Dynamic Cyclic Fatigue Resistance and Bending Stiffness of Blue-Treated NiTi Rotary Instruments at Simulated Intracanal Temperature: A Comparative In Vitro Study.

BACKGROUND: To compare the dynamic cyclic fatigue resistance, bending stiffness and phase transformation characteristics of blue heat-treated instruments, VDW Rotate (VR) and Azur Blue (AzB), at a simulated intracanal temperature of 34 ± 1°C. METHODS: Fifty-eight new instruments (n=29 per group) were allocated as follows: dynamic cyclic fatigue testing (n=12), bending stiffness evaluation (n=12), differential scanning calorimetry (DSC) (n=1), X-ray diffraction (XRD) (n=2) and energy-dispersive X-ray fluorescence (EDXRF) (n=2). Dynamic cyclic fatigue testing was conducted in an artificial canal (18 mm length, 1.4 mm diameter, 45° curvature, 5 mm radius) at 34 ± 1°C. Time to fracture was recorded and converted to the number of cycles to failure, and fractured segment lengths were measured. Bending stiffness was evaluated with a 3 mm deflection range at 34 ± 1°C. Differential scanning calorimetry, energy-dispersive X-ray fluorescence, X-ray diffraction and scanning electron microscopy were used for metallurgical and fractographic analyses. RESULTS: Differential scanning calorimetry revealed austenitic finish temperatures (AuF) of 35.8°C for VDW Rotate and 39.4°C for AzB. Azur Blue showed significantly higher cyclic fatigue resistance (2039 ± 250 vs. 1609 ± 222 cycles), lower bending stiffness (0.85 ± 0.07 vs. 1.07 ± 0.08 N) and longer fractured fragment length (4.87± 0.44 vs. 3.27 ± 0.75 mm) than VDW Rotate (P < .05). CONCLUSION: Azur Blue showed higher cyclic fatigue resistance, whereas VDW Rotate showed higher bending stiffness. These differences were more consistently associated with phase transformation behaviour than with external geometry alone.