I. LIDS

SURGICAL ANATOMY OF THE LIDS

The eyelids are thin folds of skin, muscle, and fibrous tissue that serve to protect the delicate structures of the eye. The great mobility of the lids is possible because the skin is among the thinnest anywhere on the body. Fine hairs, visible only under magnification, are present on the eyelids. Beneath the skin lies loose areolar tissue which is capable of massive edematous distention. The orbicularis oculi muscle is adherent to the skin. It is innervated on its deep surface by the facial (VII) cranial nerve, and its function is to close the lids. It is divided into orbital, preseptal, and pretarsal divisions. The orbital portion, which functions primarily in forcible closure, is a circular muscle with no temporal insertion. The preseptal and pretarsal muscles have superficial and deep medial heads that participate in the lacrimal pump (see below).

The lid margins are supported by the tarsi, rigid fibrous plates connected to the orbital rim by the medial and lateral canthal tendons. The orbital septum, which originates from the orbital rim, attaches to the levator aponeurosis, which then joins the tarsus. On the lower lid, it joins the inferior border of the tarsus. The septum is an important barrier between the eyelids and the orbit. Behind it lies the preaponeurotic fat pad, a helpful surgical landmark. An additional fat pad lies medially in the upper lid. The lower lid has two anatomically distinct fat pads beneath the orbital septum.

Deep to the fat lies the levator muscle complex-the principal retractor of the upper eyelid-and its equivalent, the capsulopalpebral fascia in the lower lid. The levator muscle originates in the apex of the orbit. As it enters the eyelid, it forms an aponeurosis that attaches to the lower third of the superior tarsus. In the lower lid, the capsulopalpebral fascia originates from the inferior rectus muscle and inserts on the inferior border of the tarsus. It serves to retract the lower lid in downgaze. The superior and inferior tarsal muscles form the next layer, which is adherent to the conjunctiva. These sympathetic muscles are also lid retractors. Conjunctiva lines the inner surface of the lids. It is continuous with that of the eyeball and contains glands essential for lubrication of the cornea.

The upper lid is larger and more mobile than the lower. A deep crease usually present in the mid position of the upper lid in Caucasians represents an attachment of levator muscle fibers. The crease is much lower or is absent in the Asian eyelid. With age, the thin skin of the upper lid tends to hang over the lid crease and may touch the eyelashes. Aging also thins the orbital septum and reveals the underlying fat pads.

The lateral canthus is 1-2 mm higher than the medial. Because of loose tendinous insertion to the orbital rim, the lateral canthus is elevated slightly with upgaze.

10.1036/1535-8860.ch4

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