cyclic fatigue resistance and fractographic analysis of race and protaper rotary niti instruments

introduction: this study investigated the fatigue and fracture modes of race and protaper rotary instruments. materials & methods: fatigue resistance was evaluated by rotating the files 30 ° or 60 ° and with 2 or 5mm radius of curvature. race taper 06 size 25 and protaper f1 files (n=40) were used. the number of rotations to failure was analyzed by two-way anova and independent sample     t-test. fracture surfaces were examined under a scanning electron microscope. results: both files exhibited significantly more resistance to fracture when angle severity was reduced and increase in curvature radius (p=0.000). protaper demonstrated higher number of cycle of failure (p=0.0029) in one group (r=5mm, 60 º ). sem observation revealed fatigue mark/features in 17 specimens, shear characteristics in 37 samples, and tensile overload in 26 samples. conclusion: radius of curvature was the main factor in torsional and fatigue failures.