Influence of radius of curvature on the cyclic fatigue resistance of three NiTi systems at simulated body temperature

Abstract Background Instrument separation due to cyclic fatigue remains a primary clinical concern in curved root canals. Furthermore, testing at room temperature may alter the phase-dependent mechanical behavior of heat-treated nickel-titanium (NiTi) instruments compared with physiological conditions. This study aimed to evaluate the cyclic fatigue resistance of three heat-treated NiTi systems—TruNatomy (TRU), ProTaper Gold (PTG), and Orodeka ONEPLEX (ORO)—in a static artificial canal model at simulated body temperature (37 °C) under varying curvature radii. Materials and methods A total of 135 instruments (TRU Prime, PTG F2, and ORO ONEPLEX; n = 15 per brand/radius) were rotated until fracture inside custom stainless-steel artificial canals (45° angle of curvature) with three radii ( R = 4, 6, and 8 mm) in a liquid bath maintained at 37 °C. Time to fracture was recorded, and the number of cycles to failure (NCF) was calculated. Fractured surfaces were evaluated using scanning electron microscopy (SEM). Data were analyzed using two-way and one-way ANOVA followed by Tukey’s post-hoc tests (α = 0.05). Results Decreasing the curvature radius significantly reduced the NCF for all systems ( P < 0.05). A significant interaction was observed between instrument type and curvature radius ( P < 0.001). At acute curvature ( R = 4 mm), all systems recorded their lowest NCF with no significant differences among them ( P > 0.05). At moderate curvature ( R = 6 mm), TRU and PTG exhibited significantly higher NCF than ORO ( P < 0.05). At mild curvature ( R = 8 mm), ORO recorded the highest NCF, followed by TRU, while PTG showed the lowest ( P < 0.05). SEM confirmed typical flexural fatigue failure. Conclusions Within the limitations of this in vitro study, smaller curvature radii markedly compromised the cyclic fatigue life of heat-treated NiTi files at 37 °C. Under acute curvature ( R = 4 mm), severe bending stress overrides individual design advantages, leading to rapid fatigue failure across all systems. TRU and PTG provided superior fatigue resistance in moderate curves, whereas ORO demonstrated higher durability in mild curves. These system-level findings cannot be directly extrapolated to clinical performance.