The sphenoid is just one of the twenty-two bones that form the skull and essentially helps to connect the neurocranium to the facial skeleton. It is a single bone in the midline of the cranial cavity situated posterior to the frontal bone but anterior to the occipital. Its name derives from the Greek 'sphenoeides,' which means wedge-shaped. It has several foramina and fissures present in its surface that allow the entry and exit of nerves and blood vessels to and from the cranial cavity. Shaped like a butterfly or a bat, it has a central body and two lateral wings on either side. It is one of the bones that make up the orbit, in particular, forming the posterior surface. The articulations of the sphenoid are with the frontal, parietal, ethmoid, zygomatic, temporal, occipital, palatine, and vomer bones. There may be several morphological variations in the relationship between the sphenoid, the ethmoid, and the frontal bone. Together, these give rigidity and stability to the skull making it the ideal place to house the brain.[1][2]

Structure

The sphenoid broadly divides into the central body, the greater wings, the lesser wings, and the pterygoid process. The body of the sphenoid is the medial part of the bone, in between the two lateral greater wings. It contains a depression called the sella turcica, which has a central depression for the pituitary gland. The body also houses the sphenoidal sinuses. The two lesser wings of the sphenoid are present anterior to the body and are triangular. The two greater wings of the sphenoid are also triangular in shape and are lateral to the body. The pterygoid process is directed downwards and made up of two laminae.[3][4]

Ligaments

The sphenoid bone has several intrinsic ligaments, which may occasionally ossify. These are the pterygospinous, the interclinoid, and the caroticoclinoid ligaments.[5]

Openings

This cranial bone has several openings to allow entry and exit of several nerves and blood vessels to move in and out of the cranial cavity. These include the optic canal, the superior orbital fissure, the foramen rotundum, the foramen lacerum, the foramen spinosum, and the foramen ovale. The foramen lacerum is between the sphenoid, the basilar part of the occipital, and the apex of the temporal bone and often contains connective tissue.[6]

Function

Sphenoid bone has many essential functions. It helps form the base and lateral sides of the skull in combination with the orbital floor. Its many articulations with other bones give the skull rigidity. It is an attachment site for many of the muscles of mastication. A portion of the sphenoid forms the facial skeleton. Also, several fissures and foramina exist in the sphenoid, which transports several blood vessels and nerves of the skull to the head and neck.  The body of the sphenoid has a cavity with a sinus that communicates with the nasal cavity. These sphenoidal sinuses work to make the skull light-weight and to increase resonance.

The sphenoid bone is a complicated structure in terms of its embryologic origin. This bone forms by the joining together of different primordia whose embryonic origins are dissimilar. The sphenoid bone has two distinct components. The cephalic mesoderm forms the basi-post-sphenoid and the orbitosphenoid, whereas the basi-pre-sphenoid and the alisphenoid have a neural crest cell origin. Several developmental diseases have links to the sphenoid bone, such as chordomas and trans-sphenoidal encephaloceles caused by the persistence of the craniopharyngeal canal. The first centers of ossification of the sphenoid appear in the eighth week of fetal life. These alae arise from the lateral walls of the skull. Later, the basisphenoid nuclei appear in the floor of the pituitary fossa while the lingulae appear between these two.[7]

The blood vessels pass through the foramina present in the wings of the sphenoid bone. The ophthalmic artery passes through the optic canal to enter the orbit. The ophthalmic veins pass out of the orbit through the superior orbital fissure. The accessory meningeal artery passes through the foramen ovale to enter the skull. The foramen spinosum allows passage for the middle meningeal artery and vein. Only some small meningeal arterial branches and veins pass through the foramen lacerum, though cartilage may close it off.

Several nerves pass through the foramina, fissures, and canal in the sphenoid bone. The optic canal allows passage to the optic nerve (CN II). The superior orbital fissure transmits the superior and inferior divisions of the oculomotor nerve (CN III), the trochlear nerve (CN IV), the ophthalmic nerve (CN V1), the abducens nerve (CN VI), and the sympathetic fibers from the cavernous plexus. The foramen rotundum contains the maxillary division of the trigeminal nerve (CN V2). The foramen ovale allows passage to the mandibular division of the trigeminal nerve (CN V3), the motor root of the trigeminal nerve, and the lesser petrosal nerve. The mandibular nerve's meningeal branch passes through the foramen spinosum.[8]