Force Magnitude and Center of Rotation: Histologic Study in Rhesus Monkey

Abstract

It is believed that a single force, regardless of its magnitude, tips a tooth around the same center of rotation. This is a histologic investigation of this concept; and an attempt to correlate clinical findings with early and late microscopic changes in the periodontium after light and heavy force application. Five Rhesus monkeys, 3-4 years of age, were used. In one, continuous forces of 40 and 600 gm. were to move a maxillary first bicuspid distally through an edentulous space, with the first molar serving as anchor for 47 days. Both teeth tipped equally toward each other. In four monkeys, light, continuous forces, 10 and 57 gm., and heavy forces, 100 and 200 gm., were used to move the two central incisors lingually over periods of 19, 62, and 66 days. The light force had to be maintained for a period of 4-6 weeks before changing the activation. The 10 gm. force did not move incisors after 19 or 62 days. The 57 gm. force moved the tooth faster than the heavy forces. Hyalinization of the periodontal membrane was recorded during the lag period, but not later. At later stages, the heavy force caused more root resorption than the lighter one. The moment/force ratio determines the center of rotation, not the single value of each. Single forces tipped teeth at the same center of rotation, regardless of their magnitude.