The Reversion Force of NiTi Orthodontic Wires in the Temperature

The interest in using the pseudoelastic properties of NiTi wires in orthodontic practice arises chiefly from the expectation that they can afford low continuous forces at constant temperature. The reversion force developed by NiTi archwires depends, however, upon temperature since the superelastic force follows a Clausius-Clapeyron like equation. Actually the oral environment temperature is modified as a consequence of hotJcold liquid/solid intake in the temperature range 5"5S°C, as previously evaluated. In the above-mentioned temperature range first results have already been obtained on the reversion force variation. Attention is focused here on the reversion force as a function of temperature under different fixed constraints. Tensile and 3-point bending tests have been performed on NiTi orthodontic wires, adopted in orthodontic practice, starting at body temperature: loading and partial unloading have been performed in order to fix different constraint conditions corresponding to different states in orthodontic therapy. The variation of the reversion force has been studied as a function of temperature. Evidence has been obtained of the hysteretic behaviour of the reversion force as a function of temperature. The results show that after a temperature loop to high temperature the reversion force is generally changed: the overall shape of the hysteresis cycle appears dominated by the presence of the R-phase at T<37OC and by the stress-induced martensite at P37OC.