentures. It is recommended that clinical studies be carried out to confirm the longevity of clasp arms based on these data.MT201