Materials and methods

Study design and cohort selection

This retrospective cohort study utilised archival data from 40 individuals who underwent orthognathic correction. The primary focus was to evaluate airway dimensional changes by stratifying subjects into two surgical protocols: single-jaw surgery (isolated mandibular setback) versus bimaxillary orthognathic surgery. Upper airway dimensions (cross-sectional area and volume) were objectively quantified using acoustic pharyngometry (Eccovision Acoustic Pharyngometer). All pharyngometric recordings were executed by a single calibrated examiner adhering to a standardised protocol. The magnitude of surgical skeletal displacement was extracted from postoperative cephalometric analyses and surgical records.

Eligibility criteria

Participants were recruited based on the following inclusion parameters:•Chronological age between 18 and 30 years, with an equal gender distribution.•Availability of comprehensive medical, surgical, and orthodontic records.•Confirmed skeletal maturity presenting with a Class III skeletal pattern (ANB < −2°) and a reverse overjet exceeding 2 mm.•Excellent general health, with no systemic morbidities, bone metabolic disorders, or contraindications for general anaesthesia.•Orthodontic management via a non-extraction philosophy, with a tooth-size arch-length discrepancy of 5 mm or less.

Candidates were excluded if they presented with:•A history of prior maxillofacial trauma, earlier orthognathic procedures, or interceptive craniofacial treatments.•Obese body mass index (BMI ≥ 30.0 kg/m²).•Any pre-existing compromised airway condition, including but not limited to obstructive respiratory disorders, severe asthma, deviated nasal septum, macroglossia, or hypertrophic tonsils/adenoids.

Sample stratification and treatment protocol

Patients were divided into two groups with equal gender distribution to avoid gender bias:•Group 1: Single-jaw surgery (isolated mandibular setback by BSSO) under the SFA.•Group 2: Bimaxillary surgery (maxillary advancement combined with mandibular setback) under the SFA.

To quantitatively evaluate the magnitude and direction of surgical skeletal movements (Point A for the maxilla and Point B for the mandible), a specific Cartesian coordinate system was established on lateral cephalograms. The horizontal reference plane (X-axis) was defined by the Frankfort horizontal (FH) plane. The vertical reference plane (Y-axis) was defined as a line passing through the Sella (S) point and oriented perpendicular to the FH plane. The displacements of Point A and Point B along these axes were recorded to calculate the horizontal (sagittal) and vertical movements (Fig. 1).

Fig. 1: Measuring the distance from the A-point (maxillary) and B-point to the X and Y axes to access the horizontal (sagittal) and vertical movements.

Upper airway parameters were objectively assessed using acoustic pharyngometry (Eccovision Acoustic Pharyngometer) (Fig. 2). The proportional variance (percentage change) in airway dimensions was calculated between T0 (one week before surgery) and T1 (one month after surgery) to appraise the immediate surgical impact, and between T1 and T2 (one year after surgery) to assess long-term airway stability or relapse. Volumetric and cross-sectional alterations, along with the corresponding relapse rates, were further quantified as ratios per millimetre of mandibular setback to facilitate precise inter-group comparisons. The amount of mandibular setback (in mm) was extracted from treatment records.

Fig. 2: Acoustic Pharyngometry (AP) system and principles of upper airway evaluation .36 (A) Components of the Acoustic Pharyngometry setup. (B) Anatomical boundaries of the upper airway (blue area) (C) Schematic representation of the AP setup and the three breathing tasks performed. The blue-shaded region under the Pharyngogram represents the total upper airway volume, derived from the cross-sectional area as a function of the pharyngeal length.

Airway changes per millimetre of mandibular setback were calculated as:•Percentage (%) change: between T1 and T0 (postoperative narrowing) and between T2 and T1 (relapse/recovery).•Airway reduction per mm of setback at T1 = (T1 − T0) / setback (mm).•Airway recovery (relapse) per mm of setback at T2 = (T2 − T1) / setback (mm).

To eliminate operator and biomechanical confounding variables, all surgical procedures were executed under general anaesthesia by the identical maxillofacial team. Furthermore, to eliminate body mass index (BMI) as a confounding factor for airway constriction and OSA risk, obese subjects were strictly excluded from the study. Presurgical orthodontic preparation uniformly utilised a 0.022-inch slot MBT pre-adjusted edgewise bracket system with a standardised archwire sequence. Postoperatively, the surgical splint was maintained to guide the interim transitional occlusion and was subsequently removed 2 to 4 weeks after surgery. Following splint removal, the active postsurgical orthodontic phase commenced immediately to capitalise on the regional acceleratory phenomenon (RAP).

To control for measurement bias, all lateral cephalograms were digitised and analysed by a single experienced orthodontist who was strictly blinded to patient demographic information and the specific evaluation time points. The resulting Intraclass Correlation Coefficient demonstrated excellent measurement agreement (ICC > 0.90 for all cephalometric variables) in a selected subset of 20 cephalograms re-measured by the same investigator after a two-week washout period. Similarly, all acoustic pharyngometry assessments were executed by a single calibrated clinician with specialised expertise in AP instrumentation and the standard operating protocol. To further ensure objectivity, the examiner was also blinded to the primary research objectives and to the specific surgical modality assigned to each subject.

Statistical methodology

Data were aggregated in Microsoft Excel. Categorical variables are reported as frequencies (n); continuous measurements as means ± SD. After confirming data normality, inter-cohort comparisons used the independent-samples Student's t-test, and longitudinal intra-cohort changes across the three time points were assessed by repeated-measures ANOVA. All tests were two-tailed with significance at p < 0.05. Analyses were conducted using SPSS v22.0 (IBM Corp., Armonk, NY, USA).