## 4 Discussion  
In the present study, the inclination changes in the maxillary premolars and the molars following the MSE treatments is reported. The MSE is a specific MARPE appliance which is designed to produce posterior and superior force vectors by inserting the micro-implants posteriorly between the two zygomatic buttresses and superiorly engaging both the palatal and nasal cortical bones [13]. These posterior and superior expansion forces are proven to be effective against the posterior and superior resistances. The bicortical engagement of micro-implants produces a more parallel pattern of expansion [12,13]. Regarding the maxillary molar inclination changes, the mean buccal tipping values of the first and second molars were 3.1\({}^{\circ}\)\(\pm\) 2.3 degrees and 2.1\({}^{\circ}\)\(\pm\) 2.5, respectively. Because the second molars were not anchor teeth, this buccal inclination change cannot be from the dental tipping. [PERSON] et al. described the MSE expansion as a skeletal rotation of the hemifical structure with the fulcium near the frontozygomatic suture [14]. This type of skeletal rotation would change the buccal inclination of the molars because the landmarks further away from the fulcium would move more. This concept was first described by [PERSON] et al. [15]. However, the first molar could have tipped locally when the micro-implants tipped or moved laterally within the palatal bone during the expansion, because the molars were the anchor teeth. Based on the fact that the second molar inclination changed by 2.1\({}^{\circ}\) without any expansion force on it, we can conclude that the skeletal rotation caused by the MSE was the reason for this change. Assuming the 2.1\({}^{\circ}\) change with the second molar was from the skeletal rotation, the first molar tipping may be about 1\({}^{\circ}\) [3.1\({}^{\circ}\)-2.1\({}^{\circ}\)]. When analyzing the dental tipping in the coronal plane, the changes occurred from a rotation of the entire craniofacial structure and maxillary basal bone. The dental tipping has to be analyzed within the alveolar bony housing. While the existing studies show similar results for the molar inclination changes with the MARPE treatments, the studies for the premolar inclination changes are few and contradictory [19,23-26]. The MARPE devices used in these studies were of different designs, either with two or four micro-implants inserted posteriorly or laterally in the palate, with various numbers of teeth banded. Two studies reported a significant buccal tipping of the first premolars [19,25]. [PERSON] et al. [19] treated a total of eight cases with an MSE device and reported a mean buccal tipping of 5.86 \(\pm\) 5.71 for the first premolars. However, they used bands on the first premolars to stabilize the MSE and positioned \begin{table}
\begin{tabular}{c c c c c} \hline
**Tooth No** & **Pre-Expansion** & **Post-Expansion** & **Change** & **_p Value_** \\ \hline
14 & 91.8 \(\pm\) 11.1 & 91.2 \(\pm\) 10.4 & \(-\)0.6 \(\pm\) 2.7 & 0.27 \\
24 & 95.0 \(\pm\) 9.1 & 94.7 \(\pm\) 9.1 & \(-\)0.4 \(\pm\) 1.8 & 0.23 \\
15 & 92.0 \(\pm\) 7.6 & 92.6 \(\pm\) 7.4 & 0.6 \(\pm\) 1.9 & 0.17 \\
25 & 92.3 \(\pm\) 12.4 & 93.7 \(\pm\) 12.5 & 1.5 \(\pm\) 1.7 & 0.018 \({}^{\star}\) \\
16 & 108.0 \(\pm\) 6.5 & 110.8 \(\pm\) 6.2 & 2.8 \(\pm\) 2.4 & 0.0002 \({}^{\star}\) \\
26 & 110.3 \(\pm\) 7.3 & 113.8 \(\pm\) 7.9 & 3.5 \(\pm\) 2.1 & \textless{}0.0001 \({}^{\star}\) \\
17 & 114.7 \(\pm\) 7.5 & 116.6 \(\pm\) 7.3 & 1.9 \(\pm\) 2.4 & 0.004 \({}^{\star}\) \\
27 & 114.6 \(\pm\) 7.8 & 116.8 \(\pm\) 8.5 & 2.2 \(\pm\) 2.6 & 0.0008 \({}^{\star}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Mean and SD of pre- and post-expansion tooth inclination and inclination change (in degrees). A positive Mean change indicates a buccal inclination, a negative mean change indicates a palatal inclination. The _p_-values of the comparison with the zero change are tabulated. The asterisk indicates statistical significance.