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 | [Comparaison de l’écrouissage par le durcissement au froid et le traitement thermique par résistance électrique sur les propriétés mécaniques et la température de transformation de l'arc NiTi: étude in vitro]

Objectives > 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.
Methods > 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 308, 458, or 608 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.
Results > 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 308, 458, and 608 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.
Conclusions > 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.