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

METHODSTo measure and analyze the torsional moments and angles of 0.46 mm x 0.63 mm rectangular orthodontic arch wires in single 0.022 inch bracket of the maxillary central incisor with a torsion measurement apparatus.

RESULTS(1) The torsional stiffness of stainless steel arch wires, Chinese nitinol arch wires, 7.0 mm vertical loop arch wires, 3.0 mm and 6.0 mm horizontal loop arch wires is 4.909, 1.325, 4.137, 3.448 and 3.024 Nmm per degree, respectively. (2) The arch wires will be twisted 19.143, 36.832, 19.851, 21.812 and 22.418 degrees; respectively to obtain optimal force. (3) Vertical and horizontal loop decreased the torsional stiffness of the rectangular stainless steel arch wire.

CONCLUSIONThe straight rectangular stainless steel arch wire is not well suitable for the torque control of the incisor. Loop design in arch wire is capable of decreasing torsional stiffness. The horizontal loop is more effective than vertical loop.