Objective To study the properties of orthodontic rectangular arch wires in torsion. Methods By using the torsion measurement apparatus, torsion moments and angles of four kinds of orthodontic wires（0.46mm×0.63mm and 0.48 mm×0.63 mm stainless steel wires, and the same size nickel- titanium wires）were tested respectively in a 0.56 mm- slot first maxillary incisor bracket. According to the torsion moments and angles, scatter graphs were drawn to fit the load- deflection curves of four kinds of wires and their linear equations were calculated. According to the equations, the torsion stiffness and the play angles of the wires at 20 N·mm correspondingly were calculated. Results The torsion stiffness of four kinds of wires were 4.909, 6.417, 1.325 and 1.363 N·mm/°, the torsion angles of different wires were 19.14°, 15.33°, 36.83°and 35.07°at 20 N·mm respectively. Conclusion The torsion stiffness of stainless steel wires was bigger than that of the same cross- section dimensional nickel- titanium wires. The difference of torsion stiffness between two dimensional nickel- titanium wires is smaller, while the difference of torsion stiffness between two stainless steel wires is larger.
