Evidence for Reduced Fatigue Resistance of Contemporary Rotary Instruments Exposed to Body Temperature.

INTRODUCTION: The purpose of this study was to evaluate the effect of 2 different temperatures (20 degrees C and 37 degrees C) on the cyclic fatigue life of rotary instruments and correlate the results with martensitic transformation temperatures. METHODS: Contemporary nickel-titanium rotary instruments (n = 20 each and tip size #25, including Hyflex CM [Coltene, Cuyahoga Falls, OH], TRUShape [Dentsply Tulsa Dental Specialties, Tulsa, OK], Vortex Blue [Dentsply Tulsa Dental Specialties], and ProTaper Universal [Dentsply Tulsa Dental Specialties]) were tested for cyclic fatigue at room temperature (20 degrees C +/- 1 degrees C) and at body temperature (37 degrees C +/- 1 degrees C). Instruments were rotated until fracture occurred in a simulated canal with an angle curvature of about 60 degrees and a radius curvature of 3 mm; the center of the curvature was 4.5 mm from the instrument tip. The number of cycles to fracture was measured. Phase transformation temperatures for 2 instruments of each brand were analyzed by differential scanning calorimetry. Data were analyzed using the t test and 1-way analysis of variance with the significance level set at 0.05. RESULTS: For the tested size and at 20 degrees C, Hyflex CM showed the highest resistance to fracture; no significant difference was found between TRUShape and Vortex Blue, whereas ProTaper Universal showed the lowest resistance to fracture. At 37 degrees C, resistance to fatigue fracture was significantly reduced, up to 85%, for the tested instruments (P < .001); at that temperature, Hyflex CM and Vortex Blue had similar and higher fatigue resistance compared with TRUShape and ProTaper Universal. CONCLUSIONS: Under the conditions of this study, using a novel testing design, immersion in water at simulated body temperature was associated with a marked decrease in the fatigue life of all rotary instruments tested.