Subjects

CBCT images were obtained from the CBCT image library of the Stomatology Hospital of Shandong University. Only images of patients of Han Chinese ethnicity, aged 6–18 years, taken between December 2010 and December 2012, were included. Strict inclusion criteria were applied while examining the medical history and CBCT images of the study subjects.

With regard to the CBCT images, only those for subjects meeting the following criteria were included: (1) clinically symmetric; (2) class I molar relationship, normal overjet and overbite; (3) no discrepancy in centric relation/centric occlusion; (4) no history of previous orthodontic treatment; (5) reasonably aligned upper and lower incisors without severe crowding; (6) no missing permanent teeth; (7) acceptable oral hygiene without obvious periodontal disease. CBCT images in which the airway structure was not seen clearly or was not complete were excluded, as well as airways containing artefacts.

In terms of the medical history, the selection criteria included no history of any craniofacial surgery or anomaly, no congenital anomalies (such as cleft lip and palate), no dysfunction of the masticatory system, no respiratory pathology or pharyngeal pathology (such as adenoid hypertrophy, tonsillitis and adenoidectomy, or history of tonsillectomy), no history of breathing problems, and no mouth breathing habit, complaint of airway restriction, nasal obstruction, snoring, or obstructive sleep apnoea. This study was approved by the necessary research ethics committee. Finally, 254 CBCT images from 119 males and 135 females were included in this study.

CBCT process

For CBCT scanning, each patient was seated in an upright position and asked to keep their jaws in maximum intercuspation, with the lips and tongue in a resting position. The Frankfort horizontal plane (FH plane) of the patients was kept parallel to the floor. Patients were instructed to breathe normally through the nose, avoiding swallowing and moving their head or tongue during the scanning process. All of the images were acquired using a Galileos CBCT scanner (Sirona, Bensheim, Germany) at 85 kV, 7 mA, and 14 s per rotation (resolution accuracy <0.15 mm). CBCT images were then saved as DICOM (digital imaging and communications in medicine) files.

Segmentation and measurement

The DICOM files were imported into Materialism’s interactive medical image control system (MIMICS) (v.16.01; Materialise, Leuven, Belgium) to visualize the images in the axial, coronal, and sagittal planes by volume-rendering.

Once the DICOM files were imported, the three-dimensional (3D) reconstruction of the patient’s head was oriented so that the FH plane was parallel to the axial plane and the midsagittal plane was oriented to the subject’s midline. The patient’s midsagittal plane was defined as a vertical plane passing through both the anterior nasal spine (ANS) and the posterior midpoint of the spine (centrum) ( Fig. 1 A) . Thirteen landmarks were labelled in the midsagittal view: sella (S), nasion (N), basion (Ba), deepest anterior point in the concavity of the upper labial alveolar process (A), deepest anterior point in the concavity of the lower labial alveolar process (B), menton (Me), anterior nasal spine (ANS), posterior nasal spine (PNS), the tip of uvula (UT), the base of epiglottis (EB), the highest point of hyoid bone (H), the most antero-inferior point on the corpus of the third cervical vertebra (C3), and the roof of the nasopharynx (Roof) ( Fig. 1 B). The hyoid bone position was evaluated using eight linear parameters C3–Me, C3–H, H–EB, H–PNS, H–Me, H–X, H–Y, and H–(C3–Me), as well as two angular parameters, H–S–Ba and H–N–S. Facial characteristics were evaluated using three linear parameters N–Me, N–ANS, and ANS–Me, as well as three angular parameters SNA, SNB, and ANB ( Fig. 1 C).

(A) Midsagittal orientation (in the axial plane) defined by the anterior midpoint between the centre of the anterior nasal spine (ANS) and the posterior midpoint of the centrum of the cervical vertebrae (centrum). (B) Thirteen landmarks were identified in the midsagittal view: S, sella; N, nasion; Ba, basion; A, deepest anterior point in the concavity of the upper labial alveolar process; B, deepest anterior point in the concavity of the lower labial alveolar process; Me, menton; ANS, anterior nasal spine; PNS, posterior nasal spine; UT, the tip of the uvula; EB, the base of the epiglottis; H, the highest point of the hyoid bone; C3, the most inferior and anterior point on the corpus of the third cervical vertebra; Roof, roof of nasopharynx. (C) Definitions of the measurement parameters in the midsagittal view. Parameters of hyoid bone position: (1) H–EB; (2) H–Me; (3) C3–H; (4) C3–Me; (5) H–(C3–Me) (H to C3–Me line); (6) H–PNS; (7) H–X (perpendicular distance from the hyoid bone to the vertical line passing through point S); (8) H–Y (perpendicular distance from H to the horizontal line passing through S); (9) H–N–S (angle formed by H–N line and N–S line); (10) H–S–Ba (angle formed by H–S line and S–Ba line). Facial characteristic parameters: (I) N–ANS; (II) ANS–Me; (III) N–Me; (IV) SNA; (V) SNB; (VI) ANB.

To isolate the desired airway from the CBCT images, the threshold value was set to a range of −1024 to −300 Hounsfield units (HU). The layers of the airway structures were defined and colour-coded based on the minimum and maximum threshold values. Finally, the upper airway in the midsagittal plane was divided into six planes, including plane 1: the horizontal plane passing through Roof (superior border); plane 2: the vertical plane passing through PNS (anterior border); EB plane (inferior border); PNS plane: the boundary of nasopharynx (NP) and oropharynx (OP); UT plane: the boundary of palatopharynx (PP) and glossopharynx (GP); H plane: the highest point was selected as the marker point for the hyoid bone, which varied with EB ( Fig. 2 A, B ). The lateral and posterior boundaries consisted of the pharyngeal walls and the anterior boundary, the anterior wall of the pharynx, the base of the tongue, and the soft palate. Meanwhile, corresponding 3D models were also reconstructed ( Fig. 2 A′, B′, A″, B″).

Airway segmentation and reference planes. Reference planes were viewed in the midsagittal plane (A, B) and 3D model (A′, B′, A‴, B″). Plane 1: the axial plane passing through the Roof point. Plane 2: the coronal plane passing through the PNS point. PNS plane: the axial plane passing through the PNS point. UT plane: the axial plane passing through the UT point. EB plane: the axial plane passing through the EB point. H plane: the axial plane passing through the H point. NP, nasopharynx; OP, oropharynx; PP, palatopharynx; GP, glossopharynx; Hyp, the top part of the hypopharynx.

The airway dimensions were evaluated in terms of multiple parameters, including volume (VOL), length (L), cross-sectional area (CSA, the ratio of VOL/L), mean CSA, anteroposterior dimension of the cross-section (AP), lateral dimension of the cross-section (LAT), and LAT/AP ( Table 1 , Fig. 3 ).

Axial slice images of the pharyngeal airway in four defined planes: (A) PNS plane; (B) UT plane; (C) EB plane; (D) H plane. AP, anteroposterior width; LAT, lateral width.

Statistical analysis

All of the parameters were measured twice by the same investigator. Among the 254 CBCT images from the study subjects, 20 were selected randomly to assess the intra-examiner concordance by intra-class correlation test (ICC) before the correlation analysis. The average value for each parameter from the two measurement times was used for Pearson correlation analysis to detect the correlation between hyoid bone position and airway dimensions. A P -value of <0.05 was considered to be statistically significant. All analyses were performed using SPSS version 13.0 software (SPSS Inc., Chicago, IL, USA).

Assessment of intra-examiner concordance

According to the ICC test, the concordance index was greater than 0.98, indicating high intra-examiner concordance.

Correlations between airway dimensions and hyoid bone position

The length, width, area, and volume were positively correlated with C3–Me, H–Y, H–(C3–Me), C3–H, H–Me, and H–PNS, while they were negatively correlated with H–EB and H–S–Ba. Additionally, H–X and H–EB were negatively correlated with the volume, mean CSA, CSA, LAT, and AP. Of note, H–PNS had a significantly higher correlation with most of the airway dimension parameters ( Table 2 ).

Table 2

Correlations between airway dimensions and hyoid bone position ( N = 254).

Measurements	C3–Me	H–X	H–Y	H–(C3–Me)	C3–H	H–EB	H–PNS	H–Me	H–S–Ba	H–N–S
L1	0.182 **	−0.069	0.414 **	0.187 **	0.169 **	−0.112	0.317 **	0.118	−0.132	0.213 **
L2	0.296 **	0.298 **	0.519 **	0.333 **	0.187 **	−0.054	0.635 **	0.217 **	−0.177 **	0.047
L3	0.295 **	0.015	0.759 **	0.502 **	0.232 **	−0.208 **	0.808 **	0.239 **	−0.286 **	0.261 **
L4	−0.142 *	0.048	−0.112	0.092	0.008	0.332 **	−0.125 *	−0.210 **	0.135 *	0.078
L5	0.355 **	0.159 *	0.807 **	0.525 **	0.260 **	−0.179 **	0.899 **	0.275 **	−0.294 **	0.218 **
L6	0.359 **	0.109	0.812 **	0.501 **	0.274 **	−0.188 **	0.858 **	0.271 **	−0.291 **	0.252 **
VOL1	0.256 **	−0.169 **	0.434 **	0.132 *	0.206 **	−0.171 **	0.344 **	0.254 **	0.006	0.124 *
VOL2	0.560 **	−0.082	0.528 **	0.261 **	0.366 **	−0.258 **	0.591 **	0.508 **	−0.101	0.005
VOL3	0.568 **	−0.197 **	0.530 **	0.301 **	0.371 **	−0.389 **	0.594 **	0.525 **	−0.312 **	0.014
VOL4	−0.149 *	0.055	−0.129 *	0.058	−0.033	0.236 **	−0.129 *	−0.193 **	0.123 *	0.055
VOL5	0.588 **	−0.152 *	0.545 **	0.282 **	0.378 **	−0.352 **	0.613 **	0.544 **	−0.227 **	0.004
VOL6	0.542 **	−0.172 **	0.560 **	0.259 **	0.357 **	−0.331 **	0.587 **	0.507 **	−0.179 **	0.038
Mean CSA1	0.230 **	−0.172 **	0.324 **	0.042	0.184 **	−0.185 **	0.274 **	0.278 **	0.062	0.028
Mean CSA2	0.539 **	−0.209 **	0.433 **	0.178 **	0.348 **	−0.286 **	0.454 **	0.516 **	−0.055	−0.018
Mean CSA3	0.587 **	−0.257 **	0.366 **	0.161 *	0.356 **	−0.404 **	0.412 **	0.566 **	−0.273 **	−0.087
Mean CSA4	0.376 **	−0.236 **	0.309 **	0.095	0.233 **	−0.290 **	0.343 **	0.377 **	−0.285 **	−0.085
Mean CSA5	0.589 **	−0.240 **	0.389 **	0.152 *	0.357 **	−0.366 **	0.427 **	0.570 **	−0.177 **	−0.070
Mean CSA6	0.536 **	−0.255 **	0.393 **	0.118	0.336 **	−0.342 **	0.399 **	0.535 **	−0.122	−0.047
CSA I	0.368 **	−0.096	0.401 **	0.091	0.277 **	−0.190 **	0.380 **	0.356 **	0.007	0.017
CSA II	0.569 **	−0.332 **	0.307 **	0.082	0.340 **	−0.276 **	0.319 **	0.562 **	−0.137 *	−0.072
CSA III	0.534 **	−0.324 **	0.486 **	0.207 **	0.376 **	−0.367 **	0.522 **	0.466 **	−0.319 **	−0.034
CSA IV	0.527 **	−0.312 **	0.434 **	0.190 **	0.364 **	−0.369 **	0.471 **	0.472 **	−0.331 **	−0.046
LAT I	0.390 **	−0.092	0.445 **	0.128 *	0.263 **	−0.241 **	0.463 **	0.352 **	−0.162 *	0.010
AP I	0.428 **	−0.009	0.358 **	0.062	0.318 **	−0.169 **	0.402 **	0.384 **	−0.036	−0.065
LAT II	0.570 **	−0.206 **	0.492 **	0.217 **	0.312 **	−0.324 **	0.531 **	0.535 **	−0.205 **	−0.024
AP II	0.654 **	−0.306 **	0.203 **	0.025	0.424 **	−0.273 **	0.240 **	0.647 **	−0.162 *	−0.150 *
LAT III	0.356 **	−0.171 **	0.585 **	0.319 **	0.211 **	−0.278 **	0.623 **	0.329 **	−0.260 **	0.085
AP III	0.637 **	−0.266 **	0.318 **	0.055	0.414 **	−0.288 **	0.362 **	0.551 **	−0.253 **	−0.125 *
LAT IV	0.345 **	−0.212 **	0.538 **	0.292 **	0.211 **	−0.221 **	0.553 **	0.319 **	−0.262 **	0.035
AP IV	0.516 **	−0.209 **	0.230 **	0.039	0.307 **	−0.316 **	0.288 **	0.479 **	−0.184 **	−0.112
LAT I/AP I	−0.120	−0.056	0.031	0.064	−0.124 *	−0.078	−0.008	−0.105	−0.179 **	0.114
LAT II/AP II	−0.035	0.076	0.377 **	0.245 **	−0.064	−0.088	0.381 **	−0.074	−0.069	0.149 *
LAT III/AP III	−0.447 **	0.186 **	0.028	0.141 *	−0.307 **	0.139 *	0.004	−0.382 **	0.115	0.188 **
LAT IV/AP IV	−0.313 **	0.103	0.110	0.129 *	−0.188 **	0.162 *	0.047	−0.274 **	0.020	0.162 *

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