Load-deflection characteristics of superelastic nickel-titanium wires

Objective: To determine the mechanical properties of commercially available thermodynamic wires and to classify these wires mathematically into different groups. Materials and Methods: The samples examined were 48 nickel-titanium (NiTi) alloy orthodontic wires commercially available from five manufacturers. These samples included 0.016-inch, 0.016- X 0.022-inch, 0.017- X 0.025-inch, and 0.018- X 0.025-inch wires. The superelastic properties of the NiTi wires were evaluated by conducting the three-point bending test under uniform testing conditions. The group classification was made under mathematically restricted parameters, and the final classification was according to their clinical plateau length. Results: The orthodontic wires tested are classified as follows: (1) true superelastic wires, which presented a clinical plateau length of >= 0.5 mm; (2) borderline superelastic with a clinical plateau length of <0.5 mm and >0.05 mm; and (3) nonsuperelastic, with a clinical plateau length of <= 0.05 mm. The results showed that the range of products displays big variations in quantitative and qualitative behavior. A fraction of the tested wires showed weak superelasticity, and others showed no superelasticity. Some of the products showed permanent deformation after the three-point bending test. Conclusion: A significant fraction of the tested wires showed no or only weak superelasticity. The practitioner should be informed for the load-deflection characteristics of the NiTi orthodontic wires to choose the proper products for the given treatment needs.