2.1 Introduction

A thorough appreciation of the detailed anatomic relationships in the eyelids and upper face is crucial to the success of the upper facial surgeon. Here, we present functional anatomy through the surgeon’s lens. We focus on those structures most important to the preoperative planning and intraoperative technique of the surgeon addressing the eyelid and eyebrow.

The layers of the face, from superficial to deep, include skin, subcutaneous and superficial fat compartments, superficial musculoaponeurotic system (SMAS), retaining ligaments, mimetic muscles, and deep fat compartments and bone (Fig. 2.1). The forehead contains all of these layers. The forehead extends from the hairline superiorly to the superior orbital rim inferiorly, at which point it becomes the upper eyelid. The eyelid has both continuous and analogous structures to the forehead and some key distinct features.

The most important function of the eyelids is eye protection. The surgeon must ensure that eyelid elevation, removal of upper eyelid skin or muscle, or eyebrow elevation will not expose the ocular surface to excessive environmental factors and prevent eyelid closure. The surgeon who addresses both aesthetic and functional issues of the upper face must, therefore, be familiar with the complex interplay of multiple structures that contribute to eyelid position, eyelid opening and closing, and ocular surface protection. Lagophthalmos, the presence of eyelid opening upon gentle closure, can result from aggressive surgery. This condition can lead to chronic dry eye and corneal exposure, known as exposure keratopathy, which is uncomfortable and potentially sight-threatening.

2.2 Skin

The skin is the largest organ in the body in surface and weight. The functions of the skin include protection, temperature regulation, and sensation. It consists of two layers: the epidermis and dermis. Beneath the dermis lies subcutaneous fatty tissue. Here, we describe the unique relevant features of the skin of the upper face and eyelids.

The forehead skin contains the cilia that make up the eyebrows, perhaps the most distinctive and expressive feature of the forehead. The eyebrows sit at the superior orbital rim in males and above the rim in females. The eyebrow can be divided medially to laterally into a head, body, and tail, and the orientation of the eyebrow hairs varies by location. This is important, as incisions near the eyebrows should be parallel to the hair follicles to minimize damage. A telltale sign of aging is the descent of the eyebrows below their native, youthful position. Repair of brow ptosis is, therefore, a critical component of aesthetic facial surgery and the subject of later chapters.

The eyelid skin is the thinnest skin in the body and, in the non-Asian eyelid, is devoid of subcutaneous fat, unlike the skin of the brow and forehead. It is critical that the upper facial surgeon appreciates the transition between the thicker brow and thinner eyelid skin types, as measurement of the amount of eyelid skin is important to preoperative planning in upper eyelid blepharoplasty and external ptosis surgery. Typically, a fairly distinct transition is present between the two types of skin. At times, this transition correlates with the inferior extent of the brow hairs; at times, the brow hairs extend onto the eyelid skin itself, and therefore, this is not a useful metric. Another clue that may help the surgeon differentiate between the brow and eyelid skin is the presence, oftentimes, of a slight color difference between the two, with the eyelid skin lighter than the adjacent brow skin. The mobility and flexibility of the eyelid skin is crucial to allow free eyelid movement and function. Cicatrization secondary to excessive tension with wound closure, scarring, or burns carries risk of eyelid retraction or ectropion.

The eyelid crease, measured by the margin–crease distance (MCD), is an important external landmark formed by the attachment of anterior projections of the levator aponeurosis to the septae between orbicularis fibers overlying the tarsus or pretarsal orbicularis. ¹ Attention to the location of the upper eyelid crease is crucial in the examination of any patient. To obtain the best aesthetic result, review of older photographs and careful discussion with the patient are critical. The normal position of the eyelid crease is approximately 8 to 10 mm above the eyelid margin in Caucasian women and approximately 6 to 8 mm above the margin in Caucasian men. In Asian individuals, it is typically a few millimeters lower. ²

The upper eyelid fold, measured by the margin–fold distance (MFD), consists of loose upper eyelid skin and subcutaneous tissue anterior to the confluence of the levator aponeurosis and orbital septum, formed with the patient’s eyes open and at rest. In patients with significant dermatochalasis (excess skin) or herniated orbital fat, the eyelid fold may overlie and obscure the eyelid crease and eyelid margin.

2.3 Subcutaneous Tissue and Fat Compartments

Throughout the face are superficial and deep fat compartments, divided by the SMAS in the face itself or by the SMAS’ counterparts in the forehead (galea) and temporal region (temporoparietal fascia). The superficial fat pads lie just deep to the skin and serve protective functions. The implications of their changing morphology in the aging face are major. ^{3 ,​ 4} The deep fat pads’ principal functions are mechanical support and facial volume. ⁵

From medial to lateral, the superficial fat compartments in the forehead are the central fat pad, the middle forehead fat pads, and the lateral temporal-cheek fat pads (Fig. 2.2). All three are bounded superiorly by the hairline and extend inferiorly to variable degrees. The inferior boundary of the central fat pad is the dorsum of the nose, and the inferior boundary of the middle fat pad is the orbicularis retaining ligament at the superior orbital rim. The paired lateral temporal-cheek fat pads extend the farthest inferiorly, down the extent of the face to the cervical area.

There are three superficial fat compartments in the periorbital area: the superior orbital fat pad, the inferior orbital fat pad, and the lateral orbital fat pad (Fig. 2.2). The superior and inferior orbital fat pads lie in the same plane as the superficial fat pads of the forehead but are in the upper and lower eyelid, respectively; they are deep to skin but superficial to the orbicularis oculi muscle. These are bounded laterally by the lateral canthal tendon, medially by the medial canthal tendon, and at the orbital rim by the orbicularis retaining ligament. The third orbital fat compartment is the lateral orbital fat pad, which lies lateral to the lateral canthus and is bounded laterally by the lateral temporal-cheek fat pad.

2.4 Superficial Musculoaponeurotic System

The SMAS is a complex network of layers of connective tissue that envelops and joins the skeletal muscles of the face. The SMAS is important surgically as a key target in face lifting and functionally as a distributor of facial expression. The SMAS extends from the galea aponeurotica to the platysma and is connected to the dermis via vertical septae. As mentioned above, the SMAS takes on different names in different regions: galea in the forehead, temporoparietal fascia in the temple, SMAS in the face itself, and platysma in the neck. The SMAS has little significance to the eyelid surgeon but is important to the upper facial surgeon during brow lifting in its relation to the temporal branch of the facial nerve, cranial nerve VII. The plane of dissection in brow lifting is within a loose areolar tissue layer that lies between the temporoparietal fascia superficially and the deep temporalis fascia overlying the temporalis muscle deeply. At the zygomatic arch, the temporal branch of the facial nerve is a deep structure that courses along periosteum. As it travels superomedially toward the glabella, the nerve becomes superficial and lies within this temporoparietal fascia. Thus, it is superficial to the plane of dissection (see “Unique Considerations for the Upper Facial Surgeon: Facial Danger Zones: Avoiding Nerve Injury”).

2.5 Retaining Ligaments, Canthal Tendons, and Orbital Septum

In addition to the fascial network provided by the SMAS are firmer condensations of fibrous connective tissue called true retaining ligaments. They are located in constant anatomic locations and separate fascial planes and compartments. These retaining ligaments serve as facial support structures, anchoring the dermis to the underlying periosteum of the facial skeleton and stabilizing the skin, SMAS, and underlying deep fascia. As such, they are important targets in surgery. ⁶

In the lateral forehead, the deep temporal fascia, the temporoparietal fascia, and the periosteum of the frontal bone fuse together to form the conjoint tendon (also known as the conjoined tendon, superior temporal septum, frontal ligament, or temporal fusion line). ^{7 ,​ 8} The conjoint tendon extends superotemporally from the lateral brow and inferiorly along the lateral orbital rim and zygomatic arch ^{6 ,​ 7} (Fig. 2.3). Complete release of this strong fascial attachment is critical to achieving a complete, long-lasting elevation during endoscopic browplasty.

The orbicularis retaining ligament extends circumferentially around the orbit along the orbital rim, inserting 2 to 3 mm outside of the rim, peripheral to the insertion of the orbital septum (Fig. 2.3). The ligament extends from the fascial tissues underlying the orbicularis oculi muscle (the galea superiorly and the SMAS laterally and inferiorly) to the periosteum of the orbital rim, and its fusion with the orbital septum is known as the arcus marginalis. ⁹ ID#b3a626a000_10^{– 11} In its course around the orbit, the orbicularis retaining ligament varies in thickness, length, and tautness. Of relevance to the upper facial surgeon, the ligament is relatively lax laterally compared to medially, likely contributing to the phenomenon of lateral hooding often encountered in dermatochalasis. ⁹

The orbicularis retaining ligament contributes to the lateral canthal tendon, a structure that similarly anchors tissue to the periosteum. However, rather than linking superficial soft tissue to periosteum, the lateral canthal tendon anchors the tarsus to the periosteum of the lateral orbital rim at the lateral canthus, where the upper and lower eyelids meet. The tarsus is a structure that provides a majority of support to the upper and lower eyelids. In the medial canthus, the medial canthal tendon anchors the tarsus to the anterior and posterior lacrimal crests through an anterior and posterior limb, respectively (Fig. 2.4). Here, the medial canthal tendon and portions of the orbicularis oculi muscle envelop the lacrimal sac in the bony lacrimal sac fossa and form the lacrimal pump.

The orbital septum is a membranous sheet that serves as the anterior boundary of the orbit. It extends from the orbital rims to the eyelids. The septum is a barrier to infection and hemorrhage within the orbit. The eyelid protractors (depressors) lie anterior to the septum, and the eyelid retractors (elevators) lie posterior to the septum.

Whitnall’s ligament, or the superior transverse ligament, is an important structure in the orbit in its relationship to the levator palpebrae superioris muscle (Fig. 2.5). It runs transversely from the trochlea in the medial orbital wall to the lacrimal gland in the lateral orbital wall and is thought to act as a fulcrum for the levator muscle. ¹² It is white in color and often encountered in large levator advancements or resections.

False retaining ligaments are less robust bands of connective tissue that weave within the soft tissue layers of the face and are of little relevance to the upper facial surgeon.

2.6 Mimetic Muscles and Eyelid Retractors

The mimetic muscles, or muscles of facial expression, animate the face and contract to create rhytids or wrinkles. The eyelid protractor, the orbicularis oculi muscle, is responsible for eyelid closure and eye protection. The eyelid retractors, the levator palpebrae superioris and Müller’s muscle, are responsible for opening the upper eyelid.

The muscles of the forehead are the frontalis, procerus, depressor supercilii, and corrugator supercilii; these are all paired muscles except the procerus, which lies in the midline (Fig. 2.6). The frontalis elevates the eyebrow, and the remaining muscles depress the eyebrow. The frontalis originates from the galea and inserts into the more inferior musculature: the procerus, corrugator, and orbital orbicularis. The frontalis’ lateral-most insertions do not reach the tail of the brow; as such, the tail lacks an elevator. For this reason, injection of botulinum toxin in the lateral orbicularis muscle inferior to the eyebrow can be a useful adjunct to upper facial surgery to lift the brow.

The eyebrow depressors are the procerus, depressor supercilii, corrugator supercilii, and orbicularis oculi. The procerus lies in the midline, originating at the aponeurosis of the nasalis muscle and inserting over the central forehead and medial brow, where it intersects with the frontalis muscle. The depressor supercilii and corrugator supercilii muscles cause depression of progressively more lateral portions of the brow. The depressor supercilii originates just medial to the medial canthus, from the frontal process of the maxillary bone, and extends underneath the procerus to insert on the medial brow skin. The corrugator supercilii originates quite close to the depressor supercilii, though slightly superior and on the frontal bone. This muscle then divides into two heads: the oblique courses similarly to the depressor supercilii, and the transverse assumes a more acute angle, running superolaterally. This muscle head also runs more deeply, coursing within the galeal fat pad.

The orbicularis oculi is the muscle of eyelid closure. It encircles the eye and consists of concentric muscle fibers divided into three contiguous parts: pretarsal, preseptal, and orbital. These names correspond to their anatomic location over the tarsus, orbital septum, and orbital rim, respectively. Voluntary contraction of the orbital portion of the orbicularis oculi muscle is responsible for forceful eyelid closure, and involuntary contraction of the pretarsal and preseptal portions is responsible for the spontaneous blink. The superior fibers of the orbicularis also act as eyebrow depressors, along with the muscles mentioned above. In the medial eyelid, the pretarsal orbicularis forms the lacrimal pump through attachments to the medial canthal tendon and posterior lacrimal crest of the lacrimal bone. With normal contraction and relaxation of the orbicularis oculi, tears are pumped through the canaliculi and lacrimal sac into the nasolacrimal duct. Laterally, the orbicularis inserts onto the lateral canthal tendon.

The mimetic musculature is responsible for rhytids, which run perpendicular to the direction of the muscle fibers and are popular targets for botulinum toxin injections. Thus, the vertically oriented frontalis and procerus muscles cause horizontally oriented skin wrinkles, whereas the corrugator is responsible for vertically oriented skin wrinkles (11’s). The orbicularis causes horizontal wrinkles lateral to the lateral canthus (crow’s feet).

The levator muscle originates in the posterior orbit from the lesser wing of the sphenoid bone and courses anteriorly. At the level of Whitnall’s ligament, the posterior muscular portion becomes the more anterior fibrous aponeurotic structure, and its trajectory turns inferiorly toward the tarsus. Near the tarsus, the aponeurosis divides into anterior and posterior portions. The posterior portion inserts on the anterior aspect of the inferior third of the tarsus. The anterior portion inserts on septae between fibers of the pretarsal orbicularis and subcutaneous tissue to form the eyelid crease. ^{1 ,​ 13} These anterior projections are less robust in the Asian eyelid, which contributes to the absence of a well-defined crease. In cases of severe aponeurotic ptosis, the levator aponeurosis may be variably disinserted and/or attenuated and displaced superiorly, making its intraoperative identification during external levator advancement surgery challenging, as it deviates from its natural anatomic position. The identification of the preaponeurotic fat pads is again crucial, as the aponeurosis can be identified just deep to these pads. The levator aponeurosis forms a medial horn, which attaches to the medial canthal tendon, and a larger lateral horn, which inserts on the lateral canthal tendon. The lateral horn of the levator divides the lacrimal gland into orbital and palpebral lobes (Fig. 2.5).

Müller’s muscle is a sympathetically innervated smooth muscle that is responsible for 1 to 2 mm of upper eyelid elevation. It originates on the undersurface of the levator muscle approximately at the level of Whitnall’s ligament and inserts on the superior border of the tarsus. The superior palpebral arterial arcade lies between the levator aponeurosis and Müller’s muscle just superior to its insertion at the tarsus. Weak attachments exist between Müller’s muscle and the overlying levator muscle. Firm attachments exist between Müller’s muscle and the underlying conjunctiva. Resection of Müller’s muscle in the Müller’s muscle–conjunctival resection (MMCR) procedure has been shown to plicate and thus advance the levator aponeurosis. ¹⁴