Elastic responses to longitudinal torsion of single-strand, rectangular, orthodontic archwire segments

This study was undertaken to characterize elastic responses of orthodontic archwire segments in longitudinal torsion, to compare experimental results with predictions from structural engineering theory, and to examine the potential interaction between flexural and torsional responses of archwires.

Passively straight and deflected rectangular wire segments were activated in torsion to states beyond their elastic limits. The wire parameters that were controlled included: the alloy, the cross-sectional size, and the gauge length. The research design included 48 cells and 240 separate tests. From torque-twist plots, values of elastic stiffness, elastic range, and unit elastic range were obtained. Raw experimental data were subjected to analyses of variance and means to a Tukey's post-hoc test. Mean stiffness and elastic range outcomes were compared with theoretical values.

Most plots were generally characteristic of Hookean materials. All three wire parameters significantly influenced the three dependent variables; few statistical interactions emerged. Theoretical stiffness values were reasonably comparable to those obtained experimentally; however, the elastic range predictions were conservative. Torsion theory predicts unit elastic ranges independent of gauge length; the experimental data displayed a nonlinear relationship. The minor influences of flexural deformations on the responses of wire segments activated in torsion are suggested as clinically inconsequential.

Few clinically relevant, controlled studies of archwire torsion have been published. A modified or new formula is needed to predict elastic range magnitudes of archwires in torsion. When flexure and torsion exist in an archwire, it may be possible to separate them to determine overall structural response.