There are two types of paired muscles in the forehead and brow, elevators and depressors. Brow elevation is due to the paired frontalis muscles that are composed of two distinct parts, a static component and a mobile component. The frontalis muscle does not originate from or insert into the bone. The superior half of the frontalis is relatively static secondary to its close adherence to the galea aponeurotica, which serves as its origin. The inferior half of the frontalis hangs freely and inserts into the orbital portion of the orbicularis oculi. This mobile component provides the entire range of motion for the muscle resulting in eyebrow elevation. When the frontalis muscle contracts superiorly directed forces are translated across the orbicularis oculi and the lower brow skin adherent to it. The mobile soft tissue of the lower brow is pulled up into the fixed superior forehead skin and soft tissue, resulting in deep transverse lines in the planes created by the deep dermal insertions between the skin and frontalis muscle. Laterally, the frontalis muscle fuses into a dense network of fascia referred to as the zone of adherence. This region lies along the palpable superior temporal line and ends inferiorly at the zygomaticofrontal suture at a convergence of fascia known as the orbital ligament.4

Several paired muscles are found along the brow and antagonize the action of the frontalis. Collectively, these muscles are referred to as forehead depressors. These include the corrugator supercilii, the orbicularis oculi, the procerus, and the depressor supercilii muscles. The corrugator supercilii originate from the frontal bone just superior to the nasal bones and insert into the dermis of the medial brow. The corrugator is composed of two heads, the oblique and the transverse. The coordinated actions of the two heads pull the brow down and medial across the glabella, resulting in vertical glabellar lines. Thus, hyperactivity may lead to the classic “angry” appearance. The orbicularis oculi muscles are oriented at right angles to the inferior border of the frontalis muscle for much of the brow’s length. Their strong sphincter function exerts a downward pull on the frontalis, creating periorbital wrinkles or “crows’ feet.” The procerus muscles originate on the nasal bones and cartilages as a single muscle belly. Superiorly, the muscles are paired and insert into the medial aspect of the frontalis muscle and the overlying dermis. Contraction of the procerus results in depression of the medial brow and the creation of transverse skin lines along the root of the nose. The depressor supercilii is found superficial to the corrugator muscle and its origin is on the frontal process of the maxilla, inferior to the origin of the corrugators. It inserts obliquely onto the medial fibers of the frontalis muscle, superior to the medial brow. The superficial position of the depressor supercilii is important to be aware of because inadvertent transection may result in over-elevation of the medial brow and a “shocked” appearance. In unison with the oblique head of the corrugators and the medial aspect of the orbicularis oculi, the depressor supercilii opposes the action of the frontalis causing depression of the medial brow and oblique glabellar skin creases.

The facial nerve (cranial VII) supplies the motor innervation to the mimetic muscles of the forehead and brow. The frontal (temporal) branch of the facial nerve supplies the frontalis, the superior portion of the orbicularis oculi, the superior aspect of the procerus, and the transverse head of the corrugator supercilii muscles. The zygomatic branch supplies the medial orbicularis oculi, the oblique head of the corrugator supercilii, the inferior aspect of the procerus, and the depressor supercilii muscles. The frontal branches course from a point 5 mm below the tragus to a point 15 mm above the lateral brow. Over the zygomatic arch, they are found about 2.5 cm lateral to the lateral canthus, placing them halfway between the lateral canthus and the inferior helix.5

Sensory innervation to the brow is by means of branches of the ophthalmic division of the trigeminal nerve (cranial nerve V). The paired supraorbital and supratrochlear nerves supply the central and medial forehead, respectively. The supraorbital nerves exit from the supraorbital foramen an average distance from midline of 2.42 ± 0.04 cm in females and 2.56 ± 0.05 in males.6 They then split into superficial (or medial) and deep (or lateral) branches to supply the forehead and scalp. The deep division supplies the frontoparietal region and can be injured along its course from the main nerve trunk, where it runs superiorly between the galea and periosteum. It pierces the galea 2 to 2.5 cm above the orbital rim and continues superiorly within 1 to 2 cm of the temporal fusion plane. If
this nerve branch is injured, it is often secondary to traction injury with the dissector or to transection by the coronal incision and results in paresthesia over the temporoparietal scalp. The superficial branch is shorter, more medial, and less often injured in browlifts. The superficial branch pierces the frontalis muscle early in its course, running superficial to the muscle belly. It supplies the area of the lower/mid-forehead along the mid-pupil line. The lateral forehead is supplied by the auriculotemporal branch of the mandibular division (V3) of the trigeminal nerve (cranial nerve V).

The blood supply to the forehead and brow is robust. Several major blood vessels to the upper face and brow, including the superficial temporal artery and facial artery, are branches of the external carotid artery. These vessels supply the medial canthal region via the angular artery and lateral canthal region via the frontal or anterior branch of the superficial temporal artery. The majority of the forehead and mid-scalp is supplied by branches of the internal carotid artery, specifically the supraorbital and supratrochlear arteries.

The venous drainage mirrors the arterial supply with some variation. One specific vein is relatively consistent and is referred to as the sentinel vein (medial zygomatico-temporal vein). The sentinel vein travels perpendicular through the temporalis fascia approximately 2 cm lateral to above the lateral canthus.4 Because of the consistent nature of its location, the sentinel vein must be identified and care must be taken not to accidentally tear the vessel during lateral dissection. This approach will avoid post-op ecchymosis and impaired visualization of the surgical field.