Bimaxillary orthognathic surgery is part of the standard repertoire for correcting pronounced or complex facial deformities. Patients undergoing corrective orthognathic surgery show subjective and objective improvements in psychosocial and functional aspects (Rustemeyer and Gregersen, 2012). In addition to skeletal changes, there are visible, as well as invisible, soft tissue alterations. The posterior air space (PAS) is vital concerning the invisible changes. The PAS is defined as the volume that determines the airflow capacity in the upper airway. It is influenced by, if not proportional to, the individual's craniofacial growth pattern (Muto et al., 2008). Surgical expansion of the PAS by maxillo-mandibular advancement (MMA) is an effective procedure in the surgical treatment of obstructive sleep apnea syndrome (OAS)) (Zinser et al., 2013; Yamashita et al., 2017). The impact of orthognathic surgery on the PAS can be visualized two-dimensionally via lateral cephalogram (LCR), as well as three-dimensionally in computed tomography (CT) or cone-beam computed tomography (CBCT) scans (Aboudara et al., 2009; Sears et al., 2011; Zinser et al., 2013; Ayoub et al., 2019). Several studies have demonstrated the expansion of the PAS by MMA and its attenuation in the context of a mandibular setback on two- and three-dimensional images (Alcalde et al., 2019; De Ponte et al., 1999; Achilleos et al., 2000; Tselnik and Pogrel, 2000; Kawakami et al., 2005; Eggensperger et al., 2006; Kim et al., 2016).

In the past, several studies have argued that changes in PAS are unpredictable (Joss and Vassalli, 2009; Riepponen et al., 2017). Such unpredictability may be a problem in some instances: when an enlargement of PAS is desired in the presence of existing OAS, and when a decrease in PAS volume could trigger OAS. In a previous study, the authors demonstrated that the changes of the PAS in the lateral cephalogram in patients with Angle Class II malocclusion and mandibular advancement are predictable (Wiedemeyer et al., 2019). The authors could even develop a statistical model that gives reliable predictions of the expected alterations. Therefore, this study aimed to investigate whether such a model can also be established for bimaxillary orthognathic surgery.