The impact of alloy treatment on the dynamic cyclic fatigue resistance of triangular base cross-section NiTi endodontic instruments

This study evaluated the dynamic cyclic fatigue resistance and bending forces of small-sized blue heat-treated files with comparable cross sections but varying geometries at simulated intracanal temperature. For each system, Race Evo (RE), ZenFlex (ZF) and Tia Tornado Blue (TTB), 20 new files were tested (n = 20). Twelve files (n = 12) were subjected to Dynamic Cyclic Fatigue testing in an artificial canal (18 mm length, 1.4 mm diameter, 45° curvature, 5 mm radius) at 34 °C. Five files (n = 5) were used for static bending tests at 34 °C. Two files (n = 2) were used for differential scanning calorimetry (DSC) and X-ray diffraction (XRD), and one (n = 1) unused file was used for geometric evaluation using stereomicroscopy and scanning electron microscopy (SEM). TTB exhibited significantly higher fatigue resistance (1512 NCF) than ZF (1234 NCF; P < 0.05), while RE showed intermediate performance (1385 NCF). RE has the shortest fragment length at 3.25 mm, which represents a significant difference compared to ZF (4.55 mm; P < 0.05) and TTB (5.1 mm; P < 0.05), respectively. TTB required the lowest mean force, 0.304 N (P < 0.05), to bend, which was a statistically significant difference compared to RE (0.361 N) and ZF (0.425 N), respectively. ZF has been shown to lack an R-phase during heating, whereas other files have both an R-phase and an austenite phase. Blue heat treatment significantly enhanced fatigue resistance by stabilizing the R-phase, overriding the influence of pitch geometry. Heat treatment plays a more decisive role than pitch design in determining cyclic fatigue resistance.