Orthodontic force characteristics of open coil springs

Applications of open coil springs in orthodontic tooth movement are many and varied. Current descriptions of force production by open coil springs are based upon force values produced in series of discrete compression tests. It was the purpose of this study to analyze and compare open-coil-spring force production from continuous load-deflection curves, thereby assisting the clinician in the selection of the ideal spring for a given application. Load-deflection curves were generated for open coil springs of various wire diameters and lumen sizes, as well as four wire alloys from three manufactures. Testing was performed on an Instron test machine using 0.016 inch round and 0.018 by 0.022 inch rectangular arch wires. With a constant lumen size, an increase in wire size produced an increase in force at a given activation. With a constant wire size, an increase in lumen size produced a decrease in force at a given activation. The effect of the arch wire size and shape was not significantly different for the coil springs with the smaller lumen sizes. For the larger lumen sizes, the springs used with the rectangular arch wire manifested a greater linear range than those used with the smaller arch wire. The difference between the load-deflection characteristics of the coil springs from different manufacturers was attributed to the difference in the pitch of the coils.