Time-related root resorption after application of a controlled continuous orthodontic force.

The aim of this investigation was to study root resorption after application of a weekly controlled, continuous orthodontic force of 50 cN (approximately 50 gm). Fifty-six maxillary premolars in 18 boys and 38 girls (mean age 13.8 years) were moved buccally with a fixed orthodontic appliance. The contralateral premolar served as a control. The experimental periods varied from 1 to 7 weeks with eight children in each group. Root resorption was registered after 1 week and occurred in all test teeth but four. The surface extension as well as the depth of the resorption showed a marked increase after 2 weeks. Starting from the third week, eight test teeth exhibited apical root resorption that had reached half way to the pulp, or more. After 7 weeks, the test teeth showed on average more than 20 times larger mean resorbed root contour than the control teeth. Individual variations were considerable regarding both extension and depth of root resorption within each test group and were not correlated to the magnitude of tooth movement achieved. The radiographs failed to reveal any adverse tissue reactions. The great individual variations in root resorption without association to the amount of tooth displacement indicate that so far unknown individual factors are influential in these adverse tissue reactions.