OBJECTIVETo study the properties of orthodontic rectangular arch wires in torsion.

METHODSBy using the torsion measurement apparatus, torsion moments and angles of four kinds of orthodontic wires (0.46 mm x 0.63 mm and 0.48 mm x 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.

RESULTSThe torsion stiffness of four kinds of wires were 4.909, 6.417, 1.325 and 1.363 N mn/degrees, the torsion angles of different wires were 19.14 degrees, 15.33 degrees, 36.83 degrees and 35.07 degrees at 20 N mm respectively.

CONCLUSIONThe 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.