The relationship between the magnitude of a constant continuous orthodontic force and rate of bodily tooth movement was studied. In 25 young adult male beagle dogs, lower third premolars were extracted and bone markers were implanted in the mandible. Sixteen weeks later, an orthodontic appliance was placed, and elastics exerting 50, 100, or 200 gm were attached to the lower second premolar to produce bodily distalization. In each dog, different forces were used on the left and the right sides. As a control group, orthodontic appliances were placed without elastic on eight sides. Tooth movement was measured directly with a digital caliper twice a week during 16 weeks. Resulting curves could be divided in four phases. Large individual differences were found in the rate of tooth movement. Tooth movements on the left and right sides of each dog, however, were highly correlated. No significant differences in the duration of each phase nor in the mean rate of tooth movement during each phase were found between the three force groups. Maximum rate of tooth movement was about 2.5 mm per month in all force groups. There were no significant differences in the mesial movement of the anchorage unit between the force groups. It is concluded that under the circumstances of this study magnitude of force is not decisive in determining the rate of bodily tooth movement, but individual characteristics are.
