Mechanical Performance of Two <scp>NiTi</scp> Rotary Files: Effects of Alloy and Design of Fatigue, Torsion, Bending, and Fracture Behavior

This study evaluated the mechanical performance of nickel-titanium (NiTi) endodontic instruments with identical tip diameters but different alloy compositions and designs. The study compared two geometric configurations of WaveOne Gold instruments (Respectively, WO I and WO II) manufactured using three different NiTi alloys: SE-Wire (non-heat-treated superelastic NiTi), M-Wire, and gold alloy. WO I instruments have a variable taper (25/0.08) and dual convex triangular cross-section, while WO II instruments feature a reduced taper (25/0.07), square cross-section, and shorter handle. Six groups were tested for cyclic fatigue resistance, torsional strength, bending behavior, deformation, and compressive load. Fracture patterns were examined via scanning electron microscopy (SEM). A two-way ANOVA evaluated interactions between alloy type and instrument design. Alloy heat treatment and design significantly affected performance. Gold alloy instruments showed superior flexibility and the highest time-to-fracture (TTF) values, particularly in WO II. WO II files required less pressure to navigate canals and demonstrated better bending adaptability due to their reduced taper and square cross-section. In contrast, SE-Wire instruments had the lowest fatigue resistance and highest pressure loads, indicating their austenitic phase and lack of thermal treatment. WO I instruments had higher torsional strength but fractured at lower angular deflections. SEM revealed distinct fracture mechanisms based on alloy and generation. WO II instruments outperformed WO I in flexibility, cyclic fatigue resistance, and canal tracking, particularly with thermomechanically treated alloys like Gold. The relationship between alloy composition and instrument design is crucial for optimizing performance and minimizing procedural risks.