Study Of Anatomical Limitations Of Mandibular Molar Distalization In Adults With Different Skeletal Patterns And Analysis Of Influencing Factors

Objectives:To measure the size of the mandibular retromolar space,the length of the mandibular body,and the buccolingual inclination of the mandibular molars in adult with different skeletal patterns by cone-beam computed tomography,to investigate the factors affecting the anatomical limitations of mandibular molar distalization,in order to provide guidance for orthodontists in clinical treatment.Methods:A total of 200 orthodontic patients who met the inclusion criteria were selected,according to the vertical and sagittal skeletal patterns,the patients were divided into skeletal class Ⅰ hypodivergent,skeletal class Ⅰ normodivergent,skeletal class Ⅰhyperdivergent,skeletal class Ⅱ normodivergent,and skeletal class Ⅲ normodivergent groups.The CBCT data of patients were imported into Smart VPro software for 3D reconstruction.In the horizontal plane,distance between second molar to the anterior border of the ramus or lingual inner cortical bone was measured at different planes(the crown planes: occlusal plane,the plane with a depth of 2 mm from the occlusal plane;the root planes: the planes with depths of 2 mm,4 mm,6 mm,8 mm,10 mm from the cemento enamel junction)parallel to the buccal cusp line,along with the length of the mandibular body,and the buccolingual inclination of the mandibular molar.SPSS 25.0software was applied for the statistically analysis,the differences of the mandibular retromolar space,the length of the mandibular body,and the buccolingual inclination of the mandibular molars were compared according to the vertical and sagittal skeletal patterns,comparisons between groups were conducted by using One-way ANOVA,and Spearman’s test was used to analyze the relationship between the above measurement items and the skeletal patterns classification indicators.Paired samples t-test was used to compare the differences of buccal-lingual inclination of the mandibular first and second molars.Stepwise regression analysis was used to clarify the factors influencing the anatomical limitations of mandibular molar distalization.P < 0.05 represented a statistically significant difference.Results:1.There were significant differences in the size of the mandibular retromolar space among patients with different vertical skeletal patterns(P < 0.01),the mandibular retromolar space gradually decreased with the increase of the mandibular plane angle(SN-MP)and the decrease of the Posterior-anterior facial height ratio(FHI)(P < 0.01).There were significant differences in the size of the mandibular retromolar space among patients with different sagittal skeletal patterns(P < 0.01),the mandibular retromolar space gradually decreased with the increase of the A-nasion-point B angle(P < 0.01).There were significant differences in the size of the mandibular retromolar space in different measurement planes among patients with the same skeletal pattern(P < 0.01),the mandibular retromolar space gradually decreased from the crown direction to the root direction,and all measurements in the crown plane were larger than in the root plane R10(P < 0.01).2.There were significant differences in the length of the mandibular body among patients with different vertical skeletal patterns(P < 0.01),the length of the mandibular body gradually decreased with the increase of the mandibular plane angle(SN-MP)and the decrease of the Posterior-anterior facial height ratio(FHI)(P < 0.01).Differences in the length of the mandibular body in patients with different sagittal skeletal patterns(P < 0.01),the length of the mandibular body decreased with the increase of the Anasion-point B angle(P < 0.01).3.Differences in the buccolingual inclination of the mandibular first and second molars among patients with different vertical skeletal patterns(P < 0.01),the buccolingual inclination of the mandibular first and second molar gradually increased with the increase of the mandibular plane angle(SN-MP)and the decrease of the Posterior-anterior facial height ratio(FHI)(P < 0.01).Differences in the buccolingual inclination of the mandibular first and second molars among patients with different sagittal skeletal patterns(P < 0.01),the mandibular retromolar space gradually increased with the increase of the A-nasion-point B angle(P < 0.01).Differences in the buccolingual inclination of the mandibular first and second molars among patients(P< 0.01),and the buccolingual inclination of the mandibular second molar was less than the mandibular first molar.4.Buccolingual inclination of the mandibular second molar,SNB,mandibular body length,and FHI are independent influences on the anatomical limitation of mandibular molar distalization(P < 0.05).Conclusion:Differences in the size of the mandibular retromolar space,mandibular body length,and the buccolingual inclination of the mandibular molars among patients with different skeletal patterns.Skeletal patterns affect the length of the mandibular body in the sagittal direction and the buccolingual inclination of the mandibular molars in the transverse direction,which in turn affect the size of the anatomical limitation of mandibular molar distalization.The mandibular retromolar space was relatively small in hyperdivergent and skeletal class Ⅱ patients,CBCT is recommended to assess the space available for mandibular molar distention when orthodontic planning for molar distention,and the buccolingual inclination of the mandibular molars can be adjusted according to the orthodontic plan to increase the available distance for molar distalization.