Objectives: to evaluate the skeletal alterations in young adult patients with maxillary transverse deficiency by maxillary skeletal expander (MSE) appliance, using Cone Beam Computed Tomography (CBCT).

Methods: Twenty-four patients (12 females and 12 males) with transverse maxillary deficiency were treated with the MSE II (Biomaterials Korea, Inc., Seoul, Korea). Their ages ranged from 17 to 22 years, The MSE II device consists of a central expansion screw welded to four tubes that acted as guides for the mini-implants. The mini-implants have a diameter of 1.8 mm and a length of 13 mm. The appliance activation varied according to the patient's chronological age. CBCT scans were captured before treatment (T1) and immediately post-expansion (T₂). Measurements were performed to evaluate the skeletal expansion.

Results: within the limits of this study, the mid-palatal suture was separated successfully in 21 of the patients there is a highzly significant between measurements before and after the expansion treatment. The change of nasal cavity width (N-N) was also significantly different before and after expansion treatment by miniscrew-assisted rated perceived exertion (MARPE), however, the study shows there is no significant difference between males and females in the result for measurements after expansion.

Conclusion: MARPE appliances, such as the MSE II, can be used to manage transverse maxillary deficiency in young adult patients.