## 4 Discussion  
Different studies identified the importance of UFM rotation in establishing a stable occlusion and its association with the molar class relationship especially in permanent dentition [6; 11; 18; 19]. Furthermore, it was observed that UFM is able to influence the available space in the dental arch during orthodontic treatment [12; 13; 14; 15; 26]. Consequently, the present study investigated the prevalence of UFM mesiopalatal rotation and its association with anterior dental crowding and occlusal class in two groups of subjects with mixed and permanent dentition. From the present data, the prevalence of UFM mesiopalatal rotation was high in both groups, in agreement with what is reported in previous studies [6; 8; 9] that are based on subjects with permanent dentition. A low difference in prevalence between the two groups seems to suggest that the prevalence of UFM mesiopalatal rotation does not change significantly with the transition from mixed to permanent dentition. In subjects with permanent dentition, there was a statistically significant association between UFM mesiorotation and anterior dental crowding, and mesiorotated molar was present in 100% of subjects with severe crowding. Subjects with UFM mesiorotation showed an OR = 3.28 to show moderate/severe anterior dental crowding, compared to subjects without UFM mesiorotation. Thus, it could be hypothesized that a mesiorotated molar occlusal surface, occupying a larger space on the dental arch owing to its trapezoidal shape [11; 12], reduces the available space in the dental arch, promoting the development of anterior crowding besides hindering proper dental intercuspation [5; 12] and worsening the molar anteroposterior relationship [12]. Even if it is clinically renowned that the correction of UFM mesiorotation is able to increase the amount of available space during the orthodontic treatment [12; 13; 14; 15; 27], to the best of the authors' knowledge this is the first study that reports a direct association of UFM mesiorotation with the presence of anterior dental crowding. The orthodontic correction of UFM mesiopalatal rotation may provide between 1 and 2 mm of gain in arch perimeter [8]. In subjects with mixed dentition, a statistically significant association was observed with the occlusal class, differently to what was observed in permanent dentition. Subjects with molar class II showed an OR = 10.2 to present a mesiopalatal rotated UFM, compared to subjects with molar class I. Besides the present study, [PERSON] et al. did not find any association between the severity of Class II and the magnitude of UFM rotation assessed by using the Ricketts' line method, in a group of 104 class II subjects with permanent dentition [1]. Nevertheless, the present investigation simply identified the presence/absence of mesiopalatal rotation without reporting the amount of molar rotation, thus is not directly comparable with what reported by [PERSON] et al. [1]. As an association between UFM mesiorotation and anterior dental crowding was identified, it seems that subjects with mixed dentition and UFM mesiorotation should be considered at higher risk of having dental crowding. The present study has some limitations, among them the absence of measure of the angle of UFM rotation that could allow in future studies an analysis of correlation of the amount UFM mesiorotation with anterior dental crowding.