## 4 Discussion  
In this study, we evaluated the effect of maxillary expansion using clear aligners on the nasal airway in children. Studying the effect of clear aligners on the intermolar distance and nasal airway volume will help orthodontists develop better treatment plans. In some previous reports [9; 10; 20], the effects of RME on nasal volume have been studied. [PERSON] et al. used three-dimensional morphometric analysis and acoustic rhinometry to evaluate the maxillary dental arches and nasal cavities and concluded that RME resulted in a significant expansion of the palate and a significant increase in nasal airway volume [20]. [PERSON] et al. reported that RME increased the nasal airway cavity size in children [9]. Moreover, [PERSON] et al. found the effect of two RME methods of Hyrax and Haas appliances on the nasal airway [10]. Both appliances can significantly increase nasal cavity dimensions in children. Our study is the first to use clear aligners as a study to evaluate their effects on the nasal airway and upper dental arch expansion in children. Regarding the segmentation of the nasal airway model, some apical regions of the nasal airway are often connected to the maxillary sinus parts, which causes a lot of noise for the segmentation of the model. Since the lower part of the nasal airway is more affected by maxillary expansion compared to the upper part [9; 21; 22; 23]. Therefore, we removed the indeterminate apical parts of the nasal airway which could be both the sinus and the nasal airway as much as possible to control for study methods standardization. In a previous study, [PERSON] et al. reported that RME can increase by about 8% in nasal airway volume in children [9]. However, our research shows an average increase of 15.13% in the nasal airway volume in the experimental group. The data in their study include a mean age of 9.70 \(\pm\) 1.41 years, similar to our data (which include a mean age of 9.13 \(\pm\) 0.51 years). However, their treatment time was 7 months, which is less than our study's treatment time which was an average of 13.43 months. This may be one of the reasons why their volume increase is smaller than ours. The nasal airway volume in the control group showed a reduction of 3.73%. This may be attributed to the lack of timely treatment following the initial visit, leading to further deterioration of the patient's condition. However, future studies should include a larger sample size to further investigate and strengthen these conclusions. Although our study revealed important new findings, there are also limitations. Correlation analysis showed almost no relationship between nasal airway volume and intermolar distance in the experimental group, while a strong inverse correlation was observed between these two variables in the control group. There are a number of possible reasons for this, such as the fact that the craniosulfacial system in children is in a developmental stage [24], and the rate of growth and morphological changes in the skeleton may make it less likely that the localized effects of maxillary expansion treatments will show up as a direct correlation. Nasal airway volume is also affected by soft tissue (e.g., nasal mucosa, adenoid size, etc.) [25]. During puberty, children's adenoids and tonsils may fluctuate considerably due to hormonal changes, and their development has not yet reached a stable state [26]. The effect of these soft-tissue factors on nasal airway volume may mask the effect of molar expansion. Also, maxillary dentoalveolar expansion using clear aligners is not expected to perform skeletal expansion, so a positive correlation between the dentoalveolarexpansion and the nasal airway may not be expected. In addition, due to the difficulty of collecting data on untreated patients in the control group, there were 7 patients in the control group in this study. Also, the mean nasal airway volume at T1 in the control group was slightly larger than the mean nasal airway volume at T1 in the experimental group.