Mechanical Properties of New NiTi Wire for Rotary Endodontic Instruments

Rotary endodontic instruments manufactured from nickel-titanium wire (M-Wire) processed by a proprietary procedure have shown improved in vitro fatigue performance, compared to conventional instruments fabricated from superelastic (SE) wire. Objective: Compare mechanical properties of M-Wire (SportsWire LLC) and conventional SE wire (Dentsply Maillefer). Methods: Wires having 1 mm diameter were evaluated with the tension test (5 cm segments) and three-point bending test (25 mm test spans). Four specimens were used for each test and wire type. Tensile test specimens were loaded to failure, and bending specimens were loaded to 4 mm deflection. The cross-head speed was 2 mm/minute, and tests were performed at 23°C. Mechanical property measurements focused on the superelastic (SE) plateau stress and fracture stress in tension and the elastic modulus in bending. Mean values of these properties were compared for both wires, using Student's t-test. Fracture surfaces of tensile test specimens were examined with an SEM. Results: The SE plateau stress and fracture stress were 455 ± 6 and 1320 ± 13 MPa for M-Wire and 520 ± 72 and 1222 ± 119 MPa for conventional SE wire. The elastic modulus of M-Wire and the SE wire were 81.5 ± 2.6 and 79.5 ± 1.2 GPa, respectively. Values of tensile properties were significantly different for the two wires, but not the elastic modulus in bending. Both wires had ductile fracture surfaces, with M-Wire having larger grain size. Conclusions: Elastic modulus values of both wires lie in the range for engineering SE NiTi alloys. The ratio of fracture stress to SE plateau stress was greater for M-Wire (2.90) than the SE wire (2.35). Macroscopic values of elastic modulus for bending are much higher than those from a complementary nanoindenter study. Research with the transmission electron microscope is needed to understand structure-property relationships for these wires.