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On optimum orthodontic force theory as applied to canine retraction Robert J. Nikolai, Ph.D.* St. Louis, MO. T he terms threshold, light, optimum, and heavy have become familiar adjectives used to categorize the magnitudes of orthodontic force, but their meanings are not universally clear. Weinstein1 has reported that a 2-gram force successfully displaced a premolar; this might be understood as being a threshold value for the tipping of that tooth. Light and heavy forces are largely indicative of the technique and appliances employ-ctl by the clinician in treating a case and are actually more descriptive of the ranges of force magnitude rather than specific force sizes. For a number of reasons, including variations in tooth size and number of teeth affected by a given force, there can be no specific boundary value between light and heavy force ranges. In the total range of orthodontic force magnitudes, the very question of the exist,ence of optimum forces seems to remain unresolved in the minds of many practitioners. Introduction The search for optimum force values in orthodontics was perhaps begun in earnest by Scl~wartz.2 The theory of optimum forces was proposed by Storcy and Smith,“, 4 and the advent of light-wire appliance usage and light-force mechanics is generally associated with their research and that of Bcgg.” In the late 1950s and throughout most of the sixties, there appeared writings of prominent orthodontic researchers which were supportive of the concept of optimum forces.“-10 More recently, however, a number of clinical and experimental investigations have been reportctl in the literature in which the results did not wholly agree with the optimum force theory and/or its corollaries (for example, the differential force concept) as formulated to date.“-‘” These later developments have motivated me to conduct a theoretical study in an atternpt to contribute to the discussion from a somewhat different standpoint. The optimum orthodontic force has been defined as that which produces a *Professor, 290 Department of Orthodontics, Saint Louis Univrrsity Mrdic:~l (“rnter.