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Clinical Anatomy of the Orbit
Definition
The orbit is the anatomical space bounded by seven orbital bones and enclosed within the multilamellar periorbita. It is separated above from the intracranial compartment and medially and inferiorly from the paranasal sinuses. The orbit contains the eye and numerous anatomical tissue systems that subserve visual function.
Key Features
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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 eyelids.
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The orbit contains the eye and extraocular muscles, along with the nerves, vascular elements, and connective tissue support systems that support the visual system.
Introduction
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. In addition, the development of better surgical techniques requires a comprehensive knowledge of the structural relationships among the numerous anatomical systems that are crowded into the small space available.
General Organization
The human orbit is a small cavity that has the approximate shape of a pear. Within this space is 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. The entire anatomical region is bound together into a functional unit, the complexity and precision of which are unmatched elsewhere in the vertebrate body.
Osteology of the Orbit
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 cm 3 . It is composed of seven bones ( Box 12.2.1 ). 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. 12.2.1 ).
Box 12.2.1
Bones of the Orbit
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. 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 and the optic canal. 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. The convoluted frontozygomatic suture line runs approximately horizontally and crosses the superotemporal rim near the lacrimal gland fossa.
The lateral wall contains two areas of thickened bone, the posterolateral trigone in the sphenoid bone and the inferolateral basin in the zygomatic bone. Thinning of these regions has become an important component of balanced medial–lateral wall decompression surgery.
Orbital Floor
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 orbital floor is thinnest just medial to the infraorbital canal, which is the most common site for blowout fractures. 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. 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. The inferior fissure transmits structures into the orbit from the pterygopalatine fossa posteriorly and from the infratemporal fossa anteriorly.
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. 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. The medial wall ends at the optic foramen, where the sphenoid forms the medial wall of the optic canal.
Within the frontoethmoid suture line in the superomedial orbit are 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.
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. A summary of the orbital bones is given in Box 12.2.2 .
Box 12.2.2
Orbital Bones Contributing to Each Wall
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