Anatomy of the nasolabial fold: the keystone of the smiling mechanism.

The nasolabial fold is absent in the face of the newborn, disappears in the paralyzed face, but is retained in the face upon death. There is very little information in the literature on what makes up the fold. Four fresh cadavers were studied by taking a strip of facial tissue from each at right angles to the fold; all soft tissue layers through the face were included. Microscopic studies of the strips showed the fold to be made up of (1) dense fibrous tissue, (2) muscle fibers branching from the levator muscles of the upper lip, and (3) striated muscle bundles originating in the fold fascia. The studies also revealed the lip elevator muscles and the "fold muscles" coursing down the lip to traverse the orbicularis oris and insert into the dermis of the upper lip, the cutaneous vermilion junction, and vermilion. The smile is formed in stages, the first stage raising the lip to the fold by the levator muscles and the muscle bundles originating in the fold. The lip meets resistance at the fold because of cheek fat. The second stage involves the raising of the lip and fold upward by the levator muscles of the upper lip. Clinically, this study relates to reanimating the paralyzed face.