THE REVERSION FORCE OF NITI ORTHODONTIC WIRES IN THE TEMPERATURE DOMAIN 5-DIVIDED-BY-55-DEGREES-C

The interest in using the pseudoelastic properties of NiTi wires in or thodontic practice arises chiefly from the expectation that they can a fford tow continuous forces at constant temperature. The reversion for ce developed by NiTi archwires depends, however, upon temperature sinc e the superelastic force follows a Clausius-Clapeyron like equation. A ctually the oral environment temperature is modified as a consequence of hot/cold liquid/solid intake in the temperature range 5 degrees-55 degrees 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 func tion of temperature under different fixed constraints. Tensile and 3-p oint bending tests have been performed on NiTi orthodontic wires, adop ted in orthodontic practice, starting at body temperature: loading and partial unloading have been performed in order to fix different const raint conditions corresponding to different states in orthodontic ther apy. The variation of the reversion force has been studied as a functi on of temperature. Evidence has been obtained of the hysteretic behavi our of the reversion force as a function of temperature. The results s how that after a temperature loop to high temperature the reversion fo rce is generally changed: the overall shape of the hysteresis cycle ap pears dominated by the presence of the R-phase at T<37 degrees C and b y the stress-induced martensite at T>37 degrees C.