Upper airway dimensions with different skeletal malocclusions? A cross-sectional study in Vietnamese children

1. Introduction

Radiography is an indispensable tool in orthodontic practice [1, 2]. The primary function of lateral cephalometric reference lines is to describe and classify craniofacial structures and dentition. The most commonly used reference planes are the Frankfort horizontal (FH) plane, which is defined as a line drawn between the inferior orbital rim and the supratragic notch, and the anterior cranial base plane (SN plane), a horizontal plane passing through the sella turcica and nasion and represents the anterior cranial base [3]. However, two-dimensional imaging techniques are ineffective in accurately reconstructing three-dimensional (3D) structures and their related pathologies [1, 4]. However, cone beam computed tomography (CBCT) has enabled the accurate estimation of craniofacial bone volume and airway dimensions. It also facilitates evaluating the effects of various orthodontic or orthopedic treatments on airway volume, including the advancement of the mandible and the expansion of the maxilla [5, 6, 7, 8].

Recently, the relationship between respiratory function and craniofacial morphology has garnered increasing attention [1]. Previous studies have indicated a relationship between the maxillary airway and facial development [1, 9, 10]. The maxillary airway is a complex structure comprising bones, cartilage and soft tissues that adapt to functions related to respiration, swallowing and phonation. The effect of breathing patterns on facial development became a focal point for the orthodontic community in the 1970s. Mouth breathing, as opposed to nasal breathing, can negatively affect growth and development, leading to a tendency for mandibular positioning of the mandible and tongue and altering the growth direction of craniofacial structures [9].

Dodda et al. [11] used lateral cephalometric radiographs to describe the distinctive characteristics of Class II, Division 2 malocclusion, as classified by angles while Di Carlo et al. [9] studied the maxillary airway’s morphology and dimensions through 3D radiological measurements in 90 young adult patients. In the latter study, the sagittal plane was assessed, and patients were divided into three groups according to the value of the ANB angle. In another study, Sfondrini et al. [12] evaluated the maxillary airway in adult Caucasian subjects without previous orthodontic treatment. Their main objective was to measure the maxillary airway dimensions in adult skeletal Class I, II and III patients. Pop et al. [1] recently evaluated several parameters, including the morphology and volume of the maxillary airway in relation to angle Class I, II and III occlusions. That study concluded that the narrowest segment of the pharynx had the highest value in patients with angle Class III malocclusions. Furthermore, the volume of the oropharynx was found to be greater in angle Class III compared to Class II patients. However, these studies did not compare measurements obtained using two different diagnostic methods: lateral cephalometric radiographs and CBCT.

A 2022 review evaluated the scientific evidence concerning the effect of various orthodontic treatment modalities on the airway. The review highlighted the significant role of orthodontists in recognizing respiratory issues in patients and correlating these with malocclusion patterns to determine appropriate treatment strategies [13, 14]. Rodrigues et al. [15] (2024) demonstrated a significant correlation between skeletal malocclusion classification and pharyngeal airway dimensions. Their study revealed that as skeletal malocclusion progresses toward a Class III pattern, there is a concomitant increase in the pharyngeal airway’s volume and cross-sectional area. To this end, radiographic measurement techniques have gradually become standardized and indispensable tools in orthodontics as treatment goals and requirements have increased [16, 17]. In 1931, Broadbent (USA) and Holrath (Germany) introduced the standard cephalometric technique, which creates 1:1 images with the object being photographed. This technique, along with direct measurements on a model cast, has become an indispensable tool in diagnoses, treatments and research in the field of orthodontics [18, 19]. Specifically, evaluations using lateral cephalometric radiographs and CBCT have become crucial to accurate diagnoses [20]. However, few studies have been conducted in the Mekong Delta region of Vietnam that provide an overview of the distribution and characteristics of malocclusion. Therefore, this study aimed to evaluate the effect of craniofacial structures on maxillary airway morphology in patients with skeletal Class I, II and III malocclusions using lateral cephalometric radiographs and CBCT in Vietnamese children.

2. Materials and methods

2.3 Study procedure

2.3.1 Scanning device

For imaging, patients were positioned upright, with a predefined head position, where the reference horizontal plane was established by the FH plane using a contour and digital gauge. Patients were instructed not to swallow or breathe during imaging. The radiographs were obtained using a Sirona Orthophos SL (Sirona, Germany), with a voxel base size of 0.08 mm, power lines and voltages of 3.0–16.0 mA and 60–90 kV, respectively, a scanning time of 14.9 seconds, and the capacity for cylindrical (field of view) measurements of 40–40 mm, 60–60 mm or 80–80 mm (according to the as low as reasonably achievable (ALARA) guidelines) [23].

2.3.2 Imaging measurement

Scanned CBCT radiographs were processed using Mimics (Materialise NV, Belgium) on Samsung computer monitors (LF27T350FHEXXV, Korea). To isolate the airway space, the threshold value was set to a range of −1024 to −100 Hounsfield units. The pharyngeal airway volume was calculated by summing the maxilla and mandibular parts of the pharyngeal airway volume. Lateral cephalometric radiographs were analyzed using WEBCEPH software (version 1.5.0, Korea). Photos were obtained according to the manufacturer’s recommended standard techniques and posture. The evaluation indicators were determined according to the illustrated figures and tables (Fig. 1, Ref. [24]; Figs. 2,3, Table 1).

Fig. 1.Limitations of airway analyses via lateral cephalometric radiography. PNS-AD2 (mm): the distance between PNS and AD2, which is a point on posterior pharyngeal wall intersecting the H-PNS line; PNS-AD1 (mm): the distance between PNS and AD1, a point on posterior pharyngeal wall intersecting the Ba-PNS line; SPAS (mm): the line parallel to the palatal plane between the midpoint of PNS and P and the posterior pharyngeal wall; MAS (mm): the line extending to the anterior and posterior pharyngeal walls from point P; IAS (mm): the line parallel to the palatal plane passing through the anteroinferior point of CV2 and the anterior pharyngeal wall. Hormion (H): the cephalometric point located near the adenoidal tissue at the cranial base and localized where a line perpendicular to the S-Ba line crosses the sphenoid bone. The variations of this point are minimal because it is located far away from growth sites (Table 1) [24].

Fig. 2.Craniofacial measurements via lateral cephalometric radiography. SNA (°): Angle between the line Sella-Nasion, and line Nasion-point A. This angle measures the position of the maxilla relative to the skull base; SNB (°): Angle between the line point A-Nasion, and line point Nasion-point B. This angle measures the mandibular’s position relative to the skull base; ANB (°): Angle between line point A—the Nasion and line point B—Nasion, indicating the relationship between the maxilla and mandibular; SN-GoGn (°): the angle between the SN plane and Steiner’s mandibular plane (Go-Gn); FMA (°): the angle between the Frankfort (FH) plane and Downs’ mandibular plane (Go-Me). Facial axis (°): the angle between the Ba-N line and Pt-Gn line; Y-axis (°): the angle between the S-Gn and FH planes; Facial Angle (°): the angle between the FH plane and the N-Pog line; L-MP (°): the angle formed by the long axis of the mandibular incisor and the mandibular plane (Go-Me) (Table 1).

Fig. 3.Three-dimensional image of the pharyngeal airway. (A) Left; (B) Right; (C) Anterior; (D) Posterior; (E) Superior; (F) Inferior. The upper airway extends from the nasal vestibule to the glottis, encompassing the nasal cavity, nasopharynx, oropharynx and hypopharynx. Airway Boundaries: the anterior region is defined by the hard palate, mandible and soft palate; the posterior pharyngeal wall, which stretches from the base of the skull to the level of the cervical vertebrae, borders the posterior region; superiorly, it is limited by the base of the skull and the nasal cavity; the inferior region extends to the level of the epiglottis.

2.3.3 The airway analyses via lateral cephalometric radiography

PNS-AD2 (mm): the distance between PNS and AD2, which is a point on posterior pharyngeal wall intersecting the H-PNS line: the line between PNS and H point; Hormion (H): the cephalometric point located near the adenoidal tissue at the cranial base and localized where a line perpendicular to the S-Ba line crosses the sphenoid bone. The variations of this point are minimal because it is located far away from growth sites.

PNS-AD1 (mm): the distance between PNS and AD1, a point on posterior pharyngeal wall intersecting the Ba-PNS line;

SPAS (mm): the line parallel to the palatal plane between the midpoint of PNS and P and the posterior pharyngeal wall;

MAS (mm): the line extending to the anterior and posterior pharyngeal walls from point P;

IAS (mm): the line parallel to the palatal plane passing through the anteroinferior point of CV2 and the anterior pharyngeal wall.

2.3.4 Craniofacial measurements via lateral cephalometric radiography

SNA (°): Angle between the line Sella-Nasion, and line Nasion-point A. This angle measures the position of the maxilla relative to the skull base;

SNB (°): Angle between the line point A—Nasion, and line point Nasion—point B. This angle measures the mandibular’s position relative to the skull base;

ANB (°): Angle between line point A—the Nasion and line point B—Nasion, indicating the relationship between the maxilla and mandibular;

SN-GoGn (°): the angle between the SN plane and Steiner’s mandibular plane (Go-Gn);

FMA (°): the angle between the Frankfort (FH) plane and Downs’ mandibular plane (Go-Me);

Facial axis (°): the angle between the Ba-N line and Pt-Gn line;

Y-axis (°): the angle between the S-Gn and FH planes;

Facial Angle (°): the angle between the FH plane and the N-Pog line;

L1-MP (°): the angle formed by the long axis of the mandibular incisor and the mandibular plane (Go-Me).

2.3.5 Three-dimensional of the pharyngeal airway

The upper airway extends from the nasal vestibule to the glottis, encompassing the nasal cavity, nasopharynx, oropharynx and hypopharynx. Airway Boundaries: the anterior region is defined by the hard palate, mandible and soft palate; the posterior pharyngeal wall, which stretches from the base of the skull to the level of the cervical vertebrae, borders the posterior region; superiorly, it is limited by the base of the skull and the nasal cavity; the inferior region extends to the level of the epiglottis.

3. Results

There were statistically significant differences between the skeletal CI, CII and CIII groups for the SNB, ANB, SN-GoGn, FMA, mandibular length, facial axis, Y-axis, facial angle and L1-MP. Conversely, the angles (SNA) and lower anterior facial height showed no significant differences. The mandibular length progressively decreased from skeletal CIII > skeletal CI > skeletal CII groups. However, there was a significant difference between skeletal CI and skeletal CII, and between skeletal CII and skeletal CIII (Table 2).

The lateral cephalometric analysis of airway dimensions showed that the PNS-AD2, SPAS and IAS measurements significantly differed between the CI, CII and CIII malocclusion groups. Specifically, CIII exhibited the largest airway width, followed by CI and CII. However, comparing each skeletal group, there was a significant difference between skeletal CI and skeletal CII in PNS-AD2, between skeletal CII and skeletal CIII in SPAS, and between skeletal CII and skeletal CIII in IAS (Table 3).

The pharyngeal airway volume on CBCT radiographs was evaluated based on the nasopharynx, oropharynx and the total oropharyngeal volume. For the nasopharynx, we observed the largest volume in CIII, followed by CII and CIII groups, with significant differences between skeletal CI and skeletal CII, and between skeletal CI and skeletal CII. For the oropharynx, the volume also progressively decreased from CIII > CI > CII, with significant differences between skeletal CII and skeletal CIII. There was also a significant difference between skeletal CI and skeletal CII regarding airway volume. CIII had the most volume (27,237.66 ± 5699.12 mm³), with CI having 26,372.73 ± 4577.11 mm³, while Class II had the least volume (22,678.74 ± 4978.22 mm³) with significant differences between skeletal CI and skeletal CII (Table 4).

There was a negative relationship between the airway parameters on lateral cephalometric radiographs and the airway volume on CBCT radiographs. This relationship was shown by the ANB and SN-GoGn angles, the FMA and the Y-axis as they exhibited a positive correlation with mandibular body length, the facial axis and the facial angle (Table 5). There was also a positive correlation between the airway parameters on lateral cephalometric radiographs and airway volume on CBCT radiographs (Tables 6,7,8,9).

4. Discussion

Currently, aesthetic concerns are being increasingly emphasized by patients, with achieving a flawless appearance being a central focus. As a result, the number of patients seeking consultations and desiring orthodontic and facial skeletal corrections has risen. Moreover, epidemiological studies conducted across different populations and ethnic groups have indicated that the prevalence of malocclusion and the demand for orthognathic treatment are on the rise. Ishiguro’s study highlighted a correlation between craniofacial structural shape and the patency of the airway [25]. Moss famously stated, “Function decides form” a concept that has strongly influenced current perspectives in orthognathic surgery and orthodontics. For example, patients who breathe through their mouths tend to exhibit a characteristic facial morphology. Conversely, facial morphology also affects function. Hakan EI and Juan Martin Palomo, in their study on airway dimensions across different craniofacial patterns, found that the oropharyngeal airway size was smallest in CII skeletal malocclusion patients, followed by CI, with the largest airway observed in CIII patients [26]. Therefore, failing to understand the oropharyngeal airway dimensions in patients with different types of malocclusion during orthognathic treatment can pose a risk to the patency of the airway and overall respiratory function.

Lateral cephalometric radiographs are basic screening tools that offer several advantages, such as low cost, minimal radiation exposure and reliable measurement data. As a result, these films are widely used in diagnosing and orthodontically correcting dental issues, as well as assessing the oropharyngeal airway dimensions in patients with CII skeletal malocclusion, which helps orthodontists develop individualized treatment plans. However, a primary limitation of lateral cephalometric radiographs is that they only evaluate the airway in the anteroposterior dimension. Therefore, to obtain a more comprehensive and accurate assessment of airway dimensions, CBCT provides the most reliable and effective imaging [27].

To better understand which factors of malocclusion may be related to changes in the maxilla airway, we evaluated the differences in craniofacial characteristics among the patient groups. These included correlations between the maxilla and mandibular jaw (ANB), mandibular length (Go-Me), mandibular position (SNB) and the growth direction of the mandible (SN-GoGn and FMA).

Regarding the mandibular body length, assessing the correlation between mandibular length and airway dimensions, a positive correlation was observed. This finding supports the notion that mandibular length may be related to oropharyngeal airway size, which is consistent with a study by Muto et al. [28], who demonstrated that craniofacial abnormalities, including retrognathism, a short mandibular body and downward rotation of the mandible can lead to a reduction in airway size. Additionally, a study by Yu-Chuan Tseng et al. [29] indicated that airway dimensions decrease as the position of the mandible moves posteriorly. However, our study did not find a significant correlation between mandibular position (SNB) and airway size.

Clinical studies have demonstrated a relationship between the craniofacial system and the airway across different skeletal patterns. When comparing various growth patterns, significant differences in airway width and volume have been observed. Our study, conducted on different malocclusion groups classified according to the ANB angle, found an inverse correlation between the ANB angle and oropharyngeal airway dimensions. As the ANB angle increased, the airway size decreased. CII subjects, having the highest ANB angle, exhibited the smallest airway dimensions. Conversely, CIII patients with the lowest ANB angle had the largest airway dimensions. These findings are consistent with those of other studies [29, 30].

Different skeletal growth patterns may develop due to various factors, such as the growth of the maxilla and mandible, the maxillary-mandibular dental relationship, tooth eruption and tongue function. It is known that most individuals with a vertical growth pattern tend to experience simultaneous maxilla airway obstruction, often accompanied by symptoms such as snoring, daytime sleepiness and mouth breathing [31]. Therefore, the relationship between growth patterns and the airway must be assessed to create better orthodontic treatment plans. Our results indicate that airway dimensions are reduced in individuals with a vertical growth pattern. Specifically, as the SN-GoGn angle and FMA increase, the airway size decreases and vice versa. Thus, in clinical situations, it is advisable to avoid downward and posterior rotation of the mandible to prevent further reduction in airway volume.

Trenouth and Timms proposed a theory linking mandibular length to the oropharyngeal airway. They hypothesized that increased mandibular length resulted in the anterior displacement of the genioglossus and geniohyoid muscle attachments, thereby widening the pharyngeal airway [32]. One study found that the mandibular length (Go-Me) was shorter in people with skeletal Class II patterns compared to Class III patterns. That study also showed a link between Go-Me and the volume of the pharynx. The results showed that there were differences in the size of the pharyngeal airway between skeletal Class II and III malocclusions, which may support Trenouth and Timms’s theory [32, 33].

Only the nasopharynx and oropharynx of the upper airway were the focus of this research. However, the literature provides substantial evidence that individuals with skeletal disorders affect all three sections of the upper airway (nasopharynx, oropharynx and hypopharynx). Therefore, future studies should also assess the hypopharynx. Further studies should expand upon these results by integrating more thorough airway evaluations. This may include using imaging modalities such as cone-beam computed tomography (CBCT) or magnetic resonance imaging (MRI), which see the hypopharynx and the remainder of the upper airway in three dimensions. Incorporating dynamic evaluations, such as sleep studies, might be beneficial to understand how these structural variations influence function.

Incorporating dynamic evaluations and broadening the range of airway measurements enables researchers to get deeper insights into the interplay between skeletal malocclusion, airway dimensions, and the progression of obstructive sleep apnea.

This study has several limitations that should be considered when interpreting the findings. Firstly, the cross-sectional design of the study prevents us from establishing causal relationships between skeletal malocclusions and upper airway dimensions. Secondly, the sample was limited to Vietnamese children, which may reduce the generalizability of the results to other populations with different ethnic or cultural characteristics. Lastly, the study did not account for other factors such as environmental influences, respiratory conditions or genetic predispositions, which could impact the upper airway dimensions independently of skeletal malocclusions.

5. Conclusions

Craniofacial morphology can influence nasal respiratory function and the maxillary airway. There was a strong correlation between airway volume and skeletal patterns. Airway dimensions were significantly reduced in skeletal Class II malocclusion patients with a high ANB angle, retrognathic mandible and vertical growth pattern. Clinicians should exercise caution when performing mandibular retrusion in skeletal Class II patients to avoid worsening airway obstruction. It is essential to determine the most suitable treatment for each patient and avoid methods that may further reduce airway size in individuals who already have a tendency toward smaller airway dimensions. This is especially crucial for preventing the onset of obstructive sleep apnea in patients. Furthermore, this study highlights the importance of lateral cephalometric radiographs in evaluating the airway.

Availability of data and materials

The data that support the findings of this study are available from the corresponding author, upon reasonable request.

Author contributions

LNL and TTNN—collected and analyzed the data. LNL, TTNN, LTT, TTHT and PTTN—designed the research study and wrote the manuscript. LNL, TTNN, LTT, TTHT, PTTN and KVPL—reviewed and edited the manuscript. All authors read and approved the final version of the manuscript.

Ethics approval and consent to participate

This retrospective study was conducted according to the guidelines of the Declaration of Helsinki and all procedures performed were following the Ethics Committee of the University of Medicine and Pharmacy of Can Tho University in Biomedical Research No: 23.002/PCT-HDDD. Consent was obtained from the parents or guardians for the use of all images of children in the manuscript.

Acknowledgment

The authors thank the technical support provided by the Faculty of Odonto and Stomatology, Can Tho University of Medicine and Pharmacy and Can Tho University of Medicine and Pharmacy Hospital, Can Tho City, Vietnam.

Funding

This research received no external funding.

Conflict of interest

The authors declare no conflict of interest.