Effects of rapid maxillary expansion on upper airway morphology and respiratory function: a scoping review

1. Introduction

Transverse maxillary deficiency is a common craniofacial condition frequently encountered in growing patients and is often associated with posterior crossbite, dental crowding, and altered occlusal relationships [1–3]. Beyond its orthodontic implications, maxillary constriction has increasingly been recognized as a condition with potential repercussions for upper airway morphology and respiratory function [4–7]. In particular, a reduced transverse dimension of the maxilla has been linked to decreased nasal cavity width, increased nasal airway resistance, and a higher prevalence of oral breathing patterns [4–8]. Rapid maxillary expansion (RME) is a well-established orthopedic procedure aimed at correcting transverse maxillary deficiency through separation of the mid-palatal suture [1–3,9]. Since its introduction, RME has been extensively investigated for its dentoskeletal effects; however, its potential influence on the upper airway and respiratory function has progressively gained attention, especially within an interdisciplinary framework involving orthodontics, otolaryngology, and sleep medicine [10–25]. The anatomical relationship between the maxilla and the nasal cavity provides a clear biological rationale for the potential respiratory effects of RME [1,4,6,8–9]. The maxilla forms the floor of the nasal cavity and contributes to its lateral walls. Therefore, transverse expansion of the maxilla may induce changes in nasal cavity morphology, particularly at the level of the nasal floor and the inferior nasal airway, which represent regions of higher airflow resistance [10,13–17,20–25]. Even modest dimensional increases in these areas may translate into clinically relevant improvements in nasal airflow. Nasal breathing plays a crucial role in normal craniofacial growth and respiratory physiology [4–8]. Chronic nasal obstruction during growth has been associated with persistent oral breathing, altered tongue posture, vertical facial growth patterns, and functional adaptations that may negatively affect dentofacial development [4–8]. Consequently, interventions that improve nasal patency during growth may exert beneficial effects not only on occlusion but also on respiratory function and overall craniofacial development [10–25]. Over the past decades, several studies have investigated the effects of RME on nasal cavity dimensions, nasal airflow resistance, and pharyngeal airway spaces using a wide range of diagnostic tools, including two-dimensional cephalometry, cone-beam computed tomography (CBCT), acoustic rhinometry, active anterior rhinomanometry, and, more recently, computational fluid dynamics [10–25]. While many studies reported favorable changes in nasal airway morphology and function following RME, the magnitude, consistency, and clinical relevance of these effects remain debated [7,10,20–25]. In particular, the impact of RME on the pharyngeal airway has yielded heterogeneous results. Unlike the nasal cavity, the pharynx is a dynamic structure influenced by multiple factors, including head posture, tongue position, adenoidal tissue, neuromuscular tone, and body posture [11–13,20,22]. These variables complicate the interpretation of pharyngeal airway changes and may account for inconsistent findings across studies.

Similarly, the effects of RME on pulmonary function remain controversial. Although some authors have described improvements in spirometric parameters following RME, others have failed to demonstrate clinically meaningful changes [18,25]. This raises questions regarding the extent to which orthopedic maxillary expansion may influence lower airway function and whether observed pulmonary changes are primary or secondary to improved nasal breathing. Another important aspect concerns the timing of intervention. Growing patients may respond more favorably to RME due to greater skeletal adaptability and sutural responsiveness [1–3,9,22]. However, the long-term stability of airway-related outcomes and their persistence into adulthood remain insufficiently documented [23]. Moreover, differences in study design, sample characteristics, imaging protocols, and outcome measures further limit the ability to draw definitive conclusions [10–25]. Therefore, this scoping review aimed to map and synthesize the available evidence on the effects of rapid maxillary expansion on upper airway morphology and respiratory function. Specifically, this review evaluates morphological changes of the nasal cavity and pharyngeal airway, functional outcomes related to nasal airflow and resistance, pulmonary function parameters, and patient-reported respiratory outcomes, with particular attention to growing patients and mouth breathers [10–25].

2. Materials and methods

3. Results

Sixteen studies investigating the effects of rapid maxillary expansion (RME) on upper airway morphology and respiratory function were included in this scoping review and are summarized in Tables 1 and 2. As shown in Table 1, the evidence comprised randomized clinical trials, prospective and retrospective clinical studies, and pilot and case-control investigations. Most samples included growing patients with transverse maxillary deficiency and frequently involved mouth breathers and/or subjects with baseline nasal obstruction. Imaging-based evaluations (2D/3D, mainly CT/CBCT) consistently indicated that the most reproducible morphological outcome after RME was enlargement of the nasal cavity, with changes often described at the level of the nasal floor and in the inferior/anterior nasal airway regions (Cappellette Jr et al. [15,16], Niu et al. [20], Ronsivalle et al. [22], Motro et al. [13], and Izuka et al. [12]). In contrast, findings regarding pharyngeal airway morphology were heterogeneous: some studies reported increases in upper and/or lower pharyngeal airway dimensions or volumes (Langer et al. [10], Aloufi et al. [11]), whereas other investigations reported minimal or non-significant modifications (Table 1), suggesting a less predictable response of the pharyngeal airway across different study designs and measurement protocols.

Functional outcomes are summarized in Table 2. Rhinomanometry-based investigations consistently reported reductions in nasal airway resistance and/or improvements in nasal ventilation after RME (Langer et al. [10], Cremonini et al. [24], Di Vece et al. [17], Iwasaki et al. [19]), with more evident functional benefits in cohorts characterized by mouth breathing and/or pre-treatment nasal obstruction when such baseline features were reported. Computational fluid dynamics analyses also suggested improved airflow behavior after expansion, including more favorable airflow distribution and reduced turbulence (Sakoda-Iwata et al. [21]), supporting the functional relevance of morphological nasal changes. Pulmonary function was assessed in a smaller subset of studies using spirometry, showing variable and generally modest changes in parameters such as FVC, FEV1, and PEF (Abate et al. [18] and Akbulut et al. [25]). Additional functional outcomes were reported in selected studies, including improved nasal mucociliary clearance (Babacan et al. [14]) and patient-reported improvements in breathing-related quality of life or perceived nasal breathing. Long-term evidence remains limited; a retrospective study assessing functional breathing after maxillary skeletal expander treatment suggested partial stability of breathing improvements over time (Combs et al. [23]), but robust long-term controlled data following expansion procedures are still scarce.

4. Discussion

The present scoping review suggests that rapid maxillary expansion (RME) is most consistently associated with favorable changes in the nasal airway. In contrast, its effects on the pharyngeal airway and pulmonary function appear more variable across the available evidence. Overall, the studies included in this review support the concept that RME primarily modifies nasal airway morphology and nasal breathing. In contrast, downstream or more distal respiratory outcomes are less consistently documented.

A biologically plausible and recurrent finding across imaging-based investigations is nasal cavity enlargement after RME, with increased nasal dimensions and/or volume reported using CT/CBCT approaches and other radiographic assessments. In mouth-breathing growing patients, increases in nasomaxillary complex volume and nasal cavity dimensions were described after expansion, supporting measurable anatomical modifications of the nasal airway following treatment [15–16]. Volumetric CBCT evidence further reported an increase in nasal cavity volume after RME, with age-related differences suggesting that growth and maturation may influence the magnitude of the skeletal and airway responses [22]. Additional CBCT investigations described rhinological or nasal cavity changes after expansion [13], and validated 3D analyses reported increases in nasal cavity volume, along with changes in pharyngeal airway volume, in certain cohorts [20]. Moreover, immediate post-expansion CBCT findings were accompanied by patient-reported improvements in quality-of-life/breathing-related outcomes in growing mouth breathers, reinforcing the potential clinical relevance of nasal airway modifications in selected patients [12]. Taken together, these studies indicate that anatomical changes at the nasal level represent the most reproducible airway-related effect of RME, consistent with the close anatomical relationship between the maxilla and the nasal cavity.

Functional outcomes largely mirrored the morphological nasal findings. Rhinomanometric assessments consistently reported reductions in nasal airway resistance and/or improved nasal ventilation after RME, including prospective evaluations demonstrating reduced nasal resistance and increased nasopharyngeal space [10], studies focused on oral breathers showing improved nasal resistance after active anterior rhinomanometry [24], and pilot clinical evidence combining rhinomanometry and endoscopic assessment reporting improved nasal patency/airflow after treatment [17]. Randomized clinical evidence also supported improved nasal ventilation after RME compared with controls, strengthening the functional interpretation of the morphological nasal changes [19]. In addition, computational fluid dynamics analysis provided complementary insight into airflow behavior, describing improved airflow distribution and reduced turbulence after RME in children with nasal mucosal hypertrophy and adenoids, thereby linking structural expansion to potentially more efficient nasal airflow patterns [21]. Beyond airflow and resistance, nasal physiology may also be affected: one prospective study reported improved nasal mucociliary clearance following RME, suggesting a possible positive impact on nasal function beyond purely mechanical resistance measures [14].

In contrast to nasal outcomes, evidence regarding the pharyngeal airway remains less consistent. Some studies reported increases in pharyngeal airway dimensions or space following RME, including rhinomanometric evidence associated with increased nasopharyngeal space [10], cephalometric findings showing increased upper and lower pharyngeal airway spaces [11], CBCT investigations reporting increases in upper airway dimensions in growing mouth breathers [12], and volumetric increases in pharyngeal airway volume in certain samples [20]. However, the variability of populations, imaging protocols, and measurement approaches across studies may limit direct comparability and likely contribute to heterogeneous findings. Given the dynamic nature of the pharyngeal airway and its sensitivity to factors such as head posture and soft tissue conditions, current evidence does not support a uniformly predictable pharyngeal airway response to RME across all patient groups. Pulmonary function outcomes were assessed in fewer studies and showed inconsistent findings. Spirometric investigations reported improvements in selected parameters, such as FVC, FEV1, and PEF, after RME in some cohorts [25], whereas other clinical evidence suggested only mild short-term improvements in breathing function [18]. Overall, the limited and heterogeneous spirometric evidence suggests that any pulmonary function changes are not as consistently documented as nasal airway improvements and, when present, may be modest in magnitude. Long-term respiratory stability after expansion remains insufficiently documented. A long-term retrospective study evaluating functional breathing after maxillary skeletal expander treatment suggested persistent or partial stability of breathing improvements over time [23]; however, long-term controlled data following expansion procedures remain scarce [18,23,25]. Consequently, while the nasal morphological and functional changes after RME appear relatively consistent in the available evidence, future longitudinal studies using standardized imaging, standardized functional testing, and clinically meaningful patient-centered outcomes are needed to clarify the durability and clinical relevance of respiratory benefits, particularly beyond the nasal airway and into longer follow-up periods.

5. Conclusion

Within the limits of the available evidence, rapid maxillary expansion consistently produces measurable improvements in nasal airway morphology and nasal breathing, including enlargement of the nasal cavity and reductions in nasal airway resistance, as assessed by objective functional tests. These benefits appear most clinically relevant in growing patients with transverse maxillary deficiency who present with mouth breathing and/or baseline nasal obstruction. In contrast, evidence regarding pharyngeal airway changes remains heterogeneous, and spirometric pulmonary function outcomes are inconsistent and generally modest, suggesting that respiratory benefits beyond nasal airway use are less predictable. Overall, RME should be primarily indicated for the correction of transverse maxillary deficiency. At the same time, potential respiratory improvements, especially at the nasal level, may be considered an important adjunctive benefit in appropriately selected patients. Future standardized longitudinal studies integrating harmonized imaging protocols, validated functional assessments, and patient-reported outcomes are needed to define the durability and clinical significance of airway-related changes after RME.

Abbreviations

Funding:

This study was partially funded by the Italian Ministry of Health - Current Research IRCCS.

Institutional Review Board Statement:

Not applicable, because this scoping review used data from previously published studies and did not involve new data collection from human participants.

Informed Consent Statement:

Not applicable.

Data Availability Statement:

All data are derived from published studies included in this review. The extracted data supporting the findings are available from the corresponding author upon reasonable request.

Conflicts of Interest:

The authors declare no conflicts of interest.