The soft tissues of the forehead can be divided into five aesthetic subunits: central forehead, lateral temporal units, and eyebrows. The bony landmarks of the zygomatic arches, orbital rims, and nasal root represent the lower anatomic boundaries, while a natural hairline represents the upper limits. The temporal line divides the lateral forehead from the temporal regions, and the orbital rim serves as a consistent marker in the evaluation of brow ptosis.

An understanding of subunit interrelation is essential for conceptual planning as well as surgical outcome. The central forehead is a direct extension of the scalp and is layered, from superficial to deep, with skin, connective tissue, galea aponeurotica, loose areolar tissue, and periosteum. The first three layers of the central forehead are tightly held together, in contrast to the loosely attached skin and fascia of the temporal region. Within the soft tissues overlying the supraorbital ridges is a confluence of muscular insertions, which include the paired frontalis, orbicularis oculi, corrugator supercilii, procerus, and depressor supercilii muscles. The interplay of these muscles is responsible for the wide array of brow expressions as well as the associated observed changes with aging.

The galea aponeurotica separates along the superior origin of the frontalis muscle to form the superficial and deep galeal planes. These planes envelop the muscles along their anterior and posterior surfaces and extend to the lower forehead. Along the brow region, numerous fibrous septa from the frontalis muscles penetrate the thin superficial galea and interdigitate into the orbicularis oculi, procerus, and the overlying dermis. The frontalis muscle is the primary elevator of the brow, and contraction of this muscle produces transverse forehead rhytides.

There are numerous eyebrow depressors: the orbicularis oculi, the depressor supercilii, the procerus, and the corrugator supercilii, all of which are superficial to the frontalis. The orbicularis oculi muscle serves as a powerful lateral brow depressor due to the lack of a corresponding muscular elevator to oppose it. It is located just deep to the skin, making it a very superficial muscle, particularly the palpebral portion, which underlies the thinnest skin of the body. Repeated contraction of the orbicularis results in thin lateral rhytides, which are often referred to as “crow’s feet.” More medially, a smaller muscle—which some consider to be a part of the orbicularis oculi—is the depressor supercilii, a brow depressor that is frequently the target of chemodenervation in order to provide a “chemical brow lift.” Deep to the orbicularis oculi at the glabella, the procerus muscle originates from the nasal bones and upper lateral cartilages and has vertically oriented fibers that insert into the dermis. The procerus causes inferior and medial displacement of the medial eyebrow with resultant horizontal rhytides in the glabella and upper nasal radix. The corrugators superciliorum are paired muscles originating from the superomedial orbital rims and lying just deep to the procerus. Their fibers are obliquely oriented and insert into the medial eyebrow dermis. They pull the brow medially and inferiorly, resulting in the vertical and oblique glabellar rhytides commonly termed “frown lines.” Deep to the corrugators are the terminal fibers of the frontalis, which overly the pericranium. The periorbita and the pericranium merge to form the orbital septum, which originates from a fibrous ring around the periphery of the orbit known as the arcus marginalis. This ligamentous structure serves to anchor the periorbital soft tissues to the underlying bone, limiting their mobility when acted upon by muscles such as the frontalis.