Material and methods

Records from 85 patients who visited a private orthodontic office in Himeji, Japan, to receive orthodontic treatment were screened for this longitudinal retrospective study. Because airway volume is influenced by head posture, craniocervical inclinations of all subjects were examined to ensure that their inclinations were between 90° and 105°. The criteria for selection included (1) Class II skeletal relationship, (2) no previous orthodontic treatment, (3) no craniofacial or growth abnormalities, and (4) no enlarged adenoids or tonsils. Forty-eight patients met these selection criteria.

CBCT data were taken before and after RME treatment (RME group) or at corresponding times but without RME treatment (control group). The RME group consisted of serial CBCT images of 28 subjects (13 boys, 15 girls) with mean ages before and after RME of 9.96 ± 1.21 and 11.23 ± 1.12 years, respectively. They required approximately 5 mm of maxillary expansion as part of their orthodontic treatment. No passive retention appliance was used before full orthodontic treatment. The mean treatment time with the RME appliance was 5.5 ± 1.0 months. The control group consisted of serial CBCT images of 20 subjects (8 boys, 12 girls) with no history of RME appliance treatment. Control CBCT images were taken at age 9.68 ± 1.02 years (corresponding to before RME) and at age 11.13 ± 1.31 years (corresponding to after RME). The control subjects were approximately matched by sex, age, and dentition with the RME subjects.

This study was reviewed and approved by the ethics committee of the Graduate School of Medical and Dental Sciences, Kagoshima University, Kagoshima, Japan.

Each subject was seated in a chair with his or her Frankfort horizontal plane parallel to the floor. Each subject was asked to hold his or her breath after the end of expiration, without swallowing, because the pharyngeal airway caliber when awake is smallest at this time. Breath holding at this moment provides a static pharyngeal airway size that can be recorded consistently in all CBCT scans, thereby reducing variations caused by changes in pharyngeal airway caliber during the respiratory cycle. This position is stable and has high reproducibility for measurement. A CBCT device (CB MercuRay; Hitachi Medical, Tokyo, Japan) was set to maximum 120 kV, maximum 15 mA, and exposure time of 9.6 seconds. Data were sent directly to a personal computer and stored in digital imaging and communications in medicine (DICOM) format.

We made morphologic evaluations of the airways (nasal, intraoral, and pharyngeal) ( Figs 1 and 2 ). Volume rendering software (INTAGE Volume Editor; CYBERNET, Tokyo, Japan) was used to create the 3-dimensional (3D) volume data of the airways. Because the airway is a void surrounded by hard and soft tissues, inversion of the 3D rendered image is required: ie, converting a negative value to a positive value and vice versa. Threshold segmentation was used to select the computed tomography units in the airway. The inverted air space has a significantly greater positive computed tomography unit than do the denser surrounding soft tissues. The distinct high-contrast border produces a clean segmentation of the airway. By modifying the threshold limits, an appropriate range defined the tissues of interest in the volume of interest for a particular scan. By using this concept, a threshold of computed tomography units was selected to isolate all empty spaces in the airway region. Subsequently, by using an appropriate smoothing algorithm with a moving average, the 3D model was converted to a smoothed model without losing the patient-specific character of the airway shape. The rendered volume data was in a 512 × 512 matrix with a voxel size of 0.377 mm.

Evaluation of nasal airway obstruction from 3D nasal airway forms in 3 subjects (top image, superior view; bottom image, lateral view): A, obvious complete obstruction ( red arrow ); B, rhinostenosis, but the presence or absence of complete obstruction cannot be determined ( yellow arrow ); C, no rhinostenosis or obstruction.

Measurement of airway volumes. A, Landmarks and planes for the axial section of the airway: 1 , Palatal plane; 2 , soft palatal plane (parallel to the palatal plane passing through the soft palatal plane); 3 , epiglottis plane (parallel to the palatal plane passing through the base of the epiglottis); 4 , soft palatal plane (inferior-most point on the uvula); 5 , base of the epiglottis. B, Parts of the airway: nasal airway; RAv , Retropalatal airway volume, between the palatal and soft palatal planes; OAv , oropharyngeal airway volume, between the soft palatal and epiglottis planes; IAv , intraoral airway volume, between the palate and the tongue.

The nasal airway (from the external nares to the choanae, including the paranasal sinuses) is shown in Figure 1 . When the continuity of the bilateral nasal meatus was broken, a 3D obstruction was assumed ( Fig 1 , A ).

The intraoral and pharyngeal airways are shown in Figure 2 . Intraoral airway volume between the tongue and palate was measured as an indication of vertical tongue position. Pharyngeal airway volumes were also measured.

The cross-sectional planes ( Fig 2 ) included (1) the palatal plane, a plane parallel to the hard palate passing through the posterior nasal spine; (2) the soft palatal plane, a plane parallel to the palatal plane passing through the inferior-most point on the uvula; and (3) the base of the epiglottis plane, a plane parallel to the palatal plane passing through the base of the epiglottis.

The following pharyngeal airway volumes ( Fig 2 ) were measured: (1) total pharyngeal airway volume, the airway between the palatal plane and the epiglottis plane; (2) retropalatal airway volume, the airway between the palatal plane and the soft palatal plane; and (3) oropharyngeal airway volume, the airway between the soft palatal plane and the epiglottis plane.

We then evaluated nasal airway ventilation conditions. Computed fluid dynamics were used to determine the presence of any functional obstruction of the nasal airway ( Fig 3 ). This method has been shown to provide a more accurate estimate of any obstruction than CBCT images alone. The constructed 3D images for the nasal airway were exported to fluid-dynamic software (PHOENICS; CHAM-Japan, Tokyo, Japan) in stereolithographic format. This software can simulate and evaluate various kinds of computed fluid dynamics under a set of given conditions. The simulation estimated airflow pressure and velocity.

Steps in the evaluation of nasal airway ventilation by computed fluid dynamics: A, extraction of the nasal airway data; B, volume rendering and smoothing; C, construction of the stereolithographic model and numeric simulation; D, evaluation of the nasal airway ventilation condition.

In our simulation, air flowed from the choana horizontally, and air was exhaled through both nostrils. The flow was assumed to be a newtonian, homogeneous, and incompressible fluid. Elliptic-staggered equations and the continuity equation were used in the study. The computed fluid dynamics of the nasal airway were used under the following conditions with PHOENICS: (1) the volume of airflow with a velocity of 200 m per second, which is the rate of respiration of a subject of this age at rest ; (2) the wall surface was nonslip; and (3) the simulation was repeated 1000 times to calculate the mean values. Convergence was judged by monitoring the magnitude of the absolute residual sources of mass and momentum, normalized by the respective inlet fluxes. The iteration was continued until all residuals fell below 0.2%.

When the 3D CBCT reconstructions indicated nasal airway obstruction, computed fluid dynamics was not used. When computed fluid dynamics indicated a maximal pressure of more than 100 Pa (with an inflow rate of 200 mL/sec) and a maximum velocity of more than 10 m per second, an obstruction was assumed.

In 1 analysis, the RME subjects were divided into 2 groups by their nasal airway condition before and after RME: (1) the obstruction group included patients in whom a nasal obstruction was detected with the 3D images or the computed fluid dynamics evaluation, and (2) the nonobstruction group included patients in whom no nasal obstruction was found with either method ( Fig 4 ).

Classification of nasal airway obstruction by using 3D models and computed fluid dynamics ( CFD ) (3D obstruction, red arrow ; CFD obstruction; yellow arrow ).

In a separate analysis, the RME subjects were classified into 3 groups by the changing pattern of their nasal airway obstruction after RME: (1) the nonimprovement group, with nasal airway obstructions both before and after RME; (2) the improvement group, with nasal airway obstruction before RME but not after RME; and (3) the ventilation group, with no nasal airway obstruction before or after RME.

Results

After RME, the intraoral airway volume decreased significantly in the RME group ( Table I ), whereas total pharyngeal airway volume, retropalatal airway volume, and oropharyngeal airway volume all increased significantly in the RME group. In the control group, total pharyngeal and oropharyngeal airway volumes both increased significantly from before to after RME. However, intraoral and retropalatal airway volumes did not change significantly. The intraoral airway volume of the RME group (1212.9 mm ³ ) was significantly greater than that of the control group (415.1 mm ³ ) before RME. The intraoral airway volume treatment change in the RME group (−933.3 mm ³ ) was significantly less than that of the control group (155.1 mm ³ ). As a result, there was no significant difference for intraoral airway volume between the 2 groups after RME.

The total pharyngeal airway volume treatment change in the RME group (3015.4 mm ³ ) was significantly greater than that of the control group (1226.3 mm ³ ), and the retropalatal airway volume treatment change of the RME group (1413.9 mm ³ ) was significantly greater than that of the control group (327.8 mm ³ ). However, the oropharyngeal airway volume treatment change of the RME group (1601.4 mm ³ ) did not differ significantly from that of the control group (898.5 mm ³ ).

Among the patients without a morphologic obstruction, the pressure after RME (89.08 ± 72.27 Pa; n = 27) was significantly lower than before RME (120.91 ± 84.18 Pa; n = 22) ( Table II ). Similarly, the velocity after RME (10.72 ± 6.38 m/sec; n = 27) was significantly lower than before RME (12.27 ± 5.99 m/sec; n = 22). There were no significant maximum pressure or velocity changes in the control group at the 2 measurement times. The maximum pressure of the RME group was significantly greater than that of the control group both before and after RME, and the treatment change was also significantly larger. The maximum velocity of the RME group was significantly greater than that of the control group before RME.

Table III shows the correlations between nasal airway ventilation conditions and intraoral airway volume. Before and after RME, maximum pressure and velocity were significantly correlated with intraoral airway volume. The treatment changes in maximum pressure and velocity were significantly correlated with intraoral airway volume. However, at each interval and treatment change, maximum pressure and velocity were not significantly correlated with any other pharyngeal airway volume (total, retropalatal, or oropharyngeal). At each interval and treatment change, intraoral airway volume was not significantly correlated with any other pharyngeal airway volume.

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