Differential Relapse of Proximal and Distal Segments after Mandibular Setback Surgery

This study aimed to evaluate the differential positional changes of the proximal and distal segments during mandibular setback surgery relapse. Thirty patients with mandibular prognathism who underwent bilateral sagittal split ramus osteotomy were included. Skull models from pretreatment, postsurgery, and post-treatment cone-beam computed tomography were superimposed to evaluate condylar displacement and rotational changes, and to determine the center of rotation (CRot) in the proximal and distal segments. During postsurgical relapse, the proximal segment rotated 2.4 ± 2.1° counterclockwise, with the CRot located near the mandibular condyle. The distal segment rotated 2.0 ± 2.3° counterclockwise, resulting in forward and upward movement of the chin. The displaced condyle partially returned to its original position during postsurgical orthodontic treatment. A vertical bony step (VBS) between the proximal and distal segments commonly occurs during mandibular setback surgery. As the VBS increases, the CRot of the distal segment shifts posterosuperiorly, following a predictable pattern outlined by regression equations. These findings suggest that relapse after mandibular setback surgery consistently occurs in an anterosuperior direction, with translatory movement becoming more prominent as the VBS increases. Including these regression equations in presurgical planning can enhance the precision of relapse simulations and help clinicians more accurately anticipate postsurgical relapse.