THE EFFECT OF 2ND-ORDER COUPLE ON THE APPLICATION OF TORQUE

Complex combinations of linear forces, moments, and couples are develo ped by the arch wire during orthodontic treatment. For instance, appli cation of torque to a canine during distal driving may create force in teractions if the tooth tips distally toward the extraction site. This investigation studied the effect of second-order couples and bracket angulations on the application of torque to a single tooth. By using a test apparatus to simulate application of torque to a single tooth, 0 .016 x 0.022 inch stainless steel wires were tested in longitudinal to rsion simultaneous to fixed amounts of second-order couples or fixed d egrees of second-order bracket angulation. Application of a second-ord er couple through a bracket to a longitudinally twisted arch wire prod uces a third order couple, since the bracket slot walls exert forces o n the wire, tending to detwist it. This third-order couple will usuall y be small as the distance between the two couple members is short. Ne vertheless, it may have a restraining effect on the third-order wire-b racket interaction. The results show that application of second-order couples or bracket angulations lead to an increase in exerted torque f or angles of twist below 22 degrees. Because of torsional play, a wire twisted 18 degrees in a 0.018-inch bracket slot did not exert any tor que unless it was subjected to a second-order couple. Thus, in an in v ivo situation where farces interact, the actual torsional play may be substantially less than predicted from theoretical models only regardi ng third-order mechanics, The restraining effect of second-order coupl es tapered when the torque created by longitudinal twisting became muc h larger than the torque exerted by the second-order couple, Second-or der couples of biologically acceptable magnitudes had little effect on the level of torque after the third-order clearance had been eliminat ed.