## 5 Discussion  
The purpose of the present study was to evaluate the changes in upper molar position and to assess the predictability of digital planning in a sample of growing patients with late mixed dentition or early permanent dentition with mesio-rotated upper molars treated with Invisualign CA. In 1956, [PERSON] stated, \"The upper first permanent molars in the malocclusions are rotated in 83% of the cases, and the axis of rotation passes through the palatal root and the mesiolingual cusp\" [10]. Several studies reported a high percentage of rotated maxillary first molars in Class II malocclusion. The upper first molar rotation existed in 85% of all orthodontic conditions, 90% of all Class II division 1 cases, and 100% of all maxillary constrictions [12]. It appeared that a transverse deficiency of the maxillary arch is a typical finding in growing subjects with upper molar mesial rotation [13]. Maximillary first molar rotates mesiolingually on its long axis, with the mesiolingual cusp and the lingual root acting as a center or axis rotation [13]. From the anatomical aspect, the upper first molar is prone to rotation due to its trapezoidal configuration and its triple roots, and thus the mesipalatal cusp is a pivotal axis around which the upper first molar rotates [14]. For this reason, when digital planning the new molar position, it is important to apply the mesial-out buccal rotation, allowing for a distal rotation on the mesiopalatal cusp as a center of rotation. Moreover, derotation of the maxillary first molar while applying a simultaneous expansion force can provide centers of rotation at various points migrating from the molar center of resistance toward the distal margin ridge. The greater the expansion, the more the center of rotation will migrate distally. Therefore, the primary objective of accommodating a functional intercuspation is to expand and derotate at the same time [12]. Our results showed that there were statistically significant differences between T2-T1 maxillary first molar rotation (Figure 3). In the present study, the upper molars were mildly mesial rotated since the study group involved was in mixed dentition, before the exfoliation of second deciduous molars, and with a Class II edge-to-edge malocclusion. This meant that our treatment protocol required molar disto-rotation without distalization. The main  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Measurements** & **Post-Treatment T2 (\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ }}}}}}}}}}}}}}}\) & ** ** ** **T2 ClinCheck (\(\mathbf{\mathbf{\mathbf{\mathbf{  
derotation observed was of about six degrees. [PERSON] et al. [8] reported that in the case of extreme mesial rotation of the upper permanent molar, i.e., when the molar is included in the dental arch perimeter with its oblique diameter instead of its mesiodistal diameter, maximal molar derotation leads to a corresponding maximal gain of space of about 2.5 mm. According to our results, orthodontic molar derotation (about six degrees) led to a mean gain of arch space of about 1 mm. Our findings agree with [PERSON] et al. [12], who reported a value of gain in arch perimeter of 2.5 mm with an upper molar derotation of about 20 degrees. It is important to emphasize that the gain in arch perimeter following molar derotation occurred anteriorly to the derotated tooth, as the buccal cusps moved distally by about 1 mm, with a simultaneous improvement in the molar occlusal relationship. Our findings indicated that there were statistically significant differences between T2 effective rotation and T2 ClinCheck programmed outcomes. ClinCheck software overestimated the value obtained in planning molar derotations. To test the predictability of the ClinCheck treatment planning software, measurements taken at the end of treatment (T2) were analyzed in comparison with the amount of movement predicted by the software at the end of the first phase of aligners (T2 ClinCheck).