Abstract: BACKGROUND:  In orthodontic treatment, there will be different degrees of external apical resorption. Severe root resorption will reduce the root/shoot ratio, reduce the stability of teeth, and even cause teeth to loosen and fall out.
OBJECTIVE: To compare the difference of external apical root resorption between high torque self-locking bracket and traditional straight wire bracket in orthodontic treatment of bimaxillary protrusion patients. 
METHODS: Forty-nine patients with bimaxillary protrusion, aged 13-16 years, who were treated in the Hospital of Stomatology of Southwest Medical University from January 2016 to December 2019, were enrolled in this study. The patients were divided into the high torque self-locking bracket group (n=24) and the traditional straight wire bracket group (n=25). Cone beam CT was taken before and after orthodontic treatment. The root morphology and length of maxillary central incisors and lateral incisors were observed by CS 3D Imaging Software. The amount of external apical root resorption between maxillary incisor and lateral incisor was calculated. This study was approved by the Medical Ethics Committee of Hospital of Stomatology of Southwest Medical University. 
RESULTS AND CONCLUSION: (1) The two groups after orthodontic treatment had achieved a good correction effect. The course of treatment was shorter in the high torque self-locking bracket group than that in the traditional straight wire bracket group (P < 0.05). (2) Cone beam CT showed that after treatment, the root morphology of the maxillary incisors had some changed. Most of them only showed the change of contour, and the apices of some teeth became shorter. (3) External apical root resorption occurred in all incisors after treatment. No cases with root resorption more than 4 mm were found. The amount of external apical root resorption of four maxillary incisors was less in the high torque self-locking bracket group than in the traditional straight wire bracket group (P < 0.05). The amount of external apical root resorption of maxillary central incisor was less in the high torque self-locking bracket group than in the traditional straight wire bracket group (P < 0.05). No significant difference was found in the amount of external apical root resorption of maxillary lateral incisor between the two groups (P > 0.05). In the same appliance group, the amount of external apical root resorption of the maxillary central incisors was less than the lateral incisors (P < 0.05). (4) Results indicate that compared with the traditional straight wire bracket, the high torque self-locking bracket may have the advantages of short treatment course and low risk of apical absorption in the orthodontic treatment of bimaxillary protrusion.

Key words: materials">, oral">, orthodontic">, self-locking bracket">, straight wire bracket">, root apex absorption">, bimaxillary protrusion