In vitro comparison of force decay between three orthodontic sliding retraction methods

Objective: The purpose of this in vitro study was to determine if there is a difference in force decay between three sliding retraction methods under a standardized force delivery system (200 gm at 25 mm stretch) at 2, 4, 6, and 8 weeks. Background: In order to achieve proper esthetics, occlusion and stability, orthodontic treatment may require extractions. Elastomeric chains, Nickel Titanium (NiTi) coils, and active ligatures are commonly used to close these extraction spaces. Methods: Twenty samples of each retraction method (elastomeric chains, NiTi Coils and active ligatures) were evaluated under standardized conditions (200 gm at 25 mm). The force of each retraction method was measured at 0, 2, 4, 6, and 8 weeks on a customized force gauge test stand (Shimpo FGV-1XY force gauge; Shimpo Instruments, Itasca, IL). Ten control samples were evaluated at 0 weeks and left un-stretched until the final measurement at 8 weeks. All samples were stored in a bath of Fusayama/Meyer artificial saliva (Pickering Laboratories, Mountain View, California) at 37°C in order to simulate the oral cavity. Results: At 2 weeks, the NiTi coils maintained their force while both the elastomeric chains and active ligatures experienced a statistically significant decrease in force over time. At 4, 6, and 8 weeks, the force of the elastomeric chains and active ligatures continued to decay and demonstrated a statistically significant decrease in force as compared to the NiTi coils and each other. At 8 weeks, the NiTi coils, elastomeric chains and active ligatures maintained 94.0%, 66.8% and 50.9%, respectively. This signifies a hierarchy of force decay with NiTi coils maintaining the largest amount of force, followed by the elastomeric chains and then the active ligatures. Conclusion: There is a significant difference in the amount of force decay of the three retraction methods over time under a standard initial force delivery of 200 gm over a 25 mm stretch. NiTi coils provide the light and constant force desired for efficient and biologically compatible tooth movement. The elastomeric chains maintained a larger amount of force than expected and have proven to achieve comparable tooth movement to NiTi coils in clinical studies. Active ligatures do not appear to be an effective means of force delivery. A force gauge is recommended to evaluate all forces placed clinically