Periocular Anatomy

Juan C. Jiménez-Pérez

1.1 Eyelids

The main function of the eyelid is to protect the eye. The eyelids attach to orbital bones via the medial canthal tendon (MCT) and lateral canthal tendon (LCT) forming two opposite arches. The lateral canthus is usually approximately 2 mm higher than the medial canthus. The horizontal length of the eyelid is about 30 mm and the vertical aperture height (palpebral fissure) is about 10 mm. The adult upper eyelid rests about 1.5 mm below the superior limbus of the cornea, while the lower eyelid rests at the level of the inferior limbus. The peak of the upper eyelid is at the medial border of the pupil, and the lowest point of the lower eyelid margin is the lateral aspect of the pupil. The eyelid margin can be conceptually divided into anterior (skin and orbicularis muscle) and posterior (tarsus and conjunctiva) lamellae for the purposes of eyelid reconstruction.¹^,²

1.2 Eyelid Layers

The layers of the upper eyelid from anterior (superficial) to posterior (deep) are skin, orbicularis oculi muscle, orbital septum, preaponeurotic fat, levator palpebrae superioris muscle, Müller muscle, tarsus, and conjunctiva (▶ Fig. 1.1). It is important to understand that the different layers of the eyelid are not always present in all eyelid sections and vary by vertical height within the eyelid.

1.2.1 Skin

The eyelid skin is the thinnest in the body. It is composed of keratinized stratified squamous epithelium and has no subcutaneous fat. With aging, skin loses elasticity and becomes thinner due to breakdown of collagen. The upper eyelid crease is formed from levator aponeurosis attachments to the skin and pretarsal orbicularis oculi muscle. The crease is usually higher in women and Caucasians compared to men and Asians. The crease is approximately at the same height as the superior tarsal border in Caucasians. In Asians, it is about 4 mm above the eyelid margin due to a lower fusion of the orbital septum with the levator muscle, allowing the preaponeurotic fat to fall more anteriorly and inferiorly in the upper eyelid.¹^,²

1.2.2 Orbicularis Oculi Muscle

The orbicularis oculi muscle is the protractor of the eyelid. It extends from the upper and lower eyelid margin in a circumferential fashion to the superior and inferior orbital rims, respectively. It is anatomically divided into the palpebral (pretarsal and preseptal) and orbital portions (▶ Fig. 1.2). Innervation by the zygomatic and temporal branches of facial nerve (seventh cranial nerve) allows the muscle to contract and close the eyelids. The involuntary closure or blinking is mediated by the palpebral portion, whereas the voluntary closure is mediated by the orbital portion of the muscle. A portion of the orbicularis, the muscle of Riolan, is visualized along the middle of the eyelid margin, forming the gray line.¹^,²

1.2.3 Orbital Septum

The orbital septum is a thin, multilayered, inelastic, fibrinous tissue that demarcates the anterior extent of the orbital soft tissues. The orbital septum arises from the arcus marginalis, a dense periosteal fusion of orbital septum, periorbita, and pericranium that circumferentially lines the orbital rim (▶ Fig. 1.3). It inserts onto the upper and lower eyelid retractors and provides support. Aging causes septum attenuation allowing the orbital fat to move forward via pseudoherniation. The orbital septum is an important anatomical landmark that divides the superficial skin and orbicularis muscle from the deeper structures of the orbit and functions as a barrier to infections and spread of tumors.¹^,²

Fig. 1.1 Cross-section of the upper and lower eyelid anatomy. (From Chen WP, Oculoplastic Surgery: The Essentials. New York: Thieme Medical Publishers; 2001.)

1.2.4 Preaponeurotic Fat

The preaponeurotic fat is a relatively avascular collection of adipose tissue situated between the orbital septum anteriorly and levator palpebrae superioris muscle posteriorly. In the upper eyelid there are two fat pads: the medial and the central. They are each enclosed in a thin fibrous sheath. The medial fat pad is usually paler in color and smaller compared to the distinctly yellow central fat pad.³ The lacrimal gland fills the analogous space in the lateral upper eyelid. The lower eyelid contains three fat pads: the medial, central, and lateral (▶ Fig. 1.4). When fat is visible through a traumatic eyelid laceration, it indicates that the injury has extended to a depth beyond the orbital septum. In such instances, after proper wound cleaning, exploration, and repositioning of fat, the septum must not be sutured so as to avoid cicatricial eyelid retraction.⁴

1.2.5 Retractor Muscles

Upper Eyelid Retractors

Opening of the upper eyelid is mediated by two muscles: the levator palpebrae superioris and Müller muscles. The levator muscle is responsible for the voluntary elevation of the upper eyelid and is the primary retractor muscle. It originates in the orbital apex from the periorbita of the lesser wing of the sphenoid bone just above the superior rectus muscle origin in the annulus of Zinn. The muscular portion measures 40 mm in length, while the aponeurosis is 14 to 20 mm in length. The muscle traverses in the superior orbit from the apex in an anterior direction until it interfaces with Whitnall

ligament, where it changes to an inferior direction. The aponeurosis inserts onto the anterior tarsal surface with additional loose attachments to the skin creating the lid crease. As the aponeurosis travels inferiorly, it spreads out creating medial and lateral horns. The medial horn of the levator aponeurosis is thinner and more delicate than the lateral horn. The lateral horn divides the lacrimal gland into palpebral and orbital lobes (▶ Fig. 1.5). The muscle is innervated by the superior division of the oculomotor nerve (third cranial nerve).¹^,²^,⁵

Fig. 1.2 The eyelid protractors. (From Watanabe K, Shoja MM, Loukas M, Tubbs RS. Anatomy for Plastic Surgery of the Face, Head, and Neck. New York: Thieme Medical Publishers; 2016.)

Fig. 1.3 The orbital septum. (From Wobig JL, Dailey RA. Oculofacial Plastic Surgery: Face, Lacrimal System, and Orbit. New York: Thieme Medical Publishers; 2004.)

Fig. 1.4 The preaponeurotic fat. (From Chen WP, Oculoplastic Surgery: The Essentials. New York: Thieme Medical Publishers; 2001.)

Müller muscle (superior tarsal muscle) is a smooth muscle innervated by sympathetic nerve fibers contributing approximately 2 mm of upper eyelid involuntary elevation. It originates from the deep surface of the levator aponeurosis at the level of Whitnall ligament and inserts at the superior tarsal border (▶ Fig. 1.5). The conjunctiva is deep to Müller muscle, and the peripheral vascular arcade is located superficial to the muscle above the superior tarsal border.¹^,²

Fig. 1.5 The upper and lower eyelid retractors. (From Watanabe K, Shoja MM, Loukas M, Tubbs RS. Anatomy for Plastic Surgery of the Face, Head, and Neck. New York: Thieme Medical Publishers; 2016.)

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