Pseudoelasticity of NiTi Orthodontic Wires Modified by Current Methodologies : A Critical Comparison

Abstract
It is well known that applications of NiTi alloys in orthodontics rely on the pseudoelastic properties developed at temperatures higher than A_f. That implies as a necessary condition that the body temperature (T_b) be higher than A_f ; the level of reversion stress, σ_r depends moreover upon the difference between T_b and A_f. Nowadays the interest is directed to obtain an archwire with distinct σ_r in different segments : that means to modify locally the properties of the wire by local thermal treatments. Current methodologies exploit either the joule effect or a hot air thermal treatment : both have been here used on NiTi archwires to modify (T_b - A_f) and, as a consequence, the expected σ_r. The properties detected along the archwires by means of Differential Scanning Calorimetry, have been found quite disuniform when the active procedure of the resistive heat treatment was used, whilst good uniformity has been found when adopting the passive hot air heat treatment method. At the light of the present results, the hot air treatment method, allowing a better local temperature control, appears more reliable than the resistive heat treatment method.