Effect of Sodium Hypochlorite Exposure on Cyclic Fatigue Resistance of Two Heat-Treated Nickel-Titanium Rotary Instruments: An In Vitro Study

IntroductionHeat-treated nickel-titanium (NiTi) rotary instruments are widely used for root canal preparation; however, their resistance to cyclic fatigue may be affected by alloy characteristics and exposure to endodontic irrigants.This study aimed to compare the cyclic fatigue resistance of ProTaper Gold (PTG) and EdgeTaper Platinum (ETP) instruments following single and repeated exposures to 5.25% sodium hypochlorite (NaOCl). Materials and methodsThis in vitro experimental study included 48 new rotary instruments, comprising 24 PTG instruments (50.0%) and 24 ETP instruments (50.0%).Each system was divided equally into distilled water control, single NaOCl exposure, and repeated NaOCl exposure groups, with eight (16.7%)instruments in each subgroup.The instruments were tested in a standardized 60 degree curved artificial canal at 37°C.The primary outcome was the number of cycles to failure (NCF).Data were analyzed using two-way analysis of variance, Tukey's honestly significant difference test, and independent-sample t-tests. ResultsETP demonstrated significantly greater cyclic fatigue resistance than PTG (main effect, p < 0.001; partial η² = 0.769).Exposure condition also significantly affected cyclic fatigue resistance (p < 0.001; partial η² = 0.431), whereas the instrument system × exposure condition interaction was not significant (p = 0.696; partial η² = 0.017).Compared with the control condition, single NaOCl exposure significantly reduced NCF (p = 0.003; Cohen's d = 1.21), as did repeated exposure (p < 0.001; Cohen's d = 2.04).The difference between single and repeated NaOCl exposures was also statistically significant (p = 0.002; Cohen's d = 0.95).ETP demonstrated significantly greater cyclic fatigue resistance than PTG under all three exposure conditions (all p < 0.001). ConclusionETP showed greater cyclic fatigue resistance than PTG, whereas 5.25% NaOCl exposure adversely affected the fatigue resistance of both systems.