Functions of the Orbit and Eyelids

Introduction

The cranium protects the brain and provides scaffolding for facial structures. During primate evolution, the orbits were enlarged and reoriented towards the front of the face. This, along with the gradual flattening of the face, allowed for improved binocular vision due to overlapping visual fields. Along with morphologic skull changes, facial mimetic musculature evolved in primates as a means of close-proximity non-vocal communication. It is in this context that the function of the eyelids to protect, lubricate, and cleanse the ocular surface, is most fully appreciated. This chapter will examine anatomy of the eyelids, orbits, and related facial structures.

Orbit osteology

Derived from cranial neural crest cells, the bony orbit consists of seven individual bones combining into four walls that surround the globe, extraocular muscles, nerves, fat, and blood vessels. The orbital bones include the sphenoid (greater and lesser wings), frontal, ethmoid, maxillary, zygomatic, palatine, and lacrimal bones. Posteriorly approaching the apex, this four-sided pyramid becomes three-sided with the gradual merger of the medial wall and orbital floor after the floor is cut off by the inferior orbital fissure. The adult orbit has a volume of 25–30 mL, with the globe filling approximately 7 mL or 25% of the space. The orbit depth as measured from the center of the orbital margin to the apex is approximately 45 mm. The widest diameter of the orbit occurs 1 cm behind the orbital rim. The lateral walls of the orbit are oriented 90 degrees from one another and run approximately 40–45 mm, while the medial walls are parallel to one another ( Fig. 14.1 ). Due to this bony orientation, the eyes tend to diverge, and thus are tonically held in adduction by the medial rectus muscles to achieve ocular alignment.

The orbital rim provides a hard tissue shield to surround and protect the eye. It is a thick discontinuous spiral that begins medially at the anterior lacrimal (maxillary bone) crest and coils to end at the posterior lacrimal (lacrimal bone) crest ( Fig. 14.2 ). The lacrimal fossa, which contains the lacrimal excretory sac, lies between these two crests. Formed by the zygomatic bone and the zygomatic process of the frontal bone, the lateral orbital rim is the strongest and thickest portion of the rim. While the posteriorly directed concavity of the lateral orbital rim allows for a wide visual field, it also makes the eye prone to injury from objects approaching from a lateral direction. The superior orbital rim is formed by the frontal bone. Medially, the superior rim contains the supraorbital notch through which the supraorbital nerve and artery pass. In some individuals, a supraorbital foramen replaces the notch. The infraorbital nerve and artery exit through the infraorbital foramen approximately 4–6 mm below the inferior orbital rim medially.

The orbital roof is comprised of the orbital plate of the frontal bone along with a minor contribution from the lesser wing of the sphenoid posteriorly. The frontal bone is the strongest component of the craniofacial skeleton, withstanding between 800 and 2200 pounds of force before fracturing. This is equivalent to the force achieved in a frontal collision at 30 mph for an unrestrained adult passenger. Orbital roof “blow-in” fractures may be associated with a number of ocular and neurologic injuries including proptosis, ptosis, optic nerve contusion, orbital hematoma, as well as contusive and hemorrhagic injuries to the ipsilateral frontal and parietal lobes. Important bony landmarks of the orbital roof include the lacrimal gland fossa temporally and another small depression 3–5 mm behind the orbital rim anteromedially known as the trochlear fossa. Here resides the fibrocartilaginous trochlea through which the superior oblique tendon passes.

The frontal sinus begins to develop between 1 and 2 years of age through invagination of the frontal bone by frontoethmoidal air cells. The filling of air or pneumatization of the sinus begins at about 5–8 years old and continues into adulthood. Therefore, there is a relative absence of frontal sinusitis in very young children. Yet, orbital complications due to frontal sinusitis such as orbital cellulitis and abscess most often occur between the ages of 5 and 10 years. It has been hypothesized that direct extension through the relatively thin orbital roof in children or congenital bony dehiscences (gaps) posterior to the trochlear and supraorbital notch may account for these findings.

The lateral wall is comprised of the zygomatic bone anteriorly and the greater wing of the sphenoid posteriorly. The vertically oriented zygomaticosphenoidal suture represents the thinnest portion of the lateral wall, and is a convenient breaking point during lateral orbitotomy. The posterior boundaries of the wall are marked by the superior and inferior orbital fissures. The frontosphenoidal suture forms the boundary between the lateral wall and orbital roof. About one-third of individuals have a meningeal foramen just superior to this suture which transmits the recurrent meningeal artery (a branch of the external carotid system) to form an anastomosis with the lacrimal artery (a branch of the internal carotid system). This collateral system has potential importance if the primary internal carotid vascular supply becomes compromised. An important bony prominence of the lateral wall is Whitnall's lateral tubercle, a small rounded protuberance of the zygomatic bone 3–4 mm inside of the lateral orbital wall and approximately 11 mm below the frontozygomatic suture. ^,

Fractures to the lateral wall may result from blunt trauma to the zygomatic bone and lateral orbital rim. Due to its multiple bony articulations, fractures of the zygoma may disrupt the wider anatomy and are often referred to as zygomaticomaxillary complex fractures or ZMC fractures ( Fig. 14.3 ). Clinical signs include lateral canthal dystopia (inferior displacement of the lateral canthal angle), cheek depression and trismus (difficulty opening the mouth owing to spasm of the masticatory muscles or impingement of the coronoid process of the mandible). Hypoesthesia may be noted at the lateral midface from disrupted branches of the zygomaticotemporal and zygomaticofacial neurovascular bundles (V1 branches traveling through the zygomatic bone), as well as in the distribution of the infraorbital nerve (V2 branch).

The orbital floor consists of the maxillary bone, the zygoma anterolaterally, and the palatine bone posteriorly. It is triangular in shape and extends from the maxillary-ethmoid buttress (the relatively dense bone strut at the union of the orbital floor and medial wall) to the inferior orbital fissure. The shortest of the walls, it travels posteriorly from the rim 35–40 mm and ends before the orbital apex at the pterygopalatine fossa. The infraorbital neurovascular bundle passes in the infraorbital groove and infraorbital canal of the orbital floor as it travels towards infraorbital foramen. The floor remains strong laterally, but thins posteromedially with expansion of the maxillary sinus. This thinned maxillary bone is where the floor usually fractures during trauma and is also a convenient site to initiate an inferior wall decompression. The infraorbital nerve is often contused during an orbital floor fracture but is rarely severed, thus, initial hypoesthesia in the V2 distribution will commonly resolve over several months’ time.

The medial wall consists of the maxillary, lacrimal and ethmoid bones, as well as the lesser wing of the sphenoid bone. The two medial walls are parallel to one another and extend 45–50 mm from the anterior lacrimal crest to the orbital apex. The lamina papyracea of the ethmoid bone is an extremely thin portion of bone that has a honeycombed structure. Giving support to the medial wall, the multiple bullae that form this structure develop secondarily to pneumatization of the ethmoid bone. This helps to explain why the medial wall fractures less often than the thicker orbital floor. Landmarks important to the orbital surgeon include the anterior and posterior ethmoidal foramina that reside at the frontoethmoidal suture and convey respective branches of the ophthalmic artery and nasociliary nerve. The anterior ethmoidal foramen is located approximately 24 mm posterior to the anterior lacrimal crest and the posterior ethmoidal foramen 36 mm posterior to the rim. The orbital foramen is located approximately 6 mm behind the posterior ethmoidal foramen. The frontoethmoidal suture may also be used surgically to approximate the level of the floor of the anterior cranial fossa.

Providing support for the inferomedial wall and helping to maintain globe position is the thickened region of bone known as the inferomedial strut. The strut is constructed using elements of multiple orbital and facial bones including the maxillary bone, ethmoid bone, and palatine bone posteriorly. ^, Preservation of the anterior portion of the inferomedial strut during orbital decompression has been shown to reduce the incidence of post-operative globe dystopia.

As previously mentioned, the lacrimal sac fossa lies between the anterior (maxillary bone) crest and posterior (lacrimal bone) crest of the orbital rim ( Fig. 14.4 ). The relative contribution of these two bones may vary. The lacrimal bone at the lacrimal sac fossa has a mean thickness of 106 µm, which allows easy penetration during dacryocystorhinostomy surgery (surgical procedure to restore the normal flow of tears into the nose from the lacrimal sac by bypassing the nasolacrimal duct). The maxillary bone is considerably denser than the lacrimal bone, and may require the surgeon to create a more posterior osteotomy during dacryocystorhinostomy. The nasolacrimal canal directs the nasolacrimal duct to the inferior meatus of the nose under the inferior turbinate.

The orbital apex

Many important neural and vascular structures pass through the orbit apex including cranial nerves II through VI, the origins of all the extraocular muscles except the inferior oblique, and arterial and venous blood supplies ( Fig. 14.5 and Box 14.1 ). Pathology in this crucial location of the orbit may lead to the orbital apex syndrome with characteristic hallmarks of visual loss from optic neuropathy and ophthalmoplegia.

The optic foramen conveys the optic nerve and ophthalmic artery from the optic canal into the orbit. The canal is housed in the lesser wing of the sphenoid bone with a contribution from the inferomedial optic strut. It runs 8–10 mm in length and is 5–6 mm in diameter. The optic canal and foramen attain adult dimensions by 3 years and are symmetric in most persons. A canal/foramen that is larger in diameter by at least 1 mm than the contralateral side may be considered abnormal.

The superior orbital fissure is located just lateral to the optic canal. Approximately 20–22 mm in length, the fissure divides the lesser and greater wings of the sphenoid bone. The superior ophthalmic vein, as well as the lacrimal, trochlear, and frontal nerves, passes through the superolateral portion of the fissure outside the annulus of Zinn, a fibrous ring formed by the common origins of the rectus muscles. The annulus is further subdivided by the oculomotor foramen through which the superior and inferior divisions of the oculomotor nerve, the abducens nerve, the nasociliary nerve (a terminal sensory branch of the ophthalmic division of the trigeminal nerve), and sympathetic fibers all pass. ^, Also passing through the annulus of Zinn are the optic nerve and ophthalmic artery. As the extraocular rectus muscles proceed forward to their insertions on the globe, they form a conoid enclosure known as the intraconal space. This compartment is useful radiologically ( Fig. 14.6 ). Intraconal pathology of the fat, vessels, and optic nerve-sheath complex may be distinguished from extraconal pathology of the lacrimal gland, bony orbit, and remaining extraconal fat. ^,

The inferior orbital fissure divides the greater wing of the sphenoid laterally from the maxillary bone of the orbital floor inferomedially. The fissure measures approximately 20 mm in length. It communicates with the pterygopalatine fossa which rests behind the maxillary sinus. Traveling through the fissure are the maxillary division of the trigeminal nerve, branches from the sphenopalatine ganglion, and the inferior ophthalmic vein. Branching away from the fissure and entering the infraorbital groove and canal are the infraorbital (V2) and the terminal branch of the internal maxillary artery.

Orbital soft tissues

Periorbital fascia

The periorbital fascia is a single interconnecting network emanating from the periosteal lining of the orbital walls, globe (Tenon's capsule), and extraocular muscles ( Figs 14.7 & 14.8 ). There are also check ligament extensions from the extraocular muscle fascia that attach to the bony orbit as well as sheaths that extend between the rectus muscles. The periorbita is firmly attached at the suture lines, foramina, fissures, arcus marginalis, and the posterior lacrimal crest. Elsewhere, it is loosely attached to the bone which creates a potential space for accumulation of blood, pus, or tumor growth. It is most firmly attached along the arcus marginalis. The orbital septum is a thin, multilayered extension of the periorbita and is the anterior soft tissue boundary of the orbit. It functions as a physical barrier to pathogens and contributes to the normal posterior position of the orbital fat pads. In the upper eyelid, the fibrous lamellae of the orbital septum gradually blends with those of the levator aponeurosis on average 3.4 mm above the superior tarsal border (with a range of 2–5 mm). In the lower eyelid, the septum inserts onto the inferior border of the tarsus after joining with the inferior retractors 4–5 mm below the tarsus. Laterally the septum fuses with the lateral canthal tendon and attaches 2–3 mm posterior to the rim at the lateral orbital tubercle (Whitnall's tubercle). Medially the septum splits and inserts onto the anterior and posterior lacrimal crest. Finally, it is anchored anteriorly to the orbicularis muscle by multiple fibrous attachments.

Tenon's capsule, or fascia bulbi, is a fibroelastic membrane that extends anteriorly from the dural sheath to encircle the globe and fuse with the conjunctiva just behind the limbus. Posteriorly it separates orbital fat from the globe and muscles. Openings within the fascia allow passage of the extraocular muscles which are in turn surrounded by their own muscular fascia. Anteriorly the muscular fascia thickens and connects with the orbital wall to form the check ligament of the extraocular muscles, preventing overaction of the muscle from which they extend. The strongest of these is the lateral check ligament that inserts on the lateral orbital tubercle, while the medial rectus check ligament inserts behind the posterior lacrimal crest.