An anatomically based study of the mechanism of eyebrow ptosis.

The development of eyebrow ptosis with aging is commonly attributed to progressive laxity of scalp and forehead soft tissues. If the change in eyebrow position with aging resulted entirely from this basic mechanism of tissue stretching, uniform lowering of the medial and lateral eyebrow segments should occur. Clinical observations show, however, that the lateral eyebrow segment usually becomes ptotic earlier than the medial segment, indicating that a more complex mechanism exists. To clarify this process, anatomic studies were done on 20 (40 half-head) fresh cadaver specimens. Histologic studies also were performed to complement the gross anatomic findings. These studies confirm that the mechanism producing eyebrow ptosis has a relatively greater effect on the lateral eyebrow segment. The lateral eyebrow has less support from deeper structures than the medial eyebrow, and the balance of forces acting on the eyebrow selectively depresses the lateral segment. Structures that may promote mobility and gravitational descent of the eyebrow, especially the lateral eyebrow segment, are (1) the galea fat pad, (2) the preseptal fat pad, and (3) the subgalea fat pad glide plane space. Three forces that act on the lateral eyebrow are (1) frontalis muscle resting tone, which suspends that eyebrow segment medial to the temporal fusion line of the skull, (2) gravity, which causes the soft-tissue mass lateral to the temporal line to slide over the temporalis fascia plane and push the lateral eyebrow segment downward, and (3) corrugator supercilii muscle hyperactivity in conjunction with action of the lateral orbicularis oculi muscle, which can antagonize frontalis muscle activity and directly facilitate descent of the lateral eyebrow. The axis point for these forces is the temporal fusion line of the skull near the superior orbital rim. The interaction of those structures and forces contributing to the mechanism producing eyebrow ptosis is discussed. Derived concepts are applied to the execution of the forehead lift procedure.