Effect of heat sterilization on the torsional properties of rotary nickel-titanium endodontic files

The purpose of this study was to determine whether heat sterilization adversely effects the torsional properties of rotary nickel-titanium files, making them more prone to fracture under torsional stress. Nine hundred sizes 2 through 10 Profile Series 29 .04 taper files were divided into groups of 10 files each and sterilized 0, 1, 5, or 10 times in the steam autoclave, Statim autoclave, or dry heat sterilizer. Then, they were subjected to torsional testing in a Torquemeter Memocouple. Complete data were collected for sizes 2 through 7, but not for sizes 8 through 10 because their torque resistance exceeded the testing limits of the Torquemeter Memocouple. A one-way analysis of variance was used to compare all experimental groups in sizes 2 through 7 with their unsterilized controls (p < 0.05). Fifty-four comparisons were made for torsional strength and 54 for rotational flexibility. Ten significant changes occurred for torsional strength and 10 for rotational flexibility. Eight of 10 changes in torsional strength were increases. Fifty-two of 54 (96.3%) comparisons for torsional strength and 47 of 54 (87%) for rotational flexibility showed a significant increase or no change. Therefore, heat sterilization of rotary nickel-titanium files up to 10 times does not increase the likelihood of instrument fracture.