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The human smile is a complex coordinated activity of mimetic muscles predominantly recognizable by a superolateral pull at the commissure and elevation of the upper lip. The aim of this study was to revisit the muscles of facial expression responsible for these motions, evaluate their relational anatomy and orientation, and relate this to optimal positioning of free muscle transfer in smile reanimation.
Nineteen hemifaces from fresh cadaveric specimens were dissected. A subsuperficial muscular aponeurotic system skin flap was elevated to expose the zygomaticus major, zygomaticus minor, levator labii superioris, and levator labii superioris alaeque nasi. Muscle location, length, width, angle of pull, and any anatomic variation were noted.
All specimens had zygomaticus major, levator labii superioris, and levator labii superioris alaeque nasi muscles present bilaterally. Conversely, the zygomaticus minor was present in only 10 of 19 hemifaces. There was no significant difference in muscle length, width, or line of pull between specimen sides. Of all the assessed muscles, the zygomaticus minor had the most transverse line of pull, at 31.6°; the zygomaticus major was more oblique with a line of pull of 55.5°; and the levator labii superioris and levator labii superioris alaeque nasi were oriented almost vertically with angles of 74.7° and 79.0°, respectively.
The mimetic muscle vector is quite vertical in comparison to the angle obtained by traditional dynamic smile reconstruction surgeries. A more vertical vector, especially at the upper lip, should be considered in smile reconstruction.

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