Third-order torque and self-ligating orthodontic bracket-type effects on sliding friction.

OBJECTIVE To examine the influence of third-order torque on kinetic friction in sliding mechanics involving active and passive self-ligating brackets. MATERIALS AND METHODS Wire-slot frictional forces were quantified and compared across five sets of brackets and tubes within a simulated posterior dental segment with -15 degrees , -10 degrees , -5 degrees , 0 degrees , +5 degrees , +10 degrees , and +15 degrees of torque placed in the second-premolar bracket; a working archwire was pulled through the slots. RESULTS Increasing the torque from 0 degrees to +/-15 degrees produced significant increases in frictional resistance with all five sets of brackets and tubes. At 0 degrees and +/-5 degrees of torque, generally less friction was created within the passive than within the active self-ligating bracket sets, and the conventional bracket sets with elastomeric ligation generated the most friction. At +/-10 degrees of torque, apparently with wire-slot clearance eliminated, all bracket-and-tube sets displayed similar resistances, with one exception at +10 degrees . At +/-15 degrees of torque, one passive set and one active set produced significantly larger frictional resistances than the other three sets. CONCLUSIONS Third-order torque in posterior dental segments can generate frictional resistance during anterior retraction with the archwire sliding through self-ligating bracket slots. With small torque angles, friction is less with passive than with active self-ligating brackets, but bracket design is a factor. Frictional forces are substantial, regardless of ligation if the wire-slot torque exceeds the third-order clearance.