Sphenoid bone anatomy: body, sella turcica, wings and pterygoid processes

What it is

A butterfly-shaped bone at the base of the skull.

Location

Lies in the middle of the skull base, articulating with the frontal, parietal, temporal, occipital and ethmoid bones and with the vomer, palatine and zygomatic bones.

Function

Forms parts of the cranial floor and orbit and houses the pituitary gland in the sella turcica.

Body system

Skeletal · single, midline structure

FMA ID

FMA52736 (Foundational Model of Anatomy)

Key facts

• Has greater and lesser wings, pterygoid processes and a sphenoid sinus.
• Many nerves and vessels pass through its foramina.
• Its foramina include the optic canal (optic nerve), superior orbital fissure (CN III, IV, V1, VI), foramen rotundum (V2), foramen ovale (V3) and foramen spinosum (middle meningeal artery).

Clinical relevance

Fractures of the skull base can involve it.

The sphenoid is one of the eight cranial bones. It is unpaired and lies in the middle of the cranial base. It contributes to the middle cranial fossa, the lateral wall of the skull (where its greater wing appears at the temple) and the floor and sides of both orbits. Its name comes from the Greek for wedge, and its shape is often compared to a butterfly. (TeachMeAnatomy: The Sphenoid Bone.)

Radiopaedia notes an alternative story for the name: some sources claim it was meant to be sphecoid, from the Greek sphex (wasp), and was miscopied. Kenhub likewise calls it the wasp bone because of its shape. The name was first used by Galen in the 2nd century. (Radiopaedia: Sphenoid bone.)

The sphenoid has four parts: a central body, two lesser wings that point forwards and outwards, two greater wings that spread to the sides, and two pterygoid processes that hang downwards. Kenhub describes it as one of the most complex bones of the body, whose main job is to provide canals and foramina for nerves and vessels entering and leaving the cranium. (Kenhub: Sphenoid bone.)

The body is the cube-shaped centre of the bone. It is almost hollow because it contains the two sphenoid sinuses, separated by a septum. On its upper surface is the sella turcica (Turkish saddle). Its front wall is the tuberculum sellae, its deepest part is the hypophyseal fossa, where the pituitary gland sits, and its back wall is the dorsum sellae. In front of the tuberculum is the chiasmatic groove, related to the optic chiasm. (TeachMeAnatomy: The Sphenoid Bone.)

The anterior clinoid processes arise from the lesser wings and the posterior clinoid processes from the dorsum sellae. Both give attachment to the tentorium cerebelli, the fold of dura mater that roofs the posterior cranial fossa. (TeachMeAnatomy: The Sphenoid Bone.)

The pituitary fossa is lined by dura mater and roofed by the diaphragma sellae, which has an opening for the pituitary stalk. Its relations are clinically important: the sphenoid sinus lies in front and below, the cavernous sinus on each side, the optic chiasm and hypothalamus above, and the clivus, basilar artery and brainstem behind. On a lateral skull X-ray the adult fossa measures roughly 4 to 12 mm high and 5 to 16 mm long, though normal values vary between populations. (Radiopaedia: Pituitary fossa (sella turcica).)

The sphenoid sinuses lie in the body of the sphenoid, in front of and below the sella turcica. The two halves are separated by a septum that is often off the midline. Each opens into the sphenoethmoidal recess at the roof of the nasal cavity. Pneumatisation (air filling) begins at about 2 years of age and proceeds more slowly than in the other paranasal sinuses. (Radiopaedia: Sphenoid sinus.)

The sinus walls can be ridged by the pituitary gland, the optic nerve and the internal carotid artery, which run just outside them. Its sensory nerve is the posterior ethmoidal nerve, a branch of the nasociliary nerve (V1), and it is supplied by the posterior ethmoidal artery and nasal branches of the sphenopalatine artery. (Radiopaedia: Sphenoid sinus.)

The lesser wings arise from the front of the body and pass upwards and outwards. They form the ridge that separates the anterior and middle cranial fossae. Each lesser wing forms the lateral border of the optic canal, which carries the optic nerve and ophthalmic artery, while the body forms its medial border. The gap between the lesser and greater wings is the superior orbital fissure, through which many structures pass into the orbit. (TeachMeAnatomy: The Sphenoid Bone.)

The greater wings extend laterally, upwards and backwards from the body and form the front of the floor of the middle cranial fossa. Three foramina pierce each greater wing near the body: the foramen rotundum (maxillary nerve), the foramen ovale (mandibular nerve) and the foramen spinosum (middle meningeal vessels). (TeachMeAnatomy: The Sphenoid Bone.)

On the outside of the skull, the lateral surface of each greater wing forms the infratemporal surface, and its anterior surface forms part of the lateral wall of the orbit. The superior orbital fissure transmits the oculomotor, trochlear and abducens nerves, the ophthalmic nerve and its branches, and the superior ophthalmic vein. (Kenhub: Sphenoid bone.)

The two pterygoid processes descend from the junction of the body and greater wing. Each splits into a medial and a lateral pterygoid plate, so there are two processes but four plates. The space between the plates is the pterygoid fossa. The lateral plate forms the medial boundary of the infratemporal fossa, and the medial plate is linked with the pharynx. (Radiopaedia: Pterygoid processes.)

The medial pterygoid plate ends below in a small hook, the pterygoid hamulus. Its lower end gives origin to the superior pharyngeal constrictor, the scaphoid fossa at its root gives attachment to tensor veli palatini, and it also anchors the pharyngobasilar fascia and the pterygomandibular raphe. The lateral pterygoid plate gives origin to the lateral pterygoid (from its lateral surface) and the medial pterygoid (from its medial surface), two muscles of mastication. (Radiopaedia: Pterygoid processes.)

The pterygoid canal runs through the root of the process and carries the nerve formed by the greater and deep petrosal nerves. The lateral plate is wider and shorter than the medial plate. (Kenhub: Sphenoid bone.)

The sphenoid articulates with 12 bones. Four are single: the frontal, ethmoid, vomer and occipital bones. Eight are paired: the zygomatic, parietal, temporal and palatine bones. (Radiopaedia: Sphenoid bone.)

Its main sutures are the sphenofrontal, sphenoparietal and sphenosquamosal sutures and the spheno-occipital joint. The sphenoparietal suture is part of the pterion. Behind the body, the sloping clivus is formed with the basilar part of the occipital bone. The sphenoid and occipital bones fuse at the spheno-occipital joint around puberty, and Kenhub gives closure of this suture by about 25 years of age. (Kenhub: Sphenoid bone.)

The body and lesser wings ossify in cartilage (endochondral ossification). The greater wings are unusual among skull bones because they ossify partly in cartilage and partly in membrane. (Kenhub: Sphenoid bone.)

The body forms from a presphenoid and a postsphenoid part, each with several ossification centres. The lesser wings develop from the orbitosphenoids and the greater wings from the alisphenoids. A persistent hypophyseal (craniopharyngeal) canal running from the pituitary fossa to the nasopharynx is a rare remnant of development. (Radiopaedia: Sphenoid bone.)

Because the pituitary gland sits directly above the sphenoid sinus, surgeons can reach it through the nose and the sinus. Endoscopic transsphenoidal surgery is the approach of choice for pituitary adenomas and avoids a larger craniotomy. (TeachMeAnatomy: The Sphenoid Bone.)

Before sinus surgery, radiologists report how far the sphenoid sinus is pneumatised and whether the bone over the optic canal or carotid canal is thin or missing (dehiscent), because these structures lie right against the sinus walls. (Radiopaedia: Sphenoid sinus.)

Pterygoid plate fractures are typically part of Le Fort fractures of the midface, but about one third occur without them, for example with zygomaticomaxillary complex or mandibular fractures. (Radiopaedia: Pterygoid processes.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Pituitary gland · endocrine
• Trochlear nerve · nervous
• Ophthalmic nerve · nervous
• Short ciliary nerve · nervous
• Optic nerve · nervous
• Anterior communicating artery · cardiovascular
• Posterior communicating artery · cardiovascular
• Ethmoid · skeletal
• Communicating branch of nasociliary nerve with ciliary ganglion · nervous
• Palatine bone · skeletal