Cyclic Fatigue Resistance of Nickel-titanium Rotary Instruments according to the Angle of File Access and Radius of Root Canal

Introduction: The aim of this study was to compare the influences from different access angles and curvature radii on cyclic fatigue resistance of nickel-titanium rotary files. Methods: Two file systems (2Shape [TS; MicroMega, Besancon, France] and HyFlex CM [HCM; Coltene/Whaledent, Allstatten, Switzerland]) were used. A total of 192 instruments of TS #25/.04 (TS1), TS #25/.06 (TS2), HCM #25/.04, and HCM #25/.06 were evaluated at 3 insertion angles (0 degrees, 10 degrees, and 20 degrees) and 2 radii (5 mm and 3 mm) in 16-mm stainless steel artificial canals with a 60 curvature. Cyclic fatigue resistance was determined by the number of cycles to failure (NCF) using a customized testing device. Data were analyzed statistically with the significance level established at 95%. Results: In the 3-mm radius canal, the instruments showed lower cyclic fatigue resistance than in the 5-mm radius canal (P < .05). HCM #25/.06 and all .04 taper instruments had a significant NCF reduction at 20 and 10 in the 3-mm radius canal (P < .05), whereas TS2 showed no significant differences. In the 5-mm radius of curvature, although .06 taper instruments had no significant NCF reduction for each angle tested, .04 taper files exhibited significant NCF reduction when tested at 20 (P < .05). Comparing the same size instruments, HCM had higher NCF than TS (P < .05). Instruments with a .04 taper exhibited higher NCF than the .06 ones with the same heat treatment (P < .05). Conclusions: An inclined insertion into the canals decreased cyclic fatigue resistance of thermal-treated instruments with a .04 taper at all radii of curvature tested. The synergistic effect of a small radius of curvature and access angulation of heat-treated instruments decreases their fatigue resistance.