A Cone-Beam Computed Tomographic Study On The Distolingual Root Of The Permanent Mandibular First Molars

BackgroundPulpitis and periapical periodontitis are the most common oral clinical bacterial infections.Root canal therapy(RCT)is an effective method for pulp treatment,which can prolong the retention time of the affected teeth.A comprehensive understanding of root canal morphology by dentists is the basis of successful root canal therapy.The complexity of root canal system determines the difficulty of root canal therapy.Otherwise,The root canal system of mandibular first permanent molar is complex and the number of root is various.One of the main structural variations is the existence of extra third root---distolingual root.Cone-beam computed tomography(CBCT)is a three-dimensional radiography technology,which can provide three-dimensional images of axial,sagittal and coronal sections,and overcome some shortcomings of traditional periapical X-ray films,such as geometric deformation and structural superposition.It is an ideal method for root canal system research.ObjectiveThe purpose of this study was to study the incidence,the anatomy of the pulp floors and the curvature of the distolingual roots of the permanent mandibular first permanent molars by cone-beam computed tomography(CBCT).This study examined the changes of root canal anatomy and morphology of distolingual root canal of mandibular first permanent molars which provided reliable theoretical basis for clinical treatment.MethodsThis study was utilized the CBCT images of mandibular first molars from the Department of Radiology,Stomatological Hospital of Shandong University from August 2015 to April 2018.A total of 1164 CBCT images of mandibular first molars of 582 patients were evaluated.Recording the name,gender,age and dental location of patients.The following information was analyzed:1.the incidence of distolingual roots;2.the incidence of three-rooted and two-rooted mandibular first permanent molars with four root orifices;3.the distance of root canal orifices and angle of disto-buccal canal orifice-disto-lingual canal orifice-mesio-lingual canal orifice(DB-DL-ML)of three-rooted molars and two-rooted molars with four root orifices;4.the curvature of distolingual roots.Chi-square test was used to compare the the gender groups and bilateral groups differences of the following data:1.the incidence of distolingual roots;2.the incidence of three-rooted and two-rooted mandibular first permanent molars with four root orifices.The antimeric correlations for occurrence of the trait were analyzed with the Pearson's rank order correlation test.T test was used to compare the following data:1.the average distance of each root canal orifices between three-rooted molars and two-rooted molars with four root orifices;2.DB-DL-ML angles between three-rooted molars and two-rooted molars with four root orifices;3.root canal curvature between left and right sides;4.the curved parts between mesio-distal and buccal-lingual views.Results1.The results showed the prevalence of three-rooted mandibular first molars was 25.60%(298/1164 teeth)calculated by teeth,and 21.10%(181/582 individuals)calculated by individuals.The incidence of distolingual root of right was higher than that of left(?2 1.276,P<0.001).There was no significant difference between male and female(P>0.05).The bilateral concurrence of distolingual root of mandibular first permanent molars was correlated.2.The incidence of mandibular first permanent molars with four root canals was 48.28%(562/1164 teeth)calculated by teeth,and 58.07%(338/582 individuals)calculated by individuals.There were significant statistical differences in the incidence of the right between the three-rooted and two-rooted mandibular first permanent molars with four root orifices(?2=16.023,P<0.001).The three-rooted molars with four root orifices were more common in the right side(?2=11.276,P<0.001),and the two-rooted molars with four root orifices were more common in the left side(?2=5.663,P=0.017).The DL-DB orifices distances of the two-rooted mandibular first molars and the three mandibular first molars were 2.54±0.56 mm and 3.13±0.56 mm.There was significant difference between them(t=11.459,P<0.001).Otherwise,there was no significant difference in the distance between the other root canal orifices.There were also significant differences in DB-DL-ML angles between the two types of molars(t=-9.757,P<0.001).The angles of the three-rooted group were 77.06±10.88°,ind the angles of the two-rooted group were 86.74±10.68°.The bilateral concurrence of mandibular first permanent molars with four root canals was correlated.3.The curvature of 273 mandibular first permanent molars were measured.The average curvature of the distolingual root canal was 30.19°±13.48° in mesio-distal view(proximal view),and 11.60°±10.05° in bucco-lingual view.The difference between the two groups has statistical significance(t=18.264,P<0.001).Most of the distolingual root canal in the proximal view was severe bending(61.17%),and the bucco-lingual view was mild bending(43.59%).There was statistical difference between left and right curvature in the mesio-distal view(t=1.929,P<0.05),and there was no statistical difference between left and right curvature in bucco-lingual view(P>0.05).The distolingual root canal bending was the most common in the middle third of the root,next in the apical third of the root,and rare at crown third of the root.ConclusionThe incidence of distolingual root of mandibular first molars is high,and the incidence on the right side is higher than that on the left side.Bilateral occurrence is correlated.The location of DL root canal orifices of three-rooted molars is farther from DB root canal orifices than that of two-rooted molars with four canals.DB-DL-ML angle is sharper in the three-rooted molars and the distolingual root is curved.The curved parts are usually located in middle third,and the curvature in the mesio-distal orientation is severe.CBCT technique can reflect the anatomy of mandibular first molars.This study examined the internal anatomy of mandibular first permanent molars which provided theoretical guidance and imageology basis for clinical treatment.