Beyond S-cross section design: mechanical properties, metallurgical features and shaping ability of two nickel-titanium rotary instruments in double curved canals

OBJECTIVES: This study evaluated the metallurgical properties and shaping ability of two nickel-titanium instruments in artificial double-curvature canals. METHODS: 20 NiTi instruments (25 mm in length) with S-shaped cross-section design and different NiTi alloy were selected for analysis (n = 10, SPIN 25.06; n = 10 Rotate 25.06). The instruments were evaluated with respect to geometric design, differential scanning calorimetry, and shaping ability. Cyclic fatigue was also evaluated after three simulated clinical uses, comparing it to that of new instruments. Data were analyzed using parametric and nonparametric tests according to data distribution, with a significance level set at 5%. RESULTS: The instruments exhibited similar design features and different phase transformation temperatures. Rotate 25.06 presented lower metal mass volume and cross-sectional area than Spin 25.06 (P < 0.05). Rotate 25.06 resulted in lower canal deviation and better center ability than Spin 25.06 (P < 0.05). Spin 25.06 instruments demonstrated greater cyclic fatigue resistance (P < 0.05); however, after three simulated clinical uses, both instruments exhibited similar clinical fatigue resistance (P > 0.05). CONCLUSION: Although Spin 25.06 showed greater resistance to cyclic fatigue and better phase transformation temperature, Rotate 25.06 achieved superior shaping performance, promoting less deviation and better centering ability in the artificial double-curved root canals. CLINICAL RELEVANCE: Selection of NiTi instruments must consider cyclic fatigue resistance, shaping ability and metallurgical behavior. Our in vitro findings highlight the need of balancing instruments' properties to reduce procedural errors and optimize clinical outcomes.