Landmark movement trajectory: A simple reference to understand the dominant muscle contraction in a dynamic smile expression.

Insight into the roles of perioral muscles play in smiling is essential to the relevant esthetic and reconstructive treatment. This study analyzed specific landmarks' movement trajectories to understand the dominant muscle activity in smiling. Sixty volunteers (aged 18-45 years) were included, and standardized frontal view images were obtained by three-dimensional photogrammetry. Each image sequence frame was referenced and superimposed onto the neutral image. Six surface landmarks were digitized on each frame to form the entire movement trajectories from resting to maximal smile. The perioral muscles were divided into different groups according to the inserting position of muscular fibers and the contracting direction. The movement paths of landmarks were analyzed to find the major muscle groups involved. The dominant muscle groups throughout smiling were successfully identified by landmark trajectory analysis in all 60 volunteers. Zygomaticus major was the dominant force for the beginning of most smiles. Apart from the zygomaticus major, other perioral muscles played a dominant role throughout smile expression, including levator anguli oris, buccinator and risorius, depressor anguli oris, and the upward muscle group of the upper lip were all identified. Besides, a notable high percentage of smiles presented a mixed pattern, indicating the cooperation of several muscle groups during smiling. Landmark movement trajectory provides a simple reference to understand the dominant perioral muscles in smiling. It can help surgeons get more information for the dynamic treatment of smiles.