Sattapan et al. (2000a) speculated that fatigue fracture might play a major role in Quantec instruments, but others have reported that cyclic fatigue was a minor factor for ProFile instruments when used correctly and discarded regularly (Yared et al. 2001). Apparently, the instruments’ cross-sectional geometry does play an important role in cyclic fatigue.
However, torsional and flexural fractures are not mutually exclusive categories and cyclic fatigue simulation using metal canals cannot closely resemble clinical conditions because only minimal friction occurs between the rotating instrument and the holding device. Cyclic fatigue might occur differently if instruments are subjected to apically directed force and rotate in curved canals. Clinically, supra-threshold torque does occur for a limited time period only (Peters & Barbakow 2002) and it might be more appropriate to calculate the product of torque and time ) in order to assess the amount of stress the instruments have been subjected to.
