Patients
This study was conducted under the Institutional Review Board approval from Seoul National University Dental Hospital (CRI12036). The study procedure was performed in accordance with Helsinki Declaration revised in 2008. This study included 20 patients with skeletal class III malocclusion who underwent bilateral sagittal split ramus osteotomy (BSSRO) and Le Fort I osteotomy with or without genioplasty at the Department of Oral and Maxillofacial Surgery, Seoul National University Dental Hospital in Seoul, Korea. All patients showed < 10 mm of MS at the B point and < 4 mm of maxillary movement at the A point in the immediate postoperative lateral cephalogram. Patients were divided into two groups: an AOB group (n = 10) with < − 4 mm of preoperative overbite and a non-AOB group (n = 10) with an overbite of > 2 mm. Those who underwent BSSRO alone were excluded, and all surgeries were performed by a single surgeon. In all patients, multislice CT scans were taken 1 month before (T1) and 3–6 months after the surgery (T2). The mean age of the patients at the time of surgery was 20.1 ± 6.1 years (range 16–26 years) and the sex ratio was 11:9 (male:female). All patients received pre- and postoperative orthodontic treatment, and semirigid fixation was completed using four-hole titanium miniplates and screws.
Cephalometric analysis
To measure surgical changes and the degree of overbite, the lateral cephalograms of each patient were taken in the maximum intercuspal position at a magnification ratio of 1.1:1 prior to surgery (T0) and immediately after surgery (T1). Each cephalogram was traced on an acetate paper. Cephalometric analysis was conducted using the superimposition technique. Reference points included the sella (S), nasion (N), A point (A), B point (B), upper incision (U1), and lower incision (L1). All reference points were transferred to the lateral cephalograms taken at T1. An x- and y-coordinate system was established, wherein the x-axis was constructed by rotating S–N clockwise by 7° (SN7), while the y-axis was constructed by drawing a line through the sella, perpendicular to SN7 (SN7v). The linear parameters consisted of the x and y coordinates of A and B.
Evaluation of pharyngeal airway in CT
3D facial multislice CT scans were obtained from all patients using a CT scanner (SOMATOM sensation 10; Siemens, Munich, Germany) with the following parameters: 120 kVp, 80 mAs, and a slice thickness of 0.75 mm. Digital image files were saved in the Digital Imaging and Communications in Medicine format and imported into the Invivo Dental software program (Anatomage, San Jose, CA, USA). CT images were rendered into volumetric images. The reconstructed sagittal, axial, and coronal slices and 3D images were then obtained. To standardize the measurements and minimize errors, the Frankfort horizontal (FH) plane was constructed to reorient the 3D images. The FH plane was constructed from the right and left porions and the right orbitale.
The airway study in CT consisted of three components: (1) changes in distance and area in axial CT, (2) changes in airway volume in 3D images, and (3) changes in hyoid bone positioning.
To assess the linear distance and cross-sectional area of the posterior airway, pre- and postoperative measurements were collected at two different levels, specifically at the retropalatal level (the level of the most posterior point of the soft palate parallel to the FH plane) and the retroglossal level (the level of the most posterior point of the tongue base parallel to the FH plane). For both levels, the largest mesiodistal width (LMD), anteroposterior length (LAP), and cross-sectional area (S) were measured.
To evaluate the airway volume, a range of − 1024 to − 600 Hounsfield units was set as the threshold value of the CT image, where the pharyngeal airway could be effectively differentiated from the neighboring soft tissue. The pharyngeal airway was divided into the following two regions using cervical vertebra (CV) reference points, with the CVn plane defined as the plane parallel to the FH plane passing through the most inferior point of the CVn: (1) the oropharyngeal volume between CV1 and CV2 planes (Vo) and (2) the upper hypopharyngeal volume between the CV2 and CV3 planes (Vh). The pharyngeal airway volumes of all patients were measured by the same examiner using the Invivo Dental software program (Anatomage, San Jose, CA, USA), and postoperative volume changes were calculated (Fig. 1).
To evaluate the hyoid bone positioning before and after surgery, two reference lines were defined. The x-axis consisted of the line tangent to the inferior portion of the sella turcica and parallel to the FH plane, while the y-axis consisted of the line perpendicular to the x-axis and passing through nasion. The horizontal and vertical positions of the hyoid bone were measured using the distances between the most anterosuperior point of hyoid bone to the x-axis (Hx) and y-axis (Hy), respectively (Fig. 2).
Statistical analysis
The descriptive statistics of the preoperative and postoperative measurements were processed using the SPSS for Windows version 21 software program (IBM Corp., Armonk, NY, USA). Kolmogorov–Smirnov analysis was performed for all measured parameters attested to the normal distribution of values. The preoperative and postoperative cephalometric measurements and pharyngeal airway volume, length, and area were analyzed using the Wilcoxon signed-rank test. A paired t test was performed to compare the differences between the groups. Differences were considered to be significant at p < 0.05