‘Investigation of the activation of temporalis and masseter muscles in voluntary and spontaneous smile production.’

Publication date: Available online 6 March 2018
Source:Journal of Plastic, Reconstructive & Aesthetic Surgery
Author(s): Jessica E. Steele, Ian R. Woodcock, Adrian D. Murphy, Monique M. Ryan, Tony J. Penington, Christopher J. Coombs
IntroductionMasticatory muscles or their nerve supply are options for facial reanimation surgery but their ability to create spontaneous smile has been questioned. This study assessed the percentage of healthy adults who activate temporalis and masseter during voluntary and spontaneous smile.MethodsHealthy volunteer adults underwent electromyography (EMG) studies of temporalis and masseter muscles during voluntary and spontaneous smile. Responses were repeated three times and recorded as negative, weakly positive or strongly positive according to activity observed. The best response was used for analysis.ResultsThirty healthy adults (median age 34 years, range 25-69 years) participated. Overall, 92% of masseter muscles were activated during voluntary smile (22% strong, 70% weak). Seventy seven percent of masseter muscles were activated in spontaneous smile (12% strong, 65% weak). Temporalis muscle was activated in 62% of responses in voluntary smile (15% strong, 47% weak) and in 45% of responses in spontaneous smile (13% strong, 32% weak). No significant difference was found for males vs females or closed vs open mouth smiles. There was no significant difference in responses between voluntary and spontaneous smile for temporalis and masseter, and use in voluntary smile did not predict activity in spontaneous smile.ConclusionsOur study has shown that masseter and temporalis are active in a high proportion of healthy adults during voluntary and spontaneous smile. Further work is required to determine the relationship between pre-operative donor muscle activation and post-operative spontaneous smile, and whether masticatory muscle activity can be up-regulated with appropriate training.