The effect of thermal treatment on the resistance of nickel-titanium rotary files in cyclic fatigue

Objective. The purpose of this study was to determine the effect of various thermal treatments on the fatigue resistance of a nickel-titanium (NiTi) engine-driven endodontic file. Study design. Fifteen groups of 5 files each of ISO 30 and taper .04 were tested in this study. The cutting tip ( 5 mm from the end) of files from 14 groups were heat treated for 30 minutes in temperatures 250 degrees C, 300 degrees C, 350 degrees C, 375 degrees C, 400 degrees C, 410 degrees C, 420 degrees C, 425 degrees C, 430 degrees C, 440 degrees C, 450 degrees C, 475 degrees C, 500 degrees C, and 550 degrees C, respectively, while 1 group was used as reference. The files were placed in a device that allowed the instruments to be tested for rotating bending fatigue inside an artificial root canal. The number of rotations to breakage was recorded for each file. The mean values of all groups were statistically analyzed using 1-way analysis of variance and Student Newman Keuls multiple comparison test at alpha = .05. Results. The 430 degrees C and 440 degrees C groups showed the highest values, with fatigue resistance decreasing for thermal treatment at lower and higher temperatures. This may be the result of metallurgical changes during annealing. Conclusion. Within the limitations of the low sample size and the specific instrument size tested, it appears that the appropriate thermal treatment may significantly increase the fatigue resistance of the NiTi file tested.