Effect from the sequence of heat treatment and grinding on the mechanical properties of nickel–titanium rotary instruments

This study aimed to evaluate how the sequence of heat treatment and grinding affects the mechanical properties of nickel-titanium rotary instruments. ProTaper Gold (PTG) and three experimental FQ files with identical designs, but different manufacturing sequences, were tested. FQhg was heat-treated before grinding, FQgh was ground before heat treatment, and FQnh was not heat-treated. Cyclic fatigue resistance, torsional resistance, and bending stiffness were measured (n = 15). Scanning electron microscopy and differential scanning calorimetry (DSC) were performed. Statistical analysis was conducted using one-way anova and Tukey's test at a 95% significance level. FQhg showed the highest cyclic fatigue resistance, followed by FQgh, FQnh, and PTG (p < 0.05), while no significant differences were found in torsional strength, toughness, or distortion angle among the FQ groups. PTG showed higher torsional strength and bending stiffness, likely due to its different cross-sectional design and taper. DSC analysis revealed minor differences in austenite transformation temperatures between FQhg and FQgh. The sequence of heat treatment and grinding significantly influenced cyclic fatigue resistance, with post-heat-treatment grinding (FQhg) providing superior performance; however, its impact on other mechanical properties was limited. The clinical relevance of processing sequence appears minimal compared to factors such as cross-sectional design and heat treatment.