Superior tarsus

The superior tarsus, also known as the superior tarsal plate, is a thick, dense connective tissue structure located in the upper eyelid. It provides the upper eyelid with the necessary rigidity and shape to maintain proper form and function. The superior tarsus is significantly larger and thicker than the inferior tarsus (found in the lower eyelid) and houses important structures like the Meibomian glands, which are essential for tear film production.

Location

The superior tarsus is located within the upper eyelid, running horizontally along the eyelid margin from the medial canthus (inner corner) to the lateral canthus (outer corner). It lies just beneath the skin, orbicularis oculi muscle, and is attached superiorly to the levator palpebrae superioris muscle, which controls the elevation of the upper eyelid. The superior tarsus also lies adjacent to the palpebral conjunctiva on its inner surface, which lines the eyelid and protects the eye.

Structure and Anatomy

The superior tarsus, or superior tarsal plate, is a dense connective tissue structure that plays a crucial role in the anatomy of the upper eyelid. It provides rigidity and houses essential glandular structures that contribute to eyelid and eye health. Below is a detailed description of its anatomy.

Structure and Composition

The superior tarsus is a thick, crescent-shaped structure made up of dense fibrous connective tissue. It is significantly larger and thicker than the inferior tarsus, providing greater rigidity to the upper eyelid.

• Dense Fibrous Tissue: The superior tarsus is composed primarily of collagen and elastic fibers, which give it its structural rigidity while allowing for some flexibility during movement.
• Size and Thickness: The superior tarsus measures approximately 10 to 12 mm in vertical height at its central portion and tapers toward the medial and lateral ends. Its larger size compared to the inferior tarsus is necessary to support the weight and movements of the upper eyelid.

Location in the Upper Eyelid

The superior tarsus is located centrally within the upper eyelid, extending horizontally from the inner corner (medial canthus) to the outer corner (lateral canthus) of the eye.

• Positioning: The superior tarsus lies just beneath the skin and orbicularis oculi muscle, which encircles the eye and is responsible for closing the eyelids. It is positioned between the skin and muscle layers on the outer surface, and the palpebral conjunctiva on the inner surface.
• Attachment to Levator Palpebrae Superioris Muscle: The superior tarsus is connected to the levator palpebrae superioris muscle via its aponeurosis. This attachment is essential for the elevation and control of the upper eyelid, allowing the eyelid to open and close effectively.

Medial and Lateral Attachments

The superior tarsus is firmly anchored to the bones of the orbit at its medial and lateral edges through the medial palpebral ligament and lateral palpebral ligament.

• Medial Canthus: At the medial end, the superior tarsus is attached to the medial palpebral ligament, which connects the tarsal plate to the bone of the medial orbital rim.
• Lateral Canthus: The lateral end of the superior tarsus is secured by the lateral palpebral ligament, which connects the tarsal plate to the lateral orbital rim. These attachments ensure the stability of the eyelid and help maintain its proper alignment with the eye.

Relationship with Surrounding Structures

The superior tarsus is closely associated with several key structures within the eyelid and eye, which contribute to its function and stability.

• Orbicularis Oculi Muscle: The orbicularis oculi muscle surrounds the superior tarsus and is responsible for closing the eyelid. This muscle works in coordination with the superior tarsus to provide controlled movements of the eyelid.
• Levator Palpebrae Superioris Muscle: The levator palpebrae superioris muscle attaches to the superior tarsus via its aponeurosis. This muscle controls the elevation of the upper eyelid, allowing it to open fully when contracted.
• Palpebral Conjunctiva: The inner surface of the superior tarsus is lined by the palpebral conjunctiva, a thin mucous membrane that helps protect the ocular surface. This membrane provides a smooth surface for the eyelid to glide over the eye and is continuous with the bulbar conjunctiva that covers the white part of the eye (sclera).

Glands within the Superior Tarsus

The superior tarsus houses a significant number of Meibomian glands (tarsal glands), which are responsible for producing the lipid component of the tear film.

• Meibomian Glands: These large sebaceous glands are embedded within the superior tarsal plate and run vertically along its length. The upper eyelid contains approximately 30 to 40 Meibomian glands, more than the lower eyelid. These glands secrete meibum, an oily substance that prevents the tear film from evaporating and keeps the ocular surface lubricated.
• Glandular Ducts: The ducts of the Meibomian glands open at the eyelid margin, near the base of the eyelashes. These ducts allow meibum to be secreted onto the surface of the eye, contributing to the tear film’s lipid layer.

Blood Supply

The superior tarsus, along with the upper eyelid, receives its blood supply from branches of the ophthalmic artery and the facial artery.

• Superior Palpebral Artery: The superior palpebral artery, a branch of the ophthalmic artery, supplies blood to the superior tarsus and the upper eyelid structures.
• Angular and Facial Arteries: The angular artery, a branch of the facial artery, also contributes to the vascular supply of the upper eyelid and the superior tarsal plate.

Venous drainage from the superior tarsus occurs via the superior ophthalmic vein, which drains blood from the upper eyelid and surrounding structures.

Innervation

The superior tarsus is innervated by both sensory and autonomic nerves, which control eyelid sensation and glandular secretion.

• Sensory Innervation: Sensory innervation to the superior tarsus and the upper eyelid is provided by branches of the ophthalmic division of the trigeminal nerve (cranial nerve V). The supraorbital and supratrochlear nerves supply sensation to the skin of the upper eyelid.
• Autonomic Innervation: The autonomic nervous system innervates the Meibomian glands within the superior tarsus. Sympathetic nerves stimulate the secretion of meibum, while parasympathetic nerves likely modulate glandular activity.

Histological Structure

The histological structure of the superior tarsus consists of dense connective tissue, glandular tissue, and vascular components.

• Connective Tissue: The superior tarsus is composed of dense collagen fibers that provide rigidity to the upper eyelid while allowing flexibility. Elastic fibers are also present, contributing to the eyelid’s ability to return to its natural position after movement.
• Glandular Tissue: The superior tarsus contains multiple Meibomian glands, which are sebaceous glands embedded within the connective tissue. These glands produce and secrete meibum, an essential component of the tear film.
• Vascularity: The tarsal plate is well-vascularized, with small blood vessels coursing through the connective tissue to supply nutrients to the glandular cells and the surrounding structures.

Function

The superior tarsus, or superior tarsal plate, plays a vital role in the structural integrity and function of the upper eyelid. It contributes to eyelid mechanics, houses essential glandular structures, and ensures proper protection and lubrication of the ocular surface. Below is a detailed description of its functions.

Structural Support and Shape Maintenance

One of the primary functions of the superior tarsus is to provide rigidity and structural support to the upper eyelid.

• Maintaining Eyelid Shape: The superior tarsus is composed of dense connective tissue, which provides the upper eyelid with the necessary rigidity to maintain its shape and position. This prevents the upper eyelid from collapsing or folding during blinking and ensures that the eyelid moves uniformly.
• Support for Eyelid Movement: The tarsal plate serves as a strong foundation that supports the movements of the upper eyelid. It allows the eyelid to elevate and close properly without losing its structural integrity, contributing to normal eyelid function.

Facilitating Eyelid Elevation and Closure

The superior tarsus is integral to the process of opening and closing the upper eyelid, primarily through its connection to the levator palpebrae superioris muscle.

• Attachment to Levator Palpebrae Superioris Muscle: The superior tarsus is attached to the levator palpebrae superioris muscle via its aponeurosis. This muscle is responsible for lifting the upper eyelid, allowing the eye to open fully. The superior tarsus provides the necessary support and tension for smooth and controlled eyelid elevation.
• Assisting with Blinking: During blinking, the superior tarsus works in coordination with the orbicularis oculi muscle, which closes the eyelid. The tarsal plate ensures that the upper eyelid closes smoothly over the eye, providing even protection and lubrication to the ocular surface.

Housing and Support for Meibomian Glands

The superior tarsus serves as the structural base for the Meibomian glands, which are embedded within its tissue. These glands play a crucial role in maintaining the tear film.

• Support for Meibomian Glands: The superior tarsus provides a stable environment for the vertically arranged Meibomian glands. These glands produce meibum, the oily component of the tear film, and the tarsal plate ensures their proper alignment and function.
• Glandular Secretion Facilitation: By supporting the Meibomian glands, the superior tarsus helps facilitate the secretion of meibum onto the ocular surface. This secretion is essential for maintaining the lipid layer of the tear film, which prevents tear evaporation and protects the eye from drying out.

Contribution to Tear Film Stability

The superior tarsus indirectly contributes to the stability of the tear film by supporting the function of the Meibomian glands.

• Lipid Layer Production: The Meibomian glands housed within the superior tarsus produce the lipid (oily) layer of the tear film. This layer reduces tear evaporation, keeping the ocular surface moist and stable.
• Ensuring Tear Film Consistency: The structural integrity of the superior tarsus ensures that the glands function efficiently, allowing for a consistent release of meibum. This contributes to the tear film’s overall stability, preventing dry spots on the cornea and maintaining ocular comfort.

Protection of the Ocular Surface

The superior tarsus helps protect the eye by ensuring proper eyelid closure, which shields the ocular surface from environmental hazards.

• Barrier Against Debris and Irritants: During blinking, the upper eyelid, supported by the superior tarsus, closes over the eye, forming a barrier that prevents foreign particles, dust, and environmental irritants from entering the eye. This protects the cornea and conjunctiva from damage and infection.
• Lubrication of the Eye: The superior tarsus plays an important role in ensuring that the upper eyelid distributes the tear film evenly across the ocular surface during blinking. This helps keep the eye lubricated, reduces friction, and promotes clear vision.

Preventing Eyelid Malposition

The superior tarsus helps maintain proper eyelid alignment, preventing conditions such as ptosis (drooping eyelid) or eyelid retraction.

• Preventing Ptosis: The superior tarsus provides structural rigidity to the upper eyelid, preventing it from sagging or drooping abnormally (ptosis). The tarsal plate’s connection to the levator palpebrae superioris muscle ensures that the eyelid remains properly elevated when the eye is open.
• Maintaining Eyelid Tension: The superior tarsus also plays a role in maintaining the tension of the upper eyelid against the eye. This prevents eyelid retraction, where the eyelid is pulled too tightly over the eye, which can cause discomfort and exposure of the ocular surface.

Facilitating Smooth Eyelid Movement

The superior tarsus enables the smooth movement of the upper eyelid during blinking and eye closure.

• Reducing Friction: The tarsal plate, in coordination with the Meibomian glands, helps reduce friction between the upper eyelid and the ocular surface during blinking. This smooth movement is essential for maintaining eye comfort and preventing irritation.
• Coordinating with Lower Eyelid: During blinking, the superior tarsus works in harmony with the inferior tarsus (in the lower eyelid) to ensure that both eyelids move synchronously. This coordinated movement helps distribute the tear film evenly and protects the ocular surface.

Clinical Significance

The superior tarsus is critical for the normal function of the upper eyelid and the protection of the eye. Its dysfunction or damage can lead to several clinical conditions:

• Ptosis: Weakness or detachment of the levator palpebrae superioris muscle or its connection to the superior tarsus can result in ptosis, where the upper eyelid droops abnormally. This can obstruct vision and may require surgical correction.
• Meibomian Gland Dysfunction (MGD): Since the Meibomian glands are housed within the superior tarsus, any dysfunction or blockage of these glands can lead to MGD, contributing to evaporative dry eye syndrome due to insufficient lipid secretion in the tear film.
• Chalazion: Blockage of the Meibomian glands in the superior tarsus can result in a chalazion, a benign lump or swelling in the upper eyelid. While generally painless, chalazia may require medical or surgical treatment.
• Eyelid Malposition: Damage to the superior tarsus from trauma, surgery, or inflammation can lead to abnormal eyelid positioning, such as eyelid retraction or ectropion, where the eyelid turns outward.