Superelasticity of nickel-titanium orthodontic archwieres: metallurgical structure and clinical importance.

The purpose of this paper is to present an overview of the applications of NiTi alloys in orthodontics, analyze their metallurgical structure and summarize the phase transformation characteristics of these wires; recent evidence on the intraoral aging pattern of these materials is also presented. The implication of the metallurgical and mechanical properties of the wires in their clinical application is discussed in the light of recent evidence which explore the time variants of orthodontic therapy with the use of NiTi or alternative archwires. Evidence indicating that the load expressed by the superelastic nickel-titanium wires increases on heating and decreases on cooling is provided. Effects on the stepwise temperature changes on heating and cooling are summarized revealing that the load measured at body temperature as the final step was much higher than that measured at 37οC as an initial step. Clinical studies are reported, according to which, no significant differences among superelastic and nonsuperelastic NiTi wires and multistranded stainless steel wires have been found with respect to treatment duration.