Bending Properties of Superelastic and Nonsuperelastic Nickel-titanium Orthodontic Wires

Cantilever bending properties were evaluated for several clinically popular sizes of three superelastic and three nonsuperelastic brands of nickel-titanium orthodontic wires in the as-received condition, and for 0.016-inch diameter wires after heat treatment at 500 degrees and at 600 degrees C, for 10 minutes and for 2 hours. A torque meter apparatus was used for the bending experiments, and the specimen test-span length was 1/4 inch (6 mm). In general, the bending properties were similar for the three brands of superelastic wires and for the three brands of nonsuperelastic wires. For the three brands of superelastic wires, heat treatment at 500 degrees C for 10 minutes had minimal effect on the bending plots, whereas heat treatment at 500 degrees C for 2 hours caused decreases in the average superelastic bending moment during deactivation; heat treatment at 600 degrees C resulted in loss of superelasticity. The bending properties for the three brands of nonsuperelastic wires were only slightly affected by these heat treatments. The differences in the bending properties and heat treatment responses are attributed to the relative proportions of the austenitic and martensitic forms of nickel-titanium alloy (NiTi) in the microstructures of the wire alloys.