Innovative materials: the NiTi alloys in orthodontics.
Since ten years the NiTi alloys have gained an ever increasing place in orthodontic practice: that is due to their peculiar mechanical properties ascribed to a martensitic thermoelastic transformation which can be thermally or, in a proper temperature range, stress-induced. In the last case, when martensite is stress-induced at body temperature, the stress-strain behaviour is pseudoelastic with large deformations gained or recovered at constant stress, respectively in direct/reverse transformation: this behaviour exploited in orthodontics allowed to overcome the drawbacks intrinsic to the use of conventional alloys as stainless steel or Co-Mo alloys, where small displacements can be achieved at decreasing loads. From the phase state diagram of NiTi alloys it appears that at body temperature they are stable, but out of equilibrium: thermal treatments at intermediate temperatures can therefore modify the equilibrium state and as a consequence the transformation temperatures respect to body temperature. That allows to modify the recovery stress level according to the requirements of practice and thus disclosing new roads: the capability to foresee NiTi archwires pre-programmed in different sections, with a personalized scheme. Attention has not currently been paid to the modifications in the recovery stress induced by a temperature change inside the oral cavity. Recent results have shown that the thermal changes in the oral cavity induced by cold/hot liquid intake can considerably modify the stress level to which the dentition is exposed: though confined to the time extent connected with drinking, similar effects can be expected also for meals intake and should be taken into account for a correct procedure.
Airoldi
G
G
Dipartimento di Fisica, Universita' di Milano, Italy. airoldi@mvmidi.mi.infn.it
Riva
G
G
D001672:Biocompatible Materials; D003722:Dental Alloys; D004548:Elasticity; D006801:Humans; D009532:Nickel; D009967:Orthodontic Appliances; D009969:Orthodontic Wires; D013314:Stress, Mechanical; D013696:Temperature; D014025:Titanium
D001672:Biocompatible Materials; D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium