Eyelid Anatomy and Function
Introduction
Maintaining a healthy ocular surface starts with a good understanding of eyelid anatomy and function. The eyelids are vital in promoting the spread of tears, lubricating the corneal surface, and protecting the eye from dust and foreign bodies. A disruption in the eyelid anatomy can prove to be harmful to the integrity of the cornea and ocular surface.
Overview of External Anatomy
The eyelids comprise of an upper and lower eyelid, joined at the medial and lateral canthi. The average aperture of the eyelids measures about 30 mm in horizontal width, and approximately 10 mm in vertical height. The highest peak on the upper eyelid lies slightly nasal, and the lowest contour of the lower eyelid rests slightly lateral. The upper eyelid generally covers 1–3 mm of the upper cornea, and the lower eyelid typically rests at, or near the lower limbus. The upper eyelid crease falls 6–10 mm from the eyelid lash line. The brow is positioned anterior to the superior orbital rim.1–4
Eyelid Skin
The eyelid skin is one of the thinnest of the body, lacking subcutaneous fat, with just loose connective tissue between the eyelid skin and orbicularis oculi. The eyelid skin is less than 1 mm in thickness. The constant dynamic movement of the thin eyelid skin is thought to contribute to age-related eyelid skin laxity.
Eyelid Muscles: Protractors
The main protractor of the eyelid, which serves to close the eye, is the orbicularis oculi. It is innervated by the facial nerve, and divided into the pretarsal, preseptal, and orbital portions (Fig. 2.1). The pretarsal and preseptal portions are used in spontaneous blink, and the orbital portion is needed for forced eyelid closure. Facial nerve palsy can lead to lagophthalmos and incomplete blink.
The pretarsal orbicularis deep origins are located on the posterior lacrimal crest, with superficial origins on the anterior limb of the medial canthal tendon. The deep head or Horner’s tensor tarsi encircle both canaliculi and are important for lacrimal pump function. The pretarsal orbicularis oculi of the upper and lower lids laterally fuse together to form the lateral canthal tendon.
The preseptal portion originates on the posterior lacrimal crest, as well as the medial portion of the anterior limb of the medial canthal tendon and the lateral portion of the lateral palpebral raphe over the lateral orbital rim.
The orbital portion of the orbicularis oculi arises from the anterior limb of the medial canthal tendon and periosteum.
The corrugators are also protractors, and originate on the superonasal rim and end at head of the brows. Corrugators promote vertical glabellar furrows. The procerus is also a protractor and runs vertically from the frontal bone to the head of the brows and causes horizontal furrows.
Eyelid Muscles: Retractors
The eyelid muscle retractors serve to open the eye. The retractors of the upper eyelid are the levator palpebrae superioris and Müllers muscles, as well as the frontalis. The lower lid retractors are the capsulopalpebral muscle and the inferior tarsal/palpebral muscle.
Upper Lid Retractor: Levator
The primary retractor of the upper eyelid is the levator muscle. The levator originates on the orbital roof near the apex, in front of the optic foramen and anterior to the superior rectus muscle. The levator muscle portion is 40 mm long, and the levator aponeurosis is 14–20 mm length.
Whitnall’s ligament or superior traverse ligament is a condensation of elastic fibers of the anterior sheath of the levator muscle. It is located between the transition of the levator aponeurosis and muscle. It provides the suspension support for the upper eyelid and superior orbital tissues. It is thought to transfer the vector of force of the levator muscle from anterior–posterior to superior–inferior. It is analogous to Lockwood’s ligament in the lower eyelid. Medially it attaches near the trochlea and superior oblique tendon, and laterally, it runs through the lacrimal gland, and attaches to the inside of the lateral orbital wall, approximately 10 mm above the lateral tubercle.1–4
The levator aponeurosis divides into an anterior and posterior portion just above the superior tarsal border. The anterior portion inserts into the pretarsal orbicularis. The most superior portion of these attachments forms the eyelid crease with contraction of the levator complex (Fig. 2.2). The posterior portion inserts onto the anterior surface of the tarsus. The aponeurosis appears as a thick whitish band between Whitnall’s ligament and the tarsal plate (Fig. 2.3).
Figure 2.3 The levator aponeurosis: O, orbicularis oculi; F, preaponeurotic fat; L, attenuated levator aponeurosis.
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