| BackgroundThe extraction of impacted mandibular third molars(IMTM)is one of the most common procedures performed in Department of Oral and Maxillofacial Surgery.Inferior alveolar nerve(IAN)injury following extraction of IMTM close to the inferior alveolar canal(IAC)usually leads to sensory disturbance.It not only seriously affects life quality of patients,but also may cause medical disputes.Preoperative radiographic evaluation of the relationship between IMTM and IAC is necessary to preoperative risk assessment and intraoperative prevention of IAN injury.Conventional radiographic examination including apical X-ray and panoramic photography(PP)can reflect only the two-dimensional anatomical relationship between the IMTM and IAC,without the bucco-lingual relationship.Cone beam computer tomography(CBCT)shows the high spatial resolution and the comprehensive visibility of relationship between the teeth and the surrounding tissue.But high cost limits the popularity of CBCT.So it is important to determine the relationship between the mandibular canal and the IMTM objectively and to guide the design of operative schemes effectively with conventional radiographic examination.PurposeIn this study,we analyzed the clinical epidemiological characteristics of IMTM which was closely related to IAC confirmed by CBCT,and compared these imaging manifestations with the panoramic images to investgate the two-dimensional radiologic features of IMTM.Then we analysed the risk factors of the IAN through retrospective studies,in order to provide theoretical guidance for the clinical procedures of IMTM extraction and prevention of IAN injury in the absence of CBCT.Methods:1.A retrospective analysis was made on the patients who underwent IMTM extraction in the Department of Oral Surgery,School of Stomatology,the Fourth Military Medical University since Jan.1,2015 to Dec.31,2016.Inclusion criteria : 1)Age: Between 15 and 35;2)received CBCT examination;3)CBCT showed that the IMTM was close to the IAC(the IMTM protruding into IAC or closely contact with IAC).Exclusion criteria: jaw cystic lesions or inflammatory lesions around IMTM.The distance between IMTM and IAC was measured from the coronal plane,the horizontal plane and the sagittal plane,and the minimum value was recorded.According to the results,the subjects were divided into two groups: Class A,IMTM protrudes into IAC(no boundary);Class B,IMTM closely contacts with IAC(the distance between IMTM and IAC? 1mm).And then the IMTM was classified according to the Winter classification and the Pell & Gregory classification.The spatial anatomical relationship between the third molar and the mandibular canal was recorded according to Maegawa's classification method.2.A retrospective analysis was made on the patients who underwent IMTM extraction in one treatment unit in the Department of Oral Surgery,School of Stomatology,the Fourth Military Medical University since Jan.1,2016 to Dec.31,2016.All patients underwent preoperative PP,when PP showed maxillary IMTM and IAC overlap,additional shooting CBCT,and included in the follow-up study.Exclusion criteria: jaw cystic lesions or inflammatory lesions around IMTM.Based on the classification of Maegawa,the relationship between IMTM and IAC was divided into 3 categories: A.IMTM contact the upper edge of IAC;B.IMTM in IAC between the upper edge and the lower edge;C.IMTM more than the lower edge of IAC.According to the three-dimensional reconstruction in CBCT,the relationship between IMTM and IAC were divided into 4 groups : I,the two are unbounded;II,the minimum distance is less than or equal to 1 mm;?,the minimum distance is greater than 1 mm but less than or equal to 2 mm;?,the minimum distance greater than 2 mm.And then,a comparative analysis was made on the CBCT and PP.3.A retrospective analysis was made on the patients who underwent IMTM extraction in the Department of Oral Surgery,School of Stomatology,the Fourth Military Medical University since Jan.1,2016 to Dec.31,2016.All IMTM were extracted with minimally invasive tooth extraction(MITE).Gender,impaction type of IMTM,the surgeon and the occurrence of IAN injury were recorded to analyze the incidence and risk factors of IAN injury after IMTM extraction.The relationship between IMTM and IAC was analyzed by PP and CBCT to expose the correlation between lower lip numbness and surgical procedures,compared with the relevant information in 2010 with the splitting crown method for removal of IMTM with postoperative numbness of lower lip.Results:1.During extractions of 26128 IMTMs(17286 cases)in the Department of Oral Surgery,School of Stomatology,the Fourth Military Medical University since Jan.1,2015 to Dec.31,2016,304 IMTMs from 214 patients were included according to inclusion criteria,among which,226(74.34%)IMTMs protrude into IAC,while 78(25.66%)IMTMs contact with IAC.According to the Pell & Gregory classification,median impaction was the most common type in both the protruding group(99,43.81%)and the contacting group(40,51.28%);while in accordance with the Winter's classification,horizontal impaction was the most common type in both the two groups [(90(39.82%),34(43.59%)].Further analysis of the relative position between IMTM and IAC showed that no matter in the protruding group or the contacting group,IAC located next to the root of IMTM [118(52.21%),63(80.77%)] mostly.The median impaction was the most common type in the protruding group(Pell & Gregory classification);horizontal impaction was the most common type in groups(Winter classification).CBCT showed that the spatial position of the tooth and IAC was mostly root contact,followed by tongue contact,root and buccal contact.2.There were 1306 IMTMs(922 cases)extracted in a treatment unit in the Department of Oral Surgery,School of Stomatology,the Fourth Military Medical University since Jan.1,2016 to Dec.31,2016.PP showed 568 teeth overlapped with the IAC,including: a.140 contact with the upper edge of the mandibular canal(24.65%),grade IV 58(41.43%),grade III 50(35.71%),grade II 24(17.14%),grade I 8(5.71%).b.Located between the two bone line 359(63.20%),grade I 155(43.18%),grade ? 148(41.23%),grade II 28(7.80%),grade III 28(7.80%);c.69 are lower than the inferior margin of the IAC(12.15%),grade I 33(47.83%),grade IV 27(39.13%),grade II(7.25%)and grade III 4(5.80%).3.During 14108 IMTMs(9216 cases)extracted in the Department of Oral Surgery,School of Stomatology,the Fourth Military Medical University since Jan.1,2016 to Dec.31,2016,27 patients occurred numbness of lower lip with the incidence of 0.191%(27/14108).All sensory disturbance are temporary,the paresthesia will fades out with time going by.The shortest time to recover is 1 week,the longest is 12 months,while the mean recover time is 12.33 ± 10.86 weeks.No permanent damage occurred.CBCT displays 22 teeth were into the mandibular canal(81.48%),5 teeth is less than 1 mm to the mandibular canal(18.52%).Through the statistical analysis,it was found that the risk of lower limb numbness in the horizontal classification was the highest(0.428%),Significantly higher than the vertical impact,(0.138%)and near the impact of students(0.100%)(P <0.05).The risk of lower limb numbness(0.259%)was lower in the Pell & Gregory category than in the low-grade biopsy(0.188%)and the high resistance(0.105%).But there was no significant difference between them(P> 0.05).The rate of inferior alveolar nerve injury(0.191%)was significantly lower than that of traditional extraction(0.730%)(P <0.05).With the increased experience of doctors in the clinical,although the risk of lower alveolar nerve injury has decreased,but the difference was not statistically significant(P> 0.05)Conclusions:1.This study demonstrates the common type of IMTM next to IAC,the relative position between the IMTM and IAC and the radiologic characteristics in panoramic photography for the first time.Furthermore,it provides guidance for making the extraction plan and preventing inferior alveolar nerve damage in the absence of CBCT.2.The surgeon should pay more attention to the horizontal IMTM in clinic because of the high proportion of close-to-IAC IMTM and the high risk of postoperative lower lip numbness compared with other types.3.Minimally invasive extraction of IMTM is an effective method to reduce the incidence of IAN injury for popularization and further application. |
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