fracture resistance of roots following instrumentation with two single-file and one multi-file nickel–titanium rotary systems

Objective: this study aimed to compare the fracture resistance of human mandibular premolar roots after instrumentation with different nickel–titanium (niti) rotary systems.methods: forty extracted human mandibular premolars were decoronated and randomly assigned to four groups (n=10). group 1 served as an uninstrumented control, while groups 2–4 were prepared using different niti rotary systems according to the manufacturers’ protocols: one curve (oc) single-file system (group 2), xp-endo shaper (xps) single-file system (group 3), and protaper next (ptn) multi-file system (group 4). all instrumented canals were obturated with gutta-percha and ah plus sealer using the lateral compaction technique. vertical fracture resistance was then tested with a universal testing machine. data were analyzed using one-way anova followed by tukey’s post hoc test (α=0.05).results: anova revealed a significant difference in fracture resistance among the groups (p=0.005). the control group (402 ± 115.93 n) showed significantly higher mean fracture resistance compared to all instrumented groups (p < 0.05). among the instrumented groups, xps (304 ± 37.47 n) had the highest mean fracture resistance, followed by oc (291 ± 78.37 n) and ptn (288 ± 50.50 n), but these differences were not statistically significant (p > 0.05).conclusions: root canal instrumentation with both single-file and multi-file niti rotary systems significantly reduced fracture resistance compared to uninstrumented roots. there was no significant difference in fracture resistance between the single-file and multi-file systems, indicating that the single-file approach does not provide an advantage in preserving root strength.