RESUMO
OBJECTIVE: NiTi closed coil springs were reported to have relatively constant unloading forces. However, the characteristics of NiTi closed coil springs from various manufacturers have not been elucidated. The purpose of this study was to compare load-deflection characteristics of various NiTi closed coil springs and to find out the optimal range of extension. METHODS: Seven kinds of NiTi closed coil springs from five manufacturers were tested. Load deflection curves were obtained at extension ranges from 2 mm to 30 mm. Also, springs were kept extended during a 4 week period, and then load deflection curves were obtained again. RESULTS: Sentalloy (Tomy) and Jinsung blue (Jinsung) showed superelasticity in every extension ranges tested and showed plastic deformation of less than 1 mm. Ni-Ti (Ormco) showed superelasticity only after the springs were extended at or more than 10 mm, thereby meaning that clinicians should extend these springs at or more than 10 mm to utilize the superelasticity. Orthonol (RMO) and Nitanium (Ortho Organizers) did not show superelasticity. After 4 weeks of extension, all springs showed plastic deformation less than 1 mm when the extension was at or under 25 mm. CONCLUSIONS: The superelastic behavior of NiTi closed springs were different among various NiTi spring products, and some NiTi closed springs failed to show superelasticity.