Electromyographic Activity of the Masseter and Temporalis Muscles Following Hyaluronic Acid Injection in the Mandibular Angle: A Longitudinal Secondary Analysis of Clinical Data

Objectives: Hyaluronic acid augmentation of the mandibular angle has become a widely performed procedure for improving lower facial contour and definition; however, because of its anatomical proximity to the masseter muscle, concerns remain regarding possible functional effects on the stomatognathic system. This longitudinal study investigated whether hyaluronic acid injection in the mandibular angle could affect neuromuscular recruitment patterns of the masseter and temporalis muscles through surface electromyography assessment. Methods: Ten adults were assessed at baseline and at 15, 30, and 60 days after injection. Electromyographic activity was recorded during mandibular tasks and chewing conditions. Repeated measures ANOVA with Bonferroni correction was used for statistical analysis (p < 0.05). Effect sizes were calculated using partial eta squared (η2p), and pairwise comparisons were explored using Cohen’s d. Data are presented as means with 95% confidence intervals (95% CIs), and individual trajectories were analyzed to characterize temporal patterns and within-subject variability. Results: Most variables did not show significant changes over time (p > 0.05), with small-to-moderate effect sizes. Significant reductions were observed in the right masseter during left laterality (p = 0.03; η2p = 0.17) and in the left masseter during maximum voluntary contraction with and without parafilm (p = 0.02–0.04; η2p = 0.19–0.22). These findings were temporary and were not consistently identified among subjects. Chewing conditions remained stable across all time points (p > 0.05). Conclusions: Hyaluronic acid injection in the mandibular angle was not associated with clinically relevant electromyographic changes. The observed variations were transient, showed small-to-moderate effect sizes, and demonstrated interindividual variability, suggesting preservation of neuromuscular function and physiological adaptive responses.