Chapter 83 – Clinical Anatomy of the Orbit
JONATHAN J. DUTTON
• The orbit is the anatomical space bounded by the orbital bones and enclosed within the multilamellar periorbita.
• Anteriorly the orbit is limited by the orbital septum, which represents the anteriormost layer of the orbital septal system and separates the orbit from the eyelid.
• The orbit contains the eye and extraocular muscles, along with the nerves, vascular elements, and connective tissue support systems that subserve the visual system.
An understanding of orbital disease demands a clear concept of normal orbital anatomy and physiological function. Only with this foundation can the clinician identify and characterize pathological states. The development of better surgical techniques requires, in addition, a comprehensive knowledge of the structural relationships among the numerous anatomical systems that are crowded into the small space available.
The human orbit is a small cavity that has the approximate shape of a pear with the stem directed posteriorly. Within this defined space are juxtaposed a complex array of closely packed structures, most of which subserve visual function.   Lobules of orbital fat surrounded by connective tissue fascia completely fill the spaces between the muscles, nerves, and vascular elements. These fat lobules provide a cushion to protect these delicate structures from injury during ocular movement. The entire anatomical region is bound together in a functional unit, the complexity and precision of which are unmatched elsewhere in the vertebrate body.
OSTEOLOGY OF THE ORBIT
The bony orbit develops from mesenchyme, which encircles the optic vesicle from as early as the sixth week of the embryonic stage. The individual orbital bones arise from a complex series of primary or secondary ossifications around the evolving optic cup and stalk. Initially, the optic vesicles are positioned 170–180° apart, on opposite sides of the forebrain. Later, these begin to rotate anteriorly as the primordial orbital bones are laid down around them.
In adults the bony orbit encloses a volume of about 30?cm3 . It is composed of seven bones, simplified from a complex of dermal
Bones of the Orbit
and endochondral elements that evolved from earlier vertebrates ( Box 83-1 ). Except for a series of canals, fissures, and foramina that communicate with extraorbital compartments, the orbit is a closed compartment with a broad opening anteriorly ( Fig. 83-1 ).
The orbital roof is composed of the orbital plate of the frontal bone, with a small contribution from the lesser wing of the sphenoid bone at the apex. This bone is a thin lamina that separates the orbit from the frontal sinus anteriorly and from the anterior cranial fossa posteriorly. The roof slopes backward and downward from the orbital rim toward the apex, where it ends at the optic canal and superior orbital fissure. The optic canal measures 5–6?mm in diameter and 8–12?mm in length; it is oriented posteromedially about 35° to the midsagittal plane and upward about 38° to the horizontal plane.
Lateral Orbital Wall
The lateral wall is formed by the greater wing of the sphenoid bone posteriorly and by the zygomatic process of the frontal bone and the orbital process of the zygomatic bone anteriorly. It lies at a nearly 45° angle to the midsagittal plane. The lateral wall is bounded below by the inferior orbital fissure, and medially by the superior orbital fissure. Behind the thick lateral orbital rim, the wall becomes quite thin where the zygomatic bone joins the greater sphenoid wing at a vertical suture line. The convoluted frontozygomatic suture line runs approximately horizontally and crosses the superotemporal rim near the lacrimal gland fossa. At 5–15?mm above this line, the frontal bone widens as it passes around the front end of the anterior cranial fossa. About halfway along the anteroposterior depth of the lateral wall, in the sphenoid wing near the frontosphenoid suture, is a small canal that carries an anastomotic branch between the lacrimal and meningeal arteries. Elevation of the periorbita during lateral orbital dissection may result in brisk bleeding from this vessel. Just behind the zygomaticosphenoid suture line, the greater wing widens as it passes around the anterior tip of the middle cranial fossa. As the greater wing is removed during lateral orbitotomy procedures, the appearance of cancellous bone warns of the imminence of reaching the dura.
Figure 83-1 Bony anatomy of the orbit in frontal view. (Redrawn with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
The floor is the shortest of the orbital walls; it extends back only 35–40?mm from the inferior rim. The orbital floor is composed primarily of the maxillary bone; the zygomatic bone forms the anterolateral portion, and the palatine bone lies at the posterior extent of the floor. The surface of the orbital floor forms a triangular segment that extends from the maxillary–ethmoid buttress horizontally to the inferior orbital fissure, and from the orbital rim back to the posterior wall of the maxillary sinus. The orbital floor is thinnest just medial to the infraorbital canal, which is the most common site for blowout fractures. Despite its thinness, the floor is strengthened by one or more trabeculae in the roof of the maxillary sinus. The orbital floor shows the greatest degree of deformation when external force is applied, which explains the high rate of floor fractures associated with even minor degrees of blunt trauma.
The infraorbital groove begins at the inferior orbital fissure and runs forward in the maxillary bone. About 15?mm from the orbital rim, this groove is usually bridged over with a thin lamina of bone to form the infraorbital canal. Within this canal runs the maxillary division of the trigeminal nerve and the maxillary artery. These exit just below the central orbital rim at the infraorbital foramen.
The floor is separated from the lateral orbital wall by the inferior orbital fissure, which is approximately 20?mm in length and runs in an anterolateral to posteromedial direction. At the orbital apex just below the optic canal, the inferior fissure joins the superior orbital fissure. The inferior fissure transmits structures into the orbit from the pterygopalatine fossa posteriorly and from the infratemporal fossa anteriorly. Multiple branches from the inferior ophthalmic vein pass through this opening to communicate with the pterygoid venous plexus. The inferior fissure also transmits the maxillary division of the trigeminal nerve from the foramen rotundum to the infraorbital sulcus.
Orbital Bones Contributing to Each Wall
Lesser wing of the sphenoid bone
Frontal process of the maxillary bone
Body of the sphenoid bone
Greater wing of the sphenoid bone
Postganglionic parasympathetic secretory and vasomotor neural branches from the pterygopalatine ganglion enter the orbit through the inferior orbital fissure, where they join with the maxillary nerve for a short distance before they pass to the lacrimal gland.
Medial Orbital Wall
The medial walls of the orbits are approximately parallel to each other and to the midsagittal plane. The medial wall is composed largely of the thin lamina papyracea of the ethmoid bone. This plate is exceptionally fragile, measuring only 0.2–0.4?mm in thickness, and separates the orbit from air cells of the ethmoid sinus labyrinth. It is a frequent site of fracture in orbital trauma and is breached easily during transnasal ethmoid sinus surgery. The lamina papyracea offers little resistance to expanding ethmoid sinus mucoceles and commonly transmits inflammatory and infectious processes from sinusitis into the orbit.
Posterior to the ethmoid bone, the body of the sphenoid bone completes the medial wall to the apex. This portion of the wall is quite thick and is only rarely involved in orbital trauma or sinus pathology. The medial wall ends at the optic foramen, where the sphenoid forms the medial wall of the optic canal.
Anterior to the ethmoid is the lacrimal bone, a thin plate that contains the posterior lacrimal crest and forms the posterior half of the lacrimal sac fossa. In the midportion of the fossa the lacrimal bone joins the orbital process of the maxillary bone. The latter is a thick bone that forms the medial orbital rim. During lacrimal bypass surgery, entrance into the nose can be achieved most easily with a hemostat by applying gentle pressure on the lacrimal portion of the fossa.
Within the frontoethmoid suture line in the superomedial orbit are two openings, the anterior and posterior ethmoidal foramina. The former usually lies 20–25?mm behind the anterior lacrimal crest, and the latter about 32–35?mm behind the anterior crest and 5–10?mm anterior to the optic canal.  These foramina transmit branches of the ophthalmic artery and nasociliary nerve into the ethmoid sinus and nose. These vessels frequently are injured in orbital trauma and are the major sources of subperiosteal hematomas. These openings mark the approximate level of the roof of the ethmoid labyrinth and the floor of the anterior cranial fossa. The cribriform plate may lie up to 10?mm below this level, just medial to the root of the middle turbinate, and can be fractured during medial wall surgery.
A summary of the orbital bones is given in Box 83-2 .
CONNECTIVE TISSUE SYSTEM
In the human, an extensive system of connective tissue forms a framework for compartmentalization and support of all orbital structures. It is essential to maintain the appropriate anatomical relationships between structural components and to allow precise and coordinated ocular movements.      Some connective tissue septa are aligned with directions of force that resist displacement of extraocular muscles during contraction. Others suspend and support delicate orbital vascular and neural elements. The essential components of this system include the periorbita, the orbital septal systems, and Tenon’s capsule. 
The orbit is lined with periosteum that is loosely adherent to the underlying orbital bones. Applied to the inner surface of the periosteum are multiple layers of orbital connective tissue that are continuous with the transorbital septal systems. Together, this complex layer is known as the periorbita. It is attached firmly at the arcus marginalis along the orbital rim, at the lateral orbital tubercle, adjacent to the trochlea, around the optic foramen, and along the inferior and superior orbital fissures. Where the periorbita joins the margins of the optic canal and superior orbital fissure, it is fused to dura, so trauma or surgery in these areas may be complicated by cerebrospinal fluid leakage.
Within the orbit the periorbita serves to support the extensive septal systems and to stabilize anatomical structures. It forms the boundaries of the entire orbital compartment. At the orbital rim, the periorbita separates into its component layers. Periosteum continues over the rims and remains in contact with the outer table of the cranial bones. The inner layers of the connective tissue system separate from periosteum at the arcus marginalis and extend into the eyelids as the orbital septum. Thus the septum represents the anteriormost boundary of the orbital compartment.
Orbital Septal System
Suspended from the periorbita to form a complex radial and circumferential web of interconnecting slings are connective tissue septa.     These septa form fine capsules around the intraconal and extraconal fat lobules; they also surround the extraocular muscles, optic nerve, and neurovascular elements and suspend these structures from the adjacent orbital walls. The fascial slings provide support and maintain constant spatial relationships between these structures during ocular movements. These septa are responsible for the transmission of restrictive forces from incarcerated or hemorrhagic orbital fat to extraocular muscles after trauma, even in the absence of true muscle entrapment. Septa that encircle the optic nerve may confine hemorrhage or air, which may result in compressive optic neuropathy after trauma.
The anterior fascial system of the orbit primarily supports the globe, anterior orbital structures (such as the lacrimal gland and superior oblique tendon), and the eyelids. It consists of a number of well-developed condensations and ligaments, as well as a more diffuse system of fibrous septa. These structures include Lockwood’s inferior ligament, Whitnall’s superior suspensory ligament, the lacrimal ligaments, and the intermuscular septum. They coordinate movements between the globe and eyelids and suspend the globe so that gaze movements occur around stable axes of rotation ( Fig. 83-2 ).
The connective tissue system is best developed in the midorbit. Here it forms well-defined fascial slings and suspensory complexes associated with each of the extraocular muscles. The fascial layers involved serve to maintain constant muscle alignment, minimize vector shifts during eye movement, and reduce sideslip over the rotating globe ( Fig. 83-3 ).
In the posterior half of the orbit, the connective tissue septal system is not as well developed as in the anterior orbit. The intermuscular
Figure 83-2 The connective tissue system in cross-sectional frontal view through the anterior orbit at the level of Whitnall’s ligament. (Adapted with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
Figure 83-3 The connective tissue system in cross-sectional frontal view through the midorbit. (Adapted with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
septum is incomplete, and the extraocular muscles lie in closer proximity to the orbital walls. Thus, no true anatomical distinction exists between the intraconal and extraconal compartments.
Tenon’s capsule is a dense, elastic, and vascular connective tissue layer that surrounds the globe, except over the cornea, and invests the anterior portions of the extraocular muscles. It provides a bursa-like surface within which the globe can move easily. This structure begins at the perilimbal sclera anteriorly and extends around the globe to the optic nerve, where it blends with fibers of the dural sheath and sclera. Anterior to the insertion of the rectus muscles, Tenon’s capsule is adhered firmly to episclera.
En route from the orbital apex to the globe, the extraocular muscles must penetrate Tenon’s capsule. The four rectus muscles pierce this structure posterior to the equator of the eye. As they proceed forward, the muscles and their thin fibrous sheaths become invested by sleevelike extensions of Tenon’s capsule, which run with them to their insertions.
MUSCLES OF OCULAR MOTILITY
Six striated extraocular muscles are attached to the eye and provide for ocular movement. The four rectus muscles arise posteriorly from the annulus of Zinn, a fibrous band that is continuous with the periorbita and dura at the optic foramen.  The muscles run forward from the annulus of Zinn, and only a thin layer of extraconal fat separates them from the periorbita along the orbital walls. Each is surrounded by a sheath continuous with the orbital fascial systems. It is through these connective tissue septa that the muscles are held in position relative to the orbital walls. These fascial systems help keep the muscles in proper alignment and minimize the
Figure 83-4 Extraocular muscles. Orbital muscles of ocular motility as seen in the coronal plane. (Adapted with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
vector shifts that would otherwise be associated with ocular movement ( Fig. 83-4 ). 
The superior oblique muscle arises above the annulus of Zinn, just superior and medial to the optic foramen. It runs forward along the superomedial orbital wall to the cartilaginous trochlea, through which its tendon slides before it turns sharply laterally to insert on the superoposterior aspect of the globe. 
The inferior oblique muscle arises anteriorly from a small depression just below and lateral to the lacrimal sac fossa. It passes laterally and slightly backward to insert on the inferoposterior surface of the globe near the macula. Along its course, the sheath of the inferior oblique muscle joins that of the inferior rectus muscle and Tenon’s capsule just behind the orbital rim to form Lockwood’s inferior suspensory ligament. The capsulopalpebral fascia extends anteriorly from this ligament to the inferior tarsal plate. During surgery in the inferior orbit, care must be taken when the orbital septum is opened, since the inferior oblique muscle and Lockwood’s ligament lie immediately behind the orbital rim.
The levator palpebrae superioris muscle originates from the annulus of Zinn and lesser sphenoid wing. It runs forward along the orbital roof in close approximation to the superior rectus muscle. Fine check ligaments interconnect the levator to the superior rectus, as well as to periosteum of the frontal bone. Near the orbital rim, fine suspensory ligaments extend from the levator sheath to the superior conjunctival fornix. Also, at about this point, a horizontal condensation is seen within the muscle sheath to form the prominent transverse ligament of Whitnall.  The latter fuses to the orbital wall near the trochlea and around the lacrimal gland. Whitnall’s ligament is an important suspensory structure for the superior orbit and eyelid and should not be cut. Anterior to Whitnall’s ligament, the levator muscle passes into a thin, fibrous aponeurosis that turns inferiorly and fans out into the eyelid. It inserts onto the inferior two thirds of the anterior tarsal face.
MOTOR NERVES OF THE ORBIT
The extraocular muscles are innervated by the third, fourth, and sixth cranial nerves. The oculomotor nerve (cranial nerve III) arises from the oculomotor nuclear complex in the midbrain and exits at the medial border of the cerebral peduncle. It passes forward in the lateral cavernous sinus, where it divides into superior and inferior divisions just before these enter the intraconal space through the superior orbital fissure. The superior branch innervates the superior rectus and levator muscles. The inferior branch sends fibers to the inferior rectus, medial rectus, and inferior oblique muscles. These branches are applied to the inner surface of the muscles, where they are cushioned and protected by the fibrous muscle sheaths. With the inferior division of the oculomotor nerve run parasympathetic fibers that arise from the Edinger-Westphal subnucleus. These synapse in the ciliary ganglion, just lateral and inferior to the optic nerve at 1.5–2?cm behind the globe. They progress via the short ciliary nerves to the ciliary body and iris sphincter.  Little redundancy occurs to these nerves, so they may be injured easily during orbital dissection. This results in disturbances of pupillary function and accommodation.
The trochlear nerve (cranial nerve IV) arises in the midbrain, exits below the inferior colliculus, and passes forward in the lateral cavernous sinus. It enters the extraconal space of the superior orbit through the superior orbital fissure above the annulus of Zinn. Here it crosses over the superior rectus and levator muscle complex and runs along the external surface of the superior oblique muscle before penetrating its substance in the posterior third of the orbit. In this position against the orbital roof, the trochlear nerve is damaged easily during blunt trauma.
The abducent nerve (cranial nerve VI) arises in the pons and passes forward in the cavernous sinus below the trochlear nerve. It enters the intraconal space of the orbit through the superior orbital fissure and annulus of Zinn. The nerve runs laterally to supply the lateral rectus muscle.
Sympathetic nerves enter the orbit via a number of different pathways to innervate the vascular muscular walls, the iris, and the accessory eyelid retractor muscles of Müller ( Fig. 83-5 ). 
SENSORY NERVES OF THE ORBIT
The optic nerve is technically not a sensory nerve but a central nervous system tract that arises from the retinal ganglion cells. Nasal fibers decussate in the optic chiasm. Fibers in the optic tracts continue backward and synapse in the lateral geniculate nuclei, from which they radiate to the occipital cortex. The orbital portion of the nerve is somewhat redundant, to allow for ocular movement. It measures about 3?cm in length and takes a sinusoidal path from the globe to the optic canal. In close approximation to the nerve are the ophthalmic artery, near the orbital apex, and the superior ophthalmic vein, in the midorbit. Both these vessels lie superior to the nerve in most individuals. The central retinal artery runs along the inferolateral side of the nerve to enter the dura about 1?cm behind the globe. The short and long posterior ciliary arteries lie close to the nerve for much of its length and are highly convoluted and redundant near the globe.
Sensory innervation to the orbit is primarily from the ophthalmic division of the trigeminal nerve (cranial nerve V) ( Fig. 83-6 ). The maxillary division supplies portions of the inferior orbit. The ophthalmic division divides into branches in the cavernous sinus just as it passes into the superior orbital fissure. The lacrimal nerve enters above the annulus of Zinn and proceeds in the extraconal space just inside the periorbita along the superolateral orbit to the lacrimal gland and upper eyelid. The frontal nerve runs forward between the levator muscle and the superior periorbita and exits the orbit at the supraorbital notch. At about the level of the posterior globe, it gives rise to the supratrochlear nerve, which exits the orbit at the superomedial rim.
The nasociliary nerve is a branch of the ophthalmic division that enters the orbit through the superior orbital fissure and
Figure 83-5 Motor nerves of the orbit that serve the muscles of ocular motility, in coronal view. (Adapted with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
Figure 83-6 Sensory nerves of the orbit, in lateral view. (Adapted with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
Figure 83-7 Arterial supply to the orbit, in coronal view. (Adapted with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
annulus of Zinn. It crosses from lateral to medial over the optic nerve, after sending small sensory branches that pass through the ciliary ganglion without synapse and continue to the globe with the short ciliary nerves. As it passes to the lateral side of the optic nerve, the nasociliary nerve gives off the long posterior ciliary nerves, which extend to the posterior globe. The nasociliary nerve continues forward in the medial orbit, where it gives rise to the posterior and anterior ethmoidal nerves. It exits the anterior orbit at the superomedial rim as the infratrochlear nerve.
ARTERIAL SUPPLY TO THE ORBIT
The arterial supply to the orbit arises from the internal carotid system through the ophthalmic artery, with anastomotic connections anteriorly from the external carotid system through the superficial facial vessels. The ophthalmic artery enters the orbit through the optic canal inferotemporal to the optic nerve ( Fig. 83-7 ). In about 83% of individuals the vessel crosses over the nerve to the medial side of the orbit; in the remaining 17% it crosses below the nerve.  Shortly after it enters the orbit, the ophthalmic artery gives off a number of branches, with some variability in the sequence between individuals. The central retinal artery is usually the first branch. It runs along the inferior aspect of the optic nerve to penetrate the dura anywhere from 8 to 15?mm behind the globe. The lacrimal artery generally arises next and courses upward and forward, pierces the intermuscular septum, and runs extraconally to the lacrimal gland just above the lateral rectus muscle. It gives rise to the zygomaticotemporal artery, which penetrates the lateral wall at about the midorbit, and to the zygomaticofacial artery, which runs inferolaterally to exit through a small foramen in the zygomatic bone. Through the latter two vessels the lacrimal artery anastomoses with the external carotid system via the transverse facial and superficial temporal arteries. The lacrimal artery terminates in the lids as the lateral inferior and superior palpebral arteries.
As the ophthalmic artery passes toward the medial orbit, the supraorbital branch is given off. This passes through the intermuscular septum medial to the levator muscle and runs forward with the frontal nerve to the supraorbital notch. In the medial orbit, the ophthalmic artery gives rise to the posterior and anterior ethmoidal arteries, which enter the ethmoidal foramina. The ophthalmic artery then continues forward as the nasofrontal artery to exit just above the medial canthus. Here, it gives off the inferior and superior medial palpebral arteries to the eyelids and terminates as the supratrochlear and dorsal nasal arteries, with anastomotic connections to the angular vessels. Branches to the extraocular muscles are given off along the course of the ophthalmic artery. The branching order is summarized in Box 83-3 .
Most Common Branching Order of the Ophthalmic Artery
Central retinal artery
Lateral posterior ciliary artery
Muscular branch to superior rectus and levator muscles
Posterior ethmoidal and supraorbital arteries
Medial posterior ciliary artery
Muscular branch to medial rectus muscle
Muscular branch to superior oblique muscle
Branch to connective tissue
Anterior ethmoidal artery
Inferior medial palpebral artery
Superior medial palpebral artery
T1. Dorsal nasal artery
T2. Supratrochlear artery
VENOUS DRAINAGE FROM THE ORBIT
Venous drainage from the orbit is primarily through the superior and inferior ophthalmic veins ( Fig. 83-8 ).    The superior ophthalmic vein originates at the superomedial orbital rim from branches of the angular, supratrochlear, and supraorbital veins.  As it passes backward along the medial orbit, it is joined by branches draining the medial and superior rectus muscles and the levator muscle, and by the superior vortex veins, the anterior ethmoidal vein, and collateral branches from the inferior ophthalmic vein. At about the midorbit it crosses to the lateral orbit, just below the superior rectus muscle. Here it is joined by the lacrimal vein and continues posteriorly to enter the cavernous sinus through the superior orbital fissure.
The inferior ophthalmic vein has an indistinct origin in a plexus of small vessels in the inferior orbit. It passes backward along the inferior rectus muscle and is joined by branches draining the inferior rectus and inferior oblique muscles, the inferior vortex veins, and the lateral rectus muscle. A branch exits through the inferior orbital fissure to join the pterygoid plexus before the vessel terminates at the superior ophthalmic vein, just before it enters the cavernous sinus.
Figure 83-8 Orbital veins. Venous drainage from the orbit, in coronal view. (Adapted with permission from Dutton JJ. Atlas of clinical and surgical orbital anatomy. Philadelphia: WB Saunders; 1994.)
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