Anatomy of the Orbit
Orbital Cavity
The orbits are the bony spaces that divide the upper facial skeleton from the middle face. The bony walls of the orbit, a four-sided pyramid, consist of a mosaic of seven bones: the zygomatic bone laterally, the frontal bone superiorly, the sphenoid bone posteriorly, with its lesser and greater wing forming the optic canal and the superior orbital fissure, the orbital process of the palatine bone and the maxillary bone inferiorly, along with the lacrimal and ethmoid bone medially (see Fig. 4.1 ).
The roof (superior wall) is thin and concave, formed primarily by the orbital plate of the frontal bone joining the lesser wing of sphenoid near the apex of the orbit. The supraorbital foramen for the supraorbital nerve and vessels is presented in the middle of the supraorbital rim ( Table 4.1 ). The floor (inferior wall) is formed by the orbital surface of maxilla and zygomatic bone and the minute orbital process of palatine bone, separating the orbit from the maxillary sinus. It is traversed by the infraorbital groove that leads to the infraorbital foramen. The floor is separated from the lateral wall by the inferior orbital fissure, which connects the orbit with the pterygopalatine and infratemporal fossa ( Fig. 4.2 ). The medial wall consists of contributions from the orbital plate of the ethmoid, the frontal process of maxilla, the lacrimal bone, and a small part of the body of the sphenoid. The extremely thin ethmoid air cells form a delicate bony structure known as the lamina papyracea, and the thinnest medial wall is perforated by the anterior and posterior ethmoid canal.
| Foramina and Fissures | Contents |
|---|---|
| Supraorbital foramen | Supraorbital nerve (V1) |
| Infraorbital foramen | Infraorbital nerve (V2) |
| Superior orbital fissure | Oculomotor (II), trochlear (IV), abducens nerve (VI), opjthalmic branches (V1) |
| Inferior orbital fissure | Maxillary nerve branches (V2), zygomatic nerve, sphenopalatine ganglion branches, infraorbital artery and vein, inferior ophthalmic vein (leading to pterygoid plexus) |
| Optic canal | Optic nerve (II), ophthalmic artery |
| Ethmoidal canals | Anterior and posterior ethmoidal artery |
| ⁎ Annulus of Zinn | Superior division of cranial nerve III, nasociliary nerve (V1), sympathetic root of cervical ganglion, inferior division of cranial nerve III, cranial nerve VI, superior ophthalmic vein |
The lacrimal groove for the nasolacrimal duct is located anteriorly. The posterior portion leads to the superior orbital fissure, a dehiscence between the two wings of the sphenoid bone that provides a passage for orbital nerves and vessels and corresponds to the anterior wall of the cavernous sinus, representing a line of communication between the middle cerebral fossa and the orbit. The summit or apex of the orbit precisely coincides with the bulging portion of the superior orbital fissure. Above and medially is the exocranial foramen of the optic canal, which gives passage to the optic nerve with its meningeal sheath and the ophthalmic artery, presenting a site of communication between the orbit and the anterior cranial fossa.
At a small bony protuberance of the sphenoidal wing, the common tendinous ring (tendon of Zinn) is attached. The ring encircles the optic foramen and the central portion of the superior orbital fissure. The lateral wall is formed by the frontal process of zygomatic bone in front and by the orbital plate of the greater wing of sphenoid in the posterior two-thirds. The bones meet at the zygomaticosphenoid suture. The lateral wall is the thickest and separates the orbit from the cerebral temporal fossa and the temporal muscle.
Orbital Fascia or Periorbita
The periorbita forms the periosteum of the orbit. It is loosely attached to the orbital bony walls, except for suture lines, fissures, and foramens of the orbit. It expands forward up to the orbital rim, to which it is strongly attached, and then merges with the cranial periosteum. It also sends out extensions toward the peripheral tarsal rim to form the orbital septum, which delineates the orbit in front and separates the intraorbital fat from the orbicular muscle of the eye. On the orbital surface of the optic canal and the medial aspect of the superior orbital fissure, the periorbita thickens, forming the tendinous attachments of the four rectus muscles, the levator palpebrae superioris and the superior oblique muscles, creating a tendinous ring known as the annulus of Zinn. Medially it is attached to the posterior lacrimal crest and forms the lacrimal sac. The periorbita thus surrounds the contents of the orbit, posteriorly expanding around the optic canal and superior orbital fissure, continuous with the optic nerve sheath, and then finally ends up united with the dura mater. Throughout, it is perforated by the various vessels and nerves of the orbit and closes the inferior orbital fissure ( Fig. 4.3 ).
Orbital Contents
The orbit can be divided into two parts, an anterior part containing the globe and a posterior compartment filled with a fatty matrix, called the adipose body, providing a cushioning effect on the muscles, the vessels, and the nerves supplying the globe. The eyeball does not touch any of the walls but is suspended at a distance of 6 mm outside and 11 mm inside. From the optic nerve to the sclerocorneal junction, the eyeball is covered by a two-layer fascia (the Tenon capsule) with parietal and visceral sheets separating it from the orbital fatty tissue. There is a virtual space between the two sheets, known as the episcleral space, that forms a sort of lubricated joint system to facilitate the movements of the eye.
The fascia is merged with the capsule of the optic nerve posteriorly and with the sclera joining the cornea in the front. In its anterior part, it is perforated by the muscles of the eye. The fascia turns back over these muscles to create their aponeurotic sheath. The Lockwood ligament consists of dense connective tissue and is attached to muscles connected to the lower lid. It acts as a hammock supporting the undersurface of the globe; therefore any damage can cause lower eyelid ptosis.
The orbital septum, also known as palpebral ligament, acts as the anterior soft-tissue boundary of the orbit. It extends from the tarsus to the orbital rim, where it gets attached to the bone and becomes the periorbita inside the orbit and the periosteum outside. The orbital septum is covered anteriorly by the preseptal orbicularis oculi muscle and is a consistent feature of both the upper and the lower eyelid, separating the orbital from the lid contents. The orbital septum helps in differentiating orbital cellulitis (behind the septum) and periorbital cellulitis (in front of the septum). Its major purpose is to prevent the spread of infection as a physical barrier against pathogens. It also contains the extraconal fat that is prolapsing with age and is being reduced during blepharoplasty. Also, the annulus of Zinn, a tight fibrous ring, divides the superior orbital fissure into intraconal and extraconal spaces ( Fig. 4.4 ).
Orbital Muscles
The orbit contains seven muscles, the superior palpebrae levator muscle and six other oculomotor muscles: four rectus muscles (superior, inferior, lateral, and medial) and two oblique muscles (superior and inferior) ( Fig. 4.5 ).
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The superior palpebrae levator originates above the optic canal, where it has a fine and tendinous form, and then broadens out with a triangular form running along the roof of the orbit on top of the superior rectus muscle. It terminates with an anterior tendon into a large fascia, which becomes inserted into the skin of the upper eyelid and upper tarsal plate. This muscle is innervated by the superior division of oculomotor nerve; by its elevating action it raises the upper eyelid, thus uncovering the cornea and portion of sclera, antagonizing the orbicularis oculi muscle, which is innervated by facial nerve. The deep surface of the levator aponeurosis also contains a layer of smooth muscle known as the Whitnall or Müller muscle, receiving its nerve supply from the superior cervical ganglion via the lacrimal nerve.
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The four rectus muscles form a conical space posterior to the eyeball. They arise from the common annular tendon (tendon of Zinn), which originates from the body of sphenoid, surrounding the superior, medial, and inferior edges of the optic canal and the inferomedial part of the superior orbital fissure. The common ring subsequently splits into the four rectus muscles, which continue forward for 4 cm to terminate in tendons attached to the anterior part of the sclera and control the eye movements.
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The two oblique muscles are the superior oblique and inferior oblique. The superior oblique muscle arises as a short tendon from the upper rim of the optic foramen, passing along the superomedial angle of the orbit. Then it abruptly creates a tendinous acute angle skirting over the trochlea, to continue muscular again with a lateral direction. It passes under the superior rectus muscle to end up on the superolateral side of the posterior part of the globe. The shorter and thinner inferior oblique muscle is located on the anterior edge of the floor of the orbit. It arises from the edge of the lacrimal canal, heading laterally and upward to the lower surface of the eyeball. After passing under the inferior rectus muscle, it ends up on the inferolateral side of the posterior part of the globe.
A fibrous septa system connects all these muscles, including orbital fascia or the Tenon capsule, with neurovascular content that can be considered an important accessory locomotor system contributing to the motility of eye. The role of this septa explains some motility disturbances in blow-out fractures of the orbit.
Intraconal Space
The intraconal space of the orbit is a musculofascial cone that contains important neurovascular structures and fat ( Table 4.2 ). The base of intraconal space is formed by the posterior part of the globe, whereas the four rectus muscles and their fascia surround this space and converge on the common tendinous ring at the orbital apex. The space formed externally between the extraocular muscles and the bony walls is called the extraconal space ( Fig. 4.6 ). The superior and medial rectus muscles arise from the part of the annulus attached to the body of the sphenoid, adjacent to the optic foramen. A tendinous portion of the annulus spanning from the body of sphenoid to the greater wing gives rise to the inferior rectus. The lateral rectus muscle arises from the body of the greater wing along the lateral border of the superior orbital fissure. The Zinn ring corresponds to the bulging end of the superior orbital fissure and provides a passage for the optic nerve, superior and inferior divisions of the oculomotor nerve (cranial nerve [CN] III), the nasociliary branch of CN V1, the abducens nerve (CN VI), and the sympathetic root of the ciliary ganglion, which traverse the intraconal space. The superior ophthalmic vein can also pass through or above this opening, and the inferior ophthalmic vein may pass inside or below it. The remaining structures enter the orbital apex outside the annulus of Zinn, within the extraconal space. They include the lacrimal and frontal nerves (V1 branches), probably the superior ophthalmic vein just below them, and the trochlear nerve (IV) closely applied to the superior fibers of the annulus.
