Since the 1980s, different devices based on superelastic alloys have been developed to fulfill orthodontic applications. Particularly in the last decades several researches have been carried out to evaluate the mechanical behavior of Ni-Ti alloys, including their tensile, torsion and fatigue properties. However, studies regarding the dependence of elastic properties on residence time of Ni-Ti wires in the oral cavity are scarce. Such approach is essential since metallic alloys are submitted to mechanical stresses during orthodontic treatment as well as pH and temperature fluctuations. The goal of the present contribution is to provide elastic stress-strain results to guide the orthodontic choice between martensitic thermal activated and austenitic superelastic Ni-Ti alloys. From the point of view of an orthodontist, the selection of appropriate materials and the correct maintenance of the orthodontic apparatus are essential needs during clinical treatment. The present work evaluated the elastic behavior of Ni-Ti alloy wires with diameters varying from 0.014 to 0.020 inches, submitted to hysteresis tensile tests with 8% strain. Tensile tests were performed after periods of use of 1, 2 and 3 months in the oral cavity of patients submitted to orthodontic treatment. The results from the hysteresis tests allowed to exam the strain range covered by isostress lines upon loading and unloading, as well as the residual strain after unloading for both superelastic and thermal activated Ni-Ti wires. Superelastic Ni-Ti wires exhibited higher load isostress values compared to thermal activated wires. It was found that such differences in the load isostress values can increase with increasing residence time.
The objective of the present study was to evaluate the effect of the residence time in the oral cavity on the electrochemical and microstructural properties of nickel-titanium superelastic and thermoelastic archwires aiming at establishing optimum treatment parameters regarding materials selection and maintenance of orthodontic apparatus. The corrosion behaviour of commercial archwires of different diameters was studied after periods of use from 1 to 3 months in the oral cavity of patients submitted to orthodontic treatment. Electrochemical measurements and scanning electron microscopy (SEM) were performed. The results revealed that the stiffness of superelastic archwires increased upon use, making it unnecessary to replace archwires in the early stages of treatment with the objective of adapting the diameter of the wire. Electrochemical tests suggested extremely high corrosion resistance of the nickel-titanium alloy even in severe acid environment. SEM images revealed limited pit formation concentrated in the archwire-bracket contacts.
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