A reassessment of force magnitude in orthodontics.

The relationship between the force magnitude delivered by orthodontic appliances and the rate of orthodontic tooth movement is controversial. This paper critically reviews the experimental data base that has shaped this controversy and graphically presents four hypotheses proposed to represent the relationship between force magnitude and the rate of tooth movement. The clinical implications of each hypothesis for treatment planning and space-closing mechanics are discussed. The authors' analysis of the data from six heavily cited clinical studies in the orthodontic literature appears to support one of the four proposed hypotheses (No. 4, Fig. 2). Acceptance of this hypothesis leads the clinician to a rational approach in manipulating the final position of teeth. Anchorage can be conserved by means of treatment strategies that reduce stress magnitudes in the periodontium of the posterior teeth while maximally efficient stress is maintained on the anterior teeth. Appliances with low load-deflection rates and relatively constant moment/force ratios allow the clinician to take advantage of the type of tooth movement proposed in this hypothesis.