Method: The pre- and post-treatment Cone-beam CT (CBCT) scans of five bilateral maxillary canine retraction patients were analyzed. Segmental T-loops with specified moment/force ratios were applied to both sides for retracting the canines to the vacant 1st premolar sites. The treatment period was between 3 to 6 months. The canines and their surrounding tissues were segmented from the CBCT scans. The alveolar bone with a thickness of 0.5 mm from the periodontal ligament was selected. This shell was divided into 24 regions, which consisted of 3 equal layers along the root's long axis and 8 divisions circumferentially on each layer. The average value of Hounsfield units of each region was computed, which was converted to its BMD through a specially designed phantom. The tooth displacement direction was determined from a separate study. Mixed-model ANOVA method was applied to test the BMD changes.
Result: The canines moved primarily in the distal direction. The average BMD change of alveolar bone in the buccal-lingual direction was 10 mg/cc, which was less than 2% of the BMD before the treatment; the BMD reduction in the movement direction was 89 mg/cc, which was about 13% of the original BMD. The statistical analysis showed that the difference between BMD in the mesial-distal and buccal-lingual directions was significant, and the difference among the three layers is insignificant.
Conclusion: The canine retraction results supported the hypothesis and suggested a BMD loss of the alveolar bone in the direction of the tooth movement and a relatively consistent BMD in the direction perpendicular to the tooth movement.