A comparison of the effects of rectangular and round arch wires in leveling the curve of Spee

A prospective randomized clinical study was designed to evaluate the effects of full continuous arch wires, rectangular in cross section, on the axial inclination of lower incisors. The intention of rectangular arch wires is to counteract the labial crown moment usually produced during leveling the curve of Spee with full arch mechanics. Patients were randomly assigned to 2 groups. Group 1 (N = 12) received round arch wires throughout the leveling stage. Group 2 (N = 16) started with flat 0.016 x 0.022 nickel titanium arch wires progressing to 0.016 x 0.022 stainless steel. The preadjusted 0.018 x 0.025 edgewise appliance was used in all cases. Lateral cephalometric radiographs and mandibular study models were taken before treatment and when the curve of Spee was leveled (or in some cases when the overbite was considered clinically acceptable). In group 1, the lower incisor proclined a mean of 6.75 degrees +/- 4.85 degrees (P <.01) and in group 2 it proclined a mean of 6.10 degrees +/- 3.95 degrees (P <.01). However, no significant difference in proclination was detected between the 2 groups. Statistically significant, but low, correlations were demonstrated between change in lower incisor axial inclination and relief of crowding r = 0.45) and change in mandibular arch depth r = 0.54), which was in turn inversely correlated with change in intercanine width r = -0.45). In both groups, the lower incisors proclined with uncontrolled tipping that can probably be attributed to the intrusive force introduced by the arch wire being labial to the center of resistance of the lower incisors. The ability of the rectangular arch wires to control labial proclination following leveling of the curve of Spee, as used in this study, was not supported.