Single Versus Dual Wire Heat Treatments in N i T i Instruments of Identical Design: A Multimethod Study

AIM: To compare the geometric, metallurgical and mechanical characteristics of NiTi endodontic instruments with the same nominal design and dimensions but manufactured under three different single heat-treatment conditions and one Dual Wire heat-treatment configuration. METHODOLOGY: A total of 240 NiTi instruments were divided into four groups (n = 60): GOLD, PURPLE, BLUE, each subjected to a distinct single heat-treatment condition, and DUAL WIRE, incorporating two different heat treatments within the same instrument. Geometric and surface characteristics were assessed by optical and scanning electron microscopy. Elemental composition was evaluated by energy-dispersive X-ray spectroscopy, and phase-transformation temperatures by differential scanning calorimetry using coronal and tip segments. Mechanical performance was assessed by cyclic fatigue, torsional, bending, buckling and cutting tests. Data were analysed using one-way ANOVA and Tukey's post hoc test (α = 5%). RESULTS: The four groups showed comparable geometric and surface characteristics, including working-part length, spiral number and density, spiral orientation and symmetry, tip design and machining marks. SEM-EDS identified near-equiatomic Ni-to-Ti ratios, with no major differences among groups. DSC revealed broadly similar phase-transformation temperature ranges. BLUE showed greater cyclic fatigue resistance than all other groups (p ≤ 0.001), whereas DUAL WIRE showed greater cyclic fatigue resistance than PURPLE (p = 0.035). PURPLE exhibited higher maximum torque than DUAL WIRE (p = 0.049), while the angle of rotation did not differ among groups (p = 0.186). BLUE was more flexible than PURPLE (p = 0.001) but showed lower buckling resistance than GOLD, PURPLE and DUAL WIRE (p ≤ 0.012). GOLD exhibited greater buckling resistance than DUAL (p < 0.001), whereas DUAL WIRE showed greater cutting ability than BLUE (p = 0.036). CONCLUSION: Despite comparable nominal design, elemental composition and phase-transformation temperature ranges, different heat-treatment configurations were associated with distinct mechanical behaviours. BLUE was characterised by greater flexibility and cyclic fatigue resistance but lower buckling resistance, whereas DUAL WIRE showed greater cutting ability than BLUE and greater cyclic fatigue resistance than PURPLE. The Dual Wire heat-treatment configuration did not confer a generalised mechanical advantage across the properties assessed.