Abstract: Objective To compare alveolar bone conditions around mandibular incisors in the adult of skeletal classⅢ malocclusion before treatment and after the presurgical orthodontic treatment. Methods 15 patients treated with orthodontic treatment and orthognathic surgery (9 men, 6 women, average age: 23.6 years) took CBCT before treatment (T0) and 1 month before orthognathic surgery (T1).The amount of alveolar bone thickness, the vertical alveolar level and the root length were measured. Paired ttest was performed for the comparisons of the differences between pre and postoperative variables using SPSS15.0. Results ① The lower incisors inclination was increased and the incisors root apex was closer to the lingual side of alveolar bone (P<0.01); ② The alveolar bone thickness at apex was not statistically changed; ③ The vertical alveolar bone levels at the labial and lingual sides were both significantly reduced especially at the lingual side after presurgical orthodontic treatment (P<0.01); ④ The root length was not significantly changed. When the lower incisor inclined to the lip in a wide range, the tooth root moved closer to the lingual bone cortex, and the attachment height of alveolar bone lowered more. Conclusion In the treatment of adults with skeletal class Ⅲ malocclusion, special attention should be given to the patients’ periodontal health, the change of alveolar bone and the relationship between root and alveolar bone. A right plan should be chosen to decrease the excessive movement of lower incisors so as to avoid adverse reaction.

Key words: skeletal class Ⅲmalocclusion, CBCT, orthodontic decompensation, alveolar bone, mandibular incisor