Study of the Impact of the Treatment Period on the Surface Characteristics of Nitinol Heat-Activated Orthodontic Wires

Heat-activated orthodontic arches exhibit a shape memory property, namely, when the temperature in the mouth increases, a phase transition is induced to the austenitic phase with the corresponding restoration of the wire’s shape to the original one. The clinical experience has shown that during orthodontic treatment a wire remains in the patient’s mouth most often between 4 and 6 weeks, and less often, between 8 and 12 weeks. The different transition effects in the heat-activated nickel-titanium arch wires are determined by various processes and factors, such as the overall composition, the time for annealing and the formation of alloys. In the work presented, the influence was investigated of the treatment period on the chemical composition and the structure of heat-activated nickel-titanium wires (3M Unitek, Monrovia, Calif.) with two different profiles – round with a diameter of 0.013 in and rectangular 0.016×0.022 in. The analyses were carried out by three independent techniques, namely X-ray diffraction analysis (XRD), scanning electronic microscopy (SEM) and energy-dispersive X-ray analysis (EDX). The results indicated that the basic chemical composition of the orthodontic wires investigated did not change during a treatment period of up to 10 weeks. When the duration of treatment was increased, we observed a gradual amorphization of the material (in both arch profiles). The presence of chemical alloys and deposits is associated with the individual oral hygiene of each patient.