RESUMEN
OBJECTIVE: The purpose of this study was to evaluate the effect of force and moment produced by Nickel-titanium wires of different sizes at activation and deactivation according to differing vertical bracket displacement. METHODS: Superelastic NiTi wires of 3 different sizes (0.014", 0.016", and 0.016" x 0.022") were tied with elastomeric or 0.009-inch stainless steel ligations in a twin-bracket, 0.018-inch slot. A testing machine recorded the effects of simulated activation of 5 distances from 1 to 5 mm and deactivation of 5 distances from 4 to 0 mm, in increments of 1 mm. RESULTS: Frictional force increased the wire stiffness during loading. Ligation of 0.014-inch NiTi wire with O-ring resulted in a significant increase in the stiffness. On application of orthodontic force for 5 mm of vertical displacement of teeth, the effective displacement in the case of the 0.014", 0.016", and 0.016" x 0.022" NiTi wires was 2 mm, 3 mm, and 4 mm, respectively. CONCLUSIONS: Our results showed that movement of teeth with large vertical displacement was ineffective because of excessive friction. This finding might contribute to the understanding of the force system required for effective teeth movement and thereby facilitate the application of the appropriate light wire for leveling and alignment.