Accuracy of third-order bends of nickel-titanium wires and the effect of high and low pressure during memorizing heat treatment

Abstract

This study evaluated the accuracy of third-order bends of nickel-titanium wires and determined the effect of high and low pressure for maintaining the wire shape during memorizing heat treatment. A computer-aided bending machine was used to incorporate 200 randomly determined torsional angles between 0° and 60° into 30 linear 0.016 × 0.022-in NeoSentalloy F80 (GAC International, Central Islip, NY) wires. The torsional bendings were randomized into 2 groups. Bends assigned to group 1 (n = 100) received heat treatment of 1.6 MPa (16 bar) pressure, and bends assigned to group 2 (n = 100) received heat treatment of 50 MPa (500 bar) pressure. Cross-sectional cuts from the bent wires were prepared by using standard metallurgical techniques, and the torsional angles were analyzed under computer control. The results of our study show that third-order bends ≤ 30° can be made with adequate clinical accuracy with an error of 1.89° ± 1.75° (0° to 10°) and 3.57° ± 1.57° (20° to 30°), irrespective of the pressure applied. With bends > 30° but ≤ 40°, the method with the higher pressure offers fundamentally greater precision than that with the lower pressure. With torque bends > 40°, the bending error with both methods is clinically unacceptable. In addition to the variability in the dimension and composition of nickel-titanium wires, the scale of the incorporated plastic deformations makes a substantial contribution to the bending error. As far as permitted by the clinical situation, then, the bend should be distributed over the maximum possible wire length. Third-order bends in the first rectangular pseudoelastic nickel-titanium wires represent an efficient means of effecting torque at an early stage. This individualization allows the full therapeutic potential of these archwires to be exploited right from the initial phase of treatment.