RESUMO
This study investigated the fatigue and fracture modes of RaCe and ProTaper rotary instruments. 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 Fl 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. 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°]. SEM observation revealed fatigue mark/features in 17 specimens, shear characteristics in 37 samples, and tensile overload in 26 samples. Radius of curvature was the main factor in torsional and fatigue failures