Environmental Temperature Drastically Affects Flexural Fatigue Resistance of Nickel-titanium Rotary Files.

INTRODUCTION: The aim of the present study was to analyze how a low environmental temperature can affect the fatigue life of instruments made by different types of heat-treated nickel-titanium alloys. METHODS: The flexural cyclic fatigue of 40 new specimens for each of the following systems was tested for cyclic fatigue resistance: ProTaper Universal F2 (Dentsply Maillefer, Ballaigues, Switzerland), ProTaper Gold F2 (Dentsply Tulsa Dental Specialties, Tulsa, OK), Twisted Files SM2 (SybronEndo, Orange, CA), Mtwo #25.06 (VDW, Munich, Germany), and Vortex Blue #30.04 and #40.06 (Dentsply Tulsa Dental Specialties). Instruments were tested at 2 different environmental temperatures: 20 degrees C (+/-2 degrees C) for room temperature (RT) group and -20 degrees C (+/-2 degrees C) for the cooled environment (CE) group (n = 20). The number of cycles to failure (NCF) and the length of the fractured fragment (FL) were recorded. The means and standard deviations of NCF and FL were then calculated; NCF data were statistically analyzed using a paired t test between groups RT and CE for each instrument tested (P < .05), whereas FL data were analyzed using analysis of variance (P < .05). RESULTS: The mean NCF values measured were significantly higher for the CE groups than the RT groups in all the systems tested (P < .05). The increase in cyclic fatigue resistance varied from 274%-854%. No differences in FL were registered among the different groups (P < .05). CONCLUSIONS: A low environmental temperature determines a drastic increase in the flexural fatigue resistance of NiTi endodontic instruments manufactured with traditional alloy and different heat treatments.