Effects of third-order torque on frictional force of self-ligating brackets

Objective: To investigate the effects of third-order torque on frictional properties of self-ligating brackets (SLBs). Materials and Methods: Three SLBs (two passive and one active) and three archwires (0.016 × 0.022-inch nickel-titanium, and 0.017 × 0.025-inch and 0.019 × 0.025-inch stainless steel) were used. Static friction was measured by drawing archwires though bracket slots with four torque levels (06, 106, 206, 306), using a mechanical testing machine (n = 10). A conventional stainless-steel bracket was used for comparison. Results were subjected to Kruskal-Wallis and Mann-Whitney U-tests. Contact between the bracket and wire was studied using a scanning electron microscope. Results: In most bracket-wire combinations, increasing the torque produced a significant increase in static friction. Most SLB-wire combinations at all torques produced less friction than that from the conventional bracket. Active-type SLB-wire combinations showed higher friction than that from passive-type SLB-wire combinations in most conditions. When increasing the torque, more contact between the wall of a bracket slot and the edge of a wire was observed for all bracket types. Conclusions: Increasing torque when using SLBs causes an increase in friction, since contact between the bracket slot wall and the wire edge becomes greater; the design of brackets influences static friction.