A Study of the Discrepancy in Upper and Lower Basal Arch Widths Related to Inclination of the Upper and Lower Molars in Jaw Deformity Patients

Abstract

There have been many studies on orthognathic surgery of class III patients with mandibular excess. In such patients, it is necessary to achieve the proper transverse relationship between the upper and lower jaws, as well as incisors, in the upright position, and alignment of both arches before surgery. However, it is difficult to diagnose discrepancies in transverse widths related to the inclinations of the upper and lower molars. The purpose of this study was to determine the proper transverse relationship by evaluating basal arch width discrepancies.
Ten patients were diagnosed as skeletal class III characterized by mandibular excess. They underwent presurgical orthodontic treatment, with extraction of bilateral upper first premolars, to reduce the labial inclination of the upper incisors, and to obtain an adequate basal arch width relationship between the mandible and maxilla. Then they were treated by bilateral sagittal split ramus osteotomy and postsurgical orthodontic correction. Lateral cephalometric radiographs before and after orthognathic surgery (T1 and T2, respectively) were used for Wits appraisal. Dental models were also taken before and after orthognathic surgery. Measurements in dental models included 1) bucco-lingual inclinations of the upper and lower first molars to the occlusal plane, and 2) transverse widths of the upper and lower basal bones (basal arch width).
Improved transverse relationships between the mandible and maxilla were achieved in all cases after treatment, although three of the ten cases showed crossbite on the first molars after orthognathic surgery. Inclinations of the upper or lower molars are correlated with discrepancy in the basal arch widths between upper and lower jaws. Discrepancy in the basal arch widths less than -0.5mm (minus means the lower jaw is wider than the upper jaw) has limited the position of the first molars in the proper bucco-lingual relationship.