Introduction
The objectives of this study were to assess the changes in right vs left nasal cavity volumes and minimum cross-sectional width, nasopharyngeal, and oropharyngeal volumes of the upper airway in response to rapid maxillary expansion (RME).
Methods
Pretreatment and posttreatment cone-beam computed tomography scans of 28 patients with a mean age of 9.86 ± 2.43 years and 20 age- and sex-matched controls were digitized and linear, angular, and volumetric measurements were obtained.
Results
Nasopharyngeal volume, right, and left nasal cavity volumes, and minimum cross-sectional widths increased significantly 2 years post RME ( P <0.05). These measurements did not show any significant increase in the control group ( P >0.05), whereas the oropharyngeal volume increase for both groups was comparable ( P = 0.92). In the experimental group, the right and left nasal cavity volumes were not significantly different at baseline or posttreatment. However, the change that occurred was significantly larger for the left nasal cavity. This change for the control group was more significant for the right nasal cavity. Maxillary right and left molar inclinations were positively correlated to the nasal cavity volume, showing that the more buccally inclined the maxillary molars were, the smaller the nasal cavity volume.
Conclusions
Nasopharyngeal and right and left nasal cavity volumes and minimum cross-sectional widths increase significantly after RME in young children. Expansion decreases the degree of difference in volume between the right and left nasal cavities. The buccal inclination of maxillary molars is correlated with nasal cavity volume.
Highlights
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Maxillary expansion reduces the volume difference between the nasal cavities long term.
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Two years posttreatment, significant increases in nasal volume were found.
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Minimum cross-sectional width and nasopharyngeal volume also increased.
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The increase in oropharyngeal volume did not differ significantly from the control group.
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Increased inclination of maxillary molars was correlated with smaller nasal cavity volume.
Rapid maxillary expansion (RME) is the treatment of choice for constricted maxillary arches in growing patients. It is routinely used to correct unilateral and bilateral crossbites, alleviate tooth size-arch length discrepancies and dental impactions. To resolve the maxillary transverse deficiency, a tooth-borne expander transmits force to the maxilla and its neighboring bones, causing the separation of several cranial and circummaxillary sutures. Resistance to the separation of maxillary halves lies mainly in the zygomatic and sphenoid bones leading to a pyramidal pattern of opening with outward tilting of maxillary halves and causing greater opening at the dentoalveolar level inferiorly and anteriorly. , , Considering this pattern of expansion and the anatomic proximity of the maxilla and nasal cavity, it can be anticipated that maxillary expansion moves the external walls of the nasal cavity laterally, increasing nasal width, volume, and the cross-sectional area mostly at the levels of inferior turbinates. , Several studies have shown that the increase in these nasal parameters through maxillary expansion reduces nasal airway resistance and improves nasal respiration.
A number of anatomic and nonanatomic features have been recognized as contributing factors to the narrowing of the airway that may predispose the individual to sleep-related breathing disorders (SRBDs). , Obesity and craniofacial abnormalities, including retrognathic mandible, small and constricted maxilla, and low position of the hyoid bone, have been indicated as contributing anatomic features. Maxillary deficiency has been shown to affect nasal resistance, tongue posture, and upper airway dimensions. , , , Although a causal relationship between dentofacial form and nasal breathing cannot be confirmed, nasopharyngeal obstruction has been associated with deficient facial growth contributing to increased facial height, narrow maxillary arch, steep mandibular plane angle, and increased craniocervical angulation. , Therefore, addressing these craniofacial abnormalities at a younger age while maxillary suture opening is more feasible can lead to a more pronounced and stable gain in nasal width and minimum cross-sectional area, a major contributor to nasal airway resistance, and may help improve breathing and possibly prevent the development of SRBDs in the future. , , As recommended by the American Association of Orthodontists’ recent white paper on obstructive sleep apnea and orthodontics, the primary objective of RME is to normalize maxillary transverse deficiency and improve occlusion, whereas a secondary positive impact of increasing upper airway volume and reducing nasal resistance may make it a plausible treatment modality in children with SRBDs. ,
Bilateral structures have been shown to grow to different degrees. Such laterality is seen throughout the body, and craniofacial structures are no exception when it can be presented as normal asymmetry, dental and skeletal midline deviations, and other right and left size differences. Genetic and environmental etiologic factors have been suggested for this phenomenon. To the best of our knowledge, there was only one study that evaluated the effect of RME on the right vs left nasal cavities using conventional tomography. Several other studies have reported on the short-term effects of RME on nasal cavity , and the nasopharyngeal and oropharyngeal , , airway volumes, and the majority of these studies evaluated the adolescent population.
The objectives of this study were to (1) evaluate the effect of RME on the right and left nasal cavities in terms of volumetric and minimum cross-sectional width changes, (2) evaluate the impact of RME on nasal cavity and nasopharyngeal and oropharyngeal airway volumes in young children over the long term in comparison with a control group, and (3) evaluate the relationship between maxillary molar divergence and nasal cavity volume.
Material and methods
This retrospective study was approved by the Boston University Institutional Review Board (no. H-34714). A deidentified cone-beam computed tomography (CBCT) repository and coded medical and dental records (no. H-32515) were screened on the basis of the following inclusion criteria: (1) diagnostic preorthodontic and postorthodontic treatment CBCT images (2) nonsyndromic patients, (3) no history of adenotonsillectomy, (4) nonsurgical expansion, and (5) successful skeletal maxillary expansion as verified by measuring the distance between right and left greater palatine foramina on CBCT images. All subjects completed treatment involving RME using a banded hyrax expander cemented to the maxillary first molars. The activation protocol followed was 1 turn per day (0.25 mm/turn) until overcorrection was achieved. The expander was maintained for 3 months postexpansion, and subsequent orthodontic treatment was carried out with edgewise appliances. The same repository was screened for selection of the control group, which consisted of CBCT scans of age- and sex-matched patients who did not present with maxillary deficiency and were not deemed to benefit from orthopedic expansion, taken before the start of orthodontic treatment and after appliance removal. The demographic information for the experimental and control groups can be found in Table I .
| Demographics | Experimental group | Control group | P |
|---|---|---|---|
| Gender, % | 0.77 ∗ | ||
| Male | 39.29 | 45.00 | |
| Female | 60.71 | 55.00 | |
| Age, mean (SD) | 9.86 (2.43) | 10.41 (1.60) | 0.1 ∗∗ |
The sample included 28 subjects (11 males, 17 females) with a mean age of 9.86 ± 2.43 years at the time of the initial scan. The control group consisted of 20 subjects (9 male, 11 female) with a mean age of 10.41 ± 1.60 years at the time of the initial scan. All CBCT scans were taken using the same iCAT machine (Imaging Sciences International, Hatfield, Pa) operated at 120 kVp, 5 mA, and 0.5 mm nominal focal spot size, rendering a 17.0 cm × 23 cm field of view with a 0.3 mm voxel size image. Patients were seated in a chair and were instructed to hold their heads in natural head position and avoid swallowing. The acquired scans were then exported as Digital Imaging and Communications in Medicine files and were processed and segmented by 2 of the authors (C.D. and C.S.) using Mimics software (version 20; Materialise, Leuven, Belgium). First, predetermined and/or custom threshold limits were selected for soft and hard tissue masks to be created. Empty spaces were delineated as the airway mask, and the connection with the outer air was eliminated.
A 3-dimensional (3D) rendering of the airway was then created by the software. Soft and hard tissue landmarks ( Table II ) were digitized on their corresponding masks, and their appropriate locations were verified on axial, sagittal, and coronal slices. These landmarks were used to construct reference planes and dissector planes as described in Table III . The dissector planes were used to segment the upper airway into the nasal cavity, nasopharynx, and oropharynx. Right and left nasal cavity separation was evident through anatomic hard tissue between the 2 compartments, as shown in the Figure . The volume of each segment was then calculated. By screening every slice on the coronal view, the narrowest portion of the left and right nasal airway was determined. To verify this narrowest portion, 5 ventral and 5 dorsal slices were remeasured and the slice with the narrowest width was selected. The number of slices from point pronasale to this narrowest width was recorded on the pretreatment scans to be consistent in measuring the same position on the posttreatment scans. Maxillary molar inclinations on initial scans were measured after previously established methodology by Miner et al, measuring the angle between the long axis lines of the maxillary right and maxillary left first molars and the functional occlusal plane.
| Point | Description |
|---|---|
| Anterior nasal spine | Most anterior and midline point of anterior nasal spine |
| Posterior nasal spine | Most posterior and midline portion of palate |
| Ala right | Most outside portion of soft tissue ala |
| Ala left | Most outside portion of soft tissue ala |
| C3 | Most anterior inferior and medial portion of C3 vertebrae |
| Greater palatine foramen right | Most anterior and inferior portion of right greater palatine foramen |
| Greater palatine foramen left | Most anterior and inferior portion of left greater palatine foramen |
| Infraorbital foramen right | Inferior and mid infraorbital foramen |
| Infraorbital foramen left | Inferior and mid infraorbital foramen |
| Midnasal bone | Midway between nasion and nasal tip using “measure over surface” function in Mimics |
| Nasion | Intersection of nasal and frontal suture at its midpoint |
| Aperture piriformis right | Widest portion of aperture piriformis right |
| Aperture piriformis left | Widest portion of aperture piriformis left |
| Pronasale | Middle most tip of soft tissue of nose |
| Nasal tip | Tip of nasal bone |
| Zygomaticotemporal suture superior right | Most superior portion of suture |
| Zygomaticotemporal suture superior left | Most superior portion of suture |
| Zygomaticotemporal suture inferior right | Most inferior portion of suture |
| Planes | Description |
|---|---|
| Reference planes | |
| Frankfort Derivative plane (FD) | A plane passing through infraorbital foramen left and right and most inferior point on the right zygomaticotemporal suture |
| Vertical nasal plane | A plane passing through nasion and the right and left piriform apertures |
| Dissector planes | |
| Superior border | A plane through midnasal point and most superior point on right and left zygomaticotemporal sutures |
| PNS plane (inferior border) | A plane through PNS point parallel to FD |
| PNS vertical plane | A plane passing through PNS point parallel to vertical nasal plane |
| Pronasale plane | A plane passing through pronasale and right and left ala |
| C3 | A plane passing through C3 point parallel to FD |
To assess intrarater and interrater reliability, a random sample (10% of the overall sample) was remeasured by the same 2 operators (C.D. and C.S.) approximately 4 weeks after initial measurements were made. For all measurements, the intraclass correlation coefficient values were >0.80 (ie, indicating good reliability), and using paired t test, no measurement was found to be significantly different at the P <0.05 level.
Paired t tests were used to compare the initial and postexpansion volumetric and minimum cross-sectional width changes after RME procedure and the initial left and right molar angulation differences before RME treatment. Student t tests were also used to analyze changes in volume and minimum cross-sectional width between the experimental and control groups. Pearson correlation test was utilized to test the relationship between initial molar angulation and initial nasal cavity volume. All statistical analysis was completed using SAS software (version 9.4; SAS, Cary, NC). Statistical significance was set at the 0.05 level.
Results
All subjects and controls were growing patients who had received orthodontic treatment with (experimental group) or without (control group) RME. The average time interval between initial and final scans for the control and experimental groups were 20.6 ± 2.14 months and 24.4 ± 10.79 months, respectively. The difference is 3.8 ± 2.1 months which was not significant ( P = 0.13). In addition, there was no statistically significant difference between the 2 groups with respect to age and sex distribution, as shown in Table I .
The successful skeletal expansion was verified by the increase in linear distances between most anterior inferior points on the right and left greater palatine foramina of 2.41 ± 1.03 mm ( P <0.01) as measured on the CBCT scans. Volumetric analysis of right and left nasal cavity, total nasal cavity, nasopharyngeal and oropharyngeal segments of the upper airway all showed a significant increase 2 years posttreatment, with the greatest percentage change seen in the nasopharynx with a 43.92% increase and the least seen in the right nasal cavity with a 26.53% increase ( Table IV ). In the control group, the only significant increase in volume occurred in the oropharyngeal airway ( P = 0.03), whereas the increase in other compartments was not statistically significant ( Table V ). Comparison of the volumetric changes in all compartments between the experimental and control groups showed the difference between the 2 groups to be significant in all segments except the oropharyngeal compartment ( P = 0.92) ( Table VI ).
| Volumetric variable | T1, mean ± SD (mm 3 ) | T2, mean ± SD (mm 3 ) | T2-T1, mean ± SD (mm 3 ) | 95% CI (mm 3 ) | P | Percent increase |
|---|---|---|---|---|---|---|
| Total nasal cavity | 7971.6 ± 1801 | 10082.90 ± 2551.73 | 2249.6 ± 2102.5 | 1361.8-3137.4 | <0.0001 | 30.82 |
| Right nasal cavity | 4094.90 ± 1079.66 | 5063 ± 1323.3 | 968.8 ± 1082.7 | 549-1388.6 | <0.0001 | 26.53 |
| Left nasal cavity | 3813.10 ± 1138.28 | 4970.3 ± 1564.43 | 1197.3 ± 1587 | 569.5-1825.1 | 0.0006 | 38.82 |
| Nasopharynx | 2815.88 ± 1037.34 | 3816.44 ± 1053.21 | 1000.6 ± 917.7 | 629.9-1371.2 | <0.0001 | 43.92 |
| Oropharynx | 7645.22 ± 2311.72 | 9994.40 ± 3511.89 | 2349.2 ± 2520.8 | 1308.6-3389.7 | <0.0001 | 33.76 |
| Volumetric variable | T1, mean ± SD (mm 3 ) | T2, mean ± SD (mm 3 ) | T2-T1, mean ± SD (mm 3 ) | 95% CI (mm 3 ) | P | Percent increase |
|---|---|---|---|---|---|---|
| Total nasal cavity | 7655.0 ± 2037.27 | 8027.34 ± 1807.87 | 372.3 ± 1456.1 | −309 to 1053.8 | 0.27 | 7.38 |
| Right nasal cavity | 3954.0 ± 1375.53 | 4304.0 ± 1421.46 | 349.9 ± 826.7 | −37.0 to 736.8 | 0.073 | 11.78 |
| Left nasal cavity | 3701.0 ± 1113.24 | 3723.39 ± 947.17 | 22.44 ± 1313.4 | −592.2 to 637.1 | 0.94 | 6.42 |
| Nasopharynx | 2716.90 ± 11,371.24 | 2908.30 ± 1256.0 | 191.4 ± 855.9 | −209.2 to 592.0 | 0.33 | 23.97 |
| Oropharynx | 8307.0 ± 3383.73 | 10551.0 ± 3680.72 | 2244.0 ± 4345.1 | 210.0 to 4277.6 | 0.03 | 41.56 |
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