Photoelastic analysis of forces generated by T-loop springs made with stainless steel or titanium-molybdenum alloy Luiz Guilherme Martins Maia, a Mila Leite de Moraes Maia, b Andr e da Costa Monini, a Alexandre Prot asio Vianna, a and Luiz Gonzaga Gandini, Jr c Araraquara, S~ ao Paulo, and Aracaju, Sergipe, Brazil Introduction: The purpose of this study was to use photoelastic analysis to compare the system of forces gen- erated by retraction T-loop springs made with stainless steel and titanium-molybdenum alloy (TMA) (Ormco, Glendora, Calif) with photoelastic analysis. Methods: Three photoelastic models were used to evaluate retrac- tion T-loop springs with the same preactivations in 2 groups. In group 1, the loop was constructed with a stainless steel wire, and 2 helicoids were incorporated on top of the T-loop; in group 2, it was made with TMA and no he- licoids. Results: Upon using the qualitative analysis of the fringe order in the photoelastic model, it was observed that the magnitude of force generated by the springs in group 1 was significantly higher than that in group 2. However, both had symmetry for the active and reactive units related to the system of force. Conclusions: Both springs had the same mechanical characteristics. TMA springs showed lower force levels. (Am J Orthod Dentofacial Orthop 2011;140:e123-e128) O rthodontists have used many devices to close the remaining spaces after tooth extractions. One of them is the T-loop spring suggested by Burstone in 1982. 1 Among its advantages, the low load-deflection ratio (L/D) is significant. Because of the shape of the spring, which incorporates much wire in its form, and with 0.017 3 0.025-in titanium-molybdenum alloy (TMA) wire, it is possible to work with a greater activa- tion for a longer time and a relatively low force, when compared with other springs with high L/D. 1-3 The moment-to-force ratio (M/F) established by the T-loop spring relating to the center of resistance of the tooth and the force can be changed by the orthodontist, according to the preactivated bends. This results in better control of the axial movement and makes it possible to select different types of movements. 4 The force system generated by the T-loop spring is controlled by the integration of the preactivated bends, the activation amount, and the position of the spring in the interbracket distance. Hoenigl et al 5 evaluated a centralized T-loop force system by first activating it at the maximum level and then deactivating it gradually until it reached the lowest level of deactivation. They concluded that the force system generated by this type of spring provides movements from a controlled crown tip-back to a radicular correction. The efficiency of this type of spring was already shown by clinical evidence, 6 and its power system was evaluated by mechanical testing 4,5,7-9 and software. 10-12 Photo- elastic analysis is a widely used optical technique for examining and measuring stress distribution in structures exposed to internal or external forces. In this technique, polarized light is transmitted through a photoelastic active material, which is doubly refractive when stressed. When the emergent light waves are viewed through an analyzer filter, the stress patterns appear as fringes or bands of color. 13 In dentistry, photoelasticity was introduced by Zak 14 in 1935 during the study in which he assessed the type of tooth movement, strength, and point of application of the forces. In orthodontics, this technique has been used to examine the stresses induced during canine re- traction with other devices that do not use a T-loop spring and with lingual appliances to evaluate stresses a Postgraduate student, Faculdade de Odontologia de Araraquara, Universidade Estadual Paulista, Araraquara, S~ ao Paulo, Brazil. b Postgraduate student, Federal University of Sergipe, Aracaju, Sergipe, Brazil. c Professor, Faculdade de Odontologia de Araraquara, Universidade Estadual Pau- lista, Araraquara, S~ ao Paulo, Brazil; adjunct clinical professor, Baylor College of Dentistry, Dallas, Tex, and Saint Louis University, St Louis, Mo. The authors report no commercial, proprietary, or financial interests in the prod- ucts or companies described in this article. Reprint requests to: Luiz Gonzaga Gandini, Jr, Av Casemiro Perez, 560, Vila Har- monia, Araraquara, S~ ao Paulo, Brazil, CEP 14802-600; e-mail, luizgandini@uol. com.br. Submitted, November 2010; revised and accepted, March 2011. 0889-5406/$36.00 Copyright Ó 2011 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2011.03.020 e123 ONLINE ONLY