Early tooth movement pattern after application of a controlled continuous orthodontic force. A human experimental model.

This clinical study was designed to investigate early orthodontic movement of human premolars subjected to a controlled, continuous, horizontally directed force. The maxillary right, first or second premolar in 56 children, 18 boys and 38 girls (mean age 13.8 years), was moved buccally with a fixed orthodontic appliance with a frontal bite-block disengaging the occlusion. The contralateral premolar served as control. A weekly controlled and reactivated force of 50 cN (approximately 50 gm) was applied. The force declined on average about 24% per week. The patient material was subgrouped to comprise eight patients in each of seven groups, for which the experimental periods varied from 1 to 7 weeks. Tooth displacement was studied on dental casts, with a coordinate measuring machine (Validator 100, TESA SA, Renens, Switzerland), which made it possible to measure the displacement in three dimensions. The movement pattern of the test teeth was found to be a combination of horizontal and vertical displacements, including tipping. The horizontal movement of the tooth crown was on average 0.8 mm during the first week and 3.7 mm after 7 weeks. The vertical movements were intrusive in 45% and extrusive in 55% of the teeth. In some tipping movements, the crown was "intruded" at the same time as the apex was "extruded." The individual variations in tooth displacements were considerable. The clinical experimental model used permits detailed study of orthodontic tooth movement under controlled force conditions, as well as detailed histologic analysis of related root resorption.