Analysis Of The Mandibular Ramus-body As A Donor Site On Cone Beam Computed Tomography

Objective: This study aimed to obtain the three-dimensional imaging data in the posterior mandible by cone-beam computed tomography(CBCT),to further measure and statistically analyze the related anatomical structures in this area,and help surgeons reduce complications in bone harvesting procedures when removing bone grafts from the this area.Methods: Cellected 60 patients’ CBCT images from January 2020 to November 2020 in the second Hospital of Hebei Medical University for a retrospective study.We selected every 5mm from the back of the mental foramen to the mandibular foramen as a reference plane,For each reference plane,linear measurements were taken of cross-sectional scans to record lateral distance from the MC to the external surface of the mandibular body,lateral thickness of the cancellous bone and cortical bone,vertical distance from the MC to the bone surface and vertical thickness of the cancellous bone and cortical bone.The distance from the root and root midpoint of the molars to the buccal surface of the mandible and the thickness of cancellous bone and cortical bone were measured in each CBCT image.SPSS Statistics 25.0software was used for statistical analysis.Results:1.In the body of the mandible,the average lateral distance from the MC to the external surface of the mandibular body is 3.68 ±0.97 mm to 7.14 ±0.99 mm,from the mental foramen to the distal region,the average lateral distance gradually increases,the max distance is 9mm.the average lateral thickness of the cancellous bone is 1.70 ±0.90 mm to 3.32 ±0.71 mm,the average lateral thickness of the cortical bone is 1.98 ±0.51 mm to 3.66±0.53 mm.The average vertical distance from the MC to the bone surface is11.52 ± 3.27 mm to 17.24 ±2.95 mm.In the molar area,the vertical distance from the MC to the bone surface is smaller,in the premolar area,the distance is larger,the minimum distance is only 7.2mm,and the maximum distance can reach 25.9mm.2.In mandibular ramus,the average lateral distance from the MC to the external surface of the mandibular body is 3.11 ±0.92 mm to 4.28 ±1.17 mm,from near part to the distal part of the mandibular ramusand the distance gradually decreases,and the minimum distance is only 0.9mm.the average lateral thickness of the cancellous bone of the MC is 0.33 ±0.31 mm to 1.65±0.28 mm.the average lateral thickness of the cortical bone is 2.22 ±0.48 mm to 2.63 ±0.43 mm,the average vertical distance from the MC to the bone surface is 9.7 ±0.92 mm to 11.49 ±1.17 mm,the average vertical thickness of the cancellous bone is 5.73 ±2.63 mm to 8.79 ±3.62 mm,and the average vertical thickness of the cortical bone is 2.7 ±2.17 mm to 4.1 ±2.92 mm.Near the mandibular foramen,there is a high content of cortical bone.3.The average distance for the middle point of the root in the first molar to the external surface of the mandibular body is only 1.4 ±0.9mm,and the minimum distance is only 0.7mm.There is enough bone at the he second molar,while the minimum distance from the root midpoint of the third molar to the external surface of the mandibular body is 0.5mm,at the root,the minimum distance to the external surface of the mandibular body is 1.8mm,while the maximum distance is 7.24 mm.4.Whether there has the third molar or not,no significant difference was found in the lateral distance from the MC to the external surface of the mandibular body,lateral thickness of the cancellous bone,lateral thickness of the cortical bone,vertical distance from the MC to the bone surface,vertical thickness of the cancellous bone,vertical thickness of the cortical bone(P>0.05).Conclusions:1.We should keep the maximum cutting depth within 3.68 mm when harvesting autogenous block grafts from the mandibular molar area.In this area,the mandibular ramus bone can be obtained with lower risk by using appropriate bone extraction instruments and schemes.2.When the autogenous bone graft is taken from the mandibular molar area,we should keep the osteotomy line below the middle level of the first molar root.At the second molar area it is relatively safe.When the third molar is involved,the detailed anatomical data should be analyzed on CBCT before operation.3.During the preparation of autogenous bone graft in the mandibular ramus area,the osteotomy line should be placed above the mandibular nerve canal as far as possible.If the osteotomy line is lower than the level of the mandibular nerve canal,the author suggests that the safe distance should be3 mm,and we should assess the morphologic contours and dimensions of the donor site by visual examination and digital palpation before operation,then determine the position of the mandibular canal in this area by CBCT.To avoid alveolar nerve injury,an inferior alveolar nerve block was avoided;in most cases,only local vestibular and lingual infiltration was performed.This retained minimal sensation to warn the surgeon when approaching the mandibular nerve.4.The bone pieces cut from the mandibular ramus are basically cortical bone,with almost no cancellous bone.Therefore,when the bone from the mandibular ramus is removed,the problem of vascular reconstruction in the bone graft area after autologous bone transplantation should be considered to ensure good healing in the bone graft area.5.Due to the high variability of the mandibular nerve canal in the mandible,all surgeries must be performed following a three-dimensional investigation to evaluate the exact position of the mandibular nerve canal and its distance from the bone walls.