Abstract: Objective: To compare the effect of pre-load on surface microstructure and cyclic fatigue resistance of K3XF and K3. Methods:Forty-eight K3XF or K3 instruments #25/.06 were randomly divided into 4 groups (n = 12). The instruments from group 1 were not pre-loaded, and the group 2 to 4 underwent pre-load for 5s, 10s and 20s by using a simulated metal root canal with curvature of 60° and radius of 3.5 mm, respectively. The surface microstructure of two instruments randomly selected from each group was observed using SEM. Then the remaining 10 instruments were submitted to the cyclic fatigue test using the before mentioned apparatus. The time till fracture was recorded as well as the number of cyclic fatigue (NCF) and the fragment length. The topographic features were analyzed using SEM. The data were analyzed with the Two-Way ANOVA analysis at a significance level of P<0.05 by using SAS 9.3. Results:SEM observation demonstrated obvious flute abrasion, numerous cracks and irregularities on both of the files. However, this kind of destruction was less on K3XF than that on K3. Cyclic fatigue resistance of K3XF significantly decreased from (210±59) to (156±37) after pre-loaded for 20 s (P<0.05). Cyclic fatigue resistance of K3 decreased from (145±38) to (110±27), (101±31) and (62±31) after pre-loaded for 5s, 10s and 20s, respectively (P<0.05). Significant difference was observed between groups (P<0.05) except that between 5s and 10s groups. Conclusion: K3XF exhibits better resistance against pre-load compared to K3.

Key words: Pre-loaded, Cyclic fatigue , Nickel-titanium instrument , SEM R-phase