Fatigue Resistance of Nickel-titanium Instruments Exposed to High-concentration Hypochlorite

Introduction: The purpose of this study was to introduce a new fatigue test model that simulates the clinical situation for evaluating the corrosion effect of 5.25% sodium hypochlorite (NaOCl) on nickel-titanium (NiTi) files and to evaluate the effect of 3 different temperatures (22 degrees C, 37 degrees C, and 60 degrees C) on the cyclic fatigue of these files. Methods: Three NiTi files (size 25/.04), K3 (SybronEndo, Orange, CA), K3XF (SybronEndo), and Vortex (Dentsply Tulsa Dental Specialties, Tulsa, OK), were subjected to cyclic fatigue tests inside a novel artificial ceramic canal with a curvature of 60 degrees and a 5-mm radius. A 19-mm-long file segment from the tip was introduced into the canal and immersed in water or 5.25% NaOCl at 3 different temperatures, and the number of revolutions to fracture (N-f) was recorded. The fracture surface of all fragments was examined by a scanning electron microscope. Data were analyzed using univariate analysis of variance with the significance level at 0.05. Results: The N-f of Vortex files was the highest followed by K3XF and K3 (P < .05) at all conditions. The Nf of all files was highest at 22 degrees C and lowest at 60 degrees C (P < .05). However, no difference in N-f was detected in Vortex files between 22 degrees C and 37 degrees C. The N-f of all files in 5.25% NaOCl was shorter than that in water although there was no statistically significant difference. No pitting or crevice corrosion was observed on the fracture surface. Conclusions: NaOCl, 5.25%, does not significantly affect the fatigue behavior of NiTi files. The fatigue resistance should be tested under specific temperature conditions. The austenite finish temperature of a file is important in determining the fracture risk at body temperature.