目的: 探索运用锥形束CT(cone-beam computed tomography，CBCT)三维重建技术测量牙根体积的方法，研究骨性Ⅲ类错患者在正畸正颌联合治疗过程中，上下颌中切牙的牙根长度、牙根体积变化规律和牙根吸收的特点。方法: 选取20例因骨性Ⅲ类错行正畸正颌联合治疗的患者，于正畸去代偿治疗前1周内(T0)、正畸去代偿治疗后正颌手术前1周内(T1)及术后正畸治疗结束后1周内(T2)三个时间点拍摄CBCT，应用三维重建技术测量上下颌中切牙的牙根长度、牙根体积(包括牙根总体积、根颈部及根体部体积)，计算长度减小百分比和体积减小百分比，并测量正畸治疗前后牙齿移动的距离。数据用SPSS 20.0软件进行统计分析，服从正态分布的组间两两比较采用最小显著性差异法(least significant difference，LSD)，不服从正态分布的组间比较采用非参数检验。比较上下颌中切牙牙根长度和牙根体积变化的差异，分析其变化规律和牙根吸收的特点。结果: 骨性Ⅲ类错患者在正畸正颌联合治疗过程中，上下颌中切牙的牙根长度和牙根体积均明显减小(P < 0.05)，根颈部和根体部的牙根体积均明显减小(P < 0.05)，根体部牙根体积的减小量更加显著，上中切牙根体部牙根体积减小量百分比为(30.51±23.23)%，下中切牙为(23.24±11.96)%。与上中切牙相比，下中切牙牙根体积的减小量和减小量百分比均较小，且上中切牙根体部的体积减小量大于下中切牙，差异有统计学意义(P < 0.05)。术前正畸阶段去代偿治疗上颌中切牙以控根移动方式腭向移动，下颌中切牙移动方式为唇向倾斜移动。结论: 骨性Ⅲ类错患者在正畸正颌联合治疗过程中，上下颌中切牙的牙根长度和牙根总体积均减小，变化规律相似。牙根体积测量提示牙根根颈部也存在牙根吸收。上下颌中切牙牙根吸收量的差异可能与牙齿移动的距离和方式相关。利用CBCT三维重建体积测量评估牙根吸收可以弥补牙根长度测量的局限性。

Objective: To explore the method of measuring root volume with cone-beam computed tomography (CBCT) three-dimensional reconstruction technology, and to study root length and root volume of upper and lower central incisors in patients with skeletal Class Ⅲ malocclusion treated by surgical orthodontic treatment. Methods: Twenty patients with skeletal Class Ⅲ malocclusion undergoing surgical orthodontic treatment were selected. CBCT data at three time points, before decompensation treatment (T0), after decompensation treatment (before orthognathic surgery, T1), and the end of post-operative orthodontic treatment (T2) were collected. Three-dimensional reconstruction technology was used to measure the root length and root volume of the upper and lower central incisors (including total root volume, cervical root and apical root), calculate the percentage of reduction volume, and measure the distance of tooth movement after orthodontic treatment. Data were statistically analyzed by SPSS 20.0 software. Least significant difference (LSD) method was used for pair comparison between the groups subject to normal distribution, and non-parametric test was used for comparison between the groups not subject to normal distribution. The differences of root length and root volume of upper and lower incisors were compared, and the characteristics of root absorption were analyzed. Results: Root length and root volume of the upper and lower central incisors were reduced during the surgical orthodontic treatment (P < 0.05) in cases. Both the root volume of cervical root and apical root were significantly reduced (P < 0.05), the reduction of apical root was more significant. The percentage of root volume reduction of the upper central incisor was (30.51±23.23)%, and lower central incisor (23.24±11.96)%. Compared with the upper central incisor, the root volume reduction amount and percentage of the lower central incisor were smaller, and apical root volume reduction of the upper central incisor was greater than that of the lower central incisor, which was statistically significant (P < 0.05). During pre-surgical orthodontic treatment, maxillary central incisor palatal moving was in a controlled tipping manner, and the mandibular central incisor tipped labially. Conclusion: In patients with skeletal Class Ⅲ malocclusion, root length and total root volume of upper and lower central incisors decreased during surgical orthodontic treatment. Root volume measurement indicated that the cervical root also had root resorption. The difference in root resorption of the upper and lower central incisors might be related to the distance and direction of teeth movement. CBCT three-dimensional reconstruction will compensate for the limitation of root length measurement in evaluating root resorption.