Introduction: The shape, size and volume of the bony nasal cavity is highly influenced by the width of the nasal floor, which is formed by the palatine bone and the palatal processes of the maxilla and can be orthodontically manipulated with the MSE (Maxillary Skeletal Expansion) technique. Current evaluation of changes in this area before and after expansion relay on cross-sectional areas and airflow measurements. However, these techniques are not able to truly point out the extend of bone changes and volume changes of this important structure. Materials and Method: This retrospective study had a sample of 22 patients distributed in two groups. Group 1 was treated with Hyrax and consisted of 6 growing patients (3 females and 3 males, mean age 12y2mo), group 2 was treated with MSE and consisted of 16 non-growing patients (10 females and 6 males, mean age 20y7mo) who met inclusion criteria. Analysis of CBCT scans before and after expansion treatment was conducted in OnDemand3D software, with automated superimposition on the cranial base. Three reference planes were established, based on reliably identifiable anatomical landmarks, anterior nasal spine (ANS), posterior nasal spine (PNS) and nasion (N). The distance between ANS and PNS in the AP plane was divided into 4 sections, and the distance between ANS and N was divided into 5 sections, excluding the most superior one. Transverse linear measurements at the resulting cross-points between the vertical and horizontal sections resulted in a total of 32 measurement points along the lateral walls of the nasal cavity.Results: The MSE group showed overall statistically significant larger movements of the lateral walls of the nasal cavity (p < 0.05). The bony structures followed the overall expansion pattern of the maxilla, though bending at the orbital rims and areas of higher bone density occured. Conclusion: MSE produces not only larger maxillary skeletal transverse movements, but also shows more expansion effects on the lateral walls of the nasal cavity compared to tooth-borne expansion
