###### Abstract  
Background: The aim of this study was to evaluate three-dimensional (3D) changes in nasal septal deviation (NSD) and related nasofacial structures after maxillary skeletal expansion (MSE). Methods: This retrospective study evaluated 28 patients aged 12.0-48.4 years (mean age, 20.4 \(\pm\) 7.3 years; 12 males, 16 females) diagnosed with transverse maxillary deficiency and treated with MSE. Cone-beam computed tomography (CBCT) images were taken at pre-expansion (T1), post-expansion (T2), and 6-months after MSE (T3) and were reoriented. Three-dimensional coordinates (x,y,z) were constructed using nasion (N) as the reference point (0,0,0). A paired-sample \(t\)-test and an independent sample \(t\)-test were performed to investigate and compare the 3D changes of the NSD and nasofacial structures after MSE, depending on the direction and amount of NSD and the amount of midpalatal expansion (MPE). Results: NSD was alleviated at T3-T1 in the coronal plane; nasal septal length increased significantly. The absolute nasal floor (NF) angle (2.06\({}^{\circ}\) at T1) decreased at T2-T1 (\(p\) > 0.05) and increased at T3-T2 (\(p\) < 0.05). The midface moved forward and downward, and pogonion (Pog) and menton (Me) moved downward and backward. There were no significant differences between the higher and lower NSD groups and greater and lesser MPE groups. Conclusions: Consequently, NSD was alleviated with variable positive nasofacial changes after MSE in both the short and long term. Therefore, MSE can be used to improve or camouflage facial deformities.  
nasal septum deviation; transverse maxillary deficiency; maxillary skeletal expansion; nasofacial structures; cone-beam computed tomography; Cartesian coordinate system; 3-dimensional coordinates +
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proposed that both RME and SARME did not change the position of the NS. On the other hand, [PERSON] et al. [8] reported that RME improved NSD and increased NS length. [PERSON] et al. [10] suggested a potentially positive effect of RME on NSD during childhood but no significant change during adolescence. Recently, [PERSON] et al. [11] concluded that rapid palatal expansion (RPE) and microimplant-assisted RPE (MARPE) significantly improved NSD in the short and long term.  
RME is widely used to correct maxillary transverse deficiencies. RME separates the midpalatal suture and expands the maxilla skeletally and dentally, thereby increasing the maxillary width, arch perimeters, and nasal cavity volume, thus reducing nasal airway resistance to improve nasal breathing [7; 12; 13]. However, a conventional RME has several side effects, including dentoalveolar tipping and a lack of long-term stability [14; 15]. In order to improve stability and increase the skeletal effect, a MARPE has recently been developed and is being widely used [11; 16; 17; 18].  
MARPE rendered almost parallel palatal expansion antero-posteriorly but showed an asymmetric nasomaxillary expansion at the rate of 30.3-50.0% [16; 19; 20]. Asymmetric nasomaxillary expansion after MARPE was influenced by initial facial asymmetry with chin deviation, initial asymmetric position of the mid-palatal suture, unilateral crossbite, asymmetric loosening of the circummaximillary suture, and discrepancies in zygomatic bone density and morphology, but it was impossible to predict the amount of asymmetric expansion initially [16; 17; 19; 20].  
The three-dimensional analysis of cone-beam computed tomography (CBCT) images have allowed 3D changes of NSD or nasofacial structures to be analyzed after maxillary skeletal expansion (MSE) using direct measurements of angles and lengths on the sectional images, and 3D superimpositions [11; 16; 17; 18; 19; 20; 21; 22], while some studies have evaluated the 3D changes using a Cartesian coordinate system [18; 23; 24]. However, there was no study to evaluate the NSD using a 3D coordinate system. Therefore, the aim of this study was to use 3D coordinates to evaluate the 3D changes in NSD and the related nasofacial structures after MSE.