Viscoelastic properties of an aesthetic translucent orthodontic wire

The objective of this study was to evaluate the time-dependent viscoelastic properties of an aesthetic orthodontic archwire. The wire is based on a recently developed translucent polyphenylene thermoplastic, whose rigid molecular structure provides high strength. While the wire has good instantaneous mechanical properties, over time all polymers may relax so it is important to understand the potential impact of the relaxation on orthodontic force systems. Four samples of 0.020 inch round and six samples of 0.021 x 0.025 inch rectangular wire were loaded in tension to a range of initial stresses, and relaxation of the stress was monitored for 7 days. Sixty-three additional samples were maintained in edgewise bracket pairs with vertical displacement for up to 6 weeks. The deformation of these wires was measured immediately after removal from the brackets and for 2 days as the samples recovered. Tensile stress decayed about 10-30 per cent over 24-48 hours depending on the initial stress. The relaxation behaviour was proportional to the initial tensile strain and therefore these data were combined into a single curve using regression. Deformation of the samples placed in the bracket pairs increased with increasing vertical displacement and time, evaluated with analysis of variance, but 19-100 per cent of the deformation was recoverable. The force systems from polyphenylene wires could vary with time and activation, but this behaviour is predictable.