Effect of stress relaxation in two different preactivation methods of beta-titanium T-loops: bends vs. continuous curvature

Abstract

Objective Assess the changes in the force system of T-loop springs (TLSs) preactivated by bends and curvature, when secured in place with a constant strain over time. Materials and Methods A total of 120 TLSs with dimensions 6 x 10 mm, produced out of 0.017 × 0.025-inch TMA, were randomly divided into two groups of 60 TLSs, each according to what preactivation was going to be used, concentrated bends or gradual curvature. Subgroup 1 was tested immediately and the remaining subgroups were tested after they were secured at 5-mm activation for different periods of time in an interbracket distance of 23 mm. Subgroups 2, 3, 4, 5, and 6 were kept activated for 24, 48, and 72 hours, 1 and 2 weeks, respectively. Using a moment transducer coupled to a digital extensometer indicator adapted to a universal testing machine, the amount of horizontal force, moment, and moment-to-force (MF) ratios were recorded. Results Both groups showed a decrease in force levels over time (P < 0.001 and 0.005 for bend and curvature preactivation, respectively), but with different behaviors. The group preactivated by bends showed a reduction in moments over time (P < 0.001), whereas the group preactivated by curvature did not (P = 0.410). The MF ratio has not changed over time in either group. Conclusions The stress-relaxation effect is more evident on group preactivated by bends.