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):

1. maxillary

2. zygomatic

3. frontal

4. lacrimal

5. sphenoid

6. palatine

7. ethmoidal

Figure 1-2 Anatomy of the orbit. A, Bony anatomy of the right orbital apex. The optic canal transmits the optic nerve, ophthalmic artery, and oculosympathetic nerves. The superior orbital fissure, between the greater and lesser wings of the sphenoid bone, transmits CNs III, IV, and VI; the ophthalmic division (CN V1) of CN V; oculosympathetics; and the superior ophthalmic vein. B, Intracranial view of the left optic canal. Within the lesser wing of the sphenoid bone is the optic foramen, which leads to the optic canal. The optic strut separates the optic canal from the superior orbital fissure. C, Anatomy of the orbital apex. The 4 rectus muscles arise from the annulus of Zinn. CNs II, III (superior and inferior branches), and VI and the nasociliary nerve all course through the annulus of Zinn. CN IV, the frontal and lacrimal nerves, and the ophthalmic veins are located outside the annulus. D, Anatomical dissection just anterior to the superior orbital fissure.

(Parts A and C illustrations by Dave Peace; parts B and D courtesy of Albert L. Rhoton Jr, MD.)

Figure 1-3 Bony anatomy of the right orbit. A, The orbital roof is composed of 2 bones: (1) the frontal bone and (2) the lesser wing of the sphenoid bone. The frontal sinus lies within the anterior orbital roof. The supraorbital notch, located within the medial one-third of the superior orbital rim, transmits the supraorbital nerve, a terminal branch of the frontal nerve of the ophthalmic division of CN V. Medially, the frontal bone forms the roof of the ethmoidal sinus and extends to the cribriform plate. B, The lateral orbital wall is formed by the zygomatic bone and the greater wing of the sphenoid bone. The junction between the lateral orbital wall and the roof is represented by the frontosphenoid and frontozygomatic sutures. Posteriorly, the wall is bordered by the inferior and superior orbital fissures. The sphenoid wing makes up the posterior portion of the lateral wall and separates the orbit from the middle cranial fossa. Medially, the lateral orbital wall ends at the inferior and superior orbital fissures. C, The orbital floor is composed of 3 bones: (1) the orbital plate of the maxillary bone, (2) the maxillary process of the zygomatic bone, and (3) the palatine bone. The nasolacrimal duct sits in the anterior medial area of the orbital floor, medial to the origin of the inferior oblique muscle. D, The medial orbital wall is formed by 4 bones: (1) maxillary (frontal process), (2) lacrimal, (3) sphenoid, and (4) ethmoidal. The largest component of the medial wall is the lamina papyracea of the ethmoidal bone. Superiorly, the anterior and posterior foramina at the level of the frontoethmoidal suture transmit the anterior and posterior ethmoidal arteries, respectively. The anterior medial orbital wall includes the lacrimal sac fossa, formed by the maxillary and lacrimal bones. The lacrimal bone is divided by the posterior lacrimal crest. The anterior part of the lacrimal sac fossa is formed by the anterior lacrimal crest of the maxillary bone.

(Illustrations by Dave Peace.)

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.

Figure 1-4 Coronal (A), sagittal (B), and axial (C) views of the anatomical relationship of the 4 paranasal sinuses to the orbit.

(Illustrations by Dave Peace.)

Figure 1-5 Coronal section, anterior view into the sphenoid sinus demonstrating the relationship of the internal carotid artery and optic nerve within the lateral wall of the sinus.

(Courtesy of Albert L. Rhoton Jr, MD.)