Impact of Repeated Use on Cyclic and Torsional Fatigue of 3 Rotary Files: Implications for Clinical Safety.

Introduction: This study aimed to compare the cyclic and torsional fatigue of S2, Logic, and Flat File NiTi files, new and after repeated use by an undergraduate student.

Methods: Ninety (n = 90) artificial mandibular molars were prepared using S2, Logic, and Flat File systems. For each system, 10 new instruments were tested mechanically, and the other 10 were tested after preparing three prototyped molars with curved mesial roots (60°). Cyclic fatigue tests were conducted in a custom device simulating a curved canal (60° curvature angle, 5 mm radius), while torsional strength tests followed ISO 3630-1 (1992) standards. Data were analyzed by analysis of variance and Tukey's tests (P = .05).

Results: Among the new #25-diameter instruments, Logic exhibited the highest number of cycles to fracture, while Flat File had the lowest (P < .05). For the used #25-diameter instruments, Logic and S2 demonstrated superior performance than Flat File regarding cycles to fracture (P < .05). The used Flat File #25-diameter instruments demonstrated a significant decrease in cycles to fracture after repeated use, and the lowest maximum torque until fracture (P < .05). All #25-diameter instruments showed a significant increase in angular deflection after repeated use (P < .05). For the #35-diameter instruments, S2 and Logic outperformed Flat File (P < .05).

Conclusions: The Logic #35-diameter instruments displayed higher maximum torque and angular deflection than the new instruments, with only Logic showing significant reductions in angular deflection after use (P < .05). The repeated use negatively affected the mechanical properties of the endodontic instruments with #25 and #35 diameters, particularly the Flat File and Logic instruments.