Asymmetric molars' mesial rotation and mesialization in unilateral functional posterior crossbite and implications for interceptive treatment in the mixed dentition

Introduction: Symmetric transverse expansion is the main outcome of the early treatment in subjects with unilateral functional posterior crossbite. The aim of this study was to analyse mesial rotation and mesialization of upper first molars as sagittal parameters to be corrected in the treatment of these patients during the mixed dentition. Methods: Digital dental cast measurements (rotation and mesialization) were performed in a sample of 48 subjects with unilateral posterior crossbite (UPXB; 19 males and 29 females, mean age 10.2 +/- 1.2 years) and in a control group of 35 subjects with normal Class I occlusion (17 males and 18 females, mean age 9.9 +/- 1.3 years). An independent sample t-test, the Mann-Whitney test, Fisher's exact test, and Pearson correlation were used for statistical comparison. Results: The amount of upper molar rotation was significantly greater in the experimental group when compared with the control group. A clinically significant 'upper molar rotation' (UMR) was present in 66.7 per cent of the subjects with UPXB versus 5.7 per cent of the control group. The UMR group presented also a significant mesialization of upper first molars when compared with the control group. In the experimental group, there was a significant difference between rotation and mesialization in the right and left side and a correlation has been found between these two variables and the amount of Class II molar relationship at the crossbite side. Limitations: This is an epidemiological case-control study and the discussed effects of an early correction of the asymmetric upper molars' migration are only speculations based on an association relationship. Conclusions: The findings of this study show an asymmetric upper first molars' migration (rotation and mesialization) in unilateral functional posterior crossbite versus a control group. An early evaluation and correction of the molars' migration during the mixed dentition should be considered in order to obtain a correct inter-occlusal sagittal molar relationship, space for an adequate eruption of permanent teeth, and perhaps reduce the need of a following fixed appliance treatment in the permanent dentition.