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.