The biomechanical effects of a sagittal split ramus osteotomy on the relationship of the mandible, the hyoid bone, and the cervical spine.

The effects of a sagittal split ramus osteotomy in correcting mandibular retrognathism and prognathism were studied cephalometrically in a group of 25 patients. The patients were divided into two groups, 18 patients with retrognathic mandibles and 7 patients with prognathic mandibles to be corrected by means of a sagittal split ramus osteotomy according to Obwegeser Dal Pont. The major complications of this type of surgical intervention have been documented in literature and are known as (1) a certain degree of relapse, i.e., mandibular instability and (2) the development of a permanently disturbed head balance. The displacement of the dental, skeletal, and neuromuscular components within the craniomandibular complex requires a large adaptive capacity and suggests that the patient will develop a forward head posture (anteroposition of the head) after both types of surgical intervention. In mandibular advancement, however, the initiating mechanism to develop a forward head posture starts in the cervicothoracic transition (C6-C7-T1-T2). In mandibular set-back, the discussed mechanism will start in the upper cervical area (C0-C1-C2-C3). The different pattern of movement of C2 and C3 in mandibular advancement and mandibular set-back, respectively, confirmed the conclusion that the initiating mechanisms, may be found in the cervicothoracic transition and the upper cervical region, respectively.