Mechanical and Metallurgical Evaluation of Four Rotary Nickel-Titanium File Systems: A Multimethod Study

To compare four rotary nickel-titanium (NiTi) file systems (Endostar E3 Azure, Endostar E3, VDW.ROTATE, and Mtwo) using a multimethod approach to evaluate their mechanical and metallurgical properties.

Endostar E3 Azure, Endostar E3, VDW.ROTATE, and Mtwo (all size 25/0.06 with an S-shaped cross-section; 52 instruments per system) were evaluated. Mechanical performance was assessed through static and dynamic cyclic fatigue tests (each treated as a separate and independent test model) together with torsional resistance, bending and buckling tests; the number of cycles to fracture (NCF) was used to compare cyclic fatigue resistance across systems operated at different rotational speeds. Metallurgical characterisation included scanning electron microscopy (SEM) for fractographic and surface analysis, energy-dispersive X-ray spectroscopy (EDS) for surface elemental composition, and differential scanning calorimetry (DSC) for phase-transformation temperatures.

Endostar E3 Azure and VDW.ROTATE recorded the highest cyclic fatigue resistance and did not differ from each other within the fatigue models; both surpassed the conventional-alloy instruments (p < 0.05). In the static model, Endostar E3 and Mtwo did not differ from each other (p > 0.05). Endostar E3 yielded the highest maximum torque (0.63 ± 0.01 Ncm), bending force (13.84 ± 1.79 gF) and buckling load (14.37 ± 4.13 gF; p < 0.05), whereas VDW.ROTATE showed the greatest angular deflection (577.03° ± 98.59°; p < 0.05). DSC revealed R-phase-mediated multi-step transformations in the heat-treated files, with austenite finish (Af) temperatures near body temperature; the conventional-alloy files exhibited a single low-intensity peak consistent with a wholly austenitic state. EDS detected surface oxygen on the thermomechanically processed files, indicating the presence of a TiO₂ layer.

Thermomechanically processed systems exhibited greater flexibility and longer fatigue life, whereas conventional NiTi instruments demonstrated higher rigidity and torque capacity. DSC-based phase characterisation was consistent with the divergent mechanical outcomes observed among groups and offered a plausible metallurgical explanation, although the contributions of alloy processing and instrument design could not be separated within the present study design.