CBCT plays a vital role in diagnosing oral and maxillofacial morphological anomalies. Interestingly, it has the advantages of low radiation, low cost, easy acquisition, short scanning time, and accurate delineation of the cavity structure boundaries[7]. Schendel et al.[8] demonstrated that the 3D upper airway volume measurements obtained from CBCT images could accurately signify the anatomical structure and spatial size. However, airway measurements on lateral cranial radiographs can only provide 2D measurements but cannot evaluate the upper airway volume and minimum cross-sectional area, which poses certain limitations.
Jadhav et al.[9] reported that the width of the upper and lower airways in patients with hyperdivergent skeletal Class II was significantly smaller than that in those with hyperdivergent skeletal Class I. Moreover, Oz et al.[10] found that the oropharyngeal segment size in patients with hyperdivergent skeletal Class II group was significantly smaller than that of the normal angle skeletal Class I group. Among the three vertical skeletal Class II groups, the size of the upper airway oropharyngeal segment in the high-angle group was significantly reduced compared to that in the low-and normal-angle groups. Furthermore, Mao et al.[11] showed that the upper airway of patients with hyperdivergent skeletal Class II is narrower and longer than that of those with hyperdivergent skeletal Class I. The study’s result also showed that the mandible length positively correlates with the sagittal and coronal diameters of the velopharyngeal segment; the insufficient mandible length is more likely to affect the velopharyngeal airway morphology. Our study showed that Hv and sagittal L1 were significantly different between the skeletal Class II high-angle and the control groups, respectively, before treatment, which indicates that the velopharyngeal segment of the patients with skeletal Class II was narrower and longer than that of those with the hyperdivergent skeletal Class I. Notably, the results were similar to those of the above mentioned studies. Therefore, we hypothesize that the reasons for the narrow and long velopharyngeal segment airway are majorly associated with two aspects. First, the pressure changes in the airway lumen when the mandible is rotated posteriorly and inferiorly. This limited change is related to passive compression and stretching of the pharyngeal wall, which may protect against severe airway obstruction. Second, it is associated with the insufficient width of the upper and lower jaws and the posterior segment of the dental arch in patients with hyperdivergent skeletal class II. A possible cause could be that the width is inadequate to limit the inherent oral volume to prevent the tongue from extending forward, and the tongue falls back; therefore, the back of the tongue causes the velopharyngeal segment to be narrowed and elongated. Kim et al.[12] found that after treating patients with hyperdivergent skeletal Class II with implant anchorage, for every 1 mm maxillary molar intrusion, the mandibular plane rotated counterclockwise by 2°, and the chin was moved forward by 2.3 mm. Koyama et al.[13] employed implant anchorage to treat patients with a high angle and discovered that the mandibular plane angle reduced by an average of 1.5° while the maxillary molar height decreased by 0.7 mm after treatment. In this study, implant anchorage was employed to actively intrude the bilateral maxillary posterior segments and upright mandibular molars and move them mesially as a whole. Consequently, the occlusal plane remained flat, the mandibular plane had a certain degree of counterclockwise rotation, the MP-SN angle decreased by an average of 1.64 ± 0.17° (P < 0.01), and the patient's profile was also significantly improved.
The sagittal U-MPW and PAS of the upper airway near the mandible were significantly altered after treatment, which is consistent with Germec-Cakan et al.[14], who suggested the increase in the sagittal size of the upper airway near the mandible was due to the mesial movement of the molars. However, no significant change in the hyoid bone position after treatment was observed, which was consistent with Shi et al.[6]and Zhang et al.[4]. Moreover, LaBanc et al.[15]supposed that the change in the mandibular position would stretch the muscles and tendons attached to the hyoid bone, increase the anterior abdominal region of the digastric muscle, and eventually return the hyoid bone to the pre-treatment position. Therefore, the hyoid bone position did not change significantly, which may be attributed to the instability in the adaptive remodeling of the muscle and its rebound and pulling effect. Compared to the patients with hyperdivergent skeletal Class I, the tongue position is low in those with hyperdivergent skeletal Class II, and the tongue body is small; the backward drop of the tongue can lead to a narrower upper airway [16]. This study’s results showed that the tongue body position did not change significantly after treatment, possibly due to the limited significant alterations in the hyoid bone position. Moreover, Aras et al.[17] reported that the mandible’s forward motion did not significantly change the tongue body and hyoid bone positions and believed that the tongue posture was associated with the hyoid bone position. Santos et al.[18] reported that the hyoid bone can play a role in fixing the hyoid muscle and that its position affects the tongue body’s position, size, and shape. The volume of the upper airway segments and total volume did not change significantly after the treatment, which aligned with the findings of previous studies [4, 6, 19–21] that proposed a negligible effect of orthodontic treatment on the size of the upper airway space in adults. However, no significant changes occurred in Hv, Hg, S1, S2, sagittal L1, and transverse L1, contrary to the findings of Hu et al.[22] and Sun et al.[23]. They believe that large upper anterior teeth are retracted to cause the tongue to move backward, compress the soft palate, and reduce S1 and S2. Conversely, this study’s results showed that the sagittal L2 and transverse L2 were significantly increased and reduced, respectively, although the minimum cross-sectional area of the glossopharyngeal segment did not change significantly. During the treatment, the upper airway was only self-regulated to permit ventilation, with no actual size change. This finding is consistent with that of Zhang et al.[4], who believed that the minimum cross-sectional area of the oropharyngeal segment remained stable in adults with hyperdivergent skeletal Class II who received strong anchorage retraction of the upper anterior teeth with the assistance of MIA. In addition, the volume of each upper airway segment did not change significantly. Therefore, it is speculated that the upper airway only undergoes adaptive changes after treatment.
Furthermore, no significant correlation was observed between the ANB angle of the sagittal change of the jaw and that in U-MPW and PAS, which corresponds with the findings of Chokotiya et al.[24]. Moreover, a significant correlation existed between the vertical changes in the jaws’ MP-SN angle, S-Go/N-Me (%), and that in the sagittal L2 rather than that in the transverse L2, indicating that the mandibular position change did not cause a change in S2. However, this result was inconsistent with that of Shi et al.[6], who reported that an increase in the minimum cross-sectional area of the upper airway was significantly linked to changes in the mandibular position. Additionally, the result showed that U-MPW, PAS, sagittal L2, and transverse L2 changes were not connected to the significant changes in the dentition, U1-SN, and overjet, which is consistent with Valiathan et al.[20], and contrary to Chen et al.[25], who found that the extraction of four premolars and massive retraction of anterior teeth reduced the S2 and considered that it was majorly attributed to the reduction of oral volume caused by retraction of the upper anterior teeth, which affected the tongue position. However, no significant changes in tongue position were found in this study, but only the self-regulation of the Vg occurred, without actual size changes. The Hv and sagittal L1 were significantly different from the individual normal groups, indicating that even after treatment, the velopharyngeal segment of patients with skeletal Class II high angle was narrower and longer than that of the control group, which was shown by the differential analysis of the 3D direction measurement indices between the experimental and control groups. However, this was considered due to the tongue body position, which did not change significantly after treatment, but still fell. Although the S2 of the experimental and control groups did not differ significantly before treatment, the sagittal L2 increased before and after treatment, and the S2 showed a significant difference compared to the control group after treatment, (P = 0.049, P < 0.05). Furthermore, the transverse L2 decreased, while the change in S2 was insignificant, which was presumed to associated with self-regulation and adaptive changes of the glossopharyngeal airway.
This study had some limitations. First, this was a single-center study with small sample size. Therefore, a multi-center study should be conducted in future studies, and the sample size should be increased. Additionally, performing a joint multidisciplinary survey with ENT and other disciplines is crucial to verify the size and changes in the relationship between the upper respiratory tract and respiratory function.