A rationale for the selection of orthodontic wires

A logical basis for the sequence of wires chosen during orthodontic treatment with fixed appliances is presented. This utilizes a new more realistic model for the radial deflection of a segment of the archwire in which the wire is restrained by the ligatures on the misaligned tooth, contacts the inner edges of the brackets either side, and is restrained by the outer ligatures on these brackets. The model, analysed using simple beam theory, allows the determination of the upper and lower bounds for the range of deflection over which orthodontic forces within the optimum range are produced without distortion for any given wire providing its flexural rigidity (EI) and yield stress in bending (sigma o) are known. The same model can be applied to vertical deflections. Some wire sequences recommended on clinical experience appear to conform to this more pragmatic approach, although to cater for even moderate misalignment the forces predicted by the model are higher than normally considered desirable.