| [Objective]To measure and analyze the relevant bony and dental factors associated with the impaction of the mandibular second molar by cone beam computed tomography,and to provide certain reference for clinical diagnosis and treatment planning.[Material and methods]53 patients with impacted mandibular second molars were selected as study group from the Orthodontic Department of Nanjing Stomatological Hospital,Medical School of Nanjing University from September,2017 to June,2019,and CBCT examinations were taken with their approvals.There were 34 samples of unilateral impaction and 19 samples of bilateral impaction,16 males and 37 females,and aged from 12~23 years old.Another 53 patients with normal eruption of mandibular second molars were chosen as control group.They were 17 males and36 females,and aged from14~21 years old.Original image data were imported into Mimics 19.0 software through DICOM format.Seventeen landmarks were pointed through multi-planer reconstruction way and the corresponding linear distances and angles were described as the three-dimensional craniomandibular features.Sagittal and vertical indexes of the jaw were measured such as SNA,SNB,ANB,and MP-FH.Mandibular morphological indexes were analyzed such as the length of the mandibular body and the retromolar space.The posterior arch crowding was calculated after the measurements of mesiodistal widths of the mandibular second and third molar.Buccal and lingual cortical bone thicknesses at the depths of 4,6,8,and 10 mm under the crest and the corresponding buccolingual alveolar bone widths were also measured.Independent or paired t test and binary linear regression analysis were carried out by SPSS 22.0software.The test level was α=0.05.[Results]1.Comparing the impaction group and normal eruption group,we found that sagittal and vertical indexes of the jaw showed no significant differences in the two groups such as SNA,SNB,ANB and MP-FH.Mandibular morphological indexes such as the length of the mandibular body had no significant differences in the two groups either.Retromolar space was(13.54±2.94)mm in the impaction group,and significantly smaller than(14.93±3.20)mm in the normal eruption group(p<0.05).Mesiodistal width of mandibular second molar was(11.74±0.89)mm in the impaction group,and statistically bigger than(11.26±0.66)mm in the normal eruption group(p<0.01).The crowding accompanied with the mandibular third molar was(9.93±3.85)mm in the impaction group,and statistically bigger than(7.55±3.49)mm in the normal eruption group(p<0.05).Regression analysis result showed that mandibular second molar impaction had correlation with the retromolar space,the mesiodistal width of the lower second molar,and the posterior crowding.Moreover,the buccal cortical bone thicknesses in the mandibular first molar area were bigger in the impaction group than in the normal eruption group(p<0.05).The lingual cortical bone thicknesses in the mandibular second molar area were smaller in the impaction group than in the normal eruption group(p<0.05).The buccolingual alveolar bone widths under 6 and 8 mm of the crest in the mandibular second molar area were bigger in the impaction group than in the normal eruption group(p<0.05).2.In the 34 samples of unilateral impaction group,mandibular morphological indexes such as the length of the mandibular body had no significant differences in the impacted side and the nonimpacted side.The retromolar space was(14.02±3.05)mm in the impacted side,which was smaller than(15.59±2.46)mm in the nonimpacted side(p<0.01).Mesiodistal width of the mandibular second molar was(11.61±0.99)mm in the impaction side and showed no significance compared with(11.63±0.74)mm in the nonimpacted side.The posterior crowding was(1.36±8.31)mm in the impacted side,which was bigger than(-0.47±7.38)mm in the nonimpacted side(p<0.01).In the mandibular first molar area,the buccal cortical bone thicknesses and the alveolar bone widths at the depth of 4,6,and 8 mm under the buccal crest were bigger in the impacted side than in the nonimpacted side(p<0.05).In the mandibular second molar area,the alveolar bone width at the depth of 6 mm under the buccal crest were bigger in the impacted side than in the nonimpacted side(p<0.05).3.Comparing 34 samples of unilateral impaction and 19 samples of bilateral impaction,sagittal and vertical indexes of the jaw showed no significant differences in the two groups such as SNA,SNB,ANB and MP-FH.The length of the mandibular body was(80.04±3.27)mm in the bilateral impaction group,which was statistically smaller than(83.63±5.65)mm in the impacted side of the unilateral impaction group(p<0.05).Retromolar space was(14.02±3.05)mm in the impacted side of the unilateral impaction group,and showed no significant difference with(12.71±2.63)mm in the bilateral impaction group.Mesiodistal width of the mandibular second molar was(12.00±0.67)mm in the bilateral impaction group and showed no significance compared with the impacted side of the unilateral impaction group.The posterior arch crowding with no third lower molar was(-2.51±2.62)mm in the bilateral impaction group,which was bigger than(-2.02±2.11)mm in the impacted side of the unilateral impaction group(p<0.01).The buccal cortical bone thicknesses and the alveolar bone widths in the mandibular first and second molar area were no significantly different in the bilateral impaction group and the impacted side of the unilateral impaction group.The lingual cortical bone thicknesses in the mandibular second molar area were smaller in the impacted side of the unilateral impaction group than the bilateral impaction group(p<0.01).[Conclusion]1.Mandibular second molar impaction has correlation with the shorter retromolar space,the bigger mesiodistal width of the mandibular second molar,and the bigger posterior crowding.It is suggested to reduce the posterior crowding by tooth extraction or moving the mandibular first molar forward to prevent the mandibular second molar impaction to some extent.2.When the mandibular second molar is impacted,the lingual cortical bone thickness is significantly smaller in the impaction group than the normal eruption group,and smaller in the impacted side of the unilateral impaction group than the nonimpacted side.So it is suggested to pay attention to the buccal bone resistance in the impacted mandibular second molar area during the uprighting process. |
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