Actual versus theoretical torsional play in conventional and self-ligating bracket systems

The aim of this study was to assess the amount of torsional play in 32 commercially available self-ligating and conventional 0·018-inch and 0·022-inch bracket systems in relation to 0·017×0·022-inch and 0·019×0·025-inch stainless steel wires, respectively, and compare the results with the theoretical amount of play for the given bracket/wire combinations.

Torque moments were measured in a mechanical force testing system by twisting straight pieces of stainless steel wire seated in the bracket slot in increments of 0·5° until a full torsional expression was registered. Five upper central incisor brackets from each of the 32 different bracket systems were selected for the study.

The result from the laboratory testing clearly showed that wire/slot play was larger than anticipated from the nominal values, especially regarding the 0·022-inch brackets and particularly in relation to the passive self-ligating brackets. The play ranged from 19·8 to 36·1° of play for the most imprecise bracket system.

The result does not favour the use of self-ligating brackets when focussing on torque control. The actual play is larger due to oversized slots and the inability of self-ligation brackets to press the archwire into the bottom of the slot. In conventional brackets, the initial torque moment is generated by the steel ligatures pressing the arch wire against the bottom of the slot. The oversize of the slot is thus less critical in relation to the conventional than in relation to the passive self-ligation bracket.