Orbital Roof

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. The roof slopes backward and downward toward the orbital apex, where it ends at the optic canal and superior orbital fissure (SOF) (Fig. 1.2). In the anterior superomedial corner of the orbit, a small depression in the frontal bone houses the cartilaginous trochlea. During surgery on the medial wall and roof, this can safely be elevated along with periorbita. In the lateral anterior roof, a shallow fossa houses the lacrimal gland. A notch or, less commonly, a foramen is situated on the superior orbital rim approximately 25 to 30 mm from the facial midline, which transmits the supraorbital neurovascular bundle.

On computed tomography (CT) coronal images, the orbital roof appears as a thin bone density line arching over the orbit, separating it from the anterior cranial fossa. On the cranial side, the surface may be gently undulating, reflecting the sulci and gyri of the overlying frontal lobe. Anteriorly, the paired frontal sinus can be seen within the frontal bone. Mass lesions in the superior orbit often deform the roof shape if long standing or can erode the roof, forming a dehiscence into the anterior cranial fossa. Bone destruction is a sign of malignancy or significant inflammatory reaction. The roof is also a common site for penetration of orbital foreign bodies and is best evaluated with CT.⁵

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 along the lateral rim (Fig. 1.3). It is bounded below by the inferior orbital fissure and medially by the SOF. Behind the lateral orbital rim, the wall becomes quite thin where the zygomatic bone joins the greater sphenoid wing. During lateral orbitotomy surgery, cutting the rim through to this thin plate allows easy outward fracture of the bone and access to the intraconal space. The convoluted frontozygomatic suture line runs approximately horizontally and crosses the superotemporal orbital rim just below the lacrimal gland fossa. Separation of this suture is sometimes seen following orbital trauma associated with a tripod fracture. About 5 to 15 mm above this suture line, the frontal bone widens into a thickened region, called the trigone, and passes around the front end of the middle cranial fossa and the temporal lobe of the brain.

Just inside the central lateral rim on the zygomatic bone is a small elevation termed Whitnall’s tubercle. This allows complex attachments of the lateral posterior head of Lockwood’s inferior orbital ligament, the lateral retinaculum of the lateral canthal ligament, the lateral horn of the levator aponeurosis, and some fibers of Whitnall’s superior transverse orbital ligament. These structures can be elevated along with periosteum during lateral orbital surgery and then repositioned without injury to their function.

In 30% to 40% of individuals, there is a small foramen (Hyrtl’s or meningolacrimal foramen) situated in the greater sphenoid wing 5 to 10 mm lateral to the SOF. This carries the meningolacrimal artery, an embryonic remnant of the primitive orbital circulation. Disruption of this vessel during lateral orbitotomy surgery can cause brisk bleeding, but it usually stops spontaneously.

Pathology involving the lateral wall is uncommon, but it can be eroded by tumors of the lacrimal gland or show defects from congenital lesions such as a dermoid cyst.⁶ The lateral wall or rim also can be distorted by fractures, which may impinge on the lateral rectus muscle. The trigone is the area of the greater sphenoid wing that thickens over the tip of the lateral lobe of the brain. It is fairly vascular and can be a site for metastases.

Orbital Floor

The floor is the shortest of the orbital walls, separated from the orbital apex by the pterygopalatine fossa. It is composed primarily of the maxillary bone, with the zygomatic bone forming the anterolateral portion and the palatine bone lying at its posterior extent but usually fused to the maxillary bone in adults. The floor ends at the posterior limit of the maxillary sinus and, therefore, does not extend to the orbital apex (Fig. 1.4). Laterally, the floor is bounded by the inferior orbital fissure, which separates it from the lateral wall. This fissure is usually bridged over by a thin plate of bone in the anterior half of the floor to form the infraorbital canal. This houses the maxillary division of the trigeminal nerve (V₂). The latter exits the orbit anteriorly through the infraorbital foramen 4 to 5 mm below the central orbital rim to provide sensory innervation to the lower eyelid and upper cheek.

The orbital floor is thinnest just medial to the canal and thus is the most common site for blowout fractures.⁷ Maxillary sinusitis or tumors may erode upward through the floor into the inferior orbit.

Medial Orbital Wall

The medial walls of the orbits are approximately parallel to the midsagittal plane and are separated by the ethmoid sinuses. They are composed largely of the very thin lamina papyracea of the ethmoid bone, which separates the orbit from the ethmoid sinus air cells (Fig. 1.5). Anteriorly, the thick frontal process of the maxillary bone lies at the inferomedial orbital rim. It contains the anterior lacrimal crest that forms the anterior portion of the lacrimal sac fossa. The anterior crus of the medial canthal ligament inserts here. The lacrimal bone is a small thin plate just posterior to the maxillary bone and contains a vertical ridge, the posterior lacrimal crest. The lacrimal sac fossa containing the nasolacrimal sac lies between the anterior and posterior crests. During dacryocystorhinostomy procedures, initial creation of the surgical osteum is easiest through the thin lacrimal bone, where it can be punctured with a hemostat, but in 8% of cases, the maxillary–lacrimal suture line is more posterior so that the thick maxillary bone underlies most of the fossa. In such cases, bone removal is more difficult and may require a burr.

Behind the lacrimal bone, the lamina papyracea of the ethmoid bone forms most of the medial orbital wall. It joins the frontal bone superiorly along the frontoethmoid suture line. This marks the upper limit of safe bone removal during orbital decompression surgery without risk of damage to the cribriform plate. The anterior and posterior ethmoidal foramina are located within or sometimes just above this suture, through which pass the ethmoidal arteries and nerves (Table 1.1). There is great variability in the position of these foramina, and the posterior foramen may be absent, or both may be multiple. The anterior ethmoidal foramen is located 22 mm (range 14–30 mm) behind the anterior lacrimal crest. The posterior foramen is 33 mm (range 25–41 mm) behind the anterior crest and 4 to 15 mm anterior to the entrance of the optic canal. The positions of these foramina are clinically important since they relate to important structures such as the cribriform plate and optic canal. The ethmoidal vessels may be the source of subperiosteal hemorrhage from orbital trauma or surgical manipulation.

The medial wall is a common site of fracture from trauma, where the lamina papyracea and orbital tissues may be displaced into the ethmoid sinus.⁸ The medial rectus muscle can be entrapped within these bony fragments, resulting in horizontal motility restriction. The lamina offers little resistance to expanding ethmoid sinus mucoceles, which slowly expand into the medial orbit, displacing the medial rectus muscle laterally and sometimes even the optic nerve. However, the medial wall provides greater resistance to fractures from blunt trauma compared with the orbital floor, largely as a result of the honeycombed bony laminae surrounding the air cells that are not present in the floor. Inflammatory sinusitis can be transmitted through the lamina papyracea or the ethmoidal foramina, forming a subperiosteal abscess in the medial orbit. In orbital decompression surgery for thyroid eye disease, the lamina papyracea is removed along with some air cells to allow the medial rectus muscle and orbital fat to prolapse into the sinus, thus reducing proptosis.^9,10

Optic Canal

The optic canal is located at the medial orbital apex within the sphenoid bone (Fig. 1.2). It measures 8 to 12 mm in length and is directed posteriomedially and upward at about 35 degrees to the midsagittal plane. The canal transmits the optic nerve and immediately below it the ophthalmic artery.

Muscles of Ocular Motility

The eye is an approximate sphere normally measuring about 24 mm in diameter and is situated in the anterior one-half of the orbit. Attached to it are the six striated extraocular muscles of ocular motility (Fig. 1.6). Four rectus muscles arise at the orbital apex from the annulus of Zinn, a fibrous ring continuous with periorbita and dura at the optic foramen and SOF. The muscles run forward, with a layer of extraconal fat separating them from periorbita along the orbital walls. Anteriorly, they pass through the Tenon capsule to insert onto the sclera. The distance from the anterior corneal limbus to the insertion of each tendon is variable, generally increasing around the globe from the medial rectus muscle (5.3 mm) to the inferior, lateral, and superior rectus muscle (7.9 mm).¹ An imaginary line drawn through these insertions is known as the spiral of Tillaux.

On both CT and magnetic resonance imaging (MRI) scans, the rectus muscles appear as slightly fusiform straps of moderate tissue density, thickest at their bellies and thinnest at the tendons of insertion.⁴ Structures within the imaginary cone formed by the four rectus muscles are said to be intraconal, and those between the muscles and the orbital walls are extraconal.

The superior oblique muscle arises above the annulus, just superior and medial to the optic foramen. It runs forward along the superomedial orbital wall to the cartilaginous trochlea at the superomedial corner of the orbit. The tendon of this muscle slides through the trochlea before turning sharply posterolaterally to insert on the superoposterior aspect of the globe.

The inferior oblique muscle originates in the anterior orbit from a small depression on the maxillary bone, below and lateral to the lacrimal sac fossa. It passes laterally and slightly backward beneath the inferior rectus muscle to insert on the inferoposterior surface of the globe near the macula. Anteriorly, the sheath of the inferior oblique muscle joins that of the inferior rectus muscle and the Tenon capsule just behind the orbital rim to form Lockwood’s inferior suspensory ligament. During surgery in the inferior orbit, care must be taken not to injure this muscle, since it lies immediately behind the orbital rim and may lie at or even anterior to the rim in proptotic eyes.

Numerous diseases can affect the extraocular muscles. In thyroid eye disease, proptosis from muscle enlargement, along with fat proliferation, is an early sign. Multiple muscle bellies are usually thickened by edema and infiltration with glycosaminoglycans, but the tendons of insertion are typically normal. Orbital myositis is an idiopathic disorder characterized by inflammatory infiltration into one or more muscles, usually involving both the belly and the tendon. Metastatic tumors frequently localize in the extraocular muscles because of their extensive vascular supply and may appear as a focal nodular enlargement in one or sometimes several muscles.¹¹ Within orbital wall fractures, the muscle often shows a more rounded contour from edema or hemorrhage. It can also be entrapped and pulled into the bony fragments, where it appears elongated toward the fracture site.⁴

Motor Nerves of the Orbit

The extraocular muscles are innervated by the third, fourth, and sixth cranial nerves¹² (Fig. 1.7). The oculomotor nerve (cranial nerve [CN] III) arises in the somatic portion of the oculomotor nucleus of the midbrain. It enters the cavernous sinus and travels forward in the dura of its lateral wall. In the anterior cavernous sinus, it is joined by sympathetic fibers from the internal carotid artery sympathetic plexus. The oculomotor nerve enters the orbit from the cavernous sinus through the annulus of Zinn as two branches. The superior branch innervates the superior rectus and levator muscles. The inferior branch sends fibers to the inferior rectus, the medial rectus, and the inferior oblique muscles on their intraconal surfaces (Table 1.2).

The trochlear nerve (CN IV) arises from the trochlear nucleus at the caudal end of the oculomotor nuclear complex in the midbrain. The nerve extends forward, pierces the dura of the cavernous sinus, and runs within the dura of its lateral wall just beneath the oculomotor nerve. It enters the orbit through the SOF above the annulus of Zinn. It crosses over the superior rectus and levator muscle complex, in the extraconal space adjacent to the orbital roof, 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 more easily damaged during blunt trauma and can be injured with displacement of periorbita during deep superior orbital surgery.

The abducens nerve (CN VI) arises in motor nuclei of the pons and runs a tortuous intracranial course to the cavernous sinus, where it runs within the body of the sinus, adjacent to the carotid artery, medial to the trigeminal nerve. The abducens nerve enters the orbit through the SOF and annulus of Zinn and runs in the lateral intraconal space to supply the lateral rectus muscle.

Pathology or surgery in the posterior orbit can affect any of these nerves to the extraocular muscles. These can result in motility disturbance with characteristic ocular deviations, depending on the nerves involved. Injury to the superior branch of the oculomotor nerve often causes ptosis of the upper eyelid. All three nerves are usually affected simultaneously from lesions located in the orbital apex or within the cavernous sinus.^13,14

Sensory Nerves of the Orbit

The optic nerve (CN I) is technically not a sensory nerve but a central nervous system tract arising from the retinal ganglion cells. Nasal fibers from each eye decussate in the optic chiasm and continue in the optic tracts to synapse in the lateral geniculate nuclei. From here, they radiate to the occipital cortex. The orbital portion of the optic nerve measures approximately 3 cm in length and is somewhat undulating and redundant to allow for ocular movement. It passes back through the optic canal in close approximation to the ophthalmic artery. On CT and MRI images, the optic nerve runs through the canal to the optic chiasm just posterior to the pituitary stalk. Since the nerve measures only about 3 mm in diameter, high-resolution thin-section scans are necessary to image the optic nerve adequately. Lesions involving the nerve can cause enlargement, and the contour is often, although not always, suggestive of specific pathology. Fusiform enlargement is typically seen with optic nerve gliomas and tubular thickening with optic nerve sheath meningiomas.¹⁵ In the latter case, the central nerve continues to be of low density, whereas the sheath enhances to produce the classic tram-tracking, or railroad track, sign. Inflammatory perioptic neuritis appears as a more irregular thickening with a shaggy surface.

Sensory innervation to the orbit is primarily from the ophthalmic division of the trigeminal nerve (CN V). The ophthalmic division divides within the cavernous sinus just as it passes into the SOF. These branches are the lacrimal, frontal, and nasociliary nerves (Fig. 1.8). The lacrimal nerve enters above the annulus of Zinn into the extraconal space and proceeds forward 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 or foramen at the superior orbital rim. The nasociliary nerve is the only branch that enters the orbit through the annulus of Zinn into the intraconal space. It crosses from lateral to medial over the optic nerve after sending small sensory branches that pass through the ciliary ganglion to the globe as the short posterior ciliary nerves. As the nasociliary nerve passes to the medial side of the orbit, it gives off the long posterior ciliary nerves that extend to the posterior globe. The nasociliary nerve continues forward in the medial orbit, where it gives off the anterior and posterior ethmoidal nerves, and then exits the orbit anteriorly as the infratrochlear nerve. Sensory nerves can be affected by inflammations or tumors. Paresthesias of the upper eyelid and forehead, especially when associated with ocular motility deficits, should alert the clinician to the possibility of an orbital apex or cavernous sinus lesion. Within the orbit, neural tumors such as neurofibromas and schwannomas are typically seen in the superior orbit, where they more commonly affect sensory nerves. Cutaneous tumors arising on the eyelids and forehead may spread along sensory nerves into the deep orbit and the cavernous sinus.

Autonomic Nervous System

Sympathetic nerves to the eye and orbit arise within the hypothalamus, descend in the spinal cord to the paravertebral sympathetic chain and synapse in the superior cervical ganglion. Fibers destined for the orbit extend along the external and internal carotid arteries to the region of the Gasserian (trigeminal) ganglion and cavernous sinus. The exact relationships within the sinus remains controversial, but these fibers probably extend along one or more routes, including the ophthalmic artery, the trigeminal nerves, and the orbital motor nerves or directly through the SOF to the ciliary ganglion.¹ Sympathetic fibers also enter the orbit via the pterygopalatine fossa along the maxillary artery and nerve.

Some sympathetic nerves pass through the ciliary ganglion without synapse and course to the globe in the short posterior ciliary nerves. Others bypass the ganglion, enter the long posterior ciliary nerves and enter the globe to mediate vasoconstriction and the iris dilator muscle. Other orbital branches arise from a diffuse sympathetic plexus in the orbital apex to innervate various orbital structures, including the lacrimal gland and the sympathetic muscles of the eyelids.

Parasympathetic nerve fibers arise in the Edinger-Westphal nucleus of the oculomotor nuclear complex and run within the oculomotor nerve through the SOF to the orbit. They then pass to the ciliary ganglion via a small sensory branch, synapse within the ganglion, and extend to the globe in the short posterior ciliary nerves to innervate the ciliary and pupillary sphincter muscles.

Other parasympathetic fibers run a more circuitous course to reach the pterygopalatine fossa, where they synapse in the pterygopalatine ganglion. Branches destined for the orbit accompany the maxillary and infraorbital arteries and infraorbital nerves and pass through the inferior orbital fissure to the inferior orbit. Secretory parasympathetic fibers reach the lacrimal gland via branches of the zygomatic nerve.

Arterial Supply to the Orbit

The arterial supply to the orbit is from the internal carotid system through the ophthalmic artery (Fig. 1.9). The ophthalmic artery enters the orbit through the optic canal inferotemporal to the optic nerve and gives off branches to orbital structures.^16,17 In approximately 80% of individuals, the optic nerve crosses from lateral to medial over the optic nerve, and in 20%, it passes under the nerve. There is some variation in branching pattern to orbital structures especially between these two pathways. The central retinal artery is usually the first branch (Table 1.3). It runs along the inferior aspect of the optic nerve to penetrate the dura of the nerve about 1 cm behind the globe. The lacrimal artery generally arises next and courses upward and forward extraconally to the lacrimal gland. Branches continue to the lateral upper eyelid, where they anastomose with the superficial temporal artery of the facial system. The lateral and medial long posterior ciliary arteries arise on either side of the lacrimal artery and run forward to the globe, parallel to the optic nerve.¹⁸

The ophthalmic artery crosses over the optic nerve in the posterior orbit and gives rise to the various muscular arteries that enter the conal surfaces of the rectus and oblique muscles. The supraorbital branch passes to the superior extraconal space to exit the orbit via the supraorbital foramen or notch. The ophthalmic artery exits the medial muscle cone and continues forward in the medial extraconal space as the nasofrontal artery and terminates as the supratrochlear and dorsal nasal arteries. These anastomose with the angular and other facial arteries from the external carotid system.

On a normal CT or MRI scan, the ophthalmic artery is seen as a vessel entering the orbit just lateral and inferior to the optic nerve. It crosses over the optic nerve from lateral to medial near the orbital apex. It can sometimes be seen running anteriorly just medial to the medial rectus muscle. The ophthalmic artery only rarely is involved directly with orbital pathology. Aneurysms are occasionally seen, as are arteriovenous malformations that appear as a tangle of dilated vessels best seen on angiography.

Venous Drainage From the Orbit

Venous drainage from the orbit is through the superior and inferior ophthalmic veins and their tributaries that drain backward into the cavernous sinus¹⁹ (Fig. 1.10). The superior ophthalmic vein originates at the superomedial orbital rim from branches of the angular, infratrochlear, and supraorbital veins of the facial system. As it passes backward crossing from medial to lateral over the optic nerve in the superior mid orbit, it is joined by the anterior ethmoidal vein, the vortex veins, collateral branches from the inferior ophthalmic vein, branches from the extraocular muscles, and the lacrimal vein. It continues posteriorly to enter the cavernous sinus through the SOF.

Ophthalmic veins not uncommonly show pathologic processes such as orbital varices that appear on imaging as irregular masses, sometimes with dilated vascular channels. A dilated superior ophthalmic vein is suggestive of a carotid–cavernous or dural–cavernous fistula with increased venous pressure and often correlates with cork-screw episcleral vessels and congested extraocular muscles.²⁰

The Eyelids

The eyelids serve an important function by protecting the globe. They help distribute tears evenly over the surface of the eye and propel them to the medial canthus, where they flow into the lacrimal drainage system. The eyelashes sweep airborne particles from in front of the eye, and voluntary and reflex eyelid blinking protects the cornea from injury and glare.

The eyelids provide significant components to the precorneal tear film, adding to secretions from the lacrimal gland. These additions are produced from the accessory lacrimal glands of Wolfring and Krause, goblet cells in the conjunctiva, and Meibomian glands in the tarsal plates.²¹ The ciliary glands of Moll and Zeis empty into the adjacent lashes. Both secrete lipids that lubricate the lashes, but they also add to the superficial layer of the tear film, retarding evaporation.

In the adult, the eyelids form a fissure centered over the cornea measuring about 8 to 10 mm vertically and 30 to 31 mm horizontally. The upper and lower eyelids meet at an angle of approximately 60 degrees at the medial and lateral canthi, with some ethnic variability.²² In the primary position of gaze, the upper eyelid margin lies at the superior corneal limbus in children and 1.5 to 2.0 mm below it in the adult. The lower eyelid margin normally rests at the inferior corneal limbus.

The margin of each eyelid is about 2 mm thick. Posteriorly, the marginal tarsal surface is covered with conjunctival epithelium, interrupted by Meibomian gland orifices. Anteriorly, the margin is covered with cutaneous epidermis, from which emerge the eyelashes (Fig. 1.11). The anterior and posterior lamellae are separated by a visible linear depression, the gray line that is an important surgical landmark in some marginal lid-splitting procedures.

The upper eyelid crease is a horizontal fold caused by attachments of superficial levator aponeurotic fibers into orbicularis intermuscular septa (Fig. 1.12). It lies about 8 to 11 mm above the eyelid margin centrally in the eyelids of non-Asian persons. If this crease is disrupted during upper lid surgery, care must be taken to reform it by removal of a spindle of orbicularis muscle or through use of deep fixation sutures to maintain normal cosmetic appearance and to prevent downward displacement of preaponeurotic fat or overhang of eyelid skin. In Asians, the upper lid crease may be single, low, or double as a result of prolapse of the orbital septum and fat over the levator aponeurosis²³ and is also likely to relate to other variations in eyelid and orbital rim anatomy.²⁴