Superelasticity and Shape Memory Effect in NiTi Orthodontic Wires

The superelasticity of NiTi shape memory alloys is exploited in orthodontics to provide a constant force in the correction of malocclusions, allowing both to nillfill the requirements of the "opumaTorthodontic force and to considerably shorten the therapy. Though some little drawbacks (superelasticity prevents formability, few choices of the force are available), which can however be overcome by the so-called "Shape & Force user Programmable Orthodontics", the great benefits offered by superelasticity have dictated the success of NiTi wires in orthodontics. Since superelasticity follows the Clausius-Clapeyron like equation, changes in the superelastic force can be expected as a consequence of a temperature change, induced by oral environment temperature modifications. In order to investigate the change in the superelastic force at body temperature (Tb) as a consequence of temperature loops, 3-point bending measurements have been here undertaken on NiTi orthodontic wires. The results have shown that, at a fixed deflection on the reversion plateau, simulating a state in the orthodontic therapy, the superelastic force is generally increased as a consequence of a temperature loop. However, as the unloading proceeds, at T=37°C, the superelastic force, expected at Tb , is nearly recovered. In order to enlighten these results, tensile tests have also been performed. The 3-point bending results have been discussed at the light both of the tensile tests results and of the inhomogeneous stress-strain fields which set in along the bent wire.