1 Anatomy of the Eyelids, Orbit, and Lacrimal System
This chapter reviews anatomy of the eyelids, lacrimal system, and orbit. Anatomic knowledge is the basis of ophthalmic plastic and reconstructive surgery. By thoroughly understanding the relationship between the various layers and structures, a surgeon can often preserve functionality and cosmesis of the orbit and periocular region.
The eyelid structures are herein described in an anterior-to-posterior approach (Fig. 1‑1).
Eyelid skin is extremely thin, and notably lacking in subcutaneous fat. The upper eyelid skin crease, found 8 to 10 mm above the lid margin in non-Asian eyelids, is composed of fine attachments of the levator aponeurosis through overlying orbicularis muscle to the skin. 1 Asian eyelids may not have an upper eyelid crease, or may have a significantly lower crease.
The medial and lateral canthal angles are the angles created by the joining of the medial and lateral upper and lower eyelids, respectively.
1.1.2 Orbicularis Muscle
The orbicularis oculi muscle is a protractor, or muscle of eyelid closure, and is divided into three portions: orbital, overlying the orbital bone; preseptal, overlying the septum; and pretarsal, overlying the tarsus. As it approaches the eyelid margin, orbicularis muscle is termed “the muscle of Riolan,” visualized as the gray line. It is innervated by cranial nerve 7, the facial nerve (Fig. 1‑2).
Tissue in this plane onwards is considered part of the anterior lamella of the eyelid. Tissue posterior to this plane is considered posterior lamella of the eyelid.
Orbital septum is a connective tissue layer arising from the bony orbital margin (arcus marginalis) and inserting onto the levator aponeurosis superiorly, and onto the inferior edge of the tarsus inferiorly (Fig. 1‑3). It can be relatively thick in children, but is typically thin in adulthood. It is a crucial zone of differentiation between pre- and postseptal anatomy and processes.
1.1.4 Fat Pads
There are two fat pads superiorly, and three pads inferiorly; all are postseptal, or orbital, and are considered contiguous with extraconal fat. Given their location relative to the levator aponeurosis, the central upper fat pads are typically referred to as “preaponeurotic fat.” The upper and lower eyelids have medial and central fat pads. The lower eyelid has a lateral fat pad.
1.1.5 Lacrimal Gland
The orbital portion of the lacrimal gland can be found lateral to the central preaponeurotic fat pad (Fig. 1‑3). It will be further described in the Orbit section. It is assisted in tear production by the fornix-based accessory lacrimal glands of Krause and Wolfring. 2
1.1.6 Eyelid Retractors
The levator palpebrae superioris is the major muscle of upper eyelid retraction (Fig. 1‑3). It arises from the lesser sphenoid wing and projects anteriorly until Whitnall ligament just posterior to the orbital rim, at which point the vector is changed to a vertical system of retraction. 1 At approximately the same point of change, the muscle transitions to its aponeurosis, which ultimately inserts onto the anterior face of the tarsus. As previously described, various muscle fibers also travel anteriorly, ultimately forming the upper eyelid skin crease. This muscle is striated and innervated by cranial nerve 3, the oculomotor nerve.
Müller muscle arises from the condensation of the levator palpebrae superioris and Whitnall ligament, and inserts upon the upper tarsal border. 1 Unlike the levator muscle, Müller muscle is a smooth, sympathetically innervated muscle.
The capsulopalpebral fascia is the major retractor of the lower eyelid. It is a fibrous sheet extending from Lockwood ligament, traveling parallel to the inferior rectus and around the inferior oblique prior to inserting along the inferior tarsal border. 1
Tarsus is a dense tissue plate found just underneath the lid margins (Fig. 1‑3). It is up to 10 mm in maximal vertical height in the upper lid, and up to 4 mm of maximal vertical height in the lower lid. 1
The medial and lateral canthal tendons arise from the medial and lateral tarsal plates, and are further strengthened by orbicularis oculi muscle heads.
The eyelid margin is an epithelialized platform noted along the upper and lower eyelids. Eyelash follicles lie deep to the eyelid margin anterior to the tarsus, and eyelashes exit the margin anteriorly. The gray line demonstrates the location of orbicularis oculi muscle. Meibomian gland orifices are microscopically visualized in line with the deeper tarsus, which envelops them.
The interpalpebral fissure, or space between the eyelid margins, spans approximately 1 cm vertically, and 3 cm horizontally. The upper eyelid usually falls approximately 1 mm below the superior corneal limbus; the lower eyelid usually skirts the inferior limbus.
The conjunctival mucous membrane lines the posterior surface of the eyelids, forms the fornices, and is contiguous with the bulbar conjunctiva.
The eyelids are heavily vascularized, with multiple anastomoses passing between the anterior and posterior lamellar vascular supply. In general, the transverse facial, superficial temporal and angular artery branches of the external carotid artery supply the anterior lamellae. The nasal and lacrimal branches of the ophthalmic artery supply the majority of the posterior lamellae. A marginal arcade supplies the upper and lower eyelid margins. A peripheral arcade is found in the upper eyelid just superior to the tarsal plate. Vascular drainage is both orbital and facial. 1
1.2 Lacrimal Drainage System
Tear drainage begins at the puncta (Fig. 1‑3). The upper and lower lids each contain a punctum medially; the punctum is 5 mm from the canthal angle superiorly, and 6 mm from the canthal angle inferiorly. Each punctum drains into a canaliculus. The canaliculi travel vertically for 2 mm, then horizontally for an additional 8 mm. The superior and inferior canaliculi fuse to create a common canaliculus 90% of the time. Then, either in the form of a common canaliculus or as two separate canaliculi, the canalicular structure empties into the lacrimal sac. The valve of Rosenmüller is a fold of tissue that serves as a one-way valve, helping prevent tear reflux from the lacrimal sac into the canalicular apparatus. The lacrimal sac, 1.2 to 1.5 cm in length, sits in the lacrimal sac fossa, cradled by the anterior and posterior lacrimal crests. The anterior lacrimal crest is part of the maxillary bone, whereas the posterior lacrimal crest is part of the lacrimal bone, with a suture line in the fossa. Periorbital tissue spans the lacrimal sac from the anterior to the posterior lacrimal crest. In addition, the medial canthal ligament and orbicularis oculi muscle fibers split, hugging the lacrimal sac and inserting onto the anterior and posterior lacrimal crests, thereby allowing the orbicularis-derived Horner muscle and muscle of Riolan to contribute to the lacrimal sac pump function. 3
Inferiorly, the lacrimal sac transitions into the nasolacrimal duct, which travels approximately 1.2 cm through the lacrimal canal. 3 The nasolacrimal canal is surrounded by maxillary bone, with the exception of the superomedial portion, which is ethmoidal. The canal comprises the lacrimal ridge intranasally, which is found anterior to the middle turbinate. The nasolacrimal duct empties into the nasal cavity via the inferior meatus. The valve of Hasner sits just at the opening of the meatus, helping to prevent reflux. 3