A lip protrusion mechanism examined by magnetic resonance imaging and finite element modeling

The lips are regarded as a paired organ for audiovisual speech communication that modulates speech sound and facial shape together in speech production. Despite the long research, the mechanism of lip protrusion is still an open question. This study aims at exploring physiological mechanisms of lip protrusion based on muscle visualization and finite element simulation of lower lip deformation. A special focus is placed on the peripheral part of the orbicularis oris among the lip muscles because it is located closer to the lip’s oral surface. This geometry suggests enhancement of lip tissue convexity by medial shortening the teeth-side lip tissue disproportionately. This hypothesis is examined by a simulation using a finite element model of the lower lip built based on high-resolution magnetic resonance imaging (MRI) data. Lower lip deformation obtained by the simulation conforms to the hypothesis of lip tissue advancement with enhanced anterior convexity.