Abstract
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.
Highlights
Ni-Ti alloys are known as important shape memory alloys (SMA) and have many applications in the medical field
In the orthodontic treatment using conventional alloys, wires are usually replaced every 30 days to account for losses in stiffness and resilience which decreases the forces applied to the teeth
Opposite experimental trends were observed as a function of clinical residence time
Summary
Ni-Ti alloys are known as important shape memory alloys (SMA) and have many applications in the medical field. They are widely used as orthodontic wires and endodontic instruments in dentistry as a result of their pseudoelastic behavior[1,2]. Each tooth and its root are progressively moved to the correct position with minimum discomfort to the patient, reducing the risks of tissue hyalinization and root resorption[3,4] This is especially due to detwinning of the microstructure that accompanies deformations on iso-stress conditions resulting from austenitic-to-martensitic transformations[2]. In the case of martensite thermal activated Ni-Ti alloys, with increase of stress the SMA wire experiments a phase change from
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