Introduction

Orthognathic surgery (OGS) combined with orthodontic treatment is widely used for correcting severe skeletal Class III malocclusions, simultaneously restoring masticatory function and facial harmony.1,2 The conventional orthognathic approach (COA) has been employed to achieve presurgical dental decompensation and a stable postoperative occlusion, but is associated with prolonged treatment duration and a transient yet marked deterioration of facial aesthetics during the presurgical phase.1 In recent years, the surgery-first orthognathic approach (SFA) has been proposed as an alternative.3 By bypassing presurgical orthodontic preparation, the SFA shortens overall treatment duration—capitalising on the regional acceleratory phenomenon (RAP)—while providing immediate aesthetic gratification for the patient.1,4,5

Despite these advantages, the impact of the SFA on upper airway dimensions remains a critical concern. Mandibular setback—a common surgical option for Class III prognathism—alters the hyoid bone's position and displaces the tongue base posteriorly, thereby inducing notable constriction and narrowing of the posterior airway space (PAS).6, 7, 8, 9 The resulting reduction in total volume and minimal cross-sectional area (CSAmin) within the oropharyngeal and hypopharyngeal compartments serves as a latent predisposing factor for obstructive sleep apnea (OSA).6,10,11 Because presurgical dental decompensation is omitted in SFA, a larger mandibular setback and greater clockwise mandibular rotation may be required to achieve transitional occlusion.10,12,13 Agarwal et al. (2020) corroborated this by demonstrating that an SFA-driven mandibular setback precipitates a more drastic immediate postoperative decline in airway volume (a reduction of 1.06 cc per millimetre of setback, contrasting with 0.56 cc/mm in the COA), with a significantly higher rate of airway rebound at the one-year follow-up, primarily attributed to initial occlusal instability.4 Compounding this issue, postoperative kinematic adaptations of the tongue—specifically decreased tongue length and increased tongue height—show an inverse correlation with the PAS, further exacerbating the risk of airway compromise.14,15

To mitigate the risk of postoperative airway compromise and OSA, bimaxillary surgery combining maxillary advancement with mandibular setback has been increasingly advocated.2,16 Maxillary advancement effectively stretches the palatal and pharyngeal soft-tissue musculature, thereby counteracting the airway constriction induced by mandibular setback. Extensive literature on COA cohorts has shown that bimaxillary interventions provide a safer airway profile than isolated single-jaw procedures.2,9,17,18 However, there is currently a paucity of comprehensive quantitative investigations evaluating volumetric and cross-sectional spatial parameters and directly comparing airway changes between isolated single-jaw and bimaxillary surgeries performed exclusively under the SFA paradigm. Additionally, despite established physiological disparities in baseline airway dimensions between males and females, gender-specific response and recovery trajectories of the upper airway following SFA interventions remain underexplored.2,15

In terms of methodology, traditional upper airway evaluations predominantly rely on two-dimensional (2D) lateral cephalograms, which fail to capture transverse dimensions and volumetric changes.19,20 While three-dimensional (3D) cone-beam computed tomography (CBCT) provides excellent spatial resolution, it involves radiation exposure, rendering it less suitable for repeated short-interval postoperative monitoring. Alternatively, acoustic pharyngometry (AP) demonstrates reproducibility and a strong correlation with CBCT volumetric analyses (r ranging from 0.75 to 0.94). This robust correlation validates AP as a highly dependable, radiation-free complementary tool for continuous, dynamic airway monitoring, effectively overcoming the static and radiological limitations inherent to CBCT.20, 21, 22 By utilising the physical principle of acoustic reflection, AP can non-invasively and rapidly quantify pharyngeal volume and minimal cross-sectional area (mCSA) in a dynamic state.23,24 The mCSA parameter derived from AP has been validated as a significant independent predictor for the presence and severity of OSA.8 Because it is entirely free of ionising radiation, AP also safely permits continuous monitoring and real-time dynamic evaluation of airway collapsibility throughout various treatment phases.8,25

To date, limited literature has directly compared upper airway changes between isolated mandibular setback and bimaxillary surgery within the SFA protocol using acoustic pharyngometry. Standardising airway alterations per millimetre of mandibular setback may also allow more clinically relevant comparisons between surgical movements. Therefore, this study aimed to compare short-term and 1-year postoperative changes in upper airway volume and minimal cross-sectional area between these two surgical approaches in patients managed with the SFA, and to evaluate the relative impact of each modality per millimetre of mandibular setback.