The sphenoid bone is an unpaired bone situated in the middle of the cranial base. It articulates with the adjacent temporal, parietal, frontal, occipital, ethmoid, zygomatic, palatine, and vomer bones and its intricate microanatomy includes numerous foramina. This bone is the center of attention in endonasal skull base surgery.
The sphenoid bone is made up of several parts: a central body that contains the sella turcica, and two greater wings and two lesser wings laterally. The greater wings make up the anterior portions of both middle fossae and the lesser wings make up the posterior portion of the anterior cranial fossa. The clinoid processes are important features of the sphenoid bone in skull base surgery.
The anterior clinoid processes are very prominent ends of the lesser wing of the sphenoid bone and extend toward the Sylvian fissure. The middle clinoid processes are eminences forming the anterior border of the sella turcica. The posterior clinoid processes form the ends of the dorsum sellae, and their size and form vary greatly in individuals. The tentorium cerebelli attaches to the posterior clinoids. The optic canals, which transmit the optic nerves and the ophthalmic arteries, are located at the junction of the body and the lesser wings. A groove in the midline of the sphenoid body creates the optic groove, posterior to which is the tuberculum sellae.
The cleft created between a greater and lesser wing forms the superior orbital fissure, which transmits the oculomotor nerve (III), trochlear nerve (IV), the lacrimal, nasociliary, and frontal divisions of the ophthalmic nerve (V1), abducens nerve (VI), superior and inferior divisions of the ophthalmic vein, and the sympathetic fibers from the cavernous sinus. Each greater wing contains the foramen rotundum, which transmits the maxillary nerve (V2); foramen ovale, which transmits the mandibular nerve (V3), accessory meningeal artery and often times the lesser petrosal nerve; and foramen spinosum, which transmits the middle meningeal vessels and the recurrent branch of the mandibular nerve.
Inferiorly, the sphenoid bone contains two pterygoid processes, made up of a medial and lateral plate, to which the medial and lateral pterygoid muscles attach, allowing for jaw movement. When looking at the sphenoid bone from the anterior direction, the pterygoid or Vidian’s canal can be noted inferomedial to the foramen rotundum. The Vidian’s nerve, artery, and vein are transmitted through this canal. Vidian’s nerve is formed by the union of the greater petrosal nerve and the deep petrosal nerve within the canal.