## 4 Discussion
A long-term SARPE study by [PERSON] and [PERSON] [40] illustrated that only 41% of total expansion was the skeletal component, and the rest of the expansion completely relapsed in 49 months. A meta-analysis by [PERSON] and [PERSON] looked at young Hyrax patients, and the long-term results indicated that the final width change was comparable to that of normal growth, suggesting a 100% relapse [41]. [PERSON] et al. [42] reported, in a longitudinal study of SARPE patients using a pre-fabricated Hyrax appliance, that the mean skeletal expansion of 1.3 mm had relapsed by 0.4 mm during the retention and post-retention periods, which represents a 30.77% relapse. Our findings suggest that MSE expansion may produce more stable results in non-growing patients, compared to both young Hyrax patients and mature SARPE patients. In the current study, an overexpression based on bone width was not exercised, and expansion produced a slightly larger maxilla in order to produce a sound occlusal relationship; however, clinicians may consider the over-expansion of the MSE appliance by at least 10% in anticipation of some skeletal relapse in the long term. The alveolar bone bending and dental tipping components varied greatly between T1 and T2, with alveolar bone measurements decreasing by 328.57% (0.69\({}^{\circ}\)) and 1568.75% (2.51\({}^{\circ}\)) and dental tipping measurements decreasing by 568.75% (0.91\({}^{\circ}\)) and 384.21% (0.73\({}^{\circ}\)) on the right and left, respectively. The significantly larger decrease in the alveolar and dental components post-treatment compared to the skeletal component can be attributed to the dental decompensation of the dentition during the orthodontic treatment after expansion. However, even after a relatively larger decrease in the FAA and FDA angles, the change in these angles from T0 to T2 is still significant (FAA = 2.11\({}^{\circ}\) R and 2.15\({}^{\circ}\) L; FDA = 2.05\({}^{\circ}\) R and 1.61\({}^{\circ}\) L), suggesting that true skeletal expansion was achieved and maintained. Thus far, there have been limited data on the long-term stability of MSE expansion and MARPE in general. The MSE protocol used in our study involved keeping the MSE appliance passively installed for 6 months post-expansion for retention during bone formation, similar to the methods used in many previous studies [21; 28; 29; 32; 33]. However, perhaps a longer period of retention with mini-screws may improve expansion stability, allowing more time for bone formation. Furthermore, due to MARPE being a relatively new technology, the current literature lacks any data regarding long-term stability beyond one year after MARPE expansion. Our study followed patients over a mean time period of around 19.5 months post-expansion. Although the results over the 19.5 months were promising, future studies are necessary to evaluate stability after 5 years or longer, for the accurate assessment of the long-term outcomes. The limitation of the current study was the small sample size, due to the difficulties of acquiring a final CBCT, although the sample size was deemed to be adequate for statistical analysis. A robust study with a much larger sample size would be desirable. Another limitation was the lack of a true control group. The study group was compared to a 0 value, assuming that any changes without treatment would be a null value in non-growing patients. Further study with a prospective randomized control study would greatly improve the level of evidence.