Abstract: Objective    To analyze the three-dimensional position relationship between the impacted mandibular third molar and mandibular canal by cone beam CT （CBCT），in order to provide basis for the risk assessment before the extraction of mandibular third molar and the formulation of the surgical plan. Methods    Totally 732 patients（1063 affected teeth）who underwent impacted mandibular third molar extraction at the Department of Stomatology，Songjiang District Central Hospital，Shanghai，China，from January 2017 to February 2022 were retrospectively analyzed. All patients′orthopantomography showed the root of impacted mandibular third molar was in contact with or overlapped with the mandibular canal. The position and angle of the observation axis of CBCT were adjusted to form a coordinate axis，and in the coronal plane，the positional relationship between the mandibular canal and the impacted mandibular third molar was classified according to the position of the mandibular canal in coordinates，including buccal side，inferior buccal side，buccal underside，middle underside，lingual underside，inferior lingual side and lingual side，and each category could be subdivided into contact and non-contact subcategories. The contact rate and distance difference between mandibular canals and impacted mandibular third molars were measured and compared. Results    The contact rate between root and mandibular canal of 1063 impacted mandibular third molars was 53.6%，and the difference was statistically significant（χ2 = 352.945，P < 0.001）. The contact rates of lingual side and inferior lingual side were 97.2% and 96.7%，respectively，which were significantly higher than those of other categories except lingual underside.The contact rates of lingual underside，middle underside，and buccal side were 79.2%，70.2%，and 60.0%，respectively，which were higher than those of buccal underside and inferior buccal side，respectively（P < 0.05）. There were 493 impacted mandibular third molars in non-contact subtype. The distance between the impacted mandibular third molar and mandibular canal of various non-contact subtypes was statistically significant（H = 80.330，P < 0.001）. The distance between the impacted third molar and the mandibular canal of buccal underside and inferior buccal side was > 1.0 mm，which was significantly larger than the distance of middle underside and lingual underside（P < 0.05）. Conclusion    By using the cross observation axis of CBCT as the axis of classification，the position between the mandibular canal and impacted mandibular third molar can be relatively accurately determined. The operation is simple and the standard is consistent，which can provide a reference for the risk assessment before the extraction of the impacted mandibular third molar. 

Key words: cone beam CT, orthopantomography, impacted mandibular third molar, mandibular canal

摘要： 目的    应用锥形束CT（cone beam CT，CBCT）分析下颌阻生第三磨牙与下颌管的三维位置关系，为下颌阻生第三磨牙拔除术前风险评估和制定手术方案提供依据。方法    选取2017年1月至2022年2月就诊于上海市松江区中心医院口腔科需行下颌阻生第三磨牙拔除术的患者732例（1063颗患牙）。所有患者曲面体层片显示下颌阻生第三磨牙牙根与下颌管接触或重叠，调整CBCT十字观察轴的位置和角度形成坐标轴，在冠状面根据下颌管在坐标中的位置，对下颌管与下颌阻生第三磨牙的位置关系进行分类，包括颊侧、颊侧偏下、下方偏颊、下方居中、下方偏舌、舌侧偏下和舌侧，每类可再分为接触、非接触亚类。测量并比较各类下颌管与下颌阻生第三磨牙的接触率和距离差异。结果    1063颗下颌阻生第三磨牙牙根与下颌管的接触率为53.6%，各类接触率比较，差异有统计学意义（χ2 = 352.945，P < 0.001）。其中，舌侧和舌侧偏下的接触率分别为97.2%、96.7%，明显高于除下方偏舌外的其他分类接触率；下方偏舌、下方居中及颊侧的接触率分别为79.2%、70.2%、60.0%，均高于下方偏颊和颊侧偏下的接触率；差异均有统计学意义（均P < 0.05）。非接触亚类下颌阻生第三磨牙共493颗，各类非接触亚类下颌阻生第三磨牙与下颌管之间距离比较，差异具有统计学意义（H = 80.330，P < 0.001）。其中，下方偏颊和颊侧偏下非接触亚类的下颌阻生第三磨牙与下颌管之间距离均大于1.0 mm，且大于下方居中和下方偏舌非接触亚类，差异均有统计学意义（均P < 0.05）。结论    采用CBCT十字观察轴作为坐标轴进行分类，可相对准确判断下颌管与下颌阻生第三磨牙的位置关系，且其操作简便、标准一致，可为下颌阻生第三磨牙拔除术前的风险评估提供参考依据。

关键词: 锥形束CT, 曲面体层片, 下颌阻生第三磨牙, 下颌管