Eyelid & Orbit 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 protect them. Our eyelids protect those eyes and serve many purposes, including shielding the eyeball from injury, controlling the amount of light entering the eye to prevent damage 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.

Anatomy of the eyelid

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.

Layers & Components of the Eyelid

Skin

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.

Blood supply

Internal and external carotid arteries supply our eyelids with 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 outward 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.

Subcutaneous tissue

The subcutaneous tissue is loose connective tissue. Fat is minimal in preseptal/preorbital skin and is absent from pretarsal skin. Subcutaneous tissue is absent over the medial/lateral palpebral ligaments, where the skin is attached to the underlying fibrous tissue.

Orbicularis Muscle

Main Protractor

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.

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.

Tarus

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.

Conjunctiva

The conjunctiva consists of a thin, clear membrane that protects your eye. It covers the inside of your eyelid along with the sclera (white of your eye). The conjunctiva creates the mucus layer that forms part of your tears.

Eyelid Retractors (muscle)

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.