OBJECTIVETo measure the strength of lingual component force on first molar while retraction canine by 150 g tractive force with different sizes of arch wire and with different widths of molar offset on the wire.

METHODSWires with different sizes and different materials [0.40 mm and 0.45 mm stainless round wires, 0.40 mm and 0.45 mm Australia wires, 0.41 mm x 0.56 mm (0.016 x 0.022 inch) and 0.46 mm x 0.63 mm (0.017 x 0.025 inch) rectangular stainless wires] were ligated in ideal arch Typodont when the canine was retracted with 150 g force in the direction from the molar buccal hook to the central point of canine bracket's distal wing under different widths of molar offset (1 mm, 2mm, 3mm). the lingual force applied on the first molar was measured by a self-made dynamometer.

RESULTS1. There is lingual component force applied to the first molar while retracting canine with 150 g tractive force. 2. the bigger the cross section area of the wire and the stronger the rigidity of the wire, the smaller the lingual component force turns. The ability of the wire in deducting the lingual component force is in direct ratio with the cross section area and the rigidity of the wire. 3. While the width of wire offset increases, the lingual force produced by the tractive force applied to the first molar decreases lineally.

CONCLUSIONThere is lingual force produced on the anchorage molar when retracting canine. In clinic for avoiding the lingual movement of the first molar, increasing the size of wire or increasing the width of wire offset is suggested.