What causes asymmetric smiles when contouring the lower face?
A shallow toxin injection into the masseter muscle when contouring the lower face causes asymmetric smiles, with the most frequently reported change being a reduction in the width of the mouth upon smiling. These could be explained by the degree of toxin diffusion depending on the amount and location of the toxin injection. This is because the toxin spreads into the risorius muscle, located in front of the masseter.
To prevent toxin from diffusing into the risorius after injection into the masseter, the needle is injected deeply into the masseter until the tip is just above the mandibular surface. In particular, the recommended dosage for someone with a well-developed risorius is under 10 U at each side to avoid toxin diffusion on the masseter surface.
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