ELASTIC RESPONSES TO LONGITUDINAL TORSION OF SINGLE-STRAND, RECTANGULAR, ORTHODONTIC ARCHWIRE SEGMENTS

Objectives. This study was undertaken to characterize elastic response s of orthodontic archwire segments in longitudinal torsion, to compare experimental results with predictions from structural engineering the ory, and to examine the potential interaction between flexural and tor sional responses of archwires. Methods. Passively straight and deflect ed rectangular wire segments were activated in torsion to states beyon d their elastic limits. The wire parameters that were Controlled inclu ded: the alloy, the cross-sectional size, and the gauge length. The re search design included 48 cells and 240 separate tests. From torque-tw ist plots, values of elastic stiffness, elastic range, and unit elasti c range were obtained. Raw experimental data were subjected to analyse s of variance and means to a Tukey's post-hoc test. Mean stiffness and elastic range outcomes were compared with theoretical values. Results . Most plots were generally characteristic of Hookean materials. All t hree wire parameters significantly influenced the three dependent vari ables; few statistical interactions emerged. Theoretical stiffness val ues were reasonably comparable to those obtained experimentally; howev er, the elastic range predictions were conservative. Torsion theory pr edicts unit elastic ranges independent of gauge length; the experiment al data displayed a nonlinear relationship. The minor influences of fl exural deformations on the responses of wire segments activated in tor sion are suggested as clinically inconsequential. Significance. Few cl inically relevant, controlled studies of archwire torsion have been pu blished. 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.