RESUMEN
NiTi rotary instruments have been extensively used in the preparation of curved root canals, due to their superelastic quality. These instruments are less susceptible to unexpected fracture as compared to their stainless steel counterparts. However, unexpected fracture of NiTi instruments under complex stress within curved canals can lead to serious problems. The aim of this investigation was to identify and evaluate the various failure mechanisms which can cause fracture of NiTi rotary instruments under clinical conditions. The causes of failure of two brands of Hero and Profile rotary files have been studied. A total of 1133 instruments that failed during clinical use were collected from several dental clinics. After inspection under a stereomicroscope, all instruments were classified as either plastically deformed or fractured. The fracture surfaces of all broken instruments were examined following ultrasonic cleaning in ethanol and acetone, 30 minutes each. The fracture surfaces of more than 70 broken instruments were subjected to fractographic analysis using a scanning electron microscope. More than 68% of the 1133 specimens were fractured. Stereomicroscopic inspection showed three failure mechanisms for the studied instruments: fracture, plastic deformation due to bending and torsion and a combination of both mechanisms. Based on fractographic observations, microvoid coalescence and cyclic stable crack growth by striation formation were identified. According to these results, excessive plastic deformation and fracture may be considered as the two main mechanisms responsible for the failure of NiTi rotary instruments