| Objective:To analyze the mandibular retromolar space among normal-divergent adult patients with different sagittal skeletal patterns by cone-beam computed tomography(CBCT),and to explore the influencing factors of mandibular retromolar space,aiming to provide a reference for molar distalization in the orthodontic clinic.Methods:A total of 120 normal-divergent adult patients,who were admitted to the Department of Orthodontics,Hospital of Stomatology,Shandong University from January 2018 to August 2021,were selected.Lateral cephalometric and CBCT images were taken for each patient at their first visit.The patients were categorized into three groups according to their ANB angle:skeletal Class Ⅰ,skeletal Class Ⅱ,and skeletal Class Ⅲ group,and then divided into third molar group and no-third molar group according to the presence of the third molar.CBCT data were imported into Mimics 21.0 software for measurement.The sagittal line(a reference line that was passing through the anterior and posterior nasal spine point)and the cuspal line(a reference line that was passing through the mesial and buccal cusps of the mandibular first and second molars)were used to be the reference lines.The distances from the most lingual point of the distal root of the mandibular second molar to the inner and outer edges of the mandibular lingual cortex were measured at the level of the mandibular second molar root bifurcation and at the levels of 2,4,and 6 mm away from the root bifurcation by two reference lines,respectively.In SPSS 26.0 software package,Mann-Whitney U-test and independent samples t-test were used to compare the mandibular retromolar space between right side and left side and between the third molars group and no-third molar group;one-way analysis of variance(ANOVA)and LSD-t was used to test the differences of mandibular retromolar space and the angle between the sagittal line and the cuspal line;the chi-square test was used to compare the number of patients with the distal root of the mandibular second molar touching the lingual bone cortex of mandible between the three groups.The level of significant difference was set to α=0.05.Results:1.There was no statistical difference in the mandibular retromolar space between the left and right sides in each measurement plane(P>0.05).2.There was no statistical difference in the retromolar space between the third molar group and the non-third molar group in each measurement plane(P>0.05).3.There were statistical differences in the mandibular retromolar space measured along the cuspal line between different sagittal skeletal patterns(P<0.05).The retromolar space of the Class Ⅲ patients was significantly larger than that of Class I patients and Class Ⅱ patients(P<0.05).There was no statistical difference in angle between the sagittal line and cuspal line among different sagittal skeletal patterns(P>0.05).4.Mandibular retromolar space gradually decreased as the measurement plane moved toward the root apex,and there were statistically significant differences in the mandibular retromolar space between different measurement planes in Class Ⅰ and Class Ⅱ patients(P<0.05).5.There are 31 patients were found that their mandibular second molar’s distal roots had contact with the root of the lingual mandible:15 in Class Ⅰ group,13 in Class Ⅱ group,and 3 in Class Ⅲ group.The differences in the proportions of each group were statistically significant(P<0.05).Conclusion:1.The presence of third molars and left and right sides do not affect the mandibular retromolar space.2.Compared with Class Ⅰ and Class Ⅱ patients,Class Ⅲ patients have a larger mandibular retromolar space,which has an advantage in molar distalization.3.CBCT could accurately assess the mandibular retromolar space.For patients who need molar distalization,orthodontists should measure the available mandibular retromolar space for molar distalization through CBCT to increase the predictability of molar distalization and reduce the occurrence of pathological damage. |
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