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

The ability of heat treatment as a result of autoclave sterilization to ext end the life of nickel-titanium rotary endodontic instruments by reducing t he effect of cyclic fatigue was evaluated using 280 size 40 Lightspeed inst ruments. Instruments were cycled in artificial canals with angles of curvat ure 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 ob tain mean cycles-to-failure values for each group. In the first experimenta l 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 n ot sterilized before being cycled to failure. In the second experimental pr otocol, instruments were cycled to 25% of the mean cycles-to-failure determ ined in the pilot study, and sterilized or not sterilized. This sequence of cycling to 25% of the predetermined cycles-to-failure limit followed by st erilization was repeated until the instruments failed. No significant incre ases in cycles to failure were observed between groups for either experimen tal protocol when instruments were evaluated at a similar radius. Significa nt differences in cycles to failure were only observed when instruments cyc led 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. Scann ing electron microscopic photos showed crack initiation and propagation in all instruments that were cycled to a percentage of the predetermined cycle s-to-failure limit. It is concluded that heat treatment as a result of auto clave sterilization does not extend the useful life of nickel-titanium inst ruments.