Vertical bony step between proximal and distal segments after mandibular setback is related with relapse: A cone-beam computed tomographic study

Introduction: Vertical bony step (VBS) occurs between proximal and distal segments of the mandible during mandibular setback surgery with bilateral sagittal split ramus osteotomy. The purpose of this study was to investigate whether VBS is correlated with the relapse of mandibular setback using 3-dimensional models constructed from cone-beam computed tomography. Methods: The subjects consisted of 30 patients who underwent bilateral sagittal split ramus osteotomy for a mandibular setback. Double jaw surgery was performed in 18 patients, and isolated mandibular setback surgery was performed in 12 patients. Cone-beam computed tomography scans were taken at pretreatment (T0), postsurgery (T1), and posttreatment (T2). Treatment changes and the correlations between measurements were evaluated. Results: The mean mandibular setback was −11.9 mm, and the mean VBS was −5.6 mm. Correlations with the relapse of mandibular setback were found in the amount of mandibular setback (T1 − T0), development of VBS (T1 − T0), posterior movement of the proximal segment (T1 − T0), counterclockwise rotation of symphysis (T2 − T1), and the resolution of VBS (T2 − T1). Conclusions: The development and resolution of VBS were correlated with the relapse of mandibular setback. Minimizing VBS is recommended to reduce the relapse of mandibular setback.