Cyclic fatigue properties of cobalt-chromium alloy clasps for partial removable dental prostheses

Abstract

Cast cobalt-chromium (Co-Cr) alloy clasps have been widely used for prosthodontic treatment, but long-term retentiveness of these clasps is not clear. The purpose of this study was to investigate the residual retentive force of 3 types of cast Co-Cr alloy clasps in relation to their proportional limits. Thirty half-round straight clasps and 30 circumferential clasp assemblies were made from 3 Co-Cr alloys (Hardalloy, Regalloy, and Vera PDN) (n=10). All of the straight clasps were subjected to a compression-deformation test to obtain data for displacement/load at the proportional limit. Two undercut depths (0.25 mm and 0.50 mm, n=5) were included in a cyclic fatigue test. Each circumferential clasp assembly was subjected to an insertion/removal test on a metal abutment tooth for 7200 cycles to simulate 5 years of clinical use. The residual retentive force was measured after 360 cycles; thus, the retentive force of each clasp was recorded 21 times throughout the test. The data were subjected to repeated-measures ANOVA (α=.05) to compare the retentive force of each group of clasps for different periods of fatigue testing. The ANOVA. The Student t test (α=.05) was used for analysis of clasps fabricated with the same alloy but with different undercuts. A linear correlation analysis (α=.05) was used to test the correlation between retentive forces and cycling sequences. The mean values of displacement at the proportional limit for the 3 Co-Cr alloys varied from 0.30 mm to 0.36 mm. The mean retentive forces of each alloy group decreased dramatically after the first cycling phase (P<.05). Thereafter, the decrease was more gradual but was still significantly different compared to the original retentive forces (P<.05). Compared to clasps in the 0.25-mm undercut group, those in the 0.50-mm undercut group exhibited greater mean retentive forces before the cyclic fatigue test (P<.05), as well as a greater decrease in the retentive force at the end of the test (P<.05). A negative correlation was found between the retentive force of cast Co-Cr alloy clasps and the logarithm of cycling sequences. After a test simulating 5 years of service, cast Co-Cr alloy clasps exhibited a residual retentive force to satisfy the requirements for clinical use.