Mesial rotation of the upper first molar: Association with anterior dental crowding in mixed and permanent dentition

The position of the upper first molar (UFM) is currently considered the "key of occlusion". The purpose of this study was to evaluate the prevalence of the mesiorotated UFM, and its relationship with anterior dental crowding in mixed and permanent dentition. Intra-oral scans of upper dental arches of 180 subjects with mixed dentition and 169 subjects with permanent dentition were retrospectively analyzed to investigate the presence of UFM mesiopalatal rotation and presence of anterior crowding. The occlusal relationship and the presence of caries were also recorded. In subjects with permanent dentition, there was a statistically significant correlation between anterior crowding and UFM mesiopalatal rotation (Pearson's chi-square = 9.03; p = 0.06). Among cases with permanent dentition, subjects with UFM mesiopalatal rotation showed an OR = 3.28 (95% CI = 0.99-10.93; p = 0.042) of moderate/severe anterior dental crowding, respect to subjects without UFM rotation. In subjects with mixed dentition, there was a statistically significant correlation between molar occlusal relationship and UFM mesiopalatal rotation (Pearson's chi-square = 14.98; p = 0.002), and subjects with molar class II showed a OR = 10.2 (95% CI = 2.16-48.22; p = 0.0005) to present UFM mesiopalatal rotation, with respect to subjects with molar class I. UFM mesiopalatal rotation is associated to anterior dental crowding in permanent dentition, and to class II malocclusion in mixed dentition.