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| Study | Participants | Interventions | Comparisons | Outcomes | Study design |
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| Christie et al. | Twenty-four healthy children (mean age, 9.9 years; range, 7.8–12.8 years; 14 boys, 10 girls). | Haas-type maxillary expander. Expansion was carried out as 2 turns per day (0.2 mm per turn) until the required expansion was complete | None | From T1 to T2, mean increases in nasal width, significant increases in basal bone of the maxilla, and significant openings in the midpalatal sutures were found | Pretreatment orthodontic records (T1) and immediately after expansion (T2) CBCT images were taken for all patients |
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| Zeng and Gao | 16 children (10 male; 6 female) with a mean age of 12.731.73 years (range, 10–15 years) | All subjects used 4,6-banded hyrax expanders. The active expansion period ranged from 2 to 3 weeks according to the expansion amount (2.7–6.3 mm). The retention period lasted 3 months | None | Linear width measurements at the level of nasal cavity increased. Lower nasal volumes increased; on the other hand, oropharyngeal volumes decrease though without any significance | CBCT images were taken immediately before (T1) and three months after expansion (T2) |
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| El and Palomo | Two groups were selected, each with 35 patients (15 males, 20 females): Rapid Maxillary expansion group and a control group. | Expansion protocol consisted of twice per day screw activation until a slight amount of overcorrection was achieved. The retention period lasted 4–6 months | The control groups were matched for age, sex, and treatment duration | A statistically significant airway value was seen in both groups although only nasopharyngeal airway showed significant difference between the groups | CBCT data for pretreatment (T0) and posttreatment (T1) intervals. Linear, cross-sectional, and volumes along airway passage were assessed |
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| Zhao et al. | The experimental group consisted of 24 patients (mean age, 12.8 ± 1.88 years) with maxillary constriction who were treated with hyrax palatal expanders | Hyrax type palatal expanders turned
1 or 2 times per day until the required expansion was achieved, retention
for at least 3 months after expansion | The control was paired in age and sex; patients included were just starting regular orthodontic treatment | Only retropalatal airway volume was found to be significantly different between groups before treatment, and this difference remained after treatment. No other statistically significant differences were found for the airways measurements | CBCT scans were taken of all patients as part of both initial orthodontic treatment records and progress records |
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| Iwasaki et al. | Twenty-eight treatment subjects (mean age 9.9661.21 years) who required RME treatment had cone-beam computed tomography images taken before and after RME. | Rapid maxillary expansion with 5 mm of expansion in average | The control group consisted of serial CBCT images of 20 subjects (8 boys, 12 girls) with no history of RME appliance treatment | Intraoral airway volume (tongue posture) decreased significantly in the RME. The increase of pharyngeal airway volume in the control group was only 41% that of the RME group | CBCT data were taken before and after RME treatment (RME group) or at corresponding times but without RME treatment (control group) |
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| Chang et al. | Fourteen orthodontic patients (mean age, 12.9 years; range, 9.7–16 years) were recruited. | The activation protocol consisted of 1 activation (90 turn) of the jackscrew per day for 28 consecutive days or until resolution of the posterior crossbite | None | The cross-sectional airway measured from the posterior nasal spine to basin level was the only parameter showing a significant increase after rapid maxillary expansion | The initial CBCT scan was taken 0 to 14 days before cementation of the maxillary expander, and the progress CBCT scan was taken 3 to 4 months after completion of active maxillary expansion |
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| Ribeiro et al. | 15 mixed dentition individuals (8 females and 7 males) | All patients were treated with rapid maxillary expansion using a fixed appliance with occlusal acrylic coating | None | The nasal cavity presented a significant transversal increase at level of the lower third. No significant change occurred in the nasopharynx although they refer to retropalatal area. A significant change was noted in the oropharynx in volume immediately after the RME | Patients were evaluated before and 4 months after the RME |
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| Pangrazio-Kulbersh et al. | 23 prospectively treated patients. 13 subjects, 7 males and 6 females with a mean age of 12.6, were treated using banded expanders, and 10 subjects, 5 males and 5 females with a mean age of 13.5, were treated with bonded expanders | Each group had the same expansion activation protocol and retention time: expansion time was 4–6 weeks, with 6–10 mm of activation | The two expansion groups were compared | Both appliances equally increased the skeletal and soft tissue dimensions of the nasal cavity and maxillary sinus volume. The posterior airway volume did not significantly change with either method of expansion | CBCT images weretaken at T1 (pretreatment) and T2 at 6 months immediately after removal of the appliance |
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| da Baratieri et al., 2014 | It comprised 30 subjects (18 males and 12 females), mean ages 9 for males and 9.7 for females. Divided in expansion and control group | Rapid maxillary expansion using banded appliance | The activation group was compared with a paired age and sex, control group | All the linear (width) measures of nasal cavity increased significantly except for alveolar angulation and inferior nasal cavity area | CBCT taken at treatment onset and one year after expansion |
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