Spatial changes of the chin in the vertical and sagittal planes after superior repositioning of the maxilla.

PURPOSE: The mandible autorotates after maxillary superior repositioning. The aim of this study was to address the changes in chin position in the vertical and sagittal planes after maxillary superior repositioning. MATERIALS AND METHODS: This cross-sectional study assessed participants who had class I occlusion with vertical maxillary excess and underwent maxillary superior repositioning. Two lateral cephalograms were taken in central occlusion and natural head position. The amount of maxillary superior repositioning was documented for every participant according to lateral cephalometric indices. The distance between the most prominent point of the chin and a perpendicular line from N to the Frankfort line was used to determine the sagittal changes of the chin before and after surgery. The distance from the N point to Me was used to assess the vertical changes of the chin before and after operation. The Pearson correlation test was used to determine the correlation between the amount of maxillary superior repositioning and the vertical and horizontal changes of the chin. The linear regression model was applied to predict the changes of the chin (dependent factor) according to the vertical change of the maxilla (predictive factor). The occlusal plane angle change, mandibular length, and mandibular plane angle were considered as variable factors. RESULTS: Twenty participants were studied. Analysis of the data demonstrated a significant correlation between the maxillary superior repositioning (predictive factor) and the horizontal and vertical changes of the chin. For every 1 mm of vertical change in the maxilla, the chin could be expected to move 0.21 mm horizontally. For a standard deviation increase of 1 in the maxillary position, the chin advanced by 0.753 of the standard deviation (beta = 0.753). For every 1-mm change of the maxilla vertically, it could be estimated that the chin moved 0.71 mm vertically when the amount of maxillary impaction was 8 mm or less. For an increase in standard deviation of 1 in the position of the maxilla, the chin moved superiorly by 0.711 of a standard deviation (beta = 0.711). In maxillary superior repositioning greater than 8 mm, for every 1 mm of superior repositioning, the chin moved 0.44 mm superiorly. There was a positive correlation between the occlusal plane change, mandibular length, mandibular plane angle as well as the vertical and horizontal changes of the pogonion (P = 0.001). CONCLUSIONS: The chin position after maxillary superior repositioning can be predicted according to the amount of maxillary vertical changes. The vertical change of the chin is more predictable than the horizontal change.