FRICTIONAL FORCES RELATED TO SELF-LIGATING BRACKETS

Orthodontic tooth movement can be regarded as teeth sliding on a wire like pearls on a string, the force being supplied by springs or elasti cs. The movement implies friction between wire and bracket, taking up part of the force and leaving an uncontrolled amount to act on the tee th. The friction is likely to depend on bracket construction and wire material. Therefore, in this investigation the friction of self-ligati ng brackets and beta-titanium wires was evaluated, as opposed to more conventional configurations. Carried by low-friction linear ball beari ngs, a bracket was made to slide along an out-stretched archwire with minimal (and known) basic friction, either parallel or at an angle to the wire. Two self-ligating brackets were used in their closed positio n without any normal force. Friction was tested against four wires: st ainless steel and beta-titanium, both in round and rectangular cross-s ections. The force used to overcome friction and to move the bracket w as measured on a testing machine at 10 mm/min, and the basic friction was subtracted.The results show that round wires had a lower friction than rectangular wires, the beta-titanium wires had a markedly higher friction than stainless steel wires, and friction increased with angul ation for all bracket/wire combinations. The self-ligating brackets ha d a markedly lower friction than conventional brackets at all angulati ons, and self-ligating brackets, closed by the capping of a convention al design, exhibited a significantly lower friction than self-ligating brackets closed by a spring. The selection of bracket design, wire ma terial, and wire cross-section significantly influences the forces act ing in a continuous arch system.