The effect of cross-sectional dimensional variations of square and rectangular chrome-cobalt archwires on torsion

The purpose of this investigation was to study the control of cross-sectional dimensions and edge bevel by various manufacturers in the production of chrome-cobalt archwires and this effect on transmitting torque through an .018 inc slot bracket system. Twenty-seven different square and rectangular chrome-cobalt wires commonly used in the edgewise technique supplied by five different manufacturers were studied with respect to dimensions, edge bevel, and mechanical properties in torsion. The mechanical study simulated application of torque to an individual tooth. Standard brackets with .018 inch slot heights were used. The results show that variation in cross-sectional dimension and edge bevel leads to variable torsional play (third-order clearance). As an example, .016 x .016 wires have a mean torsional play of as much as 26.8 degrees, with a range of 21.0 to 32.3 degrees. When using .016 x .016 wires, one must apply from 34.8 to 48.6 degrees of twist to get 20 N-mm of torsional moment. This variation is primarily due to the rather wide range in torsional play. As a result, the prediction by which a predetermined torsional moment can be delivered becomes uncertain. The results also show that because the working range in torsion of chrome-cobalt wires is somewhat limited due to high torsional stiffness, precise delivery of torsional moment based on the condition present in the oral cavity is difficult. The torsional stiffness varies between manufacturers within the various dimensional groups as a result of differences in cross-sectional geometry and material properties.