Mechanical and Thermal Impact of Sequential Use-Sterilization Cycles on ProTaper Ultimate Files: An In Vitro Assessment with Practical Implications.

INTRODUCTION

Cyclic fatigue failure is an inherent feature of rotary files when used in curved canals. This study aimed to assess the effect of repeated usage and autoclave sterilization on the dynamic cyclic fatigue resistance of heat-treated nickel-titanium files used to prepare J-shaped canals.

METHODS

New ProTaper Ultimate nickel-titanium files (Slider, Shaper, F1, F2) (n = 180) were divided into 3 groups (n = 60): Group N, included new files, were groups US-2 and US-4 included files subjected to 2 or 4 cycles of usage and sterilization, respectively. Files were used to prepare a resin-simulated J-shaped canal (19 mm length, 30° curvature) at 37°C. After each use, files were sterilized in an autoclave at 134°C for 25 minutes. Dynamic cyclic fatigue testing was then performed at 37°C, and the mean number of cycles to failure (NCF) was recorded. Fractured fragment lengths were measured using a digital microcaliper. Statistical analysis was performed using one-way ANOVA and the Kruskal-Wallis test. A P-value <.05 was considered significant.

RESULTS

The Slider file consistently showed significantly higher NCF values than Shaper, F1, and F2 files in all the groups (P < .05). Repeated cycles of usage/sterilizations had an adverse effect on the cyclic fatigue resistance of all files, with a marked reduction after 4 uses.

CONCLUSIONS

Repeated usage and autoclave sterilization showed a marked adverse effect on the novel heat-treated files. A positive correlation was seen between the increased number of usage and autoclave cycles and the reduction in NCF.