[Relation of the activation force and deflection in various orthodontic springs (closing-loops)]

For ten different forms of orthodontic springs, comprising three commercially available preformed types, the relationship between load (force) and the resulting deflection was determined experimentally. The experimental set-up consisted of a micrometric microscope containing a scale which could be calibrated and an optical bench so the springs could be fixed and activated by means of a force gauge in increments of 12.5 p up to 200 p. Ten springs of each of the ten forms were measured once. The results can be summarized as follows: The load-deflection rate of orthodontic springs depends on the modules of elasticity of the utilized alloy and the geometric configuration of the spring. For a given loop configuration the load-deflection rate is proportional to the modulus of elasticity. The more wire length used for shaping the spring elements (loops), the lower the load-deflection rate. With the exception of the Ladanyi retractor and the Bull loop all the springs examined showed a linear relationship between load and deflection. Because of the higher accuracy of force application and the relative force constancy during tooth movement, it is usually preferable to choose springs with a low load-deflection rate of about 50 p/mm.