Effect of fatigue on torsional failure of nickel-titanium controlled memory instruments

The purpose of this study was to understand how fatigue affects the torsional properties of both traditional nickel-titanium (NiTi) and NiTi controlled memory (CM) files.

Typhoon (TYP; Clinician's Choice Dental Products, New Milford, CT) 25/.04 and 40/.04 rotary files in both NiTi and CM were tested to obtain the mean number of cycles of failure (Nf) using a 3-point bending apparatus. New files were then precycled to 4 conditions (0%, 25%, 50%, and 75% of the Nf), and torsional resistance tests were performed. Each file was exposed to torsional stress until failure, and at that point the torque and distortion angles were measured. The fracture surface of each fragment was examined with a scanning electron microscope.

TYP CM files had an Nf 7 times higher than that of TYP files (P < .05). No difference in torque between the CM files and the conventional NiTi files of either file size was detected (P > .05). The torque of the size 40/.04 files was significantly higher than the torque of the size 25/.04 files (P < .05). In the 40/.04 files group, TYP files in the 75% precycling group had a significantly lower torque than files in the group with no precycling (P < .05), whereas slight precycling (25%) significantly reduced the distortion angle on TYP CM files (P < .05). The CM files of both sizes had a significantly higher distortion angle than the corresponding NiTi files (P < .05). The fractured files in the precycling groups showed the typical pattern of torsional failure.

Within the same amount of precycling (25%, 50%, and 75%), the cyclic fatigue life of TYP CM instruments was significantly higher than that of the TYP instruments. However, the torque value of TYP CM was similar to TYP files. The larger instruments were not only less resistant to cyclic fatigue but were affected most by prestressing of both TYP and TYP CM files.