A Comparative Study Of Basal And Dental Arch Width In Skeletal Class Ⅲ Malocclusion And Normal Occlusion

Background and Objectives:To compare the maxillomandibular transverse basal bone discrepancies and dental arch discrepancies in the initial first molar area between skeletal Class Ⅲ malocclusion and Class Ⅰ normal occlusion groups,and to understand the morphological characteristics of basel bone and dental arch.In order to provide a new method for measuring maxillomandibular transverse basal bone discrepancies,the width of maxillary and mandibular basal bone in the orthodontic target position was measured.Materals and Methods:Pretreatment CBCT(5g;NewTom,Verona,Italy)images of thirty skeletal Class Ⅰ patients and thirty Class Ⅲ patients were randomly selected and transferred into Mimics 21.0 software for reconstruction.Using the CBCT images,initial basal bone width were measured from the zygomatic arch point for the maxillar and from the edge of the buccal cortical bone opposite the resistance center of the first molars for the mandible.Using the casts,initial dental arch widths were measured as the distance between the right and left central fossa of the first molars.The maxillomandibular transverse basal bone discrepancies and dental arch discrepancies were calculated and measured repeatedly.Using IBM SPSS Statistics version 22.0,Independent t-tests were applied for comparisons of the maxillomandibular transverse basal bone discrepancies and dental arch discrepancies values between the skeletal Class Ⅰ and Class Ⅲ groups,and Intraclass correlation coefficient(ICC)was used to determined the reliability.Statistical significance was set at the 5% level.Then,the 3D virtual models of the maxillary and mandibular first molar were reconstructed,In the skeletal Class Ⅲ group,the sagittal position of the maxillary molars were maintained,and the mandilbular first molar were moved distally along the plane composed of resistance centers until the molar relationship was neutral.The first molars were calibrated to make them upright in the basal bone.The width the basal bone in the orthodontic target position were measured as the distance between the mesial palatal apex of the molars for the maxilla and between the central fossa of the molars for the mandible.As for the skeletal Class Ⅰ group,the first molars were only made to upright in the basal bone without sagittal movements.The measurements were done by 2 examiners and repeated after 2 weeks to determine the interexaminer and intraexaminer reliabilities.Independent sample t test were were applied using SPSS22.0 and P value less than 0.05 was deemed to be statistically significant.Results:1.Initial basal bone width:the maxillary basal bone width of skeletal Class Ⅲ groups(63.96±3.78mm)was smaller than Class Ⅰ normal occlusion groups(62.26±3.12mm),while the mandibular basal bone width of skeletal Class Ⅲgroups(65.67±2.76mm)was larger the Class Ⅰ normal occlusion groups(60.29±3.15mm).The maxillomandibular basal bone discrepancies of skeletal Class Ⅲ groups(1.69±2.54mm)were smaller than Class Ⅰ normal occlusion groups(5.38±1.24mm).2.Dental arch width:there was no significant difference between skeletal Class Ⅲmalocclusion group and skeletal class Ⅰ normal occlusion group(P> 0.05).3.The basal bone width in the orthodontic target position:the maxillary basal bone width was 41.72±1.64 mm in skeletal Class Ⅲ groups,42.33±2.08 mm in Class Ⅰgroups;while the mandibular basal bone width was 46.96±2.70 mm in skeletal ClassⅢ groups,42.66±2.25 mm in Class Ⅰ groups.The maxillomandibular basal bone discrepancies was-5.24±1.64 mm in the skeletal Class Ⅲ groups and-0.34±1.0 mm in the Class Ⅰ groups.Independent sample t-test showed that there was significant statistical difference between the two groups(P<0.05)Conclusion:1.skeletal Class Ⅱ malocclusion groups tend to show maxillomandibular transverse basal bone disharmony.2.There is no significant difference in the width of the dental arch between patients with skeletal class Ⅲ malocclusion and those with skeletal class Ⅰ malocclusion.3、The method of maxillomandibular discrepancies in the orthodontic target position can effectively reveal insufficient width of the maxilla.