To evaluate changes in dimensions and volume of upper airway before and after mini-implant assisted rapid maxillary expansion (MARME) and observe correlations between changes of upper airway and vertical skeletal pattern in young adults.
In this retrospective study, 22 patients (mean age, 22.6 ± 4.5 years; 4 male 18 female) with transverse discrepancy underwent MARME. Cone beam computed tomography was taken before and 3 months after expansion. Vertical and horizontal dimensions and volume of the nasal cavity, nasopharyngeal, retropalatal, retroglossal and hypopharyngeal airway were compared before and after MARME. Correlations between changed volume and dimensions were explored, as well as the vertical skeletal pattern.
Nasal osseous width, maxillary width, volume of the nasal cavity and nasopharynx increased significantly (P < .05). Enlarged nasopharyngeal volume correlated with increased nasal width at the PNS plane (P < .05). There were no correlations between expanded volume and maxillary width. No measurements except nasal cavity volume had a correlation with Sum angle. Increased maxillary width correlated negatively with hard palate thickness (P < .05).
(1) MARME caused an increase in volume of the nasal cavity and nasopharynx, with expansion of nasal osseous width and maxillary width. (2) Enlarged nasal width at the PNS plane contributed to the increase in nasopharynx volume. Enlarged maxillary width showed no direct relation with increased volume. (3) In this study, it was unclear about the association between changes of the upper airway and vertical skeletal pattern because of complex structures. (4) Palate thickness affected skeletal expansion of the maxilla in MARME.
Rapid maxillary expansion (RME) is a common orthodontic treatment procedure to correct transverse discrepancies.1 Expansion of the midpalatal suture affects the nasal floor and the effects extend to the surrounding nasal and craniofacial structures.2 Therefore, the effect of RME on the upper airway in three dimensions was studied.3–5 In previous studies, nasal cavity volume increased and nasal resistance reduced.6,7 However, not all studies reported an increase in pharyngeal airway volume.5,8
In studies regarding the effect of conventional tooth-borne RME on upper airway, most found it contributed to an increase of nasal cavity volume.5,9 However, conventional tooth-borne RME has side effects such as buccal crown tipping.10 Additionally, there is limited skeletal expansion in late adolescence and in adults because of interdigitation of the midpalatal suture and adjacent articulations.11 To minimize these undesirable effects and potential limitations, surgically assisted rapid maxillary expansion (SARME) is used, including surgical release of the closed midpalatal suture. Several studies have reported an increase of the nasal cavity volume and poor effect on oropharyngeal volume after SARME.3,12 Nevertheless, patients tend to be reluctant to undergo surgical procedures due to trauma. Recently, mini-implant assisted rapid maxillary expansion (MARME) for mature patients has been demonstrated to provide similar skeletal expansion to SARME, reducing surgical injury and adverse dentoalveolar side effects.13,14 Increased volume and cross-sectional area of the nasal cavity have been reported after MARPE by Kim et al.15
Because the nasomaxillary complex provides anterior bony support for the upper airway and orthodontic treatment affects these structures, causing changes in the airway to some extent, dentists have the responsibility to understand the physiology of upper airway.4,5,7,16 Katyal et al.17 found that children with narrow dentoalveolar transverse width and reduced nasopharyngeal and oropharyngeal sagittal dimensions had a high risk for sleep-disordered breathing. Many studies have reported the influence of RME on the upper airway, though the results were different due to various subjects and expansion methods. However, there are few studies about changes of each segment of the upper airway after MARME.
The first aim of this retrospective study was to compare the dimensions and volume of each segment of the upper airway before and after MARME in young adults, including the nasal cavity. The second purpose was to explore correlations between changes of the upper airway and vertical skeletal patterns. The hypothesis was that the dimensions and volume of the nasal cavity and nasopharynx would be increased by MARME and vertical skeletal pattern would be correlated with the results.
MATERIALS AND METHODS
This study included 22 patients (mean age: 22.6 ± 4.5 years; range: 18–35 years; four male, 18 female), who had undergone MARME at the Department of Orthodontics, Shandong University Dental Hospital, since January 2017. The study was approved by the Ethical Commission of Shandong University Dental Hospital (No. 20190506). All patients provided written informed consent. The inclusion criteria were: (1) young adults (18–35 years old) with transverse maxillary discrepancy, and successful opening of the midpalatal suture by MARME; and (2) availability of cone beam computed tomography (CBCT) images obtained before and after expansion. The exclusion criteria were: (1) a history of orthodontic or orthognathic treatment, (2) acute rhinitis during expansion, and (3) severe craniofacial anomalies or systemic diseases. To estimate the sample size, a pilot study was conducted in 10 patients. With α = 0.05, two-tailed, and a power of 80%, 19 patients were needed.
Every patient was treated by the maxillary skeletal expansion type II appliance (BioMaterials Korea, Seoul, Korea) developed by Dr. Moon and colleagues18 (Figure 1). The appliance consisted of bands to the permanent first molars and four holes for mini-implants. Orthodontic mini-implants (1.5 mm diameter; 11 mm length, BioMaterials Korea) were placed at the center of the holes. After immediate expander activation (four turns), the expander was activated by two turns every other day to minimize periodontal damage (one turn = 0.13 mm) until maxillary skeletal width was no longer less than that of the mandible. The required amount of expansion was set according to the diagnosis and treatment objective of each patient: usually 32–48 turns. The mean duration of expansion was 38 days (range: 30–43 days). Mucosal swelling was prevented by scrupulous oral and nasal hygiene maintenance, including copious saline irrigation. Medication for reducing swelling inside the nasal cavity was not applied. The retention time was at least 3 months, allowing bone formation in the separated maxillary suture.
CBCT scans (Quantitative Radiology, Verona, Italy; 110 kV, 7.33 mA, 4.8s typical X-ray emission time; 18 × 16 field of view; standard voxel size of 0.3 mm) were performed before expansion (T0) and after 3 months' retention (T1) by the same operator. The patients were scanned in supine position with the Frankfort plane perpendicular to the floor, keeping the teeth in centric occlusion and the tongue in the position at the end of swallowing (against the palate), breathing smoothly, and no swallowing. The digital imaging and communications in medicine (DICOM) data were imported into Dolphin Imaging software (Chatsworth, CA, USA) and used for the measurements described. The lateral cephalometric image (LC) before expansion was measured.
Before landmark identification, the three-dimensional volumetric images were oriented with the Dolphin imaging software as follows: coronal plane (horizontal line through orbitale bilaterally), sagittal plane (Frankfort horizontal), and axial plane (Crista galli to basion) (Figure 2). The Dolphin software allowed automatic volume calculation after segmenting the area of interest by setting the threshold value of 55. Detailed descriptions of these landmarks and measurements are shown in Figures 3–8 and Table 1. Lateral cephalometric measurements according to the Jarabak analysis are shown in Figure 9.
One examiner performed all measurements. To estimate reliability of the method, seven randomly selected patients were re-evaluated after one week. The intraclass correlation coefficient (ICC) showed high reliability (0.91< ICC < 0.99). Data normality and homoscedasticity of variances were assessed by Shapiro-Wilk and Levene's tests, respectively. Paired t-tests were used for continuous matched pairs of normal data and Wilcoxon signed-rank test for nonparametric variables. Pearson correlation test was used to identify correlations if data were normally distributed; if not, Spearman correlation was used. P < .05 was considered statistically significant. SPSS version 20.0 (SPSS Inc., Chicago, IL, USA) was used for all statistical analysis.
W-ANS, W-mid, W-PNS, and H-PNS (Table 2, P < .001, P < .001, P < .001, P < .001, P = .023) showed a significant increase, while H-ANS/W-ANS and H-mid/W-mid (P < .001) decreased. N-ANS increased significantly (Table 2, P = .029), while ANS-PNS decreased (P = .008). In the pharyngeal cross-section at PNS plane, AP-PNS, LL-PNS, and Area-PNS showed significant enlargement (Table 2, P = .014; P = .013; P = .011).The V-NC increased by 2925.9 mm3 after expansion (Table 2, P = .014), and the V-NPA increased by 734.9 mm3 (Table 2, P = .003). No significant differences in V-RPA, V-RGA, V-HPA, or minimum cross-sectional area (MCA) were found before and after MARME (Table 2, P > .05). There was a significant expansion of nasal, maxillary, zygomatic, and temporal bone widths (Table 2, P < .001; P < .001; P = .018; P < .001).
Enlargement of V-NC showed a positive correlation with the increase of N-ANS (Table 3, r = 0.426), SGo/NMe (r = 0.51) and a negative correlation with its original volume, Sum angle, and ∠NSAr (P < .05, r < 0). The increased V-NPA was closely linked to the enlarged W-PNS (Table 3, r = 0.655). Most measurements of upper airway were not associated with Sum angle, except the original and increased V-NC (Table 4, r = 0.481, r = −0.608). Area-PNS was highly related to V-NPA (Table 5, r = 0.592). The enlargement of Area-PNS correlated positively with the expansion of maxillary width (HP) (Table 5, r = 0.443). A negative relationship was found between the expansion of maxillary width and palate thickness (Table 5, P < .05).
This study was focused on changes of the vertical and horizontal dimensions and volume of the upper airway caused by MARME. Volume of the nasal cavity and nasopharynx showed significant increases, consistent with some previous studies.4,19 Kim et al.15 demonstrated that volume of the nasal cavity increased continuously from pre-expansion to immediately after expansion, and to 1 year after expansion. They reported nasopharyngeal volume showed a significant increase 1 year after expansion compared with the initial volume.15 In the current study, volume of the nasal cavity and nasopharynx expanded significantly 3 months after MARME, but it is necessary to investigate long-term stability in the future. In addition, the increased nasal osseous width at the PNS plane contributed to the expansion of nasopharyngeal volume and the cross-sectional area of the upper airway at the PNS plane enlarged with the increase of maxillary width. However, nasopharyngeal volume showed no significant changes in several previous studies.5 These discrepancies could be attributed to subject age, differences in definition of the upper airway volume, the expansion modality, amount of expansion screw activation, amount of pierced palatal and nasal cortical bone, skeletal characteristics, and measurement tools used. The range of age was also different among studies. In this study, adults were included with stable upper airways while others evaluated children,5 growth and development also contribute to changes in volume of the upper airway. According to a previous study, the upper airway was divided into more segments in this study, resulting in significant changes.19
No changes were found in volumes of the inferior section of the upper airway and MCA, in accordance with the previous results reported.19 Soft tissue plays an important role in the volume of the upper airway.20 The location and shape of the soft palate might change due to horizontal expansion of the hard palate. Also, the position of the tongue may change due to maxillary width expansion, affecting the volume of the upper airway to some extent. In the current study, however, the retention time was so short that soft tissue might not yet have adapted to the hard tissue. Although the changes regarding MCA and volume of the hypopharyngeal airway were not significant statistically, there was still a clinical change observed to some degree. A long-term study regarding the effect of MARME on the upper airway is required.
In addition, increased maxillary width was found to be negatively related to palate thickness, which indicated that the thicker hard palate showed the larger resistance. Bony support of the hard palate to mini-implants was also critical, a more detailed study would be performed. However, both of them were not directly related to the increased volume of the nasal cavity and nasopharynx. The structure of the nasomaxillary complex and the anatomy of the nasal cavity were complicated and irregular,21 such as a deviated nasal septum.22,23 Additionally, there might have been compensatory hypertrophy of the nasal mucosa after expansion. It was hard to conclude there was any correlation between amounts of maxillary expansion and the increase of volume.
There was an attempt made to investigate whether vertical craniofacial pattern influenced the effect of MARME on the upper airway. It was not reliable to analyze the data in categories for clinical study with such a small sample. Therefore, correlation analysis was performed. Sum angle showed no link to the increase of maxillary width, as well as to the dimensions and volume of each segment of the upper airway except nasal cavity volume. The hyperdivergent pattern showed less enlargement of the nasal cavity volume, probably because there was a larger original volume in this study, inconsistent with previous studies.24 But Sum angle and other nasal measurements showed no correlations. Turbinates, the nasal septum, and the condition of the nasal mucosa contributed to a complicated nasal structure. Therefore, it was not clear regarding an association between changes of the nasal cavity and vertical facial patterns in this study. Additionally, only the boundary changes of the upper airway by automated calculation were measured without its ventilation capacity. Further study about the morphology and function of the upper airway is required.
MARME can improve nasal airflow, leading to better ventilatory function through increased upper airway volume, though the initial purpose of the procedure was to correct a narrow maxilla.25 So, it could be a therapeutic option for nasal obstruction.26 However, there was lack of a control group in this study due to ethical issues. In addition, the sample size was relatively small, and it was not reliable to analyze measurements in categories according to vertical skeletal pattern. The tongue was not at the same position because of the presence of the expander before and after MARME. Additionally, the observation period was short. In the future, it would be useful to assess the upper airway after 1 year, and again when the expander is removed. Lastly, morphometric changes would be best related to functional aspects by respiratory tests.
Transverse dimensions and volume of the nasal cavity and nasopharynx increased after MARME when maxillary width increased simultaneously. Retropalatal, retroglossal, and hypopharyngeal airway volume were not found to be changed significantly in this study.
Enlarged nasal width at the PNS plane contributed to the increase of nasopharynx volume. Enlargement of maxillary width showed no direct relationship with increased volume.
It was unclear regarding the association between vertical skeletal patterns and changes of upper airway after MARME because of the complex structures involved.
The enlargement of maxillary width by MARME was affected by hard palate thickness.
We thank Hui Chen for assistance. This work was supported by key R & D program of Shandong Province [2018GSF118240].
Postgraduate Student, Department of Orthodontics, School and Hospital of Stomatology, Shandong University; and Shandong Key Laboratory of Oral Tissue Regeneration; and Shandong Engineering Laboratory for Dental Materials and Oral Tissue Regeneration, Jinan, Shandong Province, China.
Professor, Department of Orthodontics, Universidad International de Cataluña, Barcelona, Spain.
Professor, Department of Orthodontics, School and Hospital of Stomatology, Shandong University; and Shandong Key Laboratory of Oral Tissue Regeneration; and Shandong Engineering Laboratory for Dental Materials and Oral Tissue Regeneration, Jinan, Shandong Province, China.