Low Environmental Temperature Influences the Fatigue Resistance of Nickel-titanium Files.

INTRODUCTION: The purpose of this study was to evaluate the effect of different temperatures (0 degrees C, 10 degrees C, 22 degrees C, 37 degrees C, and 60 degrees C) on the cyclic fatigue life of nickel-titanium (NiTi) files using a new fatigue test model in zirconium oxide. METHODS: Three superelastic NiTi files (EndoSequence [Brasseler USA, Savannah, GA], ProFile [Dentsply Tulsa Dental Specialties, Tulsa, OK], and K3 [SybronEndo, Orange, CA]), and 3 heat-treated (K3XF [SybronEndo], Vortex [Dentsply Tulsa Dental Specialties], and HyFlex CM [Coltene-Whaledent, Allstetten, Switzerland]) NiTi files, all size 25/.04, were subjected to cyclic fatigue tests inside a novel, artificial ceramic canal with a curvature of 60 degrees and a 5-mm radius. The model was immersed in water at 5 different preset temperatures. The number of cycles to failure (NCF) was recorded, and the fracture surface of the fragments was examined by a scanning electron microscope. The data were analyzed using 2-way analysis of variance with the significance level at 0.05. RESULTS: When the temperature was reduced from 60 degrees C to 0 degrees C, the NCF significantly increased from over 2 to 10 times for the NiTi file groups (P < .01). K3XF had the highest fatigue resistance of all files at 0 degrees C (P < .05). Vortex files had the highest NCF at 10 degrees C-60 degrees C. The NCF of heat-treated files was significantly higher than superelastic NiTi files at 10 degrees C and 20 degrees C (P < .05). There was no significant difference in the NCF of HyFlex CM at 0 degrees C and 22 degrees C. There was little difference in the fractographic appearance among different temperatures, except that the fraction area occupied by the dimple region of some instruments at 0 degrees C was slightly smaller than at 60 degrees C. CONCLUSIONS: Cooling down to low temperatures may be an interesting strategy to improve the fatigue resistance of rotary NiTi files.