目的   利用锥形束CT（CBCT）测量骨性Ⅲ类与骨性Ⅰ类下颌磨牙远中间隙，比较两者之间的差异，同时测量下颌第二磨牙远中根尖与下颌骨舌侧骨皮质接触情况，为下颌磨牙远中移动距离提供参考。 方法   选取2020~2022年在承德市口腔医院正畸科就诊的成年患者CBCT影像资料60例，其中骨性Ⅲ类均角患者CBCT资料30例，骨性Ⅰ类均角患者CBCT资料30例。将CBCT影像资料导出为DICOM格式，然后导入Mimics 20.0软件中，测量双侧下颌第二磨牙远中间隙（下颌第二磨牙牙冠远中最凸点至平面与下颌升支前缘交点的距离）。测量下颌第二磨牙远中根尖与下颌骨舌侧骨皮质接触情况。将测量结果导入SPSS 19.0统计学软件中，分析骨性Ⅲ类和骨性Ⅰ类下颌磨牙远中间隙的差异。同时分析下颌第二磨牙远中根尖与下颌骨舌侧骨皮质接触情况。 结果   下颌磨牙远中间隙的测量结果在性别间差异无统计学意义（P>0.05）。有智齿的下颌磨牙远中间隙与无智齿的下颌磨牙远中间隙的测量结果差异无统计学意义（P>0.05）。左右两侧下颌磨牙远中间隙的测量结果差异无统计学意义（P>0.05）。骨性Ⅲ类的下颌磨牙远中间隙为(16.46±1.91) mm，骨性Ⅰ类的下颌磨牙远中间隙为(15.55±1.65) mm，下颌磨牙远中间隙的测量值骨性Ⅲ类大于骨性Ⅰ类，差异有统计学意义（P<0.05）。下颌第二磨牙远中根尖与下颌骨舌侧骨皮质接触率为34.83%。骨性Ⅲ类错??畸形与骨性Ⅰ类错畸形在下颌第二磨牙远中根尖与下颌骨舌侧骨皮质接触率上差异无统计学意义（P>0.05）。 结论   骨性Ⅲ类下颌磨牙远中有一定的间隙。在临床上可设计磨牙远中移动，从矢状向扩大牙弓，为下前牙内收提供间隙，但需要评估下颌第二磨牙远中根尖与下颌舌侧骨皮质接触情况，若下颌第二磨牙远中根尖与下颌舌侧骨皮质接触，则避免设计磨牙远中移动。 

Objective  To measure the distal space between skeletal class Ⅲ and skeletal class Ⅰ mandibular molars, compare the differences, and to measure the contact situation between the distal root tip of the second mandibular molar and the lingual cortical bone of the mandible, providing a theoretical reference for the distance of mandibular molar distal movement by using cone beam CT(CBCT).  Methods   CBCT imaging data of 60 adult patients who received orthodontic treatment at Chengde Stomatological Hospital from 2020 to 2022 were selected, including 30 patients with skeletal class Ⅲ mean angle and 30 patients with skeletal class Ⅰ mean angle. CBCT image data were exported to DICOM format, then import it into Mimics 20.0 software, and measure the distal space of bilateral mandibular second molars. The contact between the distal root tip of the second mandibular molar and the lingual bone cortex of the mandible was measured. Import the measurement results into SPSS 19.0 statistical software to analyze the differences and the contact rate.  Results   There was no statistical difference in the measurement results of the distal space of mandibular molars between genders, and sides, and between mandibular molars with or without wisdom teeth (P>0.05). The distal space of mandibular molars in skeletal class Ⅲ were greater than those in skeletal class I, and the difference was statistically significant (P<0.05). The contact rate between the distal root tip of the second mandibular molar and the lingual bone cortex of the mandible was 34.83%. There was no significant difference in the contact rate between the distal root tip of the mandibular second molars and the lingual cortical bone of the mandible between class Ⅲ malocclusion and class I malocclusion(P>0.05).  Conclusion   There is a certain gap in the distal part of the skeletal class Ⅲ mandibular molar. In clinical practice, the molar can be designed to move distally, expanding the dental arch from the sagittal direction to provide a gap for the adduction of the mandibular anterior teeth. However, it is necessary to evaluate the contact between the distal root tip of the second mandibular molar and the lingual bone cortex of the mandible. If the distal root tip of the second mandibular molar contacts the lingual bone cortex of the mandible, the distal movement of the molars should be avoided.