Cyclic fatigue of nickel-titanium rotary instruments after clinical use with low- and high-torque endodontic motors

This study investigated the cyclic fatigue resistance of used nickel-titani um rotary instruments, clinically operated by a traditional high-torque mot or and a new low-torque electric motor. Fifty instruments (ProFile, Maillef er, Baillagues, Switzerland) for each of the following sizes were used: .06 -25, .06-20, and .04-25. They were divided into three groups: A = 10 new in struments (control), B = 20 used instruments operated by a high-torque moto r, and C = 20 used instruments operated by a low-torque motor. Each instrum ent was used in 10 clinical cases (at least six molars). Cyclic fatigue tes ting of new and used instruments was performed with a device that allowed t he instruments to rotate freely inside a stainless-steel artificial canal a t 350 rpm. Data were recorded and statistically analyzed. Mean (and SD) num bers of rotation-to-breakage cycles for .06-25, .06-20, and .04-25 instrume nts were, respectively: group A 463 (85), 535 (101), and 613 (105); group B 229 (64), 343 (92), and 233 (84); and group C 352 (77), 454 (89), and 466 (86). Results showed that group A instruments exhibited significantly highe r values of rotation-to-breakage cycles (two-sample t test, p < 0.01), when compared with groups B and C, respectively. Statistically higher resistanc e to cyclic fatigue was noted for group C in all sizes compared with group B (two-sample t test, p < 0.01), showing that the use of endodontic motor w ith lower torque values reduced cyclic fatigue of nickel-titanium rotary in struments.