Tarsal plates are crescent-shaped structures located within the eyelids, contributing to the integrity and function of these delicate tissues. These structures are present in both the upper and lower eyelids. They provide fundamental support, helping the eyelids maintain their proper shape and position.
Structure and Purpose of Tarsal Plates
The tarsal plates are composed of dense connective tissue, primarily consisting of collagen fibers, which provide strength and rigidity to the eyelids. They also contain some elastin fibers, contributing to their flexibility. These structures are not cartilage, but rather firm, plate-like formations molded to the curvature of the eyeball.
Each eye has a superior tarsal plate in the upper eyelid and an inferior tarsal plate in the lower eyelid. The superior tarsal plate is larger, measuring approximately 10 mm in height in its central aspect, while the inferior tarsal plate is smaller, around 3.5 to 5 mm in height. Both plates are roughly 1 mm thick and about 29 mm in length, providing consistent structural support across the eyelid.
A primary function of the tarsal plates is to offer mechanical support and maintain the eyelid’s shape, facilitating smooth movement during blinking. They also serve as attachment points for muscles responsible for eyelid elevation, such as the levator palpebrae superioris muscle. This structural integrity is also important for eye protection from foreign particles and desiccation.
The tarsal plates house modified sebaceous glands known as Meibomian glands. These glands produce an oily substance, or meibum, a layer of the tear film. This oily secretion helps to reduce tear evaporation, maintaining a healthy and lubricated ocular surface.
Conditions Affecting Tarsal Plates
Several medical conditions directly involve or are significantly influenced by the tarsal plates. A chalazion, for instance, develops when a Meibomian gland within the tarsal plate becomes blocked. This blockage leads to a localized cyst of chronic inflammation, often appearing as a painless, firm nodule that gradually forms over weeks. The retained sebaceous secretions cause enlargement and can result in local irritation.
A stye, also known as a hordeolum, is an acute bacterial infection of a gland at the eyelid margin. Unlike a chalazion, a stye is typically painful, red, and tender to the touch, developing rapidly over days. An external stye occurs when a Zeis or Moll gland at the base of an eyelash follicle becomes infected, while an internal stye affects a Meibomian gland, differentiating it from the non-infectious nature of a chalazion.
Entropion is a condition where the eyelid margin turns inward, causing the eyelashes to rub against the cornea and potentially leading to discomfort or vision problems. This inward turning is often associated with age-related changes, including a weakening or disinsertion of the lower lid retractors that help stabilize the eyelid. Studies suggest that entropion can correlate with tarsal plates that are smaller than average for a given age range, allowing the eyelid to rotate inward.
Conversely, ectropion involves the outward turning of the eyelid margin, typically affecting the lower eyelid. This condition can lead to ocular exposure and inadequate lubrication of the eye surface. Like entropion, it is frequently linked to age-related laxity of the eyelid’s horizontal structures. Research indicates that ectropion may be associated with tarsal plates that are age-normal or even larger than average, which can mechanically overcome the muscle tone responsible for eyelid positioning.