Frictional behavior of stainless steel bracket-wire combinations subjected to small oscillating displacements

In orthodontic treatment, sliding is frequently used to cause tooth movement. Inherent to this technique is the generation of a counteracting frictional force. In this pilot study, a fretting test consisting of reciprocating tangential displacements was used to investigate test parameters influencing frictional forces during sliding processes. Tests were run at a normal load of 2 N and a frequency of 1 Hz for tangential displacement strokes of 200 microm. Stainless steel orthodontic wires with cross-sections of .017 x .025 in (W17) and .018 x .025 in (W18), and brackets with slot sizes of .018 in (B18) and .022 in (B22) were used. A specific centered positioning method was developed to achieve a parallel alignment of the wire and the bracket slot. The experimental results indicated the significant role of the centered positioning method on the friction value. Implementation of the centered positioning method resulted in a friction force ranging from 0.89 N to 0.97 N at a 200 microm displacement amplitude and 1 Hz frequency, corresponding to a coefficient of friction ranging from 0.45 to 0.49 for the B18-W17 and the B22-W17 bracket-wire combinations, respectively. When the centered positioning method was not used, significantly higher values for the coefficient of friction were found for both bracket-wire combinations. The slot-filling, bracket-wire combinations (B18-W18 and B22-W22) resulted in an increased coefficient of friction and therefore are not recommended as sliding systems.