Comparison of cold-hardening bending and direct electric resistance heat treatment on the mechanical properties and transformation temperature of NiTi archwire: An in vitro study

Abstract

The aim of this investigation was (1) to compare the effects of cold-hardening bending and direct electric resistance heat treatment (DERHT) methods; and (2) to compare the effects of offsets and angulations on the mechanical properties and thermal analysis of NiTi alloy archwire. Nickel-titanium (NiTi) archwires (Ormco, Glendora, CA), 0.016×0.022-inch, were bent by cold-hardening bending and DERHT methods into 1-mm, 2-mm, or 3-mm offset for a 3-point bending test, and at angles of 30°, 45°, or 60° to the horizontal plane for testing the change in transformation temperature (Af) measured by differential scanning calorimetry (DSC). The data were analysed using ANOVA followed by the Scheffe post-hoc test. The 3-point bending test results of the cold-hardening bending and DERHT methods were not significantly different between the 1-mm, 2-mm and 3-mm offset groups (95% CI: -0.05 to 0.97; P=0.082, 95% CI: -0.65 to 0.74; P=0.983 and 95% CI: -0.61 to 0.98; P=0.813, respectively). Increasing the offset resulted in a significantly decreased force in the 3-point bending test (P<0.001). The Af temperatures of the cold-hardening bending and DERHT methods were not significantly different for the 30°, 45°, and 60° bending angulations (95% CI: -1.93 to 1.39; P=0.876, 95% CI: -1.2 to 0.87; P=0.878, 95% CI: -2.24 to 1.18; P=0.636, respectively). Af temperatures were not influenced by different bending angulations. NiTi archwire shape can be modified by using both cold-hardening and DERHT bending methods, because the mechanical properties and Af temperature are not affected. However, the bending distance has an effect on the mechanical properties.