Oropharyngeal airway changes after rapid palatal expansion evaluated with cone-beam computed tomography

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1 Original article Oropharyngeal airway changes after rapid palatal expansion evaluated with cone-beam computed tomography Ying Zhao, a Manuel Nguyen, b Elizabeth Gohl, c James K. Mah, d Glenn Sameshima, e and Reyes Enciso f Los Angeles, Calif Introduction: The aims of this retrospective study were to use cone-beam computed tomography (CBCT) to assess changes in the volume of the oropharynx in growing patients with maxillary constriction treated by rapid palatal expansion (RPE) and to compare them with changes in age- and sex-matched orthodontic patients. Methods: The experimental group consisted of 24 patients (mean age, 12.8 ± 1.88 years) with maxillary constriction who were treated with hyrax palatal expanders; the control group comprised 24 age- and sex-matched patients (mean age, 12.8 ± 1.85 years) who were just starting regular orthodontic treatment. Beginning and progress CBCT scans, taken in the supine position, were analyzed with software to measure volume, length, and minimal cross-sectional area of the oropharyngeal airway. The 2 groups were compared with paired t tests. Results: Only retropalatal airway volume was found to be significantly different between groups before treatment (P = 0.011), and this difference remained after treatment (P = 0.024). No other statistically significant differences were found relative to changes in volume, length, or minimum cross-sectional area of the oropharyngeal airway between the groups, but the molar-to-molar width after RPE increased significantly compared with the controls (P <0.001). Conclusions: Narrow oropharyngeal airways in growing patients with maxillary constriction was demonstrated. But there was no evidence to support the hypothesis that RPE could enlarge oropharyngeal airway volume. (Am J Orthod Dentofacial Orthop 2010;137:S71-8) Transverse maxillary deficiency is associated with many problems that include esthetic issues, dental crossbites, occlusal disharmony, and other functional problems. One functional problem is a narrow oropharyngeal airway resulting from the retroposition of the tongue. Prior studies suggest that maxillary constriction can also play a role in the pathophysiology of obstructive sleep apnea (OSA). 1-3 Johal and Conaghan 3 concluded that maxillary morphologic differences exist between OSA and control subjects, supporting their role as an etiologic factor. Rapid palatal From the School of Dentistry, University of Southern California, Los Angeles. a Visiting scholar, Department of Orthodontics; associate professor, Capital College of Medical Sciences XuanWu Hospital, Beijing, China. b Lecturer, Division of Endodontics, Oral Surgery and Orthodontics. c Resident, Department of Orthodontics. d Clinical associate professor, Department of Orthodontics. e Associate professor and director, Department of Orthodontics. f Clinical assistant professor, Division Endodontics, Oral Surgery and Orthodontics. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Reyes Enciso, University of Southern California, School of Dentistry, 925 W 34th St, DEN312, Los Angeles, CA ; , renciso@usc.edu. Supported by State Scholarship Fund from China Scholar Council ( ) and NIDCR grant (5 K25 DE016391). Submitted, August 2008; revised and accepted, November /$36.00 Copyright 2010 by the American Association of Orthodontists. doi: /j.ajodo expansion (RPE) as a treatment modality for OSA has been proposed based on the hypothesis that airway volume increases after maxillary expansion secondary to the tongue s repositioning more anteriorly in the oral cavity. 4,5 RPE was first described in the 1860s by Emerson Angell, who used a jackscrew with contrarotating heads. 6 Since then, RPE has become a common treatment for transverse maxillary deficiency, and its effects have been well described in the literature. Studies have documented the amount of maxillary width increase, 7,8 nasal resistance reduction, 4,9,10 and intranasal capacity increase with RPE treatment. In this regard, Pirelli et al 15 recruited 31 children with OSA, followed them for up to 4 months after RPE treatment, and found that all had a decreased apnea-hypoapnea index with a mean maxilla cross-sectional expansion of 4.32 ± 0.7 mm. The specific changes in oropharyngeal airway volume and length are not as clear. Two-dimensional imaging techniques (lateral and anteroposterior cephalometric images) and 3-dimensional (3D) imaging techniques (magnetic resonance imaging, computed tomography [CT], and cone-beam CT [CBCT]) have been used to evaluate changes in airway volume Techniques that have been used to compare the change of airway space after RPE treatment include cephalometric analysis and S71

2 S72 Zhao et al American Journal of Orthodontics and Dentofacial Orthopedics April 2010 acoustic rhinometric measurement, which is used to evaluate the change of nasal airway volume and nasal airway resistance However, acoustic rhinometric measurements cannot correctly detect constrictions and expansions less than 3 to 4 mm. 20 Lateral and anteroposterior cephalometric evaluations have been used to compare the dimensional changes in the maxilla and upper airway, but the complex shape change of the airway after RPE treatment is not represented well with 2-dimensional images. 21,22 Compared with magnetic resonance imaging and traditional CT techniques, CBCT is a better method for airway volume measurement because of its lower cost, easier access, availability to dentists, and much lower overall effective absorbed dose of radiation than CT. 16 The CBCT (NewTom 3G, QA S.R.L., Verona, Italy) imaging system used in this study has an overall effective absorbed dose of radiation of 57 μsv and scanning time of 36 seconds. 23 This compares favorably with traditional dental images such as panoramic radiographs with radiation doses of 6 μsv and a full-mouth series with a range of 33 to 84 μsv 24 to 14 to 100 μsv, 25 depending on variables such as film speed, technique, kilovolts (peak), and collimation. Therefore, the purpose of this study was to evaluate the changes of oropharyngeal airway volume after RPE treatment in adolescent patients compared with age- and sex-matched controls by using 3D images obtained by CBCT. The null hypothesis was that there is no difference in the mean changes in volumetric oropharyngeal airway (retropalatal or retroglossal) between pairs of patients with RPE and matched controls treated with ortho dontics only. We also evaluated differences in changes in the minimum cross-sectional airway, orpharyngeal airway length (retropalatal and retroglossal), and molar-to-molar width between the matched pairs. Material and methods In this retrospective study, 24 healthy patients (mean age, 12.8 ± 1.88 years; range, years; 6 boys, 18 girls) who fulfilled the following inclusion criteria were selected: (1) required RPE treatment because of a bilateral or unilateral crossbite, (2) required orthodontic treatment, and (3) had beginning and progress treatment CBCT scans. All patients who fulfilled these criteria were included in the study. Twenty-four controls who had regular orthodontic treatment only (no RPE) and had CBCT beginning and progress scans were selected to match the RPE patients in age (age difference between controls and subjects, 3.0 ± 2.54 months) and sex. The control group s mean age was 12.8 ± 1.85 years (range, years; 6 boys, 18 girls). All patients were in the late transitional or early permanent dentition stage. No patients were Fig 1. Midsagittal orientation (in the axial plane) defined by the anterior midpoint between the 2 maxillary central incisors and the posterior midpoint of the spine. planned for extraction or orthognathic treatment. This study was approved by the institutional review board of the University of Southern California. A review of the treatment records showed that the patients in the RPE group had a hyrax-type palatal expander banded on the maxillary first premolars and first molars. The patients were monitored weekly for appropriate activation of the appliance. Expanders were turned 1 or 2 times per day until the required expansion was achieved ie, slight overcorrection of the crossbite (average time, 4-6 weeks) and then were stabilized. A rapid palatal expander or a transpalatal arch was used for retention for at least 3 months postexpansion. Most patients with RPE had no orthodontic treatment until after the fixed retention period, but some patients had appliances placed (eg, bonded brackets on maxillary incisors) during the fixed retention period. The control patients started orthodontic treatment within 6 months of the experimental group. CBCT scans were taken of all patients as part of both initial orthodontic treatment records and progress records (midpoint of the total treatment time, from 8 months to 2 years; average, 15 months). The patients were scanned in supine position with the Frankfort plane perpendicular to the floor in centric occlusion. The digital imaging and communications in medicine (DICOM) data were imported into the imaging software Vwork (version 5.0, Cybermed USA, Torrance, Calif) and used for the measurements described below. All measurements were made by a blinded operator (Y.Z.). Standardization of head position between scans was achieved by measuring the angle between the line from

3 American Journal of Orthodontics and Dentofacial Orthopedics Zhao et al S73 Volume 137, Number 4, Supplement 1 A B C Fig 2. A, Midsagittal plane showing oropharyngeal airway volume. The superior border was defined as the horizontal line through the posterior nasal spine (PNS). The inferior border was defined as the horizontal line through the superior point of epiglottis. The oropharyngeal airway volume was divided into retropalatal and retroglossal airway volumes by the horizontal plane through the inferior point of the uvula. B, Retropalatal airway volume. C, Retroglossal airway volume. The number at the top right on every midsagittal image is the airway volume (mm 3 ). TE, Top of the epiglottis; U, tip of the uvula. anterior nasal spine to posterior nasal spine, and the vertical line predefined by the software on the beginning scan and matching this to the progress scan (by reslicing the volume according to the beginning head angulation). To define the oropharynx volume area of interest, an approximate square-shaped prism was defined first to outline the general area, and then, based on that outline, an anatomically well-defined hand-traced volume was created. To orient the prism angulation, the patient s midsagittal plane was defined as the anterior midpoint of the 2 maxillary incisors and the posterior midpoint of the spine (Fig 1). The total oropharyngeal airway volume was defined as the airway volume between the 2 planes as follows: the superior plane was defined on the midsagittal image as the horizontal line through the posterior nasal spine, and the inferior plane was defined as the horizontal line through the superior point of the epiglottis (Fig 2, A). 26 To define

4 S74 Zhao et al American Journal of Orthodontics and Dentofacial Orthopedics April 2010 A B Fig 3. A, Axial image showing the cross-sectional area of the airway volume. The number at the bottom right is the area (mm 2 ) of the airway cross-sectional area. B, The location of the minimum cross-sectional area shown by the 3D cursor in the 3D airway image. the anteroposterior and lateral borders of the volume area of interest, the first step was to define and select a square area containing the entire airway on an axial view. All axial views were checked to ensure that the airway was included in the selected area. Subsequently, the upper and lower borders in the airway volume area of interest were determined, and the specific borders of the segmented airway were defined manually by tracing the soft tissue-air interface at each 1-mm axial slice with the segmentation tool. Once segmentation was performed, the software automatically computed the volume of the oropharyngeal region in cubic millimeters (DICOM header contains the size of the voxel in millimeters plus the thickness of the slices). Last, the oropharyngeal airway volume was divided into retropalatal and retroglossal airway volumes by creating a horizontal plane through the inferior point of the uvula (Fig 2, B and C). The cross-sectional area in square millimeters is automatically displayed on the axial image by the software (Fig 3, A). The user can scroll through all cross-sectional images and determine the minimum cross-sectional area. Figure 3, B, shows the 3D model of the oropharyngeal airway with the location of the minimum cross-sectional area displayed by the location of the 3D cursor. The oropharyngeal airway length was measured at the midsagittal section of the airway volume by using the linear-measurement tool (Fig 4). The molar-to-molar width was measured as the distance between the lingual alveolar crests at the level of the first molars for all patients (experimental and controls), similar to the protocol described by Garib et al 27 (Fig 5). By using hard-tissue landmarks rather than softtissue landmarks, the possibility of gingival thinning or recession affecting the data was eliminated. Statistical analysis Baseline and after-treatment characteristics between the RPE and control matched pairs were compared by using the Student paired t test for continuous matched pairs of normal data and the Wilcoxon signed rank test for nonparametric variables (Tables I and II). Normality was assessed with the Kolmogorov-Smirnov test. To improve accuracy, individual volumes, areas, and lengths were measured twice, and the means were used for analysis. The mean intraclass correlation co efficients were for the controls and for the subjects. Absolute changes in the anatomic variables were defined as the difference between the measurements in the beginning and progress scans. Percentage changes in anatomic variables were computed by subtracting the beginning scan measurement from the progress scan measurement, dividing by the beginning scan, and multiplying by 100. To investigate statistical differences between the matched pairs of RPE patients and controls, paired t tests for continuous normal matched variables and the Wilcoxon signed rank test for nonparametric variables were used for statistical analysis with SAS software (version 9.1, SAS Institute, Cary, NC), with a significance level of 0.05.

5 American Journal of Orthodontics and Dentofacial Orthopedics Zhao et al S75 Volume 137, Number 4, Supplement 1 Fig 4. Midsagittal section of the volume showing oropharyngeal, retropalatal, and retroglossal airway lengths. Fig 5. Coronal section at the level of the first molars showing the molar-to-molar width measurement. Table I. Comparison between RPE and control groups at baseline and end of treatment RPE group, Parameter mean ± SD* Control group, mean ± SD Paired t test, P value Age (y) 12.8 ± ± 1.85 Sex 6 boys, 18 girls 6 boys, 18 girls Before treatment Oropharyngeal airway volume (cm 3 ) 7.6 ± ± Retropalatal airway volume (cm 3 ) 4.0 ± ± Retroglossal airway volume (cm 3 ) 3.6 ± ± Oropharyngeal airway length (mm) 40.8 ± ± Retropalatal airway length (mm) 26.1 ± ± Retroglossal airway length (mm) 14.7 ± ± Minimum cross-sectional area (mm 2 ) ± ± Molar-to-molar width (mm) 33.5 ± ± After treatment Oropharyngeal airway volume (cm 3 ) 7.7 ± ± Retropalatal airway volume (cm 3 ) 4.1 ± ± Retroglossal airway volume (cm 3 ) 3.6 ± ± Oropharyngeal airway length (mm) 41.9 ± ± Retropalatal airway length (mm) 27.0 ± ± Retroglossal airway length (mm) 14.9 ± ± Minimum cross-sectional area (mm 2 ) 99.7 ± ± Molar-to-molar width (mm) 36.8 ± ± *Data presented as numbers for sex and mean ± standard deviation for the continuous variables. P values obtained from paired t tests for the continuous normal matched variables. Results The RPE patients and the controls had been individually matched by age and sex (Table I). However, they differed at baseline in retropalatal airway volume; the controls had significantly larger retropalatal airway volumes (P = 0.011). This statistically significant difference remained after RPE treatment (P = 0.024). After treatment, no significant differences in absolute and percentage changes of total oropharyngeal airway volume, retropalatal airway volume, or retroglossal airway volume between the RPE group and the controls were found (Table II). There were also no statistical significant differences in the absolute and percentage changes of length or minimum cross-sectional area of the oropharyngeal airway between the 2 groups, but there was a significant increase of molar-to-molar width in the RPE group compared with the controls after treatment (Table II). The absolute mean molar-to-molar width increases

6 S76 Zhao et al American Journal of Orthodontics and Dentofacial Orthopedics April 2010 Table II. Airway changes in the RPE and control groups in absolute and percentage values Parameter RPE group, mean ± SD Control group, mean ± SD Paired t test, P value* Percentage changes Oropharyngeal airway volume (%) 4.6 ± ± Retropalatal airway volume (%) 7.3 ± ± Retroglossal airway volume (%) 16.0 ± (median, 10.31) 3.1 ± (median, 8.81) Oropharyngeal airway length (%) 3.1 ± ± Retropalatal airway length (%) 3.6 ± ± Retroglossal airway length (%) 11.3 ± ± Minimum cross-sectional area (%) 23.9 ± ± Molar-to-molar width (%) 10.7 ± (median, 8.2) 1.8 ± 3.74 (median, 1.04) < Absolute changes Oropharyngeal airway volume (cm 3 ) 0.1 ± ± Retropalatal airway volume (cm 3 ) 0.1 ± ± Retroglossal airway volume (cm 3 ) 0.0 ± ± Oropharyngeal airway length (mm) 1.1 ± ± Retropalatal airway length (mm) 0.9 ± ± Retroglossal airway length (mm) 0.2 ± ± Minimum cross-sectional area (mm 2 ) 7.0 ± ± Molar-to-molar width (mm) 3.3 ± 3.10 (median, 2.78) 0.6 ± 1.29 (median, 0.38) < *P values obtained from paired t tests for the normal matched continuous variables. P values obtained from Wilcoxon signed rank test for nonparametric matched variables. were 3.3 ± 3.10 mm (median, 2.78 mm) in the RPE group and 0.6 ± 1.29 mm (median, 0.38 mm) in the controls (P <0.001), and the mean percentage increases of molar-to-molar width were 10.7% ± 10.96% (median, 8.2%) in the RPE group and 1.8% ± 3.74% (median, 1.04%) in the controls (P <0.001). Discussion Cranofacial abnormalities have been recognized as part of the pathophysiology of OSA, and it is thought that these abnormalities predispose to OSA through adverse effects on upper airway dimensions. The more commonly identified abnormalities include mandibular deficiency, inferiorly placed hyoid bone relative to the mandibular plane, narrowed posterior air space, greater flexion of the cranial base, and elongation of the soft palate. 17,28 There is a concern that maxillary constriction might also play a role in the pathophysiology of OSA because maxillary constriction is associated with low tongue posture that could result in oropharynx airway narrowing, which is a risk factor for OSA. 29 The oropharynx airway can be subdivided into the retropalatal (from the level of the hard palate to the caudal margin of the soft palate) and retroglossal (from the caudal margin of the soft palate to the base of the epiglottis) regions. 30 Most traditional CT studies that have evaluated airway caliber in patients with sleep apnea during wakefulness have found narrowing in the retropalatal region. 16,31,32 Using CBCT images taken in the supine awake position, we found that subjects with maxillary constriction (RPE patients) appear to have a narrower oropharyngeal airway relative to the control group (P = 0.001). RPE is a traditional orthodontic treatment for maxillary constriction. It has been proposed as a treatment modality for OSA 4,5 and has been shown to increase the width of the maxilla and reduce nasal resistance and the apnea-hypoapnea index of OSA patients. 33 Researchers evaluated the nasal airway by acoustic rhinometry and rhinomanometry and showed that RPE is effective for widening the nasal cavities 34 and increasing internasal volume. 35 However, Malkoc et al 36 evaluated the pharyngeal airway dimensions using lateral and posteroanterior cephalometric radiographs and reported that mandibular symphyseal distraction osteogenesis alone or followed by RPE does not clinically significantly affect pharyngeal airway dimensions in adults; our results are consistent with those. They found statistically significant but clinically small changes in oropharyngeal width (+1.0 mm), tongue length ( 2.2 mm), and vertical airway length ( 2.3 mm). 36 They concluded that changes from mandibular symphyseal distraction osteogenesis or RPE might be counteracted by reflex mechanisms that act to preserve airway patency. We compared the absolute and percentage changes in the retropalatal and retroglossal airways after treatment and found no significant difference between the RPE and the control matched pairs, although maxillary width increased significantly in the RPE group.

7 American Journal of Orthodontics and Dentofacial Orthopedics Zhao et al S77 Volume 137, Number 4, Supplement 1 The protocol for expansion was to turn 1 or 2 times per day until the required amount for expansion was achieved (slight overcorrection of the crossbite). However, we could not ascertain whether the patients and their families complied strictly with this activation regimen. Since this was a retrospective study, another possible limitation was the clinical criteria used for RPE. Slight overcorrection of molar crossbite was used as the indicator for adequate palatal expansion. The average absolute mean molar-to-molar width change was 3.3 ± 3.10 mm (median, 2.78 mm). Because the controls were exactly matched by age and sex to the subjects, the possible confounding effect of growth was minimized. One limitation of this retrospective study was the absence of control over tongue position when the CBCT images were taken. The positions of the tongue and soft tissues are important anatomic factors that affect the shape and size of the oropharynx airway volume. 37 Because of exact matching by age and sex, and the standardized imaging protocol (scans taken during the same period of time by the same operator with the same instructions to the patients), possible confounding effects of tongue-position changes (changes in tongue length and height) should be minimized. Further studies should be performed to assess those changes. From our result, we found that RPE treatment for orthodontic purposes is not an effective method to increase the oropharynx airway volume of patients with maxillary constriction; this result agrees with that of Malkoc et al. 36 Whether sleep apnea patients treated with RPE combined with mandibular advancement could have increases of oropharyngeal volume was beyond the scope of this project. Conclusions 1. the oropharyngeal airway volume (retropalatal airway volume) in growing subjects with maxillary constriction (unilateral or bilateral posterior crossbite) is significantly smaller than in subjects without constriction. 2. there was no evidence to support the hypothesis that RPE treatment will enlarge the volume of the oropharyngeal airway despite the increased intermolar width after RPE treatment. References 1. Seto BH, Gotsopoulos H, Sims MR, Cistulli PA. Maxillary morphology in sleep apnoea syndrome. Eur J Orthod 2001;23: Kushida CA, Efron B, Guilleminault C. A predictive morphometric model for the obstructive sleep apnea syndrome. Ann Intern Med 1997;127: Johal A, Conaghan C. Maxillary morphology in obstructive sleep apnoea: a cephalometric and model study. Angle Orthod 2004;74: Schmidt-Nowara W, Lowe AA, Wiegand L, Cartwright R, Perez-Guerra F, Menns S. Oral appliances for the treatment of snoring and obstructive sleep: a review. Sleep 1995;18: Timms DJ. 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