Mechanical Performance of Two NiTi 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.