The orbital apex is the most posterior portion of the pyramidal-shaped orbit, positioned at the craniofacial junction. The anatomy of the orbital apex is significant for the complex association between bony, neural, and vascular elements.
Fractures of the apex rarely are isolated because they occur in the association with or as extension of fractures of the facial skeleton, base of skull, or other more anterior orbital fractures. See the image below.
Axial CT scan exhibiting a left apex fracture through the optic canal. Note associated lateral wall and medial wall fractures. This patient also required a craniotomy for a subdural hematoma.
The anatomy of the orbital apex should be reviewed briefly, with emphasis on the neurovascular structures traversing the optic canal, superior orbital fissure (SOF), and inferior orbital fissure.
Two bony roots that connect the lesser wing of the sphenoid with the body of the sphenoid form the optic canal. The inferior root separates the optic canal from the superior orbital fissure and also is referred to as the optic strut. The superior root forms the roof of the optic canal and separates it from the anterior cranial fossa. The body of the sphenoid forms the medial wall of the canal. From an anterior view, the entrance to the optic canal is the most superior and medial structure in the apex. Each optic canal passes posteromedially at an angle of approximately 35° to the sagittal and opens posteriorly into the chiasmatic groove (which terminates posteriorly at the tuberculum sellae). The canal has an intimate relationship to the sphenoid sinus, and with extensive sinus pneumatization, the optic canal may become completely surrounded by a posterior ethmoidal Onodi air cell, the sphenoid sinus, or an aerated anterior clinoid process.
In adults, the canal is 6.5 mm in diameter and about 8-12 mm in length. The canal transmits the optic nerve and the ophthalmic artery. Throughout its intraorbital and intracanalicular course, the optic nerve is surrounded by pia mater, arachnoid, and dura mater, giving the nerve a sheath. Therefore, optic nerve is a white matter tract of the brain and carries with it meningeal coverings. Within the orbit, the optic nerve is quite mobile; however, within the canal, the optic nerve sheath remains adherent to the sphenoid periosteum and thus is fixed.
The SOF is situated between the greater and lesser sphenoid wings, with the optic strut at its superomedial margin. It lies between the roof and lateral wall of the orbit. The SOF is divided at the spina recti lateralis by the annulus of Zinn, the common tendinous origin of the recti muscles. Lateral to the annulus of Zinn, the SOF transmits the lacrimal nerve, frontal nerve, trochlear nerve, the superior ophthalmic vein, and it may transmit a recurrent branch of the lacrimal artery. Within the annulus pass the superior division of III, nasociliary nerve, inferior division of III, abducent nerve, and fibers from the internal carotid sympathetic plexus.
The inferior orbital fissure lies between the orbital floor and lateral wall and communicates with the pterygopalatine and infratemporal fossae. It transmits the maxillary nerve (which continues to give the infraorbital nerve), the zygomatic nerve, the infraorbital artery, venous communications between the inferior ophthalmic vein and the pterygoid plexus, and an orbital branch of the pterygopalatine ganglion.