Three-dimensional analysis of facial soft-tissue movement during mastication in children with lip incompetence

Lip incompetence is an esthetic orthodontic problem, but its functional relevance during mastication remains unclear. This study aimed to develop an objective system for assessing three-dimensional facial dynamics during mastication and to compare facial soft-tissue coordination and lip-closure force between individuals with and without lip incompetence. Thirty-nine children aged 6-12 years were classified into a lip-incompetence group (n = 16) and a control group (n = 23). Participants chewed gum unilaterally while 3D facial images were recorded. A homologous wire-mesh model was generated for each frame, and elastic energy based on inter-point displacement was calculated for the upper and lower lips. After regional averaging and normalization, the time-integrated difference between normalized mean elastic energies of the upper and lower lips was defined as the Surface Synchrony Index (SSI), representing lip-movement coordination rather than strict temporal phase synchronization. Lip-closure force was also measured. SSI was significantly higher in the lip-incompetence group than in the control group, indicating reduced coordination between upper and lower lip movements during mastication. In contrast, no significant difference in lip-closure force was observed between groups. These findings indicate that lip incompetence was associated with impaired coordination of facial soft-tissue movements during mastication. Static observation and lip-closure force alone are insufficient to evaluate functional problems during mastication. Surface-based kinematic indices enable objective assessment of difference in facial soft-tissue coordination associated with lip incompetence and may provide clinically relevant information for functional evaluation.