[Metallurgic behavior and mechanical property of nickel-titanium endodontic files made by 3 heat treatment techniques].

Objective: To compare the phase transformation behavior, bending property and cyclic fatigue resistance of CM-wire, R-phase and M-wire nickel-titanium endodontic files treated by 3 heat treatment techniques. Methods: Hyflex CM (25 mm, 25#/0.06) made from CM wire, TF (25 mm, 25#/0.06) made from R-phase heat treatment, ProTaper Next X2 (25 mm, tip size 25#/0.06) made from M-wire, Mtwo (25 mm, 25#/0.06) and ProTaper F2 (25 mm, tip size 25#/0.08) made from conventional nickel-titanium wire were chosen. Five of each files were chosen for differential scanning calorimetric (DSC) evaluation (n=5). The specimens were prepared from adjacent portions of the shaft. The weight and length were typically (20+/-1) mg and 2-3 mm. Another eight of the 5 nickel-titanium files were undergone cantilever-bending test, respectively (n=8). Further ten of the 5 instruments were submitted to the cyclic fatigue test by using a simulated metal root canal (n=10). The data of M(s), M(f), A(s), A(f), DeltaH, bending load, number of cyclic fatigue (NCF), fragment length were analyzed using one-way analysis of variance at a significance level of P<0.05. Results: The A(f) of Hyflex CM [(60.27+/-0.94) ℃] and ProTaper Next [(51.40+/-0.32) ℃] were higher than body temperature of 37 ℃. The bending load values at 3.0 mm deflection of Hyflex CM [(0.867+/-0.074) N], TF [(1.275+/-0.146) N], Mtwo [(2.281+/-0.426) N], ProTaper Next [(3.104+/-0.252) N] and ProTaper [(4.227+/-0.483) N] increased gradually and significantly. The NCF of Hyflex CM (582+/-99), TF (427+/-85), ProTaper Next (158+/-22), Mtwo (129+/-32) and ProTaper (65+/-20) decreased gradually. Conclusions: The 3 heat treatment techniques exert tremendous influence on the phase transformation behavior and mechanical characteristics of the files, which demonstrate improved bending property and cyclic fatigue resistance than the conventional ones.