Objectives: Coil springs are used in orthodontics to deliver forces to move teeth. However, the optimal amount of activation to produce predictable, continuous deactivation forces, for the purpose of orthodontic tooth movement, is unclear. The purpose of this study was to evaluate the deactivation force characteristics of nickel-titanium open-coil springs after varying levels of activation.

Methods: Four open-coil spring products were evaluated: Dentsply GAC 100 gram and 150 gram, Orthoclassic Orthodontics Medium 150 gram, and American Orthodontics nickel-titanium springs. Open-coil springs were compressed (activation) with a universal testing machine to 20, 40, 60, or 80% of original length (15 mm; n=15/product/activation level). Measurements were conducted at 37°C to simulate intraoral temperatures. Deactivation force values at 3 mm were compared among the four activation levels within each product using ANOVA/Tukey.

Results: For all four open-coil spring products, deactivation force values at 3 mm significantly (p<0.05) decreased with greater activation/compression level. When activated to 20% of original length, the force values at 3 mm compression upon deactivation were only 53-73% of the force level when activated to 80% of original length. When activated to 40% of original length, the force values at 3 mm compression upon deactivation were 65-91% of the force level when activated to 80% of original length.

Conclusions: Deactivation forces of nickel-titanium open-coil springs at a given compression level are dependent upon the extent of prior compression. Orthodontists should be aware that force values in open-coil springs depend not only on material and structural factors, but amount of activation as well.