Effect of sterilization on cyclic fatigue of rotary nickel-titanium endodontic instruments

The ability of heat treatment as a result of autoclave sterilization to extend the life of nickel-titanium rotary endodontic instruments by reducing the effect of cyclic fatigue was evaluated using 280 size 40 Lightspeed instruments. Instruments were cycled in artificial canals with angles of curvature of 30 degrees and either 2 or 5 mm radii of curvature. In a pilot study, instruments were sterilized or not sterilized and cycled to failure to obtain mean cycles-to-failure values for each group. In the first experimental protocol, instruments were cycled to either 25%, 50%, or 75% of the mean cycles-to-failure limit determined in the pilot study, then sterilized or not sterilized before being cycled to failure. In the second experimental protocol, instruments were cycled to 25% of the mean cycles-to-failure determined in the pilot study, and sterilized or not sterilized. The sequence of cycling to 25% of the predetermined cycles-to-failure limit followed by sterilization was repeated until the instruments failed. No significant increases in cycles to failure were observed between groups for either experimental protocol when instruments were evaluated at a similar radius. Significant differences in cycles to failure were only observed when instruments cycled to failure in the artificial canal with 2 mm radius were compared with instruments cycled to failure in the artificial canal of 5 mm radius. Scanning electron microscopic photos showed crack initiation and propagation in all instruments that were cycled to a percentage of the predetermined cycles-to-failure limit. It is concluded that heat treatment as a result of autoclave sterilization does not extend the useful life of nickel-titanium instruments.