BIOANATOMY AND BIOMECHANICS
The upper eyelid begins superiorly at the superior orbital crease beneath the eyebrow and ends at the free margin of the upper lid. The lower lid begins at the infraorbital crease that corresponds roughly but not exactly to the reflection of the orbital rim. The anatomy of the eyelid is complex and three-dimensional. Prior to performing any form of eyelid surgery, a thorough study of anatomy must be undertaken (Fig. 10.1).
Periocular anatomy. (A) Structure of the eyelids. The eyelids are suspended by tendons medially and laterally. The lacrimal gland is located at the superolateral aspect deep to the orbicularis oculi. The lacrimal drainage apparatus is protected by the anterior leaflet of the medial canthal tendon, which is cut away in this diagram. In cross section, the lower lid is composed of conjunctiva, tarsal plate, orbicularis musculature, and skin. (B) Arterial supply of the eyelid. The eyelids receive rich and redundant blood supply from the internal and external carotid systems
The eyelids are suspended medially and laterally by tendons and are multilaminar free margin structures. The lids function to protect the eye from trauma and they keep the eyes moist and lubricated. The anchoring tendons of the eyelid are bifurcated structures with deep and superficial components. The medial canthal tendon originates medially from the anterior and posterior lacrimal crests and from the frontal process of the maxillary bone. The superficial portion of the medial canthal tendon protects the lacrimal drainage system that passes just deep to the tendon insertion to drain into the nasal cavity. The lateral canthal tendon is firmly attached to the zygomatic bone at Whitnall’s tubercle just within the bony orbit. Between the two tendons stretch the dense, fibrous tarsal plates, which provide structural support to the eyelids.
The lacrimal glands, the meibomian glands, and the lacrimal drainage system strike a balance between the creation of lubricants and their removal. The meibomian glands are embedded in the tarsal plate and empty onto the palpebral conjunctiva. The lacrimal glands lie within the superolateral orbit. The canalicular drainage system originates from the medial most upper and lower lids, where two punctae are visible. The canaliculi join posterior to the anterior leaflet of the medial canthal tendon and dive deep from that point medially.
The lids consist of the tarsal plate covered by laminae of mucosa, muscle, fat, and skin. The complex anatomy of the eyelids is beyond the scope of this discussion and is eloquently described elsewhere. The external lids consist of a thin, redundant, mobile epidermis and dermis overlying a circumferential track of orbicularis oculi. The skin of the eyelid is loosely attached to the orbicularis and can be extensively mobilized from the underlying muscle.
The vascular supply to the periocular area is extensive and redundant. ...