Comprehensive Overview of Eyelid Anatomy

Our eyes are our primary sensory organ that allow us to understand our environment. At Idaho Eyelid and Facial Plastic Surgery, our surgeons and support team have a full understanding of anatomy of the eye and the eyelids that protection them. Our eyelids protect those eyes and serve many purposes, including shielding the eyeball from injury, controlling the amount of light entering the eye and preventing damages from that light, and lubricating the eyeball with tears secreted by the lacrimal gland. All these functions together help maintain the structural integrity of the eyeball and protect them from external influences.

The eyelid consists primarily of skin, soft subcutaneous tissue and a thin layer of muscle called the orbicularis oculi. Under this muscle is the septum which includes the fibrous orbital septum and tarsi. The eyeball is covered by a thin layer of tissue called the conjunctiva, and fat tissue aids in protecting it. The tissues can be divided into planes by structures called the septum. The orbital septum differentiates the orbital tissue from the lid. Behind the septum are a number of different other structures, such as the anterior and posterior lamellae. The anterior lamella consists of the skin and the orbicularis oculi muscle while the posterior lamella consists of the conjunctiva and the tarsus.

The Structure of the Eyelids

The upper eyelid starts at the eye and extends upwards joining the skin of the forehead and is distinguished from the forehead skin by the presence of eyebrows. Similarly, the lower eyelid starts at the eye margin and extends to join the skin of the cheek. Upon inspection, it is evident that the lower eyelid is looser (laxity) than the upper eyelid, particularly because the tissue within the cheek that blends with the lower eyelid is more dense.

At the top of the upper eyelid is a fold in the skin called a superior palpebral sulcus (skin crease). It lies approximately 8 to 11 mm above the margin of the upper eyelid and consists of levator aponeurosis fibers. These levators lift and retract the upper eyelids. Similarly, the skin fold in the lower eyelid is called the inferior palpebral sulcus. This lower skin fold is often more prominent in children and can become less prominent as one gets older. Anatomically, the inferior skin crease is seen around 3 to 5 mm below the outer aspect of the lid margin.

The inner aspect of the eyelid is called the inner canthal region, where the nasojugal fold is located. Anatomically, this fold lies between the orbicularis oculi and the levator labii superioris. The nasojugal fold is that area of the inner aspect of the eye where tears roll down and can accumulate (also called the tear trough). Similar to the nasojugal fold, the outer region of the eyeball is called the malar fold and runs from that outer location towards the nasojugal fold.

When the eyes are open, the space between the upper and lower eyelids is typically described as ‘fusiform’, or the palpebral fissure. Typically, the palpebral fissure measures between 28 to 30 mm wide and around 9 to 10 mm in height. When we are young, upper eyelids lie slightly higher than older individuals. There are two points at which the upper and lower eyelids meet; the medial canthus (inner) and the lateral canthus (outer). When examined along a horizontal plane, the medial canthal angle is located around 2 mm lower than the lateral canthal angle in Caucasians (3 mm lower in Asians). The nose lies around 15 mm on the inside of the medial canthus.

Skin and Subcutaneous Tissue of the Eyelids

The eyelid is primarily made of skin, the thinnest skin in the body. Eyelid skin is less than one mm thick. Sebaceous glands within the eyelids secrete an oily substance called sebum. These glands are in larger numbers at the nasal aspect of the eyelid. At the junction between the eyelids and forehead/cheek, the texture of the skin changes and becomes a lot thicker. Below the skin is a layer of thin connective tissue called a subcutaneous tissue along with a thin layer of fat. Typically, subcutaneous tissue is absent at points where the skin is attached directly to underlying ligaments (medial and lateral palpable ligaments). The skin and subcutaneous tissue can be subject to certain clinical conditions such as dermatochalasis and blepharochalasis.

Orbicularis Oculi Muscle Function

The orbicularis oculi muscle is important to eyelid function and facial expression. When it contracts and relaxes, the skin over the muscle moves as well. The orbicularis oculi muscle is attached to the skin through tissue that forms the superficial musculoaponeurotic system. The orbicularis oculi muscle consists of two parts: 1) the orbital part plays a role when the eyelids need to be tightly shut and is further split into pretarsal and preseptal segments; 2) the palpebral portion plays a role in winking and blinking. The muscle is supplied by the facial nerve then splits into different branches to supply these different muscles. The facial nerves travel under those muscle groups.

The orbital part of the orbicularis oculi muscle is also linked with other muscles responsible for facial expression. It starts from the inner margin of the orbit, further attaching to the upper and inner aspect of the orbital bone. The muscle fibers interact with the surrounding facial muscles (corrugator supercilii and frontalis muscle). The preseptal portion of the orbicularis oculi muscle consists of a deeper muscle. The fibers within upper/lower eyelids join to form the lateral palpebral raphe. The pretarsal portion is similar and its fibers run under the lateral palpebral raphe, traveling to the lateral orbital tubercle through the lateral canthal tendon.

Submuscular areolar tissue

The submuscular areolar tissue lies beneath the orbicularis oculi muscle, and is a loose connective tissue. It forms an area that divides the eyelid into a front and back portion. The fibers of the levator aponeurosis pass through the submuscular areolar tissue. A portion of these fibers contribute towards the development of the upper eyelid crease. In the lower eyelid, the fibers of the orbitomalar ligament passed through this plane.

Tarsi and Orbital Septum Structure

Tarsal plates

The tarsal plate helps the eyelids maintain their shape and integrity, is dense and fibrous tissue roughly 1mm thick and 29 mm in length. The superior tarsus and inferior tarsus are the two types of tarsi. The superior tarsus is of a crescent shape and measures around 10mm vertically in its central aspect. It narrows out as it traverses towards the nose and outer eyelid. The lower part of the superior tarsus forms the back of the eyelid, next to the eyeball conjunctiva. Likewise, the inferior tarsus lies in the lower eyelid and measures 3.5 to 5mm in height at its center. It is also in contact with the conjunctiva. Each tarsi is attached to the orbit margin through the medial and lateral palpebral ligament. Within the tarsal plates are tiny glands called mebomian glands. There are 25 glands as tall as the tarsus, and they open at a point just in front of the lid margin where the conjunctiva meets the skin.

Medial and Lateral Palpebral Ligaments

Also called the medial canthal tendon (MCT), the medial palpebral ligament is a band of fibrous tissue that gives structure to the inner aspect of the tarsal plates. It is similar to the orbicularis oculi muscle and the tear ducts. The MCT is comprised of an anterior limb formed by a small part of the superficial part of the orbicularis muscle (behind the tarsus). It travels along a horizontal plane but is also attached to the frontal bone through superior extension. The deepest part of the orbicularis muscle is behind the lacrimal crest and the lacrimal sac fascia. This is why the fascia of the lacrimal sac is therefore closely related to the MCT.

Lateral palpebral ligament

Also called the lateral canthal tendon (LCT), the lateral palpebral ligament is a band of fibrous tissue that originates from the tarsus and travels under the orbital septum and to the lateral orbital tubercle. The LCT is around 10.5mm long and 6.5 mm wide. The orbital septum and the LCT are distinguished by a pocket of fat. The LCT is attached to the outer part of the orbital rim through a superficial plane of fascia. This facia is called the superficial lateral canthal tendon and helps to keep the lateral canthus fixed in place.

Orbital septum

A band of tissue that separates a structure is called a septum. This is connective tissue that unites the orbital bone at the periosteum. The septum joins the lid retractors at the lid margins and includes layers that are connected to the anterior tissue framework. The septum functional mobility is quite similar to the eyelids. Traveling laterally, the septum follows the rim of the orbit at the arcus marginalis. Toward the nose, the septum runs across the supraorbital groove in front of the trochlea and along the posterior aspect of the lacrimal crest. Thus, the septum lies in front of the medial ligament and behind the lacrimal sac and Horner muscle.

The septum passes the lacrimal sac fascia, reaches the anterior lacrimal crest, then along the lower orbital rim beyond the zygomaticomaxillary suture. Small recesses result given separation from the zygomatic bone, also known as premarginal recess of Eisler which is filled with fat. The septum finally reaches the lateral orbital margin below the Whitnall ligament. The function of the septum is to aid the levator aponeurosis, and is considered during surgery at Idaho Eyelid and Facial Plastic Surgery.

Orbicularis retaining ligament

The orbicularis retaining ligament links the orbicularis oculi muscle to the lower orbital rim. While it is strong in the lower region, it is weak in the center. It is linked with tissue formed by fusion of the outer part of the orbicularis oculi and the deeper periosteum and temporalis fascia. This fusion, or orbital thickening, covers the frontal region of the zygomatic bone. The orbicularis retaining ligament typically thins with age and stretches more in the central region.

Upper lid retractors

The upper lid retractors are comprised of a group of muscles designed to elevate the upper eyelids. The levator palpebrae superioris (LPS) starts from the bottom aspect of the sphenoid bone located within the skull. The levator muscle and the superior rectus muscle are joined together by fibrous tissue. The Whitnall ligament, a suspensory ligament to support the upper eyelids, the superior orbit and the lacrimal syste, is similar to orbital fascia and lies near the LPS muscular junction. The LPS varies in thickness, and is thinner between the upper orbital rim and the Whitnall ligament. The LPS aponeurosis forms ‘horns’ called medial and lateral horns. The lateral horn runs through the lacrimal gland and splits into the palpebral lobe and the orbital lobe. The aponeurosis eventually reaches the superior tarsal plate having fused earlier with the orbital septum. A small part of the aponeurosis attaches to the lower aspect of the anterior part of the tarsal plate. Pars of this fusion extends forwards to pretarsal orbicularis oculi muscle and skin and forms the skin crease in the upper eyelid.

Eyelid Fat Pads

There are a variety of fat pads both within and around the eyelid. One layer of fat called the pre-aponeurotic fat is found right behind the orbital septum and in front of the levator aponeurosis. In the upper eyelid region are two more fat pads that are centrally and medially (towards the nose) located. The medial fat pad is pale yellow in color and lies in front of the levator aponeurosis, while the central pad of fat is broader and a deeper yellow color. These fat pads encompass the inner aspect of the lacrimal gland, which is distinguished by pink color. The lacrimal gland is behind the orbital margin but may prolapse slightly, making it more prominent when the eye is examined.

The lower eyelid fat pads are a bit different in structure than the upper eyelid fat pads. The central fat pad is separated from the medial fat pad by the inferior oblique muscle. There is a small amount of fat that lies in front of the inferior oblique muscle as well. The inferior oblique muscle originates from a small indentation in the lower orbital floor, traveling behind the orbital margin and at the upper nasal lacrimal canal. It passes underneath the inferior rectus muscle and through the Tenon capsule, in proximity to the macula of the eye.

Blood Supply to the Eyelids

Internal and external carotid arteries supply our eyelids blood. The ophthalmic artery branches off the internal carotid artery and supplies blood to various parts of the eyelid. At the inner part of the upper eyelid, the artery splits and travels outwards to supply both the upper and the lower eyelid. The branch that supplies the lower eyelid arises from the superior marginal vessel. The superior and inferior marginal vessels form the marginal arcade, located at the front of the tarsus. The superior marginal arcade is connected to the peripheral arcade adjacent to the Muller muscle. Another branch of the internal carotid artery is the lacrimal artery. The lacrimal artery passes through the orbital septum and joins the marginal arcade. The branches of the internal carotid artery are explained above, but the external carotid artery supplies the eyelids and is part of the facial, infraorbital and the superficial temporal artery.

Lymphatic drainage

The lymphatic drainage of the eyelid is the anatomical process of fluids being removed from the eyelids and surrounding tissue. Much of upper eyelid and outer half of the lower eyelid drainage occurs through preauricular lymph nodes. Only a small portion of the middle upper eyelid and the inner half of the lower eyelid drains into the submandibular lymph nodes.

Muscles

Many muscles around the eyeball control movement. These are extraocular muscles, and more specifically include the medial rectus, lateral rectus, inferior/superior oblique, and inferior/superior rectus muscles. These muscles move the eyeball, including rotation of the eyeball. In the eyelid itself, the levator palpebrae superioris is responsible for elevation of the upper eyelid. The extraocular muscles are supplied by a variety of cranial nerves, including the oculomotor nerve, the trochlear nerve and abducens nerve.

Bones

The orbital socket orbit comes the fusion of orbital bones, namely the frontal bone, sphenoid bone, zygomatic bone, the Palatine bones, the maxillary bone, and the lacrimal bone. Blood vessels and nerves within the orbital socket are supplied through fissures. The three main conduits are the superior orbital fissure, the inferior orbital fissure, and the optic canal. While the superior orbital fissure allows for the passage of the frontal nerve, lacrimal nerve, nasociliary nerve, the inferior orbital fissure allows for the passage of the infraorbital nerve, zygomatic nerve, infraorbital artery and vein, and parasympathetic nerve supplying the lacrimal gland.

Lacrimal gland

The lacrimal gland allows us to make tears and is divided by the levator aponeurosis into the orbital and palpebral lobe. The lacrimal gland secretes tears to lubricate, protect, and moisted the surface of the eye. The tears are drained through a series of ducts, namely the lacrimal papillae, canaliculi, lacrimal sac and naso-lacrimal duct.