[Changes in root and alveolar bone before and after treatment by retracting the upper incisors]

Using cone beam computed tomography (CBCT) in image measurement on the patients with maxillary protrusion, the study aims to identify the changes in root and alveolar bone before and after treatment by upper incisor retraction.

The study was conducted on 37 patients who have received orthodontic treatment from January 2014 to December 2015. The sample comprised 17 males and 20 females, with an average age of 14.5 years. The patients underwent extraction of bimaxillary premolars and given maximum anchorage to retract the upper incisors. The adducent angle, adducent amount, and the amount of elongation of the upper incisor teeth were measured by cephalograms. The patients were scanned by NewTom VGi to obtain CBCT data before and after treatment with upper incisor retraction. Using the NewTom NNT tool, we obtained the multiple planar reconstruction and then adjusted the coronal, axial, and sagittal axis. The sagittal section of the long axis of the maxillary central incisor through the incisal edge and root apex was selected to measure the changes in the root and alveolar bone before and after incisional treatment.

Before and after retracting the upper incisors, the adducent angle of central incisor measured 12.92°±6.43°. Adducent amount of the incisors reached (5.54±2.21) mm. Incisor extension amount totaled (0.60±0.95) mm. Root absorption length was (0.81±0.46) mm. Root absorption rate was 6.80%±3.60%. Statistical differences were observed in the changes in root length before and after incisor retraction (P<0.05). After upper incisor retraction, increasing distance from the labial side alveolar ridge to the cemento-enamel junction reached (0.20±0.22) mm. After treatment, we observed that the height of the labial-side alveolar bones decreased and showed statistical difference with the height of labialside alveolar bones before treatment (P<0.05). The results show the correlation between root absorption and horizontal displacement of maxillary center incisor and the distance from the upper incisor apex to labial cortical bone. A correlation also exists between the variable quantity of the labial-side alveolar bones and adducent angle of the upper incisor, with a correlation coefficient of 0.354. The results also show significant difference (P<0.05).

After compensatory treatment of patients with maxillary protrusion, the root length of upper incisor was absorbed remarkably. The height of the labial-side alveolar bones was reduced. A greater tooth movement or beyond the anatomical limitations and alteration limits of the alveolar bone can easily lead to root resorption. A negative correlation exists between the variable quantity of the labialside alveolar bones and adducent angle of the upper incisor.