Abstract
Nickel-titanium (NiTi) rotary files have enabled efficient root canal preparations that maintain the canal center with fewer aberrations compared to hand files. However, NiTi rotary files are susceptible to fracture, which can thereby compromise root canal treatment. Therefore, NiTi files have been developed to enhance fracture resistance by modifying design and thermal treatment. The objective of this study was to compare the torsional fatigue resistance and bending resistance of NiTi files manufactured from different alloys and treatments. ProTaper NEXT X2 (PTN; M-wire), V taper 2H (V2H; controlled memory wire), NRT (heat-treated), and One Shape (OS; conventional alloy) instruments of tip size #25 were compared. Torsional fatigue was evaluated by embedding the 3 mm tip of each instrument (N = 10/brand) in resin and the repetitive application of torsional stress (300 rpm, 1.0 N·cm) by an endodontic motor with autostop when the file fractured. The number of loading cycles to fracture was recorded and analyzed by Kruskal–Wallis and Mann–Whitney U tests with Bonferroni's correction. Bending resistance of the instruments was tested using a cantilever bending test to the 3 mm point from the tip (N = 10/brand). The stress was measured when deflection of 3 mm was subjected and statistically analyzed with a one-way analysis of variance and Tukey's honest significance difference test (α = 0.05). V2H withstood the highest number of load applications during torsional fatigue testing (p < 0.05), followed by NRT, PTN, and OS, where the differences between NRT and PTN (p=0.035) and between PTN and OS (p=0.143) were not statistically significant. V2H showed the lowest bending stiffness, followed by NRT, PTN, and OS (p < 0.001). Thermal treatment of NiTi wire resulted in improved mechanical properties, and controlled memory wire provided improved flexibility and torsional fatigue resistance.
Highlights
NiTi rotary instruments have enabled easier and faster root canal preparations that maintain the canal center with fewer aberrations [1]
While conventional NiTi files, which were not subjected to specific thermal treatment, consist of austenite at room temperature, NiTi files manufactured by thermal treatment possess certain amount of martensite or R-phase structure at room temperature [7, 9]
V taper 2H (V2H; SS White, Lakewood, NJ, USA), manufactured using controlled memory (CM) wire, presented superior flexibility and resistance to cyclic fatigue fracture than V taper 2 (SS White) which is made from conventional NiTi alloy [17]
Summary
NiTi rotary instruments have enabled easier and faster root canal preparations that maintain the canal center with fewer aberrations [1]. ProTaper (PTN; Dentsply Maillefer, Ballaigues, Switzerland), made from M-wire, exhibited superior cyclic fatigue resistance compared to NiTi files made from conventional alloy [10]. V taper 2H (V2H; SS White, Lakewood, NJ, USA), manufactured using CM wire, presented superior flexibility and resistance to cyclic fatigue fracture than V taper 2 (SS White) which is made from conventional NiTi alloy [17]. E objective of this study was to compare the torsional fatigue resistance of PTN, V2H, NRT, and OS by using a torque-controlled endodontic motor that simulated their use in clinical practice. Their flexibility was compared by measuring the bending resistance of these NiTi rotary files. Their flexibility was compared by measuring the bending resistance of these NiTi rotary files. e null hypothesis was that there were no differences in torsional fatigue resistance or bending resistance between the PTN, V2H, NRT, and OS instruments
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