Quantitative Prediction of Change in Chin Position in Le Fort I Impaction

Vertical maxillary excess, a dentofacial deformity present in a large proportion of population impute an increased lower facial height due to increased maxillary height. This results in a clockwise rotation in the mandible, and the chin advances posteriorly and inferiorly. Le Fort I superior repositioning of the maxilla begets movement of pogonion point anteriorly and cranially. Cephalometric analysis helps to access change in position of chin following Le Fort I impaction. In our study, from this analysis a formula was defined to perceive the exact amount of this change in chin position along the vertical and sagittal plane as a result of autorotation.

This experimental study assessed 45 patients with vertical maxillary excess over a period of 2 years and 6 months (January 2016-May 2018) in Government Medical College Hospital, Kottayam, Kerala, India. The planned procedure was Le Fort I superior impaction for correction of vertical maxillary excess. Pre-operative cephalograph was taken initially. Post-operative cephalograph after 3 months was then compared with initial pre-operative cephalograph to assess the change in position of the pogonion and menton.

Forty-five participants were studied. The multiple regression model was applied to predict the changes in the chin (dependent factor) according to the vertical change in the maxilla (predictive factor). For every 1 mm change in the maxilla vertically, the chin was estimated to move 0.59 vertically. For a standard deviation increase of 1 in the position of the maxilla, the chin moved superiorly by 0.744 of the standard deviation. For every 1 mm of vertical change in the maxilla, the chin could be expected to move 0.22 mm horizontally. For a standard deviation increase of 1 in the maxillary position, the chin advanced by 0.273 of the standard deviation.

This study draws to a conclusive finding that the movement of maxilla in the superior direction has an effect on the repositioning of the chin in the anterior and cranial directions. This has led to a formulation that 1 mm of superior impaction of maxilla results in 0.6 mm of vertical and 0.2 mm of sagittal movement of chin. This might help to have a glance of future chin position and aid in deciding the need for mandibular surgery.