Cobalt-chromium and nickel-chromium alloys for removable prosthodontics, Part 1: Mechanical properties.

PURPOSE: This investigation compared the mechanical properties of three base metal casting alloys (Vitallium2, Neoloy N [Regular], and Regalloy T) for removable partial denture (RPD) frameworks with those for two base metal RPD alloys (Vitallium, Jelenko LG) whose mechanical properties had been previously reported. MATERIALS AND METHODS: Plastic patterns with dimensions meeting ADA specification no. 14 requirements were used with appropriate casting investments to cast tensile test bars of the five alloys. Nine specimens of each alloy were loaded to failure, and the mechanical properties of modulus of elasticity, 0.1% yield strength, 0.2% yield strength, and tensile strength were obtained from five well-behaved load-elongation plots. Values of percentage elongation were also determined, along with Vickers hardness of the work-hardened alloys from polished cross-sections of fractured tensile specimens. The fracture surfaces for representative specimens of each alloy were examined with a scanning electron microscope. RESULTS: Although the measured values of mechanical properties generally agreed with those reported by the manufacturers, differences found for Vitallium and Jelenko LG with previous investigations suggest that there may have been some changes in alloy compositions and processing by the manufacturers. The investment burnout procedure adopted for the plastic patterns and the present casting conditions may also have affected the mechanical properties of some alloys. Two alloys, Jelenko LG and Regalloy T, had mean values of percentage elongation exceeding 10%, but caution is recommended for clinicians and dental laboratories when performing clasp adjustments with all five alloys because of the substantial work hardening that can occur. CONCLUSIONS: In general, all of the alloys met the requirements in ADA specification no. 14 for base metal RPD alloys. Further research will be necessary to determine the relationships between the mechanical properties, particularly yield strength, rate of work hardening and percentage elongation, and the clinical requirements for these alloys.

Evaluation of the accuracy of cast reorientation to a surveyor by prosthodontic residents.

This study investigated the accuracy of a group of evaluators in reestablishing the orientation of a tripoded master cast to a surveyor instrument. Three prosthodontic residents served as evaluators to reestablish the tilt of a previously tripoded cast with three known reference coordinates. The distance from the known to the reference point was recorded for the three points in four trials by a judge with a machinist height gauge. Analysis of variance revealed a significant difference between the evaluator cast position and the known reference position. The investigation demonstrated +/- 0.2 mm errors at each of the three locations; 0.3-degree angulation errors occurred in reestablishing the tilt of the master cast in relation to the surveyor.