Effects of stress relaxation in beta-titanium orthodontic loops

Abstract

Introduction: This study evaluates the changes in the force system of the beta-titanium T-loop spring (TLS) caused by stress relaxation. Methods: Ninety TLSs with dimensions of 6 x 10 mm, made of 0.017 x 0.025 in beta-titanium alloy and preactivated by concentrated bends, were randomly distributed into 9 groups according to the time point of evaluation. Group 1 was tested immediately after spring preactivation and stress relief by trial activation. The other 8 groups were tested after 24, 48, and 72 hours, and 1, 2, 4, 8, and 12 weeks. By using a moment transducer coupled to a digital extensometer indicator adapted to a universal testing machine, the amounts of horizontal forces and moments and the moment-to-force ratios were recorded at every 0.5 mm of deactivation from 5 mm of the initial activation in an interbracket distance of 23 mm. Results: The horizontal forces and moments were higher (P < 0.001) for group 1 compared with the other 8 groups, which were not different among themselves. All groups produced similar moment-to-force ratios (P 5 0.600), with no influence of time. Conclusions: The TLSs preactivated by concentrated bends had progressive load decreases over time, and this effect is critical in the first 24 hours. (Am J Orthod Dentofacial Orthop 2011;140:e85-e92)