Variability of effective root torque as a function of edge bevel on orthodontic arch wires.

The effective torque delivered by a variety of arch wire-bracket combinations has been measured experimentally in terms of the deviation angle, by means of a torque-meter apparatus, and compared to theoretical values calculated from both nominal and measured bracket slot and wire dimensions. Edgewise brackets with both 0.018 and 0.022 inch slot widths from two manufacturers were used, with three different arch wire sizes for each slot size. Stainless steel, nickel-cobalt, and beta titanium wires from a total of three manufacturers were employed. The present effective torque values were less than the theoretical values previously reported by Dellinger and Creekmore, who had separately focused on manufacturer tolerances associated with the wire cross sections and slot widths, respectively. Differences between the experimental and theoretical values of deviation angle were in agreement with measurements of the wire-bracket tolerances and with microscopic observations of differences in the edge bevel at the corners of the arch wires. The average edge bevel contribution to the measured deviation angle varied from 0.2 degrees to 12.9 degrees for the various wire-bracket groups, and the average percentage contribution from 3 to 63%. The edge bevel contribution to the deviation angle was higher for stainless steel wires than for nickel-cobalt wires marketed as square-cornered. The highest deviation angles and edge bevel contributions were found for the beta titanium wires.