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Original Article

Korean J Orthod 2023; 53(6): 358-364   https://doi.org/10.4041/kjod22.187

First Published Date January 31, 2023, Publication Date November 25, 2023

Copyright © The Korean Association of Orthodontists.

Three-dimensional evaluation of the pharyngeal airway space in patients with anterior open bite

Seong-Sik Kim , Yong-Il Kim, Soo-Byung Park, Sung-Hun Kim

Department of Orthodontics, Dental Research Institute, and Dental and Life Science Institute, School of Dentistry, Pusan National University, Yangsan, Korea

Correspondence to:Sung-Hun Kim.
Assistant Professor, Department of Orthodontics, Dental Research Institute, and Dental and Life Science Institute, School of Dentistry, Pusan National University, 20, Geumo-ro, Mulgeum-eup, Yangsan 50612, Korea.
Tel +82-55-360-5164 e-mail kmule@hanmail.net

How to cite this article: Kim SS, Kim YI, Park SB, Kim SH. Three-dimensional evaluation of the pharyngeal airway space in patients with anterior open bite. Korean J Orthod 2023;53(6):358-364. https://doi.org/10.4041/kjod22.187

Received: August 26, 2022; Revised: October 12, 2022; Accepted: November 18, 2022

Abstract

Objective: This study aimed to three-dimensionally evaluate the pharyngeal airway space (PAS) of patients with anterior open bite (AOB) by using cone-beam computed tomography (CBCT) and compare the findings with those obtained in individuals with normal occlusion. Methods: The open bite group (OBG, n = 25) consisted of patients with an anterior overbite of –3 mm or less, while the control group (n = 25) consisted of age- and sex-matched individuals with an anterior overbite of 1–3 mm, Angle Class I malocclusion (1° ≤ point A-nasion-point B angle ≤ 4°), and a normodivergent profile (22° ≤ Frankfort mandibular plane angle ≤ 28°). After the CBCT data were reconstructed into a three-dimensional image, the PAS was segmented into four parts, and the volume of each part was measured. Pharyngeal airway length (PAL) and the area and transverse width of the part showing minimal constriction were also measured. Pearson’s correlation analysis was used to evaluate the correlation between changes in the PAS and the amount of anterior overbite. Results: The OBG showed a significantly narrower airway space in the nasopharyngeal, hypopharyngeal, and total airway volumes. The OBG also showed a significantly smaller area and transverse width of the part with minimal constriction. The OBG showed a significantly longer PAL, but there was no correlation between the amount of anterior overbite and the changes in PAS. Conclusions: The PAS was associated with AOB. Patients with AOB had a narrower PAS and a smaller part showing minimal constriction.

Keywords: Computed tomography, Airway

INTRODUCTION

The respiratory structures are closely related to dentofacial growth and development.1 For example, mouth breathing caused by narrow airways may lead to “adenoid facies.”2 The pharynx is one of the major anatomical structures in the upper airway.1 It is divided into several segments, including the nasopharynx, velopharynx, oropharynx, and hypopharynx.1

Anterior open bite (AOB) can occur as a result of various etiologies, including excessive vertical growth, oral habits, and crowding.3 It shows a higher prevalence in growing children than in adults because the frequency of AOB caused by oral habits, such as thumb sucking and tongue thrust, is high in growing children.4 Although self-correction is possible by discontinuing these habits,5 most patients require orthodontic treatment.

Orthodontic treatment of AOB is considered difficult because of the relapsing tendency of vertical problems and the tongue-thrust habit.6 However, since AOB causes various problems, it is also categorized as a malocclusion, indicating the need for orthodontic treatment. In addition to esthetic problems, AOB can cause functional problems such as pronunciation difficulties and reduced masticatory performance.7 Moreover, individuals with AOB may have a different oral structure from those with normal occlusion,8,9 which may change the pharyngeal airway space (PAS). Therefore, evaluation of the PAS in patients with AOB is essential.

Although a previous study10 evaluated the correlation between AOB and PAS, it used two-dimensional (2D) images. However, 2D measurement of the PAS is limited because it visualizes three-dimensional (3D) structures in two dimensions, and because 2D images do not provide information regarding the volume or cross-sectional area. Thus, 3D cone-beam computed tomography (CBCT) is appropriate for analyzing the PAS.11 To the best of our knowledge, this study is the first attempt to analyze the PAS in patients with AOB using 3D imaging.

The purpose of this study was to three-dimensionally evaluate the PAS of patients with AOB by using CBCT and to compare the findings with those obtained in age- and sex-matched individuals with normal occlusion.

MATERIALS AND METHODS

Participants

This retrospective study was conducted on individuals aged over 20 years who underwent CBCT for other purposes such as third molar impaction or oral disease at the Pusan National University Dental Hospital from 2015 to 2020. The study protocol was approved by the Institutional Review Board of Pusan National University Dental Hospital (PNUDH-202016).

The open bite group (OBG, n = 25) consisted of patients with an anterior overbite of –3 mm or less, while the control group (n = 25) consisted of age- and sex-matched individuals with an anterior overbite of 1–3 mm, Angle Class I malocclusion (1° ≤ point A-nasion-point B angle (ANB) ≤ 4°), and a normodivergent profile (22° ≤ Frankfort–mandibular plane angle ≤ 28°) (Table 1). The exclusion criteria were as follows: (1) previous airway disease or surgery; (2) maxillofacial syndromes; (3) obstructive sleep apnea; (4) a history of temporomandibular joint disorder; (5) previous orthodontic treatment; (6) recent onset of AOB; (7) Angle Class III malocclusion (ANB < 1°); (8) abnormal head posture (craniocervical angle in the second vertebra, < 90° or > 110°)12; and (9) high body mass index (BMI) (> 30 kg/m2).13

Table 1 . Descriptive statistics

ParametersOpen bite group
(n = 25)
Control
group
(n = 25)
P value
Age (yr)23.8 ± 2.422.9 ± 2.60.843*
Male13120.768
Overbite (mm)–4.6 ± 1.21.9 ± 0.8< 0.001*
ANB (°)3.0 ± 1.92.5 ± 1.20.252*
FMA (°)27.6 ± 5.125.1 ± 2.70.393*
Angle classification
Class I1725
Class II80
Class III00

Values are presented as mean ± standard deviation or number only.

ANB, Angle of nasion-A point line to nasion-B point line; FMA, angle of the Frankfort line to the mandibular plane; NS, not significant.

*Independent t test.

Chi-square test.



To detect a mean difference of 2.0 cm3 in the nasopharyngeal volume (NPV) with a standard deviation of 1.6 (obtained from a previous pilot study), a β error of 0.20, and an α error of 0.05, 25 participants were required for each group.14

CBCT data

CBCT (DCT pro; Vatech, Seoul, Korea) was performed using the protocol of the image center under the same conditions for participants of both groups. The participants were instructed to bite with maximum intercuspation in an upright position, not to swallow, and not to move the tongue while undergoing CBCT. The CBCT settings were as follows: field of view, 20 × 19 cm; scanning time, 24 seconds; tube current, 4 mA; tube voltage, 90 kVp; and voxel size, 0.3 mm. CBCT data were reconstructed using InVivo5.4 software (Anatomage, San Jose, CA, USA).

PAS measurements

The 3D images were then re-oriented. The Frankfort horizontal (FH) plane was used as the horizontal reference plane. The plane vertical to the FH plane and crossing the nasion and basion was used as the vertical reference plane (VRP). The reference plane and landmarks were determined on the basis of previous studies (Figure 1 and Table 2).15 The sagittal slice of the VRP was chosen to allow the software to evaluate the airway. The threshold range of air was set between –1,000 and –587 Hounsfield units.16

Figure 1. Landmarks and reference planes. See Table 2 for the definitions of each landmark.

Table 2 . Landmarks and descriptions

LandmarksDescription
BaBasion
SSella
SoMidpoint in the Ba-S line
PNSPosterior nasal spine
UTTip of the uvula
ETTip of the epiglottis
CV4Most anterior inferior point of body of fourth cervical vertebra
HYThe most anterosuperior point of the hyoid bone
Frankfort horizontal planePlane constructed on both sides of Porion and right Orbitale
PL1Plane passing through So and PNS
PL2Plane parallel to FH plane and passing through PNS
PL3Plane parallel to FH plane and passing through UT
PL4Plane parallel to FH plane and passing through ET
PL5Plane parallel to FH plane and passing through CV4
PALShortest distance from HY to PL2


The PAS was segmented into four parts and the volume of each part was measured. The upper limit of the NPV was the plane (PL1) connecting the posterior nasal spine (PNS) at the midpoint (So) of the sella and the basion. The lower limit of the NPV was the plane (PL2) parallel to the FH plane while passing through the PNS. The lower limit of the velopharyngeal volume (VPV) was the plane (PL3) that passed through the tip of the uvula (UT) parallel to the FH plane. The lower limit of the oropharyngeal volume (OPV) was the plane (PL4) parallel to the FH plane while passing through the tip of the epiglottis (ET). The lower limit of the hypopharyngeal volume (HPV) was the plane (PL5) parallel to the FH plane while passing through the most anteroinferior point of the body of the fourth cervical vertebra (CV4). The total airway volume (TAV), which was the volume of all four segments, was also measured. The pharyngeal airway length (PAL) was the vertical distance from PL2 to the most anterosuperior point of the hyoid bone (HY). In addition, the area, anteroposterior length (APL), and transverse width (TW) of the part showing minimal constriction in the PAS were measured (Figure 2).

Figure 2. Measurements of the pharyngeal airway space. A, Anteroposterior length and transverse width of the part showing minimal constriction. B, A three-dimensional airway model with total volume and the part showing minimal constriction.

Statistical analysis

All measurements were performed by a single investigator with more than 5 years of experience in using InVivo software. Intra-examiner reliability was measured using the intraclass correlation coefficient (ICC). Re-measurements were conducted after 1 month for all participants. Levene’s and Shapiro–Wilk tests were used to determine homogeneity and normality. An independent t test was conducted to verify significant differences between the two groups. Pearson’s correlation analysis was used to evaluate the correlation between changes in the PAS score and the amount of anterior overbite. SPSS version 19.0 (IBM Corp., Armonk, NY, USA) was used for all of the statistical analyses.

RESULTS

All ICCs were greater than 0.882 (Table 3). The OBG showed narrower volumes for all PAS segments, but only the NPV, HPV, and TAV of the two groups were significantly different (Table 4). All measurements of the part with minimal constriction were lower in the OBG, but only the area and TW showed significant differences. A statistically longer PAL was observed in the OBG. Pearson’s correlation analysis revealed no correlation between the amount of anterior overbite and changes in the PAS (Table 5).

Table 3 . Intraclass correlation coefficients for intra-examiner reliability

VariablesOpen bite groupControl group
Nasopharyngeal volume0.9210.911
Velopharyngeal volume0.9190.923
Oropharyngeal volume0.9320.882
Hypopharyngeal volume0.9270.915
Total airway volume0.9120.910
Area of most constriction part0.9340.923
APL of most constriction part0.9130.918
TW of most constriction part0.9020.892
Pharyngeal airway length0.9540.962

APL, anteroposterior length; TW, transverse width.



Table 4 . Comparison of the pharyngeal airway space

MeasuresOpen bite groupControl groupP value
Nasopharyngeal volume (cm3)8.47 ± 1.4810.34 ± 1.860.041*
Velopharyngeal volume (cm3)7.42 ± 2.269.40 ± 2.530.439
Oropharyngeal volume (cm3)5.42 ± 2.066.21 ± 2.500.357
Hypopharyngeal volume (cm3)5.52 ± 1.706.94 ± 2.480.045*
Total airway volume (cm3)26.83 ± 6.0632.79 ± 9.790.017*
Area of most constriction part (mm2)165.00 ± 41.51230.60 ± 57.080.039*
APL of most constriction part (mm)9.94 ± 3.7311.61 ± 2.960.122
TW of most constriction part (mm)24.95 ± 5.1229.72 ± 8.570.031*
Pharyngeal airway length (mm)62.32 ± 5.1758.21 ± 4.840.016*

Values are presented as mean ± standard deviation.

APL, anteroposterior length; TW, transverse width.

*P < 0.05.



Table 5 . Pearson’s correlation analysis between the amount of anterior overbite and the measured variables in the pharyngeal airway space

VariablesrP value
Nasopharyngeal volume0.1890.189
Velopharyngeal volume0.1800.212
Oropharyngeal volume0.2220.122
Hypopharyngeal volume0.2660.062
Total airway volume0.2500.080
Area of most constriction part0.2210.123
APL of most constriction part0.2130.138
TW of most constriction part0.2140.136
Pharyngeal airway length–0.2340.101

APL, anteroposterior length; TW, transverse width; r, Pearson correlation coefficient.


DISCUSSION

The present study demonstrated smaller volumes in all the pharyngeal segments in the OBG. We speculated that individuals with AOB had a different oral structure from those with normal occlusion,8,9 which may have changed the PAS. Laganà et al.8 reported that patients with AOB had a high and narrow palate, which is related to the tongue-thrust habit. In addition, the tongue positions of patients with AOB may have changed. Since patients with AOB often show a hyperdivergent profile,9 the mandible rotates clockwise, and the base of their tongues moves to the posteroinferior side. This tongue position may narrow the PAS. However, statistical significance was only observed for NPV, HPV, and TAV. This may be owing to the fact that VPV and OPV are affected by changes in soft tissues such as the uvula and epiglottis.

A significantly longer PAL was observed in the OBG. The PAL response is considered to compensate for the narrow airway.17,18 Thus, the position of the hyoid bone may reflect the efficiency of breathing. If breathing is difficult, the hyoid bone moves down to increase breathing capacity. This was consistent with the results of a previous study10 using 2D analysis. The part showing minimal constriction was also significantly narrower in the OBG. The amount of anterior overbite and changes in PAS were not correlated. Thus, the PAS did not depend on the amount of overbite but was related to the presence of AOB. Additionally, we assumed that individual variations existed within each group.

Since PAS is affected by head posture,12 sex,19 BMI,13 and age,15 the control group was selected by matching age and sex. Participants with a high BMI or abnormal head posture were excluded. Many studies have compared the differences in PAS in relation to the anteroposterior skeletal pattern, but their results are controversial. Some studies20,21 reported no differences in PAS in relation to the anteroposterior skeletal pattern, while another study15 reported that Class III malocclusion was associated with a wider PAS than Class II malocclusion. Therefore, we excluded patients with Class III malocclusion to minimize the effects of the anteroposterior skeletal pattern. The difference in PAS in relation to the vertical skeletal pattern is also a topic of controversy. One study20 reported no differences in PAS, whereas another study22 reported a difference.

At present, there is no consensus regarding the criteria for PAS segmentation.23 This study used the segmentation criteria proposed by Claudino et al.15. The nasal cavity was excluded from the measurement because it contains a variety of anatomical structures that cannot be clearly segmented.

Because the PAS has a 3D and irregular shape, it cannot be accurately measured using 2D linear measurements. In a previous study,24 3D CBCT was shown to have higher accuracy than 2D lateral cephalograms in PAS evaluation. Moreover, the InVivo5 program used in this study reduced measurement errors because it automatically measured the volume and the part showing minimal constriction only if the upper and lower limits of PAS were provided with a threshold.25 Nevertheless, there remain concerns regarding the measurement of airways using CBCT.26

A thorough understanding of the growth of the craniofacial area is crucial for orthodontists. Orthodontists must understand the growth mechanisms in patients with AOB to provide complete care to these patients.

This study had some limitations. First, consistent control of the position of soft tissues, such as the tongue, over the prolonged CBCT scan times was challenging. Second, this study only involved anatomic evaluations; however, dynamic evaluations, such as functional assessment of airflow, are also necessary. Finally, the sample size of this study was small. A larger sample size is required to obtain more reliable results. Thus, further studies on the association between AOB correction and PAS are warranted.

CONCLUSIONS

The limited evidence obtained in this study suggests that patients with AOB have a narrower PAS and smaller part showing minimal constriction. However, no correlation was observed between the amount of anterior overbite and the changes in the PAS.

FUNDING

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

AUTHOR CONTRIBUTIONS

Conceptualization: SHK. Data curation: YIK. Formal analysis: YIK. Funding acquisition: SSK. Methodology: SHK. Project administration: SBP. Visualization: SBP. Writing–original draft: SSK. Writing–review & editing: SHK.

CONFLICTS OF INTEREST

No potential conflict of interest relevant to this article was reported.

References

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  22. Ucar FI, Uysal T. Orofacial airway dimensions in subjects with Class I malocclusion and different growth patterns. Angle Orthod 2011;81:460-8. https://doi.org/10.2319/091910-545.1
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  23. Torres HM, Evangelista K, Torres ÉM, Estrela C, Figueiredo PTS, Valladares-Neto J, et al. Comparison of dimensions of the nasopharynx and oropharynx using different anatomical references: is there equivalence?. J Oral Maxillofac Surg 2019;77:2545-54. https://doi.org/10.1016/j.joms.2019.07.008
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  24. Aboudara C, Nielsen I, Huang JC, Maki K, Miller AJ, Hatcher D. Comparison of airway space with conventional lateral headfilms and 3-dimensional reconstruction from cone-beam computed tomography. Am J Orthod Dentofacial Orthop 2009;135:468-79. https://doi.org/10.1016/j.ajodo.2007.04.043
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  25. ElShebiny T, Morcos S, El H, Palomo JM. Comparing different software packages for measuring the oropharynx and minimum cross-sectional area. Am J Orthod Dentofacial Orthop 2022;161:228-37.e32. https://doi.org/10.1016/j.ajodo.2021.04.024
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  26. Zimmerman JN, Lee J, Pliska BT. Reliability of upper pharyngeal airway assessment using dental CBCT: a systematic review. Eur J Orthod 2017;39:489-96. https://doi.org/10.1093/ejo/cjw079
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Article

Original Article

Korean J Orthod 2023; 53(6): 358-364   https://doi.org/10.4041/kjod22.187

First Published Date January 31, 2023, Publication Date November 25, 2023

Copyright © The Korean Association of Orthodontists.

Three-dimensional evaluation of the pharyngeal airway space in patients with anterior open bite

Seong-Sik Kim , Yong-Il Kim, Soo-Byung Park, Sung-Hun Kim

Department of Orthodontics, Dental Research Institute, and Dental and Life Science Institute, School of Dentistry, Pusan National University, Yangsan, Korea

Correspondence to:Sung-Hun Kim.
Assistant Professor, Department of Orthodontics, Dental Research Institute, and Dental and Life Science Institute, School of Dentistry, Pusan National University, 20, Geumo-ro, Mulgeum-eup, Yangsan 50612, Korea.
Tel +82-55-360-5164 e-mail kmule@hanmail.net

How to cite this article: Kim SS, Kim YI, Park SB, Kim SH. Three-dimensional evaluation of the pharyngeal airway space in patients with anterior open bite. Korean J Orthod 2023;53(6):358-364. https://doi.org/10.4041/kjod22.187

Received: August 26, 2022; Revised: October 12, 2022; Accepted: November 18, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Objective: This study aimed to three-dimensionally evaluate the pharyngeal airway space (PAS) of patients with anterior open bite (AOB) by using cone-beam computed tomography (CBCT) and compare the findings with those obtained in individuals with normal occlusion. Methods: The open bite group (OBG, n = 25) consisted of patients with an anterior overbite of –3 mm or less, while the control group (n = 25) consisted of age- and sex-matched individuals with an anterior overbite of 1–3 mm, Angle Class I malocclusion (1° ≤ point A-nasion-point B angle ≤ 4°), and a normodivergent profile (22° ≤ Frankfort mandibular plane angle ≤ 28°). After the CBCT data were reconstructed into a three-dimensional image, the PAS was segmented into four parts, and the volume of each part was measured. Pharyngeal airway length (PAL) and the area and transverse width of the part showing minimal constriction were also measured. Pearson’s correlation analysis was used to evaluate the correlation between changes in the PAS and the amount of anterior overbite. Results: The OBG showed a significantly narrower airway space in the nasopharyngeal, hypopharyngeal, and total airway volumes. The OBG also showed a significantly smaller area and transverse width of the part with minimal constriction. The OBG showed a significantly longer PAL, but there was no correlation between the amount of anterior overbite and the changes in PAS. Conclusions: The PAS was associated with AOB. Patients with AOB had a narrower PAS and a smaller part showing minimal constriction.

Keywords: Computed tomography, Airway

INTRODUCTION

The respiratory structures are closely related to dentofacial growth and development.1 For example, mouth breathing caused by narrow airways may lead to “adenoid facies.”2 The pharynx is one of the major anatomical structures in the upper airway.1 It is divided into several segments, including the nasopharynx, velopharynx, oropharynx, and hypopharynx.1

Anterior open bite (AOB) can occur as a result of various etiologies, including excessive vertical growth, oral habits, and crowding.3 It shows a higher prevalence in growing children than in adults because the frequency of AOB caused by oral habits, such as thumb sucking and tongue thrust, is high in growing children.4 Although self-correction is possible by discontinuing these habits,5 most patients require orthodontic treatment.

Orthodontic treatment of AOB is considered difficult because of the relapsing tendency of vertical problems and the tongue-thrust habit.6 However, since AOB causes various problems, it is also categorized as a malocclusion, indicating the need for orthodontic treatment. In addition to esthetic problems, AOB can cause functional problems such as pronunciation difficulties and reduced masticatory performance.7 Moreover, individuals with AOB may have a different oral structure from those with normal occlusion,8,9 which may change the pharyngeal airway space (PAS). Therefore, evaluation of the PAS in patients with AOB is essential.

Although a previous study10 evaluated the correlation between AOB and PAS, it used two-dimensional (2D) images. However, 2D measurement of the PAS is limited because it visualizes three-dimensional (3D) structures in two dimensions, and because 2D images do not provide information regarding the volume or cross-sectional area. Thus, 3D cone-beam computed tomography (CBCT) is appropriate for analyzing the PAS.11 To the best of our knowledge, this study is the first attempt to analyze the PAS in patients with AOB using 3D imaging.

The purpose of this study was to three-dimensionally evaluate the PAS of patients with AOB by using CBCT and to compare the findings with those obtained in age- and sex-matched individuals with normal occlusion.

MATERIALS AND METHODS

Participants

This retrospective study was conducted on individuals aged over 20 years who underwent CBCT for other purposes such as third molar impaction or oral disease at the Pusan National University Dental Hospital from 2015 to 2020. The study protocol was approved by the Institutional Review Board of Pusan National University Dental Hospital (PNUDH-202016).

The open bite group (OBG, n = 25) consisted of patients with an anterior overbite of –3 mm or less, while the control group (n = 25) consisted of age- and sex-matched individuals with an anterior overbite of 1–3 mm, Angle Class I malocclusion (1° ≤ point A-nasion-point B angle (ANB) ≤ 4°), and a normodivergent profile (22° ≤ Frankfort–mandibular plane angle ≤ 28°) (Table 1). The exclusion criteria were as follows: (1) previous airway disease or surgery; (2) maxillofacial syndromes; (3) obstructive sleep apnea; (4) a history of temporomandibular joint disorder; (5) previous orthodontic treatment; (6) recent onset of AOB; (7) Angle Class III malocclusion (ANB < 1°); (8) abnormal head posture (craniocervical angle in the second vertebra, < 90° or > 110°)12; and (9) high body mass index (BMI) (> 30 kg/m2).13

Table 1 . Descriptive statistics.

ParametersOpen bite group
(n = 25)
Control
group
(n = 25)
P value
Age (yr)23.8 ± 2.422.9 ± 2.60.843*
Male13120.768
Overbite (mm)–4.6 ± 1.21.9 ± 0.8< 0.001*
ANB (°)3.0 ± 1.92.5 ± 1.20.252*
FMA (°)27.6 ± 5.125.1 ± 2.70.393*
Angle classification
Class I1725
Class II80
Class III00

Values are presented as mean ± standard deviation or number only..

ANB, Angle of nasion-A point line to nasion-B point line; FMA, angle of the Frankfort line to the mandibular plane; NS, not significant..

*Independent t test..

Chi-square test..



To detect a mean difference of 2.0 cm3 in the nasopharyngeal volume (NPV) with a standard deviation of 1.6 (obtained from a previous pilot study), a β error of 0.20, and an α error of 0.05, 25 participants were required for each group.14

CBCT data

CBCT (DCT pro; Vatech, Seoul, Korea) was performed using the protocol of the image center under the same conditions for participants of both groups. The participants were instructed to bite with maximum intercuspation in an upright position, not to swallow, and not to move the tongue while undergoing CBCT. The CBCT settings were as follows: field of view, 20 × 19 cm; scanning time, 24 seconds; tube current, 4 mA; tube voltage, 90 kVp; and voxel size, 0.3 mm. CBCT data were reconstructed using InVivo5.4 software (Anatomage, San Jose, CA, USA).

PAS measurements

The 3D images were then re-oriented. The Frankfort horizontal (FH) plane was used as the horizontal reference plane. The plane vertical to the FH plane and crossing the nasion and basion was used as the vertical reference plane (VRP). The reference plane and landmarks were determined on the basis of previous studies (Figure 1 and Table 2).15 The sagittal slice of the VRP was chosen to allow the software to evaluate the airway. The threshold range of air was set between –1,000 and –587 Hounsfield units.16

Figure 1. Landmarks and reference planes. See Table 2 for the definitions of each landmark.

Table 2 . Landmarks and descriptions.

LandmarksDescription
BaBasion
SSella
SoMidpoint in the Ba-S line
PNSPosterior nasal spine
UTTip of the uvula
ETTip of the epiglottis
CV4Most anterior inferior point of body of fourth cervical vertebra
HYThe most anterosuperior point of the hyoid bone
Frankfort horizontal planePlane constructed on both sides of Porion and right Orbitale
PL1Plane passing through So and PNS
PL2Plane parallel to FH plane and passing through PNS
PL3Plane parallel to FH plane and passing through UT
PL4Plane parallel to FH plane and passing through ET
PL5Plane parallel to FH plane and passing through CV4
PALShortest distance from HY to PL2


The PAS was segmented into four parts and the volume of each part was measured. The upper limit of the NPV was the plane (PL1) connecting the posterior nasal spine (PNS) at the midpoint (So) of the sella and the basion. The lower limit of the NPV was the plane (PL2) parallel to the FH plane while passing through the PNS. The lower limit of the velopharyngeal volume (VPV) was the plane (PL3) that passed through the tip of the uvula (UT) parallel to the FH plane. The lower limit of the oropharyngeal volume (OPV) was the plane (PL4) parallel to the FH plane while passing through the tip of the epiglottis (ET). The lower limit of the hypopharyngeal volume (HPV) was the plane (PL5) parallel to the FH plane while passing through the most anteroinferior point of the body of the fourth cervical vertebra (CV4). The total airway volume (TAV), which was the volume of all four segments, was also measured. The pharyngeal airway length (PAL) was the vertical distance from PL2 to the most anterosuperior point of the hyoid bone (HY). In addition, the area, anteroposterior length (APL), and transverse width (TW) of the part showing minimal constriction in the PAS were measured (Figure 2).

Figure 2. Measurements of the pharyngeal airway space. A, Anteroposterior length and transverse width of the part showing minimal constriction. B, A three-dimensional airway model with total volume and the part showing minimal constriction.

Statistical analysis

All measurements were performed by a single investigator with more than 5 years of experience in using InVivo software. Intra-examiner reliability was measured using the intraclass correlation coefficient (ICC). Re-measurements were conducted after 1 month for all participants. Levene’s and Shapiro–Wilk tests were used to determine homogeneity and normality. An independent t test was conducted to verify significant differences between the two groups. Pearson’s correlation analysis was used to evaluate the correlation between changes in the PAS score and the amount of anterior overbite. SPSS version 19.0 (IBM Corp., Armonk, NY, USA) was used for all of the statistical analyses.

RESULTS

All ICCs were greater than 0.882 (Table 3). The OBG showed narrower volumes for all PAS segments, but only the NPV, HPV, and TAV of the two groups were significantly different (Table 4). All measurements of the part with minimal constriction were lower in the OBG, but only the area and TW showed significant differences. A statistically longer PAL was observed in the OBG. Pearson’s correlation analysis revealed no correlation between the amount of anterior overbite and changes in the PAS (Table 5).

Table 3 . Intraclass correlation coefficients for intra-examiner reliability.

VariablesOpen bite groupControl group
Nasopharyngeal volume0.9210.911
Velopharyngeal volume0.9190.923
Oropharyngeal volume0.9320.882
Hypopharyngeal volume0.9270.915
Total airway volume0.9120.910
Area of most constriction part0.9340.923
APL of most constriction part0.9130.918
TW of most constriction part0.9020.892
Pharyngeal airway length0.9540.962

APL, anteroposterior length; TW, transverse width..



Table 4 . Comparison of the pharyngeal airway space.

MeasuresOpen bite groupControl groupP value
Nasopharyngeal volume (cm3)8.47 ± 1.4810.34 ± 1.860.041*
Velopharyngeal volume (cm3)7.42 ± 2.269.40 ± 2.530.439
Oropharyngeal volume (cm3)5.42 ± 2.066.21 ± 2.500.357
Hypopharyngeal volume (cm3)5.52 ± 1.706.94 ± 2.480.045*
Total airway volume (cm3)26.83 ± 6.0632.79 ± 9.790.017*
Area of most constriction part (mm2)165.00 ± 41.51230.60 ± 57.080.039*
APL of most constriction part (mm)9.94 ± 3.7311.61 ± 2.960.122
TW of most constriction part (mm)24.95 ± 5.1229.72 ± 8.570.031*
Pharyngeal airway length (mm)62.32 ± 5.1758.21 ± 4.840.016*

Values are presented as mean ± standard deviation..

APL, anteroposterior length; TW, transverse width..

*P < 0.05..



Table 5 . Pearson’s correlation analysis between the amount of anterior overbite and the measured variables in the pharyngeal airway space.

VariablesrP value
Nasopharyngeal volume0.1890.189
Velopharyngeal volume0.1800.212
Oropharyngeal volume0.2220.122
Hypopharyngeal volume0.2660.062
Total airway volume0.2500.080
Area of most constriction part0.2210.123
APL of most constriction part0.2130.138
TW of most constriction part0.2140.136
Pharyngeal airway length–0.2340.101

APL, anteroposterior length; TW, transverse width; r, Pearson correlation coefficient..


DISCUSSION

The present study demonstrated smaller volumes in all the pharyngeal segments in the OBG. We speculated that individuals with AOB had a different oral structure from those with normal occlusion,8,9 which may have changed the PAS. Laganà et al.8 reported that patients with AOB had a high and narrow palate, which is related to the tongue-thrust habit. In addition, the tongue positions of patients with AOB may have changed. Since patients with AOB often show a hyperdivergent profile,9 the mandible rotates clockwise, and the base of their tongues moves to the posteroinferior side. This tongue position may narrow the PAS. However, statistical significance was only observed for NPV, HPV, and TAV. This may be owing to the fact that VPV and OPV are affected by changes in soft tissues such as the uvula and epiglottis.

A significantly longer PAL was observed in the OBG. The PAL response is considered to compensate for the narrow airway.17,18 Thus, the position of the hyoid bone may reflect the efficiency of breathing. If breathing is difficult, the hyoid bone moves down to increase breathing capacity. This was consistent with the results of a previous study10 using 2D analysis. The part showing minimal constriction was also significantly narrower in the OBG. The amount of anterior overbite and changes in PAS were not correlated. Thus, the PAS did not depend on the amount of overbite but was related to the presence of AOB. Additionally, we assumed that individual variations existed within each group.

Since PAS is affected by head posture,12 sex,19 BMI,13 and age,15 the control group was selected by matching age and sex. Participants with a high BMI or abnormal head posture were excluded. Many studies have compared the differences in PAS in relation to the anteroposterior skeletal pattern, but their results are controversial. Some studies20,21 reported no differences in PAS in relation to the anteroposterior skeletal pattern, while another study15 reported that Class III malocclusion was associated with a wider PAS than Class II malocclusion. Therefore, we excluded patients with Class III malocclusion to minimize the effects of the anteroposterior skeletal pattern. The difference in PAS in relation to the vertical skeletal pattern is also a topic of controversy. One study20 reported no differences in PAS, whereas another study22 reported a difference.

At present, there is no consensus regarding the criteria for PAS segmentation.23 This study used the segmentation criteria proposed by Claudino et al.15. The nasal cavity was excluded from the measurement because it contains a variety of anatomical structures that cannot be clearly segmented.

Because the PAS has a 3D and irregular shape, it cannot be accurately measured using 2D linear measurements. In a previous study,24 3D CBCT was shown to have higher accuracy than 2D lateral cephalograms in PAS evaluation. Moreover, the InVivo5 program used in this study reduced measurement errors because it automatically measured the volume and the part showing minimal constriction only if the upper and lower limits of PAS were provided with a threshold.25 Nevertheless, there remain concerns regarding the measurement of airways using CBCT.26

A thorough understanding of the growth of the craniofacial area is crucial for orthodontists. Orthodontists must understand the growth mechanisms in patients with AOB to provide complete care to these patients.

This study had some limitations. First, consistent control of the position of soft tissues, such as the tongue, over the prolonged CBCT scan times was challenging. Second, this study only involved anatomic evaluations; however, dynamic evaluations, such as functional assessment of airflow, are also necessary. Finally, the sample size of this study was small. A larger sample size is required to obtain more reliable results. Thus, further studies on the association between AOB correction and PAS are warranted.

CONCLUSIONS

The limited evidence obtained in this study suggests that patients with AOB have a narrower PAS and smaller part showing minimal constriction. However, no correlation was observed between the amount of anterior overbite and the changes in the PAS.

FUNDING

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

AUTHOR CONTRIBUTIONS

Conceptualization: SHK. Data curation: YIK. Formal analysis: YIK. Funding acquisition: SSK. Methodology: SHK. Project administration: SBP. Visualization: SBP. Writing–original draft: SSK. Writing–review & editing: SHK.

CONFLICTS OF INTEREST

No potential conflict of interest relevant to this article was reported.

Fig 1.

Figure 1.Landmarks and reference planes. See Table 2 for the definitions of each landmark.
Korean Journal of Orthodontics 2023; 53: 358-364https://doi.org/10.4041/kjod22.187

Fig 2.

Figure 2.Measurements of the pharyngeal airway space. A, Anteroposterior length and transverse width of the part showing minimal constriction. B, A three-dimensional airway model with total volume and the part showing minimal constriction.
Korean Journal of Orthodontics 2023; 53: 358-364https://doi.org/10.4041/kjod22.187

Table 1 . Descriptive statistics.

ParametersOpen bite group
(n = 25)
Control
group
(n = 25)
P value
Age (yr)23.8 ± 2.422.9 ± 2.60.843*
Male13120.768
Overbite (mm)–4.6 ± 1.21.9 ± 0.8< 0.001*
ANB (°)3.0 ± 1.92.5 ± 1.20.252*
FMA (°)27.6 ± 5.125.1 ± 2.70.393*
Angle classification
Class I1725
Class II80
Class III00

Values are presented as mean ± standard deviation or number only..

ANB, Angle of nasion-A point line to nasion-B point line; FMA, angle of the Frankfort line to the mandibular plane; NS, not significant..

*Independent t test..

Chi-square test..


Table 2 . Landmarks and descriptions.

LandmarksDescription
BaBasion
SSella
SoMidpoint in the Ba-S line
PNSPosterior nasal spine
UTTip of the uvula
ETTip of the epiglottis
CV4Most anterior inferior point of body of fourth cervical vertebra
HYThe most anterosuperior point of the hyoid bone
Frankfort horizontal planePlane constructed on both sides of Porion and right Orbitale
PL1Plane passing through So and PNS
PL2Plane parallel to FH plane and passing through PNS
PL3Plane parallel to FH plane and passing through UT
PL4Plane parallel to FH plane and passing through ET
PL5Plane parallel to FH plane and passing through CV4
PALShortest distance from HY to PL2

Table 3 . Intraclass correlation coefficients for intra-examiner reliability.

VariablesOpen bite groupControl group
Nasopharyngeal volume0.9210.911
Velopharyngeal volume0.9190.923
Oropharyngeal volume0.9320.882
Hypopharyngeal volume0.9270.915
Total airway volume0.9120.910
Area of most constriction part0.9340.923
APL of most constriction part0.9130.918
TW of most constriction part0.9020.892
Pharyngeal airway length0.9540.962

APL, anteroposterior length; TW, transverse width..


Table 4 . Comparison of the pharyngeal airway space.

MeasuresOpen bite groupControl groupP value
Nasopharyngeal volume (cm3)8.47 ± 1.4810.34 ± 1.860.041*
Velopharyngeal volume (cm3)7.42 ± 2.269.40 ± 2.530.439
Oropharyngeal volume (cm3)5.42 ± 2.066.21 ± 2.500.357
Hypopharyngeal volume (cm3)5.52 ± 1.706.94 ± 2.480.045*
Total airway volume (cm3)26.83 ± 6.0632.79 ± 9.790.017*
Area of most constriction part (mm2)165.00 ± 41.51230.60 ± 57.080.039*
APL of most constriction part (mm)9.94 ± 3.7311.61 ± 2.960.122
TW of most constriction part (mm)24.95 ± 5.1229.72 ± 8.570.031*
Pharyngeal airway length (mm)62.32 ± 5.1758.21 ± 4.840.016*

Values are presented as mean ± standard deviation..

APL, anteroposterior length; TW, transverse width..

*P < 0.05..


Table 5 . Pearson’s correlation analysis between the amount of anterior overbite and the measured variables in the pharyngeal airway space.

VariablesrP value
Nasopharyngeal volume0.1890.189
Velopharyngeal volume0.1800.212
Oropharyngeal volume0.2220.122
Hypopharyngeal volume0.2660.062
Total airway volume0.2500.080
Area of most constriction part0.2210.123
APL of most constriction part0.2130.138
TW of most constriction part0.2140.136
Pharyngeal airway length–0.2340.101

APL, anteroposterior length; TW, transverse width; r, Pearson correlation coefficient..


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