An in vitro study of cyclic fatigue resistance and bending performance of three heat-treated nickel-titanium endodontic systems at physiological temperature

Objective. To evaluate and compare the cyclic fatigue resistance and bending resistance of three heat-treated nickel-titanium systems (2Shape, Reciproc Blue, and EdgeFile X7) at a physiological intracanal temperature (37°C). Methods. Eighty-four unused NiTi instruments were tested, with a total of 28 instruments allocated to each of the three systems. Fourteen independent instruments from each system – 2Shape (25/.06), Reciproc Blue (25/.08), and EdgeFile X7 (25/.06) – were used for each test. Bending resistance testing was conducted according to ISO 3630-1 using a universal testing machine (Instron, Canton, MA, USA) with a 45° deflection protocol at 37°C. Cyclic fatigue resistance was evaluated in a stainless-steel artificial canal with a 45° angle of curvature, a 3-mm radius of curvature, a 1.5 mm canal diameter, and a 3.5 mm curved segment. The canal was immersed in a thermally controlled water bath. Time to fracture was recorded for the reciprocating system, and NCF was calculated for the continuous-rotation systems. Data were analyzed using one-way ANOVA followed by post hoc tests (α = 0.05). Effect size was recorded. Results. Significant differences were observed among the tested groups (p < 0.05). EdgeFile X7 demonstrated the highest cyclic fatigue resistance, while Reciproc Blue showed the greatest bending resistance. Fragment length did not differ significantly among the groups (p > 0.05). Effect size analysis indicated a strong influence of instrument design and heat-treatment technology on mechanical behavior. Conclusion. The mechanical performance of the tested heat-treated NiTi instruments at 37°C is influenced by both metallurgical phase composition and geometric design. EdgeFile X7 demonstrated higher resistance to cyclic fatigue at simulated intracanal temperature, while Reciproc Blue exhibited higher bending resistance, likely associated with its greater taper and metal mass.