Objective  To observe changes in alveolar bone and length of tooth root treated by maxillary and mandibular incisor retraction via planting nail in patients with bimaxillary protrusion by means of using three-dimensional conical beam computed tomography (CBCT).

  Methods  A total of 35 adult patients with class Ⅰ bimaxillary protrusions were selected. After extraction of four first premolars, we used micro-implant anchorage to strengthen anchorage. The CBCT were used before and after orthodontic treatment to observe changes of dehiscence and fenestration of alveolar bone and root length.

  Results  The labial side fenestration rates of maxillary canines, mandibular lateral incisors and mandibular canines were decreased after treatment, and the differences were statistically significant(P < 0.05). The lingual side fenestration rates of all anterior teeth showed no significant differences (P>0.05). The labial side dehiscence rates of maxillary anterior teeth, mandibular central incisors and mandibular lateral incisors were increased after treatment, and the differences were statistically significant (P < 0.05). The lingual side dehiscence rates of all anterior teeth except mandibular central incisors were increased after treatment, and the differences were statistically significant (P < 0.05). The root lengths of maxillary central incisors, maxillary lateral incisors and mandibular anterior teeth were significantly shorter than that before treatment (P < 0.05). The root length of the maxillary canines had no significant change after the treatment(P>0.05).

  Conclusion  Risk of tooth resorption and bone defect occur in class Ⅰ bimaxillary protrusion patients when incisors are retracted by using micro-implant anchorage. Thus, torque control and risk assessment should be paid attention during the correction process.