Middle Mesial Canals In Mandibular First Molars:a Cone-beam Computed Tomographic Study

Objective Using Cone beam computed tomography（CBCT）,this paper discussed the morphology and incidence of the Middle mesial root canal（MMC）of Mandibular first molar（MFM）,and evaluated the relationship between the related factors and the incidence of MMC.Methods CBCT imaging data were collected for each patient who met the inclusion criteria,the following were recorded:age,gender,the tooth position,the distance between the mesiobuccal（MB）and mesiolingual（ML）orifices,and the existence of the radix entomolaris.When MMC was present,the root canal morphology was described and counted according to the Vertucci classification and Ahmed classification respectively.All data were analyzed by SPSS20.0 statistical software.Results CBCT images of 1000 patients met the inclusion criteria,and there were 1785MFM images in total,of which 145 contained MMC.The incidence of MMC in males（7.12%,67/941）was slightly lower than that in females（9.24%,78/844）,P>0.05.The incidence of MMC in left MFM（8.34%,76/911）was slightly higher than that in right side（7.89%,69/874）,P<0.05.The incidence of MMC was slightly higher in the radix entomolaris group（8.86%,41/463）than in the non-radix entomolaris group（7.87%,104/1322）,P>0.05.The distance between MB-ML root orifices in non-MMC group was 2.99±0.44mm,it was 3.39±0.49mm in MMC group,P<0.05.Based on age,subjects were divided into four groups:<20 years old,20-39 years old,40-59years old,≥60 years old.The incidence of MMC in<20 years old group was11.85%（25/211）.The incidence was 9.34%（57/610）in 20-39 years old group.The incidence rate in 40-59 years group was 7.43%（50/673）.The incidence rate of≥60 years old group was 4.47%（13/291）,P<0.05.The MFM containing MMC were classified according to Ahmed’s classification,and a total of 15 different types were obtained.Among them,type ²M^3-2D¹,type ³M^3-2DB¹DL¹and type ²M^3-2D^2-1were the most frequent,accounting for 62.76%（91/145）.According to Vertucci’s classification:type VIII（3-3）accounted for 10.34%（15/145）;additional type（3-2）accounted for75.17%（109/145）,the largest number;additional type（type 3-1）accounted for14.48%（21/145）.Conclusion1.The incidence of MMC in MFM was 8.12%;among them,fused MMC was the most common,while MMC with independent apical orifice was extremely rare,with an incidence of only 0.84%.2.The incidence of MMC decreased with age,with the highest incidence in young people under 20 years old,and the incidence of MMC in people older than 40 years old was significantly lower than that in people under 40years old.3.The incidence of MMC was not related to gender or the presence of the radix entomolaris,but was related to the tooth position:the incidence of MMC in left MFM was slightly higher than that in right side.4.As the distance between MB-ML decreased,the incidence of the MMC increased.