Changes in the nasal and pharyngeal airway volume with miniscrew-assisted rapid palatal expansion appliances

Sample collection

The protocol of this study was reviewed and approved by West Virginia University Institutional Review Board for human research protection (Protocol #1908679933). Permission was also granted by Wuhan University School of Stomatology to use the records from their institution. A total of 250 records of patients who completed maxillary expansion with the maxillary skeletal expander (MSE) appliance between January 1, 2015, and July 1, 2020, were reviewed. A total of 20 cases met the following inclusion criteria: Complete late mixed or permanent dentition excluding third molars; non-smoker; no gender or race predilection; and complete pre-treatment and post-treatment records, including intraoral scans; and no previous orthodontic treatment. The exclusion criteria included: No previous orthodontic or orthopedic treatment and no craniofacial anomalies. With a sample size of 20, we achieved 80% power for a two-sided test with a significance alpha level of 0.05 based on the effect size of 0.66 (calculated from data). Each subject was given a random identification number, and all protected health information identifiers were removed for de-identification purposes. The final age of the sample ranged from age 8 to 22. The cervical vertebral maturation (CVM) ranged from Stages 2 to 6.

Appliance design and activation protocol

All patients were treated with the MSE manufactured by BioMaterials Korea, Inc. The MSE appliance is one type of design of the MARPE appliance. It consists of a central expansion screw, which is held in place against or close to the palate. The design of the appliance may vary from patient to patient, but commonly the maxillary first premolar and first molar are pre-fit with orthodontic bands to allow the MSE arms to be soldered to. Four miniscrews are inserted through the guide holes surrounding the expansion screw and seated fully against the metal framework of the MSE appliance [Figure 1]. Each screw is inserted perpendicular and bicortically through the palate for maximum skeletal anchorage. The miniscrews are 1.8 mm in diameter, and although the length varies depending on the anatomical thickness of the patient’s palate, a length of 11–13 mm was routinely used.

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Figure 1:

(a-c) Maxillary skeletal expander (MSE) manufactured by BioMaterials Korea, Inc.

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Every subject began maxillary expansion 2 weeks after the placement of micro implants. Patients were instructed to turn the expansion screw one turn in the morning and one turn in the evening. Maxillary expansion was suspended if patients complained of unbearable pain; if jackscrew was too tight to turn; if one or more micro-implants were loose; and if micro-implants or linkages were wrapped by soft tissue and there was obvious localized inflammation. Patients were instructed how to activate the expansion and how to flush the palatal region after meals to keep oral hygiene. The number of turns of expansion appliance was recorded and adverse event was reported. The expansion was terminated when 2–3 mm of overexpansion was achieved.

CBCT analysis

CBCT scans were taken before pre-treatment and immediately after expansion with MSE appliance. The CBCT scans from West Virginia University were obtained with a Gendex GX-DP-700 cone-beam three-dimensional imaging scanner, and the Wuhan University CBCT scans were obtained with a NewTom VGi 9 cone beam (Imola, Italy). The chosen field of view was 17 × 13 cm with a 300 voxel size and 16 bit grayscale. Exposure components were pre-adjusted to the selected field of view: 11.30 s scan time, 85 KV, and 4.0 mA. All patients were scanned in standing supine position, upright head posture, and in maximum intercuspation. The data of each patient were reconstructed with a 0.0 mm slice thickness, and the DICOM images were assess using the Dolphin imaging software (11.95).

To standardize the image analysis procedure, all CBCT scans were oriented in three planes of space (sagittal, coronal, and transverse). Image orientation was done with the use of the Dolphin imaging software (version 11.95). Sagittal orientation was determined with a line of best fit down the skeletal midline [Figure 2]. Coronal orientation was based on Frankfort horizontal plane from the right lateral perspective. Transverse (axial) orientation was based on a line passing through the furcation of the permanent first molar from the right lateral perspective [Figure 3]. Definition of the airway compartments was adopted from Kavand et al.^[16] to allow for the comparison of results. The upper airway was divided into three compartments in the mid-sagittal plane: Nasal cavity; nasopharynx, and the oropharynx. For the nasal cavity, the sagittal view was defined as the nasal cavity was defined as the airway space bound by the lines connecting sella (S), nasion (N), the tip of the nasal bone, anterior nasal spine, posterior nasal spine (PNS), and back to sella. The frontal view was outlined avoiding the surrounding maxillary, ethmoidal, and sphenoid sinuses [Figures 4-6]. For the nasopharynx: The airway space bound by the lines connecting S, PNS, to the tip of the odontoid process, and back to S. For the oropharynx: The airway space bound by the lines connecting the tip of the odontoid process, PNS, menton, the most anterior-inferior point of the third cervical vertebrae, and back to the tip of the odontoid process [Figure 7].

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Figure 2:

Head orientation in the mid-sagittal plane of a CBCT scan.

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Figure 3:

Head orientation in the coronal and transverse planes of a CBCT scan.

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Figure 4:

Frontal dimension showing measurements for superior and inferior nasal cavity width. (a) Green line showing measurement. (b) Blue line delineating the tip of the vomer bone.

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Figure 5:

Transverse dimension showing measurements for anterior and posterior nasal cavity width at the level of the vomer tip.

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Figure 6:

Boundaries of the nasal cavity in the (a) frontal view; (b) sagittal view.

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Figure 7:

(a) Boundaries of the nasopharynx in the sagittal view; (b) Boundaries of the oropharynx in the sagittal view.

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To determine the pattern of expansion for the nasal cavity following treatment with the MSE appliance, skeletal landmarks of the nasal cavity were defined in the frontal and transverse dimensions. The landmarks chosen for this study were based on easily identifiable and repeatable structures but not currently used in the literature as cephalometric landmarks. For the frontal view: CBCT oriented to view a coronal slice through the landmark of the furcation of the permanent first molar. Superior landmark is defined as the widest point of the frontoethmoid suture. An inferior landmark is defined as the widest point of the wall of the nasal cavity passing through the tip of the vomer. For the transverse view: Orient CBCT axially through the superior tip of the vomer bone. Anterior landmark is defined as the widest point of the nasal cavity anteriorly. Posterior landmark is defined as the widest point between the walls of the pterygoid plates [Figure 8].

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Figure 8:

Example of airway volume creation for the nasal cavity. The airway boundary was defined on the cone-beam computed tomography as specified in Figures 6 and 7 with green points and lines. Seed points (yellow dots) were added to the negative space on each draft slice to calculate the airway volume (shown in pink). An airway sensitivity ranging from 20 to 50 was determined based on the examiner’s judgment to best fill the negative spaces of the specific airway compartment in question while avoiding any of the surrounding cranial sinuses.

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Statistical analysis

All statistical analyses were conducted using SAS (version 9.4, 2013, SAS Institute Inc., Cary, NC). Paired t-tests were performed to evaluate nasal and pharyngeal airway volume changes between pre- and post-measurements with the treatment of MARPE appliances. We calculated intraclass correlation coefficients to evaluate the reliability of the measurements. Pearson correlation test was utilized to assess the relationship between nasal and pharyngeal volume changes. All statistical tests were two-sided and P < 0.05 was considered statistically significant.

Intrarater reliability analysis

To assess the intrarater reliability measurements, pre- and post-expansion measurements for 10 randomly selected subjects were repeated 2 weeks apart. The intraclass correlation coefficient was high for all measurements, indicating a high level of agreement between the two sets of measurements done by the evaluator [Table 1].

Comparison of pre- and post-expansion measurements on all subjects

Seventeen of the 20 patients had a successful split of the mid-palatal suture following expansion with the MARPE appliance. Two of the three patients who did not have successful splitting of the mid-palatal suture had CVM Stage 5, and the other patient had a CVM of 6.

Significant increase in nasal cavity volume, frontal dimension inferior nasal width in the frontal dimension, nasal anterior width in the transverse dimension, and transverse nasal posterior width were found following expansion [Table 2]. No significant changes were seen for volume changes of the nasopharynx, oropharynx, and frontal nasal superior width.

The nasal cavity volume increased by 12.0% following expansion with the MSE appliance. The percent change of linear measurement of the nasal cavity is useful to describe the pattern of expansion. In the frontal plane, the inferior nasal width increased by 7.1% post-expansion. In the transverse plane, the anterior nasal width increased by 9.4%, and the posterior nasal width increased by 3.4% post-expansion.

Correlation between the changes in variables

Pearson’s correlation coefficients between the pre- and post-expansion difference of the variables presented in Table 3 showed that there were significant positive correlations between measurements (post-pre) of the nasopharynx and oropharynx volumes (r = 0.63 and P = 0.003), nasopharynx volume and frontal nasal inferior width (r = 0.55 and P = 0.01), oropharynx volume and frontal nasal inferior width (r = 0.49 and P = 0.03), frontal nasal inferior width and transverse nasal anterior width (r = 0.63 and P = 0.003), transverse nasal posterior width and frontal nasal inferior width (r = 0.46 and P = 0.04), and transverse posterior nasal width and transverse anterior nasal width (r = 0.77 and P < 0.0001).

Effect of maxillary expansion on airway compartments

Significant orthopedic separation of the mid-palatal sutures was found with maxillary expansion with the MSE appliance.

The present study showed significant differences between the pre-treatment and immediately post-treatment airway volumes for the nasal cavity (12.0% increase). These results agree with the previous studies reported by Smith et al.^[18] and Kavand et al.^[16] Significant increase in nasopharyngeal volume (8.48%) was reported by Yi et al. in a recent study.^[22] No statistically significant changes were observed for the nasopharynx and oropharynx volume following expansion, which agrees with the studies by Zhao et al.,^[19] Smith et al.,^[18] Kavand et al.,^[16] and Pereira-Filho et al.^[20]

Expansion pattern of the nasal cavity

Rapid maxillary expansion with conventional tooth-borne RPE and surgical-assisted RPE has been shown to follow a triangular or “V” pattern of expansion. The expansion pattern with the MARPE appliance varied among the subjects in this study. It was thought that since the jackscrew is closer to the center of resistance of the maxilla, the lateral forces applied with appliance activation promote a greater parallel sutural opening in the coronal view. In addition, if the jackscrew is placed more posteriorly along the palate, the lateral forces are applied closer to the pterygoid plates. Since the pterygoid plates are an area of high resistance to expansion, a posteriorly placed jackscrew can promote a more parallel sutural opening compared to a more anteriorly placed tooth-borne RPE. Thus, the pattern of maxillary sutural separation with MSE appliances can vary among patients [Figure 9].

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Figure 9:

Examples of sutural separation pattern following expansion with the MARPE appliance. (a) “V” separation pattern; (b) parallel separation; (c) reverse “V” separation pattern.

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The effect of RME on the surrounding nasal structure is not as widely studied. Tausche et al.^[21] studied the change in nasal volume following surgical-assisted RPE and concluded that the greatest percent change occurred anteriorly at the level of the nasal floor, which matched the “V”-shaped opening of the mid-palatal suture. It is probable that the pattern of nasal expansion is correlated with the shape of the mid-palatal suture opening as the soft-tissue anatomy is intimately related to the skeletal structure. With a tendency toward a more parallel sutural separation with the miniscrew-assisted expansion jackscrew, this research supported the hypothesis that the separation of the nasal cavity also tended to be more parallel.

In the majority of patients with a successful split of the mid-palatal sutures, the only nasal width measurement that showed no significant increase was the superior measurement of the frontal view. The other three nasal width measurements indicated a significant increase following expansion. These results strongly support that the anatomy of the nasal cavity was influenced by orthopedic changes of the palate. However, the pattern of expansion for the nasal cavity appears to be more triangular than parallel. In the frontal plane, the apex of the triangle is situated between the junction of the frontal and ethmoid bones which showed a 1.3% increase in width. The inferior nasal measurement increase is more than 5 times that of its corresponding measurement at a width increase of 7.1%. Similarly, the transverse plane shows a triangular pattern of expansion. The posterior nasal width increased by 3.4%, while a 9.4% change in anterior nasal width increased almost 3 times the posterior width following expansion. This triangular expansion pattern is likely due to the complex boney articulations surrounding the nasal cavity that prevents a parallel separation, unlike what can be observed at the mid-palatal suture. With greater resistance along the cranial base, the change in skeletal nasal width is restricted more superiorly and posteriorly compared to the inferior and anterior portions of the nasal cavity.

Correlation among the treatment variables

The present study found significant positive correlations between the change in nasopharynx and oropharynx volumes, nasopharynx volume and frontal nasal inferior width, oropharynx volume and frontal nasal inferior width, frontal nasal inferior width and transverse nasal anterior width, transverse nasal posterior width, and frontal nasal inferior width, and transverse posterior nasal width and transverse anterior nasal width. These positive correlations support the hypothesis that the increase in airway volume is a result of the increase in width of the maxillary expansion appliance.

Clinical implications and limitation of the present study

Rapid maxillary expansion may be a useful adjunct for the treatment of OSA in conjunction with comprehensive orthodontic treatment. The use of a MARPE appliance for maxillary expansion can increase the scope of treatment to young adults which is not possible with convention rapid palatal expansion appliance without using miniscrews as additional anchorage. The increased orthopedic effect with the MARPE appliance can be used in pubertal patients to increase nasal cavity and nasopharyngeal volume.

The present study included patients ranging from 8 to 22. According to the study by McMullen et al.,^[23] the treatment of patients with MARPE appliances is effective in growing and non-growing patients. Although greater skeletal and dental changes were observed in growing patients, a similar ratio of skeletal to dental transverse changes was observed in both groups. Due to the small sample size, the airway response of growing and non-growing patients to maxillary expansion was not studied to determine whether airway response will be greater in growing patients.

The present study also did not determine if the increase in airway volume decreases airway resistance. The present study showed an increase in nasopharyngeal airway but limited changes in the oropharynx. The point of greatest restriction (minimum axial area) for an OSA patient can be located anywhere along the upper airway. Expansion therapy is unlikely to alleviate any sleep apnea symptoms if the minimum axial area is in the oropharynx. As expected, the severity of OSAS also plays a factor in the treatment necessary to resolve symptoms. The standard of care for patients with severe OSAS is a double jaw advancement to open the airway and decrease the resistance to airflow. Future studies could investigate the extent that RME has on mild, moderate, and severe OSAS patients, although the likely effect of expansion on the oropharynx will be limited. Another limitation of this study is the lack of data on airflow dynamics. Perhaps, the most useful method to correlate changes in airway volume and airflow dynamics would be to study a group of OSAS patients prospectively and evaluate both airway volume changes with any observed improvements in airflow with 3D-computational fluid dynamics/fluid-structure interaction.