CBCT Analysis Of The Relationship Between The Mandibular Third Molar Impaction And Root Resorption Of The Adjacent Second Molar

Objective: To analyze the correlation between the impacted mandibular third molar(IMTM)and the external root resorption(ERR)of the second molar by cone-beam computed tomography(CBCT),and to explore the risk factors of mandibular second molar ERR,providing reference for clinical preventive extraction of mandibular third molar(MTM)and for prevention of ERR of mandibular second molars.Methods: A total of 327 patients(578 patients)who were treated in our hospital from January 2017 to December 2019 and were required to be hospitalized to remove IMTM were selected and divided into male and female groups according to gender and ≤20 years,21-30 years old,31-40 years old and >40 years old according to age.To analyze the related factors of IMTM causing the second molars ERR,including MTM impaction direction,impaction depth,inclination angle,and ascending ramus classification,etc.Logistic regression was used for multivariate analysis using the significant factors in the univariate analysis results.The effect of MTM impacted type on the severity of second molar ERR and the correlation between MTM impacted type and different parts of second molar ERR were further analyzed.Results:1.There was a statistical difference between the ERR of the second molars and gender(P<0.05),and the incidence of ERR in male patients was higher than that in female patients(27.9%);There was no statistical difference between the ERR of the second molars and age(P>0.05).2.There were significantly statistical difference between the second molar ERR and the different impacted directions of MTM(P<0.05).The comparison results showed that there was no statistical difference between horizontal and mesial impaction(P>0.05),but there were differences between mesial and vertical impaction,and vertical and horizontal impaction(all P<0.05).When the MTM was mesial impaction,the incidence of ERR in the second molar was the highest(32.7%).3.There was a statistically significant difference between the second molar ERR and the different inclination angles of MTM(P<0.05).The comparison results showed that there were statistical differences between ≤20° and 21°-40°,41°-60°,61°-80°and >80°(all P<0.05),but there was no statistical difference between 21°-40° and41°-60°,61°-80° and >80°(all P>0.05).The incidence of ERR in second molars was the highest(31.7%)when the MTM inclination angle was >80°.4.There was a statistically significant difference between the second molar ERR and different impacted depth of MTM(P<0.05).The comparison results showed that there was no statistical difference between impacted lower teeth and impacted medial teeth(P>0.05).But there was a statistical difference between impacted lower teeth and impacted high teeth(P<0.05),and there was a statistical difference between impacted medial teeth and impacted high teeth(P<0.05).The incidence of ERR in the second molar was the highest(38.6%)when the MTM was low impacted.5.There was a statistically significant difference between the second molar ERR and ascending ramus classification(P<0.05).There was no statistical difference between class II and class III(P>0.05).But there was statistical difference between class I and class II,and class I and class III(P<0.05).The incidence of ERR in the second molar was the highest(44.9%)when the MTM was in class III.6.Logistic multivariate analysis showed that there was no statistical significance between gender and the occurrence of ERR(P>0.05).The risk of ERR in mesial impaction is 76.212 times that of vertical impaction(P<0.05),the risk of ERR in low impaction is 2.988 times that of high impaction(P<0.05),and the risk of ERR in class III is 2.344 times that of class I(P<0.05).There was no statistical significance between the inclination angles and the occurrence of ERR(P>0.05).7.There was no significant difference between the severity of ERR and age,impaction direction and ERR location(all P>0.05),but there was a statistically significant difference between the severity of ERR and different impacted depths of MTM(P<0.05).When MTM was low impacted,the second molars had more moderate resorption(40.9%).8.There was a significant difference between the different impacted depths of MTM and the ERR site of the second molar(P<0.05).The common resorption site of low impacted teeth was the apical 1/3(54.5%).There was a significant difference between the different impacted direction of MTM and the ERR site of the second molar(P<0.05).The most common resorption site of mesial impaction was the cervical 1/3(73.8%).There was a significant difference between the different inclination angle of MTM and the ERR site of the second molar(P<0.05).When the inclination angle was >80°,the common resorption site of the second molar was the medial 1/3(57.7%).9.There was a significant difference between the ascending ramus classification and the ERR site of the second molar(P<0.05).In class I,the common resorption site was the cervical 1/3(54.7%),and in class II and III,the common resorption site was the medial 1/3,46.2% and 63.6%,respectively.Conclusion:The impacted direction,impacted depth and ascending branch classification of MTM are the influencing factors of ERR in second molars.The impaction direction,inclination angle,impaction depth and ascending branch classification of MTM have an impact on the location of ERR.When MTM is mesial impaction,low impaction and in III classification,the incidence of ERR in second molar is higher,and preventive extraction should be performed as soon as possible in clinical practice.