Respiratory problems not only cause sleep disorders but also reduce the quality of life [1]. For instance, mouth breathing is closely related to obstructive sleep apnea (OSA) [2]. During the initial orthodontic diagnosis, an evaluation of the airway could be helpful in screening for respiratory problems. Furthermore, the orthodontic treatment itself might affect respiratory function [3]. Therefore, considerable attention must be paid to the patient’s respiratory function when establishing an orthodontic treatment plan.

Typically, skeletal Class II malocclusions are caused by mandibular retrognathism rather than maxillary prognathism [4]. The dentoalveolar and skeletal effects of orthopedic treatments in growing patients with mandibular retrognathism have been extensively studied [5]. However, several studies [6,7] have found that patients with retrognathic mandibles have a reduced pharyngeal airway space (PAS), which has increased interest in the respiration effect of orthopedic treatments [8-10].

The PAS is primarily composed of the pharynx, which is held by its surrounding skeletal structures. The PAS dimensions largely depend on sex, age, and body mass index (BMI) [11,12]. However, a more important factor is the mandibular position [13]. Alteration of the mandibular position increases or decreases the dimension of the PAS by repositioning the hyoid bone, tongue, and soft palate [14]. For example, orthognathic surgery [15] or mandibular advancement (MA) device [16] modifies the mandibular position, which leads to dimensional changes in the PAS.

An accurate understanding of changes in the PAS due to mandibular movement is essential for achieving good orthodontic treatment outcomes. However, the PAS can be easily affected by other factors such as growth and head posture, making it difficult to accurately measure the sole effect of mandibular movement. To exclude other factors affecting the PAS, it is necessary to evaluate the change in the PAS by changing only the mandible at the same time point and in the same head posture. However, the previous studies [8-10,16] had difficulty excluding factors other than the mandible because the measurements were obtained while wearing the orthopedic device or with a time difference before and after treatment. Therefore, it was difficult to exclude changes in the skeletal or dentoalveolar structure caused by the use of the device, while in the case of growing patients, growth-related changes were difficult to exclude.

To date, there have been no studies on how the PAS has changed according to MA and mouth opening at the same time point in the same patient. This study was made possible by using two lateral cephalograms taken for the Fränkel maneuver [17] in growing patients with mandibular retrognathism.

This retrospective study’s goal was to assess changes in the PAS due to MA and mouth opening using lateral cephalograms and to identify the relationship with surrounding structures that cause these changes.

Subjects

A total of 27 patients with a chief complaint of mandibular retrognathism (15 males and 12 females) who visited the Pusan National University Dental Hospital from 2013 to 2021, were included (Table 1). These inclusion criteria were used to select the subjects: (1) adolescents aged 8 to 14 years; (2) BMI under 30 kg/m²; (3) no craniofacial deformity; (4) no history of orthodontic treatment; (5) while taking two consecutive cephalograms, no alterations in the craniocervical angle. The minimum sample size of 27 subjects was obtained based on the calculations with β error of 0.2, α error of 0.05, and effect size of 0.5 [18]. The Institutional Review Board of the Pusan National University Dental Hospital gave its approval to this study (PNUDH-2022-07-001).

Data acquisition

Two lateral cephalograms were obtained by requesting that only the mandibular change occurred in the same head posture. The mandible was in the centric occlusal position for the first lateral cephalogram, and the anterior edge-to-edge bite position for the second (Fig. 1).

Fig. 1. Lateral cephalogram. (A) Centric occlusion bite, (B) anterior edge-to-edge bite.

Each landmark was selected based on previous studies [19,20]. Using the V-ceph (Osstem Inc., Seoul, Korea) program, all cephalometric landmarks were traced. The immediate effects of MA and vertical mouth opening on the surrounding structures and PAS were measured (Fig. 2, Table 2). The horizontal reference plane (HRP) was designated as the line that was parallel to the Frankfort horizontal (FH) plane and passed through the posterior nasal spine (PNS). The vertical reference plane (VRP) was designated as the line that was perpendicular to the FH plane and passed through the PNS. The dimensional change in the PAS was evaluated using the distance from the posterior pharyngeal wall to the PNS (PAD1), the tip of the uvula (PAD2), tongue (PAD3), epiglottal tip (PAD4), and vallecula (PAD5). Changes in the soft palate were measured by the angle (UVA) between the HRP and the tip of the uvula. Changes in the tongue were measured using the height of the tongue (TOH) and length of the tongue (TOL). Distances from the hyoid bone’s most anterior point to the HRP (HVD) and VRP (HHD) were used to measure the change in the hyoid bone.

Fig. 2. Cephalometric landmarks. For definitions, see Table 2 for definitions of each landmark.

Statistical analysis

One examiner completed all cephalometric measures. Intraexaminer reliability tests were conducted using Dahlberg’s error formula and the interclass correlation coefficient (ICC). All datasets were re-measured after 1 month. The data were found to be regularly distributed by the Shapiro–Wilk test. Therefore, to compare changes in the surrounding structures and PAS before and after MA and mouth opening, a paired t-test was employed. To determine the correlation between the change in the mandible and the change in the PAS, Pearson’s correlation analysis was employed. All statistical analysis was performed using the SPSS software. A p-value of 0.05 or less was considered statistically significant.

Table 3 showed high ICCs. No Dahlberg’s error value exceeded 0.32 mm for the linear variable and 0.56° for the angular variable.

The average MA and vertical opening (VO) were 8.47±3.48 mm and 6.96±3.28 mm, respectively. With the exception of PAD1, all PAS variables after MA and VO demonstrated a statistically significant increase (Table 4, Fig. 3). The length of the tongue increased (p<0.001) and the height of the tongue decreased (p<0.001). Moreover, the soft palate moved anteriorly (p<0.001) and the hyoid bone moved upward (p=0.001) (Fig. 4). A statistically significant positive correlation was found only between the changes in the mandible and the changes in PAD2 (Table 5, 6). The multivariate regression equation for PAD2 was as follows: the change in PAD2=–1.362+0.343×MA+0.256×VO.

Fig. 3. Changes in the pharyngeal airway space after mandibular advancement and vertical opening. NS, not significant. **p<0.001.

Fig. 4. Changes in the hyoid bone, soft palate, and tongue position after mandibular advancement and vertical opening. Red arrow, changes in the mandible; Blue arrows, changes in the tongue, soft palate, and hyoid bone.

Various orthopedic devices are used for orthodontic treatment. However, these devices can affect the surrounding anatomical structures, especially the PAS. In some devices, the PAS can be positively impacted, while in others, it can be negatively impacted. For example, facemask device did not appear to have any further effects on the PAS [21] and cervical headgear reduced the sagittal dimensions of the PAS [22]. On the other hand, twinblock appliance increased the PAS dimensions [23]. Specifically, the MA device used in patients with skeletal Class II malocclusion had different effects on the PAS depending on the range of MA and mouth opening [24].

The pharynx is known to grow rapidly in size and volume during adolescence [25]. To fully comprehend the alterations in the PAS due to positional changes in the mandible, it is necessary to exclude growth-related changes. In addition, even a slight change in head posture [26] can alter PAS dimensions. Therefore, this study evaluated PAS change based on the movement of the mandible at the same time point in the same patient, followed by investigating the relationship with the surrounding structures.

In this study, the PAS dimensions increased after MA and mouth opening, which was statistically significant. The increase in PAS dimension can be described as the result of upward movement of the hyoid bone and anterior displacement of the tongue and soft palate. However, PAD1 was not significantly different because PAD1 was not affected by surrounding tissues. These results are similar to those of MA surgery [27-29]. Immediately after MA surgery, the PAS dimension increased, the hyoid bone moved upward, and the tongue moved forward.

The correlation between the change in the mandible and the change in the PAS was statistically significant only for PAD2. This means that MA and the VO had a positive correlation with the increase in PAS of the velopharynx region. Since patients with OSA have constriction of the PAS in the velopharynx region [30], it could be predicted that an intra-oral appliance that advances the mandible and makes a VO could help OSA.

The hyoid bone is tightly joined to the tongue and mandible by muscles and ligaments, whereas the surrounding bones are not connected to the hyoid bone. Mandibular movement can modify the hyoid bone’s position, which in turn alters the position of the tongue and the morphology of the PAS [31]. According to previous studies [32,33], an inferiorly positioned hyoid bone increases the vertical positioning of the tongue and moves the soft palate posteriorly, which narrows the PAS. In contrast, superior positioning of the hyoid bone decreases the vertical positioning of the tongue and moves the soft palate anteriorly, resulting in wider PAS. This study is in accordance with previous findings [29,30], as the dimensional increase in the PAS due to MA and mouth opening is related to the upward movement of the hyoid bone.

The PAS can be assessed using a variety of techniques. Among them, the lateral cephalogram is the preferred technique due to its benefits such as low radiation exposure, low cost, and providing information on the soft palate, but it has a disadvantage in that it lacks information about the cross-section or volume. Nevertheless, several studies [34,35] have revealed that the lateral cephalogram provided sufficient information when evaluating the PAS compared to cone-beam computed tomography.

The results of this study were limited to certain aspects. First, only two-dimensional evaluation of the PAS was made using the lateral cephalogram. A three-dimensional evaluation is necessary in the future. Second, only immediate effects of MA and mouth opening were evaluated. Muscles may adapt to this advancement with time and lowered tongue posture may not be maintained after advancement. Nevertheless, this study would serve as a basis when designing the orthopedic devices or comparing changes of the PAS before and after MA treatment. Third, this study only included patients with skeletal Class II malocclusions. Studies on patients with different malocclusions are required. Fourth, the PAS can be affected by other factors such as supine vs. upright and awake vs. sleep. Finally, the sample size of this study was small. To get more accurate results, more sample sizes are required.

In growing patients with mandibular retrognathism, MA and mouth opening increased the dimensions of the PAS. This effect was the result of the forward movement of the tongue and soft palate, and the upward movement of the hyoid bone. However, the correlation between the change in the mandible and the change in the PAS was positive only in the velopharynx.

This work was supported by a 2-year research grant of Pusan National University.

The author declares no competing interests.