Double or bifid zygomaticus major muscle: anatomy, incidence, and clinical correlation.

The anatomy of the double or bifid zygomaticus major muscle is investigated in a series of 50 hemifacial cadaver dissections. The double zygomaticus major muscle represents an anatomical variation of this muscle of facial expression. This bifid muscle originates as a single structure from the zygomatic bone. As it travels anteriorly, it then divides at the sub-zygomatic hollow into superior and inferior muscle bundles. The superior bundle inserts at the usual position above the comer of the mouth. The inferior bundle inserts into the modiolus below the corner of the mouth. The incidence of the double zygomaticus major muscle was 34% in the present study, as it was found to be present in 17 of 50 cadaver dissections. This study shows that variation in the individual morphology of the mimetic muscles can be a common finding. Clinically, the double or bifid zygomaticus major muscle may explain the formation of cheek "dimples." The inferior bundle was observed in several specimens to have a dermal attachment along its mid-portion, which tethers the overlying skin. When an individual with this anatomy smiles, traction on the skin may create a dimple due to this dermal tethering effect.

The anatomy of the labiomandibular fold.

The anatomy of the labiomandibular fold was evaluated in a series of 12 fresh hemifacial cadaver dissections. The techniques of methylene blue dye injection, histologic evaluation, and gross dissection all confirm that the labiomandibular fold has distinct anatomic boundaries. The superior boundary is formed by the cutaneous insertion of the depressor anguli oris muscle at the labiomandibular crease. The inferior boundary is determined by the mandibular ligament, which has been previously described. The cutaneous insertion of the depressor muscle and the mandibular ligament act as relative points of fixation. The dynamic forces of both aging and facial animation act about these two points to create the typical appearance of the labiomandibular fold. This anatomy is consistent with that seen in other areas of the face such as the nasolabial and nasojugal folds, where the dermal insertion of muscle and/or fascia defines an anatomically distinct region. Clinically, this anatomy may suggest that a subcutaneous plane of dissection during the face lift procedure may allow manipulation and reduction of the fat that was noted lateral to the dermal insertion of the depressor anguli muscle. Subcutaneous dissection also avoids lateral pull on the platysma muscle, which may tend to accentuate and distort the labiomandibular crease due to its intimate association with the overlying depressor muscle.