The force-displacement characteristics of three types of buccal-canine retraction spring to both horizontal (normal) and vertical displacements is derived from simple beam theory. The analyses are used to examine theoretically the effect of variations in the geometry of a given type of spring and also to compare the merits of three designs in terms of the force applied on activation, and their stability to vertical displacement. Experimental measurements on enlarged models of the force—displacement behaviour were in good agreement with the values predicted by the derived equations.
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