AIM:To observe the incidence of middle mesial canal ( MMC) in mandibular molars and to analyse the related factors. METHODS:110 mandibular molars from 110 outpatients were selected. The mesial subpulpal isthmus area of the mesial root in mandibular molars was inspected, the MMCs were searched and deoppilated using dental operating microscope. The number of MMCs was recorded, relation of the incidence rate of MMCs whith sex, age of the patient at the time of treatment, molar type (first or second mandibular molar), and presence/absence of an independent second distal ca-nal was analused. Their pathway was characterized by periapical angulated projection. Results were statistically analyzed by using the chi-square test. RESULTS:Of the 110 mandibular molars treated, 20 (18%) had negotiable MMCs. The inci-dence of MMCs was 30. 3% (10) in patients<25 years old, 19% (8) in patients of 25-50 years old, and 5. 7% (2) in patients>50 years old. Analysis of the data revealed a significant difference in the distribution of MMCs among different age groups (P<0. 05). The differences in the distribution of MMCs based on sex, molar type, and presence of a second distal canal were not significant. CONCLUSION:The incidence of negotiable MMCs is significantly higher in younger patients than that in the older ones.%目的：：评估汉族人群下颌磨牙近中中央根管( MMC)的发生率及其相关因素。方法：选取110例需行根管治疗的汉族患者的下颌第一、二磨牙110个,显微镜下探查其近中髓底峡区,并疏通MMC。记录患者的性别、年龄、牙位、是否存在远中独立舌侧根管,并进行统计学分析。结果：110个下颌磨牙中,有20个(18%)牙疏通了MMC。 MMC发生率在<25岁组为30.3%(10个),25~50岁组为19%(8个),>50岁组为5.7%(2个),各年龄组间相比均有统计学差异(P<0.05)；性别、磨牙类型和远中独立舌侧根管的存在与MMC发生率无明显相关性。结论：下颌磨牙近中中央根管的发生率及疏通率均为年轻人高于老年人。
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