The Orbit
The orbit—connected posteriorly to the parasellar region—makes up the anterior aspect of the skull and is composed of 7 craniofacial bones (Fig 1-3):
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maxillary
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zygomatic
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frontal
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lacrimal
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sphenoid
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palatine
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ethmoidal
The orbit is approximately 45 mm wide and 35 mm in maximal height. The total volume of the orbit is approximately 30 cm3. The medial wall is approximately 40 mm from the rim to the optic canal. The medial walls are roughly parallel, whereas the lateral walls form an angle of almost 90°. The orbital rim is made up of the frontal bone superiorly, which connects to the zygomatic bone (at the frontozygomatic suture) laterally. The inferior orbital rim is made up of the zygomatic bone inferolaterally and the maxillary bone inferonasally (meeting at the zygomaticomaxillary suture). Medially, the orbital rim consists of the maxillary and lacrimal bones, which join the frontal bone superiorly. An additional 3 bones contribute to the orbit: (1) the ethmoidal bone medially, (2) the palatine bone inferiorly in the posterior orbit, and (3) the sphenoid bone laterally and superiorly in the orbital apex.
The orbit is surrounded by several important structures. The 4 paranasal sinuses surround the floor (maxillary sinus) and the medial wall (ethmoidal and sphenoid sinuses) of the orbit (Fig 1-4). The frontal sinus has a variable relationship to the anterior orbital roof. The other major structures around the orbit are the anterior cranial fossa superiorly (containing the frontal lobe) and the temporal fossa laterally (containing the temporalis muscle). The roof of the ethmoidal complex, delineated by the frontal ethmoidal suture (top of the ethmoidal bone, or lamina papyracea), marks the inferior boundary of the anterior cranial fossa. It is important to realize that surgical intervention above this anatomical landmark—as occurs, for example, during endoscopic sinus surgery—can result in entry into the anterior cranial fossa or a cerebrospinal fluid (CSF) leak.
The sphenoid sinus forms the medial wall of the optic canal (Fig 1-5). Surgery within the sphenoid sinus has the potential to damage the optic nerve; alternatively, the sphenoid sinus is a surgical route facilitating decompression of the optic chiasm. In approximately 4% of patients, the bone may be incomplete, leaving only mucosa separating the sinus from the optic nerve. The pterygomaxillary area, which contains the sphenopalatine ganglion and the internal maxillary artery, underlies the apex of the orbit. This area communicates posteriorly through the foramen rotundum and the vidian canal to the middle cranial fossa, anteriorly through the infraorbital canal to the cheek and lower eyelid, and superiorly through the inferior orbital fissure to the orbit.
Canals and fissures
The orbit communicates with the surrounding areas through several bony canals and fissures. Posteriorly, the orbit is contiguous with the cavernous sinus through the superior orbital fissure (see Fig 1-2). The medial wall of the orbit continues as the lateral wall of the sphenoid bone, marking the medial extent of the cavernous sinus. Therefore, when sharp objects enter the medial orbit, they are directed through the superior orbital fissure, where they can lacerate the carotid artery.
The orbit is connected superiorly and posteriorly to the anterior cranial fossa by way of the optic canal (see Fig 1-2B), which transmits the optic nerve, ophthalmic artery, and some sympathetic fibers. Inferiorly at the apex, the orbit is connected to the pterygopalatine area—and thus the temporal and inferotemporal regions—through the inferior orbital fissure. This fissure, formed by the greater wing of the sphenoid, maxillary, zygomatic, and palatine bones, carries the maxillary branch of CN V (CN V2), infraorbital vessels, the inferior ophthalmic vein, branches from the pterygopalatine ganglion, parasympathetic fibers that innervate the lacrimal gland, and collateral meningeal arteries that help connect the external and internal carotid circulation.
Anteriorly, the orbit connects to the inferior meatus of the nose (beneath the inferior turbinate) through the nasolacrimal duct, which carries tears into the nasal cavity. In addition, numerous variable bony canals carry blood vessels that travel to and from the orbit and surrounding structures. Some of the more constant of these canals include the anterior and posterior ethmoidal foramina, which carry blood vessels connecting the internal carotid circulation (ophthalmic artery) to the external carotid circulation (terminal branches of the ethmoidal arteries) at the level of the roof of the ethmoidal complex. Additional supraorbital and zygomaticotemporal foramina carry blood vessels between the orbit and corresponding vessels of the forehead and temple.
Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. 2nd ed. Philadelphia: Saunders; 2011.
Rhoton AL, Natori Y. The Orbit and Sellar Region: Microsurgical Anatomy and Operative Approaches. New York: Thieme; 1996.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.