Modeling facial movement: I. A dynamic analysis of differences based on skeletal characteristics.

PURPOSE To introduce a novel approach to analyze and model facial movements; and to quantify variations in facial movement caused by the extent of skeletal differences between the maxilla and mandible and the middle to lower facial heights. The hypothesis was that there are differences in facial movement related to the underlying facial skeleton which may be explained by the shape of the face rather than the pattern of movement. PATIENTS AND METHODS The study sample consisted of 43 subjects (23 men, 20 women) with a mean age of 18.5 years (SD = 11.90). Measures of the facial skeletal differences were made from lateral cephalometric radiographs, and subjects were classified as Class I and Class II, and normal to decreased lower anterior face height, respectively. Facial movements were recorded by a video-based tracking system. Descriptive and inferential statistics were performed on principal component scores generated from the movement data. A linear mixed-effects model was used to test for significant differences in movement among the different skeletal types. RESULTS A dynamic modeling of facial movements was described that has numerous potential clinical applications. Also, differences in movement were found during the lip purse movement. Specifically, skeletal Class I individuals showed greater forward and upward movement during lip purse compared with individuals with severe skeletal Class II who moved their lips straight forward with less magnitude of movement. CONCLUSION For most of the movements, apart from the lip purse, differences in motion were explained by static facial shape.