Biomechanics of torque from twisted rectangular archwires. A finite element investigation

To evaluate the magnitudes of initial and subsequent sequential deactivational third-order moments generated in rectangular twisted archwires (either when the archwires are actually twisted before insertion in the standard edgewise bracket slots or when they are indirectly twisted in the preadjusted slots of the modern edgewise brackets) in order to judge their biologic acceptability.

A finite element study was carried out with the MSC Patran/Nastran interface. Three-dimensional models were constructed with 170 nodes of upper 0.017- x 0.025-inch and 0.019- x 0.025-inch archwire segments extending bilaterally from the maxillary central incisors to the first premolars. Required twists were applied at the appropriate locations to derive the applied and reactionary moments both initially and during the time needed for complete deactivation.

The results indicated that a round-tripping possibility does exist in certain clinical procedures. Furthermore, the moments produced could be quite high, thereby enhancing the possibility of root resorption.

Twists in rectangular archwires may be used only when reciprocal torque is needed on adjacent teeth. In other situations, alternative torquing methods should be considered.