Measurement And Classification On The Thicknesses Of The Buccal And Lingual Bone Walls Of The Mandibular Third Molars In The Vertical Positions With CBCT

Objective:Cone beam computed tomography（CBCT）was used to measure the thicknesses of the buccal and lingual bone walls of each measurement site of the mandibular third molars in the vertical positions,and to obtain the measured data for data processing and statistical analysis.The mandibular third molars in the vertical positions were classified according to the difference of the thicknesses of the buccal and lingual bone walls.Analyze and compare whether there were statistical differences among the thickness of the buccal and lingual bone walls,ages,genders and sides,and at the same time,measure the distance between the root tip and the mandibular canal,and analyze whether there were statistical differences among the distance between the root tip and the mandibular canal,ages,genders and sides.Therefore,these data provide preoperative guidance and theoretical basis for the extraction of the mandibular third molars in the vertical positions.Methods:The CBCT image data taken by New Tom were collected and analyzed by the clinicians,and it was confirmed that there was a mandibular third molar in the vertical position.A total of 153 samples meeting the inclusion criteria,genders,ages and left and right sides were not limited.After three-dimensional imaging with the built-in NNT Viewer software,according to the linear measurement method,the CBCT image analysis software was used to measure the thicknesses of the buccal and lingual bone walls corresponding at 0.9mm at the root of the medial alveolar crest（site O₁ and O₂）,at the midpoint of the apex（site R）and at the midpoint of the root（site P₁and P₂）.The mean value M of the difference in thicknesses of the buccal and lingual bone walls was used as the classification standard.The index of lingual position was the mean value M≥1mm,the index of central position was the mean value-1mm<M<1mm,and the index of the buccal position was the mean value M≤-1mm.Meanwhile,the distance between the root tip and the mandibular canal was measured.Results:The thicknesses of the buccal bone walls of the mandibular third molars in the vertical positions were 6.71±1.25mm;there was no correlation with ages,and there was no statistical difference within genders and sides（P>0.05）.The thicknesses of the lingual bone walls of the mandibular third molars in the vertical positions were 2.28±0.82mm;there was no correlation with ages,and there was statistical difference with genders（P<0.05）,but there was no statistical difference within sides（P>0.05）.According to the M value obtained by the linear measurement method,the mandibular third molars in the vertical position were measured in 150 cases of lingual position,and 3cases of central position.The mandibular third molar in the vertical position of the buccal position could not be measured in the selected samples.The distances between the root tips and the mandibular canals were 4.37±2.74mm;there was no correlation within ages,and there was statistical difference within genders（P<0.05）,but there was no statistical difference within sides（P>0.05）.Conclusion:The thicknesses of the buccal bone walls and the lingual bone walls of the mandibular third molars in the vertical positions and the distances between the root tips and the mandibular canals had no correlation with ages.There was no statistical difference among the thicknesses of the buccal bone walls of the mandibular third molars in the vertical positions,genders and sides（P>0.05）.There was statistical difference between the thicknesses of the lingual bone walls of the mandibular third molars in the vertical positions and genders（P<0.05）,but there was no statistical difference within sides（P>0.05）.There was statistical difference between the distances between the root tips and the mandibular canals and genders（P<0.05）,there was no statistical difference within sides（P>0.05）.CBCT image can truly reflect the three-dimensional anatomical structure around the mandibular third molar in the vertical position.CBCT was used to measure the thicknesses of the buccal and lingual bone walls of the mandibular third molar in the vertical position.Detail clinical data were obtained,and a new classification of the mandibular third molar in the vertical position was performed according to the M value.In the selected samples,the lingual position was the most,the central position was second,no the buccal position.By measuring the thicknesses of the buccal and lingual bone walls and the distances between the root tips and the mandibular canals,the physician can fully understand the anatomical relationship of the structure around the mandibular third molar in the vertical position.Thus,provide preoperative data for extraction of the mandibular third molar in the vertical position and theoretical guidance for selecting the appropriate surgical procedures.