Influence of sterilization on mechanical properties and fatigue resistance of nickel-titanium rotary endodontic instruments: An in vitro study

Abstract

Aims and Objectives: This study aimed to evaluate the fatigue resistance of nickel–titanium (NiTi) endodontic files before and after sterilization, to evaluate the effects of sterilization on NiTi rotary files that were fatigued to half of their average life, and to assess whether sterilization cycles have any effect on the microhardness of NiTi files by testing the Vickers microhardness before and after sterilization. The objective of this study was to clarify the effects of sterilization on fatigue resistance and Vickers microhardness of rotary NiTi endodontic instruments.Materials and Methods: Thirty-six NiTi files (ProTaper F2 size) were evaluated for fatigue test and hardness without sterilization and after five cycles of autoclaving. These were grouped into A, B, and C (12 per group). An artificial metallic canal that simulated a curved root canal allowed the instruments to freely rotate inside it. A “C”-shaped clamp made of cast iron was used to hold the two halves of the artificial canals together. X-Smart endomotor (Dentsply, Maillefer, USA), digital stopwatch, autoclave, light mineral oil as lubricant, and so on were also used. Statistical Analysis: Analysis of variance (one-way ANOVA) test was performed as parametric test to compare different groups. To enable multiple comparisons, Duncan’s multiple range test was used as post hoc analysis. Student’s t-test was used to compare Vickers microhardness between two groups. Results: The number of cycles to failure in group A, group B, and group C were tabulated and the mean values of each group were calculated and analyzed with variance test (one-way ANOVA test), and it did not show any statistically significant difference (P > 0.05) between the groups. In the Vickers microhardness test, results showed an increase in the microhardness of NiTi instruments of sterilized group when compared to that of unsterilized group. This was a statistically significant difference.Conclusion: This study has substantiated that autoclaving has relatively no negative impact on the fatigue failure of NiTi instruments; however, a slight increase in hardness is observed on repeated sterilization. The most common reason for failure of the files was flexural strain, which almost always corresponded to the area of maximum curvature of the simulated root canal.