Torque expression in conventional and self-ligating orthodontic brackets: An in vitro study

This study assessed torque expression at different angulations of conventional, interactive, and passive self-ligating brackets, with or without metal ligatures, and evaluated surface morphology and chemical composition (%). All brackets (Roth prescription, 0.022" x 0.028" slot) were fixed to round acrylic cylinders with cyanoacrylate adhesive. Five hundred segments of rectangular archwires (0.019" x 0.025") were bent and welded (20 mm x 10 mm), then fixed to the brackets either with or without 0.010" metal ligatures in ten experimental groups (n = 10). The assembled specimens were then mounted on a universal testing machine, and a vertical force was applied to the free end of the archwire, resulting in a 10 mm displacement. Mean torque values (N·mm) at specified torque angles of 0° to 25°, as well as the mean torque angles (°) corresponding to 5 and 20 N·mm, were analyzed using ANOVA and Tukey’s test (α = 0.05). Bracket design and chemical composition were examined by SEM/EDS. Slot angulation and dimensional measurements were evaluated using ImageJ. Passive self-ligating brackets, without and with ligatures, showed the smallest torque angles, statistically smaller than those of the other groups (p < 0.001). Bracket clips were composed predominantly of nickel and titanium. Passive self-ligating brackets, either ligated or not, showed the highest torque expression at both 5 and 20 N·mm. Overall, torque expression was influenced more by the dimensional characteristics of brackets and rectangular archwires than by the type of ligature used. Clinically, passive self-ligating brackets may facilitate earlier torque expression during orthodontic treatment.

Key Words:
Orthodontic brackets; metal brackets; torque expression; SEM/EDS