Effect of A Clinical-Replicable Cooling Protocol on the Cyclic Fatigue Resistance of Heat-Treated Nickel-Titanium Instruments

INTRODUCTION: The aim of this study was to evaluate the bending and cyclic fatigue resistance of Wave One Gold (WOG) and X1 Blue (X1B) instruments when tested at body temperature (36°C ± 1°C) with and without subjected to an alloy cooling protocol. MATERIALS AND METHODS: A total of sixty instruments (n=30) were tested. Forty instruments (n=20) were randomly selected and divided into two groups: body temperature (BT; n=20) and body temperature with cooling protocol (CP; n=20). Cyclic fatigue test was performed until fracture in a conventional stainless-steel device with water bath equipment to simulate body temperature. CP group instruments were subjected to 5 seconds of spray cooling every 30 seconds. Time to fracture was recorded in seconds. Resistance to bending at an angle of 45 degrees was evaluated using twenty instruments (n=10). Fractured surfaces were examined under scanning electron microscopy (SEM). Statistical analysis was performed at a 5% significance level. Results: There was no difference in the cyclic fatigue resistance between instruments in BT groups (P>0.05). Cooling protocol significantly increased the cyclic fatigue resistance of X1B instruments (P=0.0003) and WOG instruments (P=0.0003). RESULTS: WOG instruments had a significantly lower cyclic fatigue resistance compared to X1B instruments in CP group (P=0.0001). There were no significant differences between the values of resistance increase presented by the instruments after cooling (P>0.05). Bending test presented no statistically significant differences between the tested instruments (P>0.05). Both instruments in both groups showed typical features of cyclic fatigue behavior under SEM. CONCLUSIONS: X1 Blue #25.06 and WaveOne Gold #25.07 instruments presented similar cyclic fatigue resistance. The investigated clinical-replicable cooling protocol improved the cyclic fatigue resistance of the tested instruments, with X1 Blue #25.06 presenting a greater cyclic fatigue resistance after cooling. Both instruments presented a similar bending capacity.