Research Article| Volume 29, ISSUE 4, P186-192, December 2018

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# Anatomy of the human orbit

Published:October 12, 2018
The contents of the human orbit develop from all embryonic germinal layers to form diverse and specialized interconnected structures, whose unified function is that of providing sight. A thorough knowledge of these structures and their inter-relationships is imperative for the head and neck surgeon. This article will discuss the structures contained within the orbit from a surgical perspective in the hope of expanding the head and neck surgeon's knowledge and comfort when treatment indication necessitates entry into this space.

## Introduction

The orbit contains and protects the delicate structures that comprise the visual sensory components of our central nervous system. Even minor injury or imbalance of any one of these structures in either orbit can lead to significant and debilitating visual dysfunction. Surgical manipulation of 1 component in the orbit can, and often does, alter the function of another component. Surgery on 1 orbit can also cause asymmetry in position or function relative to the other orbit, causing a poor visual or aesthetic outcome. In some instances, staging of procedures may be required to regain normal, symmetrical function of both orbits and their contents, thus allowing the return of normal visual function. Therefore, a thorough knowledge of orbital anatomy and function is essential to yield the best outcome for patients. The outcome of any orbital surgery requires success in functional, visual, and aesthetic terms.

## Embryology

By 3 weeks of gestation, just prior to closure of the neural groove, the neuroectoderm forms a bud on either side that migrate anteriorly toward the surface ectoderm as optic vesicles, attached to the forebrain by the optic stalks. The optic vesicles contact the surface ectoderm, then invaginate to form the optic cups. As the eyes develop, they integrate portions of all 3 germinal layers. The reader is referred to the attached references for additional embryologic ocular development. This article will concentrate on orbital development. Osseous formation of the orbit begins within the first 2 months of gestation via neural crest cell migration. The orbital floor and lateral wall develop from a maxillary migration wave of neural crest cells that wrap around the developing eye, while the superior and medial walls develop from a frontonasal migration over the prosencephalon above the developing eye. The extraocular muscles form from condensations of mesodermal cells and are well developed by 4 months’ gestation. As ganglion cells in the developing retina grow, they enter the optic stalk by 7 weeks, forming the optic nerve. Mesodermal elements enter the surrounding tissue to form the septal and vascular elements of the nerve. The eyelids form from mesoderm, except the skin and conjunctival layers. Lid buds grow from above and below the optic cup, fusing at 8 weeks, separating again at the fifth month when lashes and other dermal elements form. The lacrimal gland and drainage apparatus form from ectoderm, with the nasolacrimal duct initially being a solid cord of epithelium between the maxillary and nasal processes, canalizing just prior to birth. Bony ossification begins during the third month of gestation and fusion occurs between months 6 and 7.

## Orbital bones

The orbit by definition contains the globe and associated extraocular muscles, vasculature, and nerves, along with significant fat and the lacrimal gland. The classic conception of the human orbit is described as pyramidal in shape with the base representing the opening of the orbit and the point of the pyramid corresponding to the orbital apex. For the majority of orbital surgical approaches, this conception is very adequate. The one exception is the orbital floor that is shaped more like a pear, with an upward sweep of the floor as one proceeds posteriorly, important when considering orbital floor repair (Figure 1).
The orbital roof is composed of the orbital plate of the frontal bone and the lesser wing of the sphenoid. It separates the orbit from the frontal sinus anteriorly and the anterior cranial fossa posteriorly. The roof slopes backward and downward toward the apex, ending at the optic canal and superior orbital fissure. The optic canal is vertically oval, 5-6 mm in diameter, and 10-12 mm in length. Landmarks of the orbital roof include the lacrimal gland fossa laterally, the supraorbital notch and/or foramen at the junction of the medial and middle third of the rim, and the trochlea, located about 5-mm posterior to the nasal orbital rim through which the superior oblique passes and gets redirected laterally.
The lateral orbital wall is formed by the greater wing of the sphenoid, the zygomatic process of the frontal bone, and the orbital process of the zygomatic bone. It is at a nearly 45° angle to the midsagittal plane. It is bordered by the inferior and superior orbital fissures. The anterior lateral wall is the thickest part of the bony orbit, but thins dramatically at the zygomatic-sphenoid junction. The lacrimal gland fossa is at the anterior superior most aspect of the lateral orbital wall above which the frontal bone flattens where it passes around the front end of the middle cranial fossa. Halfway along the lateral wall in the sphenoid bone is a small canal carrying an anastomotic branch between the lacrimal and meningeal arteries. When entering the orbit via a lateral approach, the structures to consider from anterior to posterior are the lacrimal gland, transmitted vessels and sensory nerves, and the structures passing through the inferior and superior orbital fissures. Whitnall's tubercle is a small projection of bone about midway up the lateral rim formed by condensation and attachment of the lateral canthal tendon and the network of suspensory ligaments, which support the orbital contents. The frontozygomatic suture lies about 1 cm above this tubercle. This wall ends about midequator of the globe so as not to interfere with peripheral vision.
The medial wall is composed largely of the lamina papyracea of the ethmoid bone. It is extremely thin and separates the orbit from the ethmoid air cells. It is vulnerable to fracture or inadvertent damage during endoscopic sinus surgery, and can allow transmission of infection from the ethmoid air cells into the orbit. Posterior to the ethmoid bone lies the body of the sphenoid bone which is much thicker. The medial wall ends at the optic canal. Anterior to the ethmoid bone is the lacrimal bone, forming the posterior lacrimal crest and the posterior half of the lacrimal sac fossa, where it meets the orbital process of the maxillary bone, which again thickens. Lacrimal bypass surgery usually infractures the lacrimal portion of the fossa, but if too posterior, usually a burr is needed to pass through the maxillary portion. Approximately 20-25-mm posterior to the posterior lacrimal crest, the anterior then posterior ethmoidal (another 8- to 10-mm posterior) arteries pass through the medial wall at the frontoethmoidal suture. This landmark is also the most common area where the roof of the ethmoidal labyrinth is found, above which lies the anterior cranial fossa. The cribriform plate can be as much as 10 mm below this line, so review of imaging is mandatory to avoid penetration into the cranial vault.
The orbital floor is the shortest of the orbital walls, extending 35-40 mm from the orbital rim. It is composed primarily of the maxillary bone, with the zygomatic bone contributing to the anterolateral portion and the palatine bone comprising the posterior most portion. The floor ends at the posterior maxillary sinus wall and therefore does not extend to the orbital apex. The floor is thinnest medial to the infraorbital canal, which begins as a groove anterior to the inferior orbital fissure, deepening as it migrates anteriorly and laterally, usually forming a bony canal about 15-mm posterior to the rim and exits about 1-cm inferior to the orbital rim. The infraorbital canal carries the maxillary division of the trigeminal nerve and the maxillary artery. Care must be taken to avoid injury to these structures during fracture repair or decompression surgery. The inferior orbital fissure separates the floor from the lateral wall, posteriorly joins the superior orbital fissure just inferior to the optic canal, and is contiguous with the foramen rotundum. This fissure allows communication with the pterygopalatine and infratemporal fossae. Branches from the inferior ophthalmic vein communicate through this fissure with the pterygoid venous plexus. The maxillary branch of the trigeminal nerve passes from the foramen rotundum into the inferior orbital fissure along with the infraorbital artery, and parasympathetic branches from the pterygopalatine ganglion pass through to join the maxillary nerve on the way to the lacrimal gland.

## Connective tissue

Within the orbit there is an elaborate system of connective tissue that compartmentalizes and supports the orbital soft tissue structures, necessary for maintaining the appropriate relationships between structural components required for precise and coordinated ocular function.
The periorbita is comprised of the bony periosteum and the connective tissue layers that attach the periosteum to nearly all other orbital structures. Much of the periorbita is loosely attached, but it is strongly attached at the arcus marginalis along the orbital rim, the lateral orbital tubercle, the trochlea, the optic foramen, and the superior and inferior orbital fissures. Where it adheres to the optic canal and superior orbital fissure, it is fused to the dura. The periorbita serves to stabilize and support orbital structures and forms a boundary around the entire orbital compartment. At the orbital rim, the periorbita fuses with the periosteum of the outer table of the orbital bones while the inner connective tissue layers extend into the eyelids to form the orbital septa, the anterior-most boundary of the orbit.
An extensive orbital septal system extends from the periorbita in radial and circumferential directions creating a web of slings. These surround the fat globules, the extraocular muscles, optic nerve, and neurovascular bundles. Alterations of these septa can cause restriction or asymmetry of movement, and create subcompartment syndromes within the orbit. Anteriorly, this fascial system supports the globe, lacrimal gland, eyelids, and oblique muscles. Specific condensations of this system include Whitnall's and Lockwood's ligaments. In the midorbit, this system comprises the intermuscular septa. Posteriorly, this system again fuses to the periosteum at the annulus of Zinn, optic canal, and orbital fissures.
Tenon's capsule is a dense, elastic, vascular tissue that envelops the globe, except for the cornea, and extends onto the anterior portion of the extraocular muscles (extending to the equator of the globe). It allows for decreased friction between the muscles and globe during eye movement. Tenon's capsule is densely adherent at the extraocular muscle insertions into the sclera, the limbal interface between the sclera and cornea, and the dural sheath of the optic nerve.

## Apertures of the orbit

The medial wall, composed mostly by the lamina papyracea, has 2 distinct openings, the anterior and posterior ethmoidal foramina, which transmit their respective arteries and, variably, veins. These are extremely important landmarks, due to the fact that they are the most common site of junction to the cribriform plate, above which lies the cranial vault.
The superior orbital fissure separates the orbital roof from the lateral wall, meaning the greater and lesser wings of the sphenoid bone. This fissure transmits cranial nerves III, IV, VI, the ophthalmic division of V, and sympathetic nerve fibers. It is also the exit for the majority of venous blood via the superior orbital vein. The superior orbital fissure is divided by the annulus of Zinn (Figure 2), a condensation of connective tissue serving as the origins of the 4 rectus muscles and the levator palpebrae superioris. Within the annulus pass the ophthalmic artery and the following nerves: CN VI, CN III and the nasociliary nerve. Outside of this annulus are located the superior ophthalmic vein, CN IV, and the lacrimal and frontal nerves.
The inferior orbital fissure separates the orbital floor from the lateral wall. It transmits the maxillary branch of CN V and the inferior ophthalmic vein. It is continuous with the foramen rotundum posteriorly, the pterygopalatine fossa and infratemporal fossa inferiorly and the infraorbital groove anteriorly.
Zygomaticofacial and zygomaticotemporal canals transmit their respective neurovascular bundles to the cheek and temporal fossa areas, anastomosing with the external facial vasculature.
The nasolacrimal canal passes through the maxilla as an extension of the nasolacrimal sac fossa in the anteromedial floor, transmitting the nasolacrimal sac to end below the inferior turbinate. Fracture of this canal can occur with facial injury (naos-orbito-ethmoidal (NOE) fracture, causing obstruction of tear drainage and epiphora).
The optic canal is located within the lesser wing of the sphenoid bone, 8-10 mm in length, and is separated from the superior orbital fissure by the bony optic strut. This canal transmits the optic nerve, ophthalmic artery, and sympathetic fibers. It is vertically oval in shape, measuring approximately 6.5 mm in greatest diameter.

## Extraocular muscles

There are 6 extraocular muscles, 4 rectus muscles and 2 oblique muscles. The 4 rectus muscles all pull in their expected direction, and are antagonistic pairs, so injury to one will create a deviation of the globe in the direction opposite the injured muscle. The medial rectus muscle pulls the eye medially toward the nose (adduction), while its antagonist, the lateral rectus muscle, pulls the eye laterally (abduction). The superior rectus muscle pulls the eye upward (supraduction) and the inferior rectus muscle pulls the eye down (infraduction). The 4 rectus muscles arise from a common tendinous ring surrounding the optic nerve (and a portion of the superior orbital fissure) called the annulus of Zinn. They continue anteriorly, separated from the periorbita by a thin layer of (extraconal) orbital fat.
The action of the oblique muscles is counter-intuitive. The superior oblique depresses the globe and creates a rotational movement (torque) in a clockwise direction, while the inferior oblique elevates the globe and creates a torque in a counterclockwise direction. The superior oblique originates at the annulus of Zinn, continues anteriorly along the superomedial orbital wall, then passes through the cartilaginous trochlea that reflects it in a posterolateral direction attaching to the superior globe beneath the superior rectus muscle. The inferior oblique muscle arises just posterior and lateral to the lacrimal duct (anteromedial orbital floor). It travels laterally and slightly posteriorly, deep to the inferior rectus muscle to attach to the inferoposterior wall of the globe, very near the macula of the retina (our true center of vision). As the inferior oblique passes deep to the inferior rectus, a condensation of connective tissue occurs, forming Lockwood's ligament. The capsulopalpebral ligament extends anteriorly from this, attaching to the tarsal plate of the lower lid. Injury to this fascial complex can create motility deficits of the globe and lower eyelid.
The levator palpebrae superioris muscle elevates the upper lid. It begins at the annulus of Zinn and lesser sphenoid wing, running just above the superior rectus muscle. At the level of the orbital rim, connective tissue fibers from the muscle sheath separate and attach to the superior conjunctival fornix. Additional condensation of this tissue at this level creates the transverse ligament of Whitnall, which attaches medially to the trochlea and laterally envelops the lacrimal gland. The muscle then continues anteriorly changing into a thin fibrous aponeurosis, turning inferiorly, and attaching to the anterior aspect of the tarsal plate. Medially and laterally, the aponeurosis joins the canthal tendons, forming horns.''
Although not technically part of the orbit, the orbicularis oculi finishes off the muscles involved in orbital function. The orbicularis muscle is anterior to the orbital septum of the eyelids and extends above the eyebrow superiorly and onto the upper cheek inferiorly. The orbicularis is the sphincter'' muscle of the eye, the contraction of which closes the eyelids. Medially and laterally, the arcuate fibers attach to ligamentous raphes. They are closely associated with the mid- and upper facial musculature and are innervated by facial nerve branches.

## Orbital nerves

The optic nerve exits the posterior wall of the globe traveling posterior and medial to the optic canal. The nerve is an extension of the brain, enveloped by pia, arachnoid and dura mater. The orbital portion of the nerve averages 30 mm in length and 4 mm in diameter. The dura fuses to the annulus of Zinn at the apex of the orbit and is continuous with the periosteum of the optic canal. The ophthalmic artery travels inferior to the nerve, piercing it about 1-cm posterior to the globe (Figure 3).
The extraocular muscles are innervated by cranial nerves. The oculomotor nerve (CN III) divides into inferior and superior branches along the lateral wall of the cavernous sinus prior to entering the orbit via the superior orbital fissure. The superior branch innervates the superior rectus and levator muscles. The inferior branch innervates the medial rectus, inferior rectus, and inferior oblique muscles. The nerve branches enter the muscles from the deep (intraconal) surface, hence are protected unless deep intraconal surgery is being performed. The inferior division also carries parasympathetic fibers to the ciliary ganglion, located about 1 cm behind the globe, innervating the ciliary body and iris sphincter. Injury to these fibers will cause disturbances in pupillary function and accommodation. The trochlear nerve (CN IV) passes through the cavernous sinus and enters the orbit via the superior orbital fissure, outside the annulus of Zinn. It travels above and across the superior rectus and superior oblique muscles entering the muscle from the external (extraconal) surface at the posterior third of the muscle. It can be easily injured by disease processes in the superior orbit or during deep superior orbital surgery. The abducens nerve (CN VI), which innervates the lateral rectus muscle, exits the cavernous sinus via the superior orbital fissure inside the annulus of Zinn, running laterally and entering the muscle from the intraconal surface, again relatively protected from extraconal surgery.
Sensory innervation of the orbit is largely from the ophthalmic division of the trigeminal nerve. The maxillary division supplies a small portion of the inferior orbit. The ophthalmic division divides into lacrimal, frontal, and nasociliary branches in the cavernous sinus. The lacrimal branch travels in the extraconal space of the lateral orbit ending in the lacrimal gland. The frontal branch travels anteromedial above the levator muscle and divides into the supraorbital and supratrochlear nerves. The supratrochlear nerve exits along the superomedial rim, while the supraorbital nerve exits slightly lateral through the supraorbital notch or foramen in the superior orbital rim. The nasociliary nerve enters the orbit via the superior fissure and annulus of Zinn, travels anteriorly above the optic nerve, sending some fibers into the ciliary ganglion. Additional fibers form the long ciliary nerves supplying sensation to the corneal surface. It then continues forward giving rise to the anterior and posterior ethmoidal nerves and finally exits the orbit as the infratrochlear nerve.

## Vascular supply

The arterial supply (Figure 4) of the orbit is derived from the ophthalmic branch of the internal carotid artery with variable amount of anastomosis via the facial vasculature with the external carotid artery. The ophthalmic artery enters the orbit through the optic canal inferotemporal to the optic nerve. It crosses over the optic nerve while traveling medially, then sprouts several branches of variable sequence. Branches go to each extraocular muscle, feeding the muscles from the intraconal surface. An additional branch goes to the lacrimal gland, giving off zygomaticotemporal and zygomaticofacial branches along its path that penetrate the lateral orbital wall, anastomosing with the external carotid vascular supply. The central retinal artery penetrates the dura of the optic nerve about 1-cm posterior to the globe, supplying the retina. A supraorbital branch travels along the levator muscle, paralleling the frontal nerve, giving rise to anterior and posterior ethmoidal arteries, then finally dividing and supplying the upper and lower lids as well as the medial canthal area and nose.
Venous drainage (Figure 5) from the orbit is through the superior and inferior ophthalmic veins. The superior ophthalmic vein originates from the medial orbit as a confluence of the angular, supratrochlear, and supraorbital veins. As it progresses posteriorly, it picks up drainage from the muscular veins, vortex veins (draining the globe), and ethmoidal veins, then crosses laterally and inferior to the superior rectus muscle, is joined by the lacrimal vein and exits the orbit through the superior orbital fissure, draining into the cavernous sinus. The inferior ophthalmic vein originates from a plexus of small collector veins in the inferior orbit, picks up drainage from muscular veins and vortex veins, exiting via the inferior orbital fissure to join the pterygoid plexus. There are variable interconnections between the superior and inferior ophthalmic veins within the orbit.

## Summary

The orbit contains an interconnected, interdependent array of structures, making orbital surgery complex. Exacting knowledge of the anatomy is essential for successful surgical outcomes. One must always keep in mind that manipulation of 1 structure nearly always has effects on other structures within the orbit, the result of which can be detrimental to the visual outcome of 1 or both eyes. Add to this the need for preservation of nonvisual function and cosmesis, and it becomes apparent that this small intricate space must be respected.

## Disclosure

The authors reported no proprietary or commercial interest in any product mentioned or concept discussed in this article.