The spontaneous smile in dynamic motion

Introduction: The purpose of this study was to record and analyze the dynamic nature of spontaneous smiles. Methods: Fifteen children (9 girls and 6 boys; average age, 10.5 years) were filmed with a hidden camera while they watched a funny cartoon video. Spontaneous smiles were recorded, and the video frames were digitized. Time-graphs of the measurements were constructed, and plots of the movement of the mouth points were drawn. Results: Facial measurements showed that the upper lip elevated by 28%, relative to the rest position, and the mouth increased in width by 27%. The corners of the mouth moved laterally and superiorly at an angle of approximately 47°. Time analysis showed that the smiles developed in a staged fashion. The first stage (attack phase) was the shortest, lasting an average 500 ms. It was followed by a sustaining phase that included waxing and waning. The smile ended with a fade-out stage. The second and third stages were of variable duration and could be interrupted by the attack phase of a subsequent smile. Conclusions: The dynamics of the spontaneous smile and the findings of this study raise concerns about the validity of a single photographic capture for esthetic assessment and treatment planning.