| Objective:The danger zone is the weak part of the root canal wall,where lateral piercing of the root canal is easy to occur.The purpose of this study was to observe the anatomical structure of the mesial root canal,the thickness of the canal wall and the location of the root canal danger zone in the mandibular first molars(MFMs).To evaluate the effect of contracted endodontic access cavity(CEC)on mesial root canal wall thickness and apical transportation of MFMs prepared with using One Curve single file system,in order to provide experimental basis for selecting better endodontic access cavity and reducing the risk of root canal lateral piercing in clinical root canal therapy.Method:1.Data collection: 20 MFMs extracted for severe periodontal disease that met the inclusion criteria,including intact coronal roots,mesial roots with two separate root canals,mesiobuccal canal(MB)and mesiolingual canal(ML),and a curvature of 0° to 20°,were selected.Then randomly divided them into 2 groups based on the endodontic access design,with 10 samples in each group: Traditional endodontic access cavity(TEC)and CEC.In the TEC group,the pulp chamber roof of the tooth was completely removed,while in the CEC group,the pulp chamber roof and pericervical dentin were preserved as much as possible.Then,the One Curve single file was adopted to conduct root canal preparation.Next,cone beam computed tomography(CBCT)was performed on extracted teeth before and after preparation,and the measurement sections were located at 0-7.0 mm below the root bifurcation of the mesial root canal at 1 mm intervals.The minimum wall thickness on the mesial and distal aspect of the root canal was measured in each section.2.Data analysis: the measurement results are inputted by Microsoft Excel.SPSS26.0was used to make statistics,and the difference was statistically significant when P<0.05.The minimum root canal wall thickness was expressed by “mean± standard deviation”(mm).The distribution ratio of root canal wall thickness and the proportion of the thinnest root canal thickness in the two groups before and after preparation were expressed by frequency(%).The experimental data were analyzed by descriptive statistical method.Chi-square test was used to analyze the validity of the counting data.Test the normal distribution of data,if it is normal distribution(P>0.05),then use independent sample t-test and paired sample t-test to analyze the validity.If it is non-normal distribution(P<0.05),rank sum test is used for validity analysis.Results:1.Prepreparation CT measurements of 20 MFMs showed that the danger zone in the range 0-4 mm under root bifurcation,a mean thickness of 1.18 mm on the mesial aspect of the MB root canal and 1.08 mm on the distal aspect.The mean thickness of the ML root canal was 1.28 mm on the mesial aspect and 1.07 mm on the distal aspect.The thickness of the root canal wall of the mesial root of MFMs decreased gradually from the 0～4mm under the root bifurcation,and the thinnest part of the root canal danger zone was located at the4 mm under the root bifurcation.2.MB root canal of MFMs: before preparation,the proportion of the minimum thickness of the root canal wall mesial aspect of MB root canal less than that of 0.5mm in TEC group was 8.75%.The proportion of CEC was 7.5%(χ2= 0.08,P=0.77).After preparation,the root canal thickness of TEC group was 16.25%,while that of CEC group was 16.25%(χ2= 0.00,P=1.00).ML root canals of MFMs: the number of sections in which the thickness of the root canal wall mesial aspect of ML was less than that of 0.5mm in TEC group was 5.0% before preparation,and that in CEC group was 2.5% before preparation(χ2= 0.69,P=0.41).After preparation it was15.0% in TEC group and 12.5% in CEC group.There was no significant difference between the two groups in the cross-sectional ratio in which the thickness of the root canal wall mesial aspect was less than that of 0.5mm(χ2= 0.21,P=0.65).3.After preparation,the apical transportation of the MB root canal in the TEC group tends to be more mesial side(median:-5.26%,quartile: 9.5%),while in the CEC group(median: 1.3%,quartile: 21.2%),it is slightly inclined to the distal side.In both two groups,the deviation trend of the mesiolingual root canal was distal side.Conclusions:1.The thickness of the root canal wall of the mesial root of MFMs decreased gradually from the 0～4mm under the root bifurcation,and the thinnest root canal danger zone was located at the 4mm under the root bifurcation,with an average of 0.78 mm.2.The probability that the mesial aspect of the mesial root canal of MFMs is a dangerous zone(18%-36%)is smaller than the distal aspect of the root canal,but the risk of penetration can’t be ignored.3.When using the One Curve preparation system,compared with the TEC,the CEC based on the minimally invasive concept did not increase the risk of perforation of the mesial root canal of MFMs.4.Clinicians should note that the use of CEC makes apical transportation of mesiobuccal root canal more likely to be distal. |
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