Root Resorption Associated With Canine Retraction Treatment

Objectives: External apical root resorption (EARR) is linked to orthodontic treatment. However, the cause of EARR is multifactorial. Study of the cause requires effective control of these factors and reliable methods to evaluate the outcomes. The objectives of this study were to develop the technologies and to clinically verify root resorption due to canine retraction.
Methods: Pre- and post-treatment maxillary cone-beam computed tomography (CBCT) scans of 18 subjects undergoing bilateral canine retraction were used to calculate EARR. Subjects were treated using calibrated segmental T-loops to deliver 124 cN retraction force and the moment-to-force ratio suitable for translating or controlled tipping of the canine. The EARR of both maxillary canines and incisors were studied. The canines had the retraction force applied, but the incisors had no orthodontic force. Target teeth were segmented digitally. The algorithm for quantifying EARR was developed. Subject age and treatment duration were recorded. Genotypes were analyzed using the subjects’ saliva samples. Rate of tooth movement was calculated. Associations of these factors with EARR data were evaluated using chi-square tests and t-tests, with significance when p<0.05. Criterion was established to identify definite EARR.
Results: Only 6/18 subjects showed definite EARR, meaning that load is not the only factor. The teeth that had no orthodontic load did not show definite EARR, meaning that the orthodontic load is necessary for EARR to occur. Subjects with EARR were G/G genotype of IL-1b rs11143634, indicating these subjects may have high risk to have EARR compared to subjects with other type of genotype. Longer treatment duration and older age also associated with EARR, indicating that these factors might also contribute to EARR.
Conclusions: Orthodontic load is necessary, but not sufficient, to cause EARR. Other factors, such as load level, genotype, age and treatment duration, should be the focus of future studies.