Force decay of elastomeric ligatures: influence on unloading force compared to self-ligation

To compare elastomeric ligatures (EL) vs self-ligating (SL) brackets in terms of their effects on the unloading force of a 0.014-inch CuNiTi aligning wire. By simulating the alignment of a lingually malposed canine and using a full-arch design, three ligation methods -- SL, EL, and "relaxed" elastomeric ligature (REL) -- were tested with 30 wires per group. Each test was conducted in a dry field within a Plexiglas case at 35 +/- 2 degrees C. Eleven brackets were affixed from first molar to first molar site on a steel plate with a space created at the location of the left canine. The wire was deflected 3 mm then was unloaded at a speed of 2 mm per minute. Data were gathered from four points along the unloading plot generated during each test and then were compared by using the Kruskal-Wallis one-way analysis of variance. Mann-Whitney tests were used to compare combinations of groups. Wires ligated with EL and REL produced an average unloading force equal to 56% and 88%, respectively, of the same wire in an SL bracket. The unloading forces produced by a wire after force decay of the elastomers are not statistically different from the forces present in self-ligating systems.