Changes in the stiffness of the ligating mechanism in retrieved active self-ligating brackets

The purpose of this study was to investigate the effect of intraoral aging on the force applied during engagement of a wire into the slot of active self-ligating brackets.

Two types of brackets were used: Speed (Speed System Orthodontics, Cambridge, Ontario, Canada) and In Ovation-R (GAC, Bohemia, NY). Ten as-received and 10 retrieved maxillary incisor brackets collected after full treatment for at least 15 months (range, 15-19 months) were included in each bracket group. The brackets were mounted on the orthodontic measurement and simulation system, and the spring was subjected to tensile stress, which induced deformation of up to 1.5 mm, with a custom-made jig and laboratory configuration at body temperature. On the loading portion of the loading-unloading curve of clips, the slope was fitted a first-degree equation curve, to determine the stiffness/deflection rate of the clip. The results of slopes and ranges were analyzed with a 2-sample t test at the .05 level of significance, with aging as the discriminating variable.

No difference was found between as-received and used brackets with respect to force exerted by the spring in 1 bracket group, whereas the other group showed extensive relaxation after use; neither group had permanent deformation. The consistency of the initial force levels varied significantly in each bracket group.

The initial force levels and the effect of intraoral conditions on the stiffness of the clip seem to vary between products, with potential implications for the archwire engagement into the bracket slot and associated mechanotherapy.