The sphenoid bone (also sphenoidal bone, sphenoid, latin: os sphenoidale) is a single bone that lies in the base of the skull between the frontal, temporal and occipital bones. The sphenoid bone has a central body, paired greater and lesser wings that extend laterally from the body of the sphenoid, and two pterygoid processes descending from the junction of the body and greater wings.
The main parts of the sphenoid bone are:
- body of sphenoid,
- lesser wing of sphenoid (2),
- greater wing of sphenoid (2),
- pterygoid process (2).
Body of sphenoid
The body of the sphenoid (or sphenoid body) is the central portion of the sphenoid bone located between the wings and processes of the sphenoid, and housing the sphenoidal sinuses. The sphenoid body has six surfaces: superior, posterior, anterior, inferior, and two lateral surfaces.
The superior surface of the body of the sphenoid presents the following landmarks:
- chiasmatic groove,
- sella turcica with
- hypophysial fossa,
- tuberculum sellae,
- dorsum sellae.
The chiasmatic groove is a groove between the right and left optic canals.
The sella turcica (turkish saddle) is a saddle-shaped depression on the body of the sphenoid, which lies above the sphenoidal sinus and contains the hypophysis (pituitary gland).
The hypophysial fossa (or pituitary fossa) is a depression on the superior surface of the sphenoid body occupied by the hypophysis.
The tuberculum sellae is a small process in front of the hypophysial fossa.
The dorsum sellae is the posterior wall of the hypophysial fossa.
Each lateral surface unites with the greater wing and the pterygoid process of the sphenoid, and has an important feature, the carotid groove.
The carotid groove is a longitudinal groove lateral to the body of the sphenoid bone that houses the internal carotid artery.
The posterior surface of the body of the sphenoid joins with the basilar part of the occipital bone.
The anterior surface of the sphenoid body presents:
- opening of sphenoidal sinus (2),
- septum of sphenoidal sinuses.
The opening of the sphenoidal sinus is an aperture that opens anteriorly into the nasal cavity, specifically, into a small space called the spheno-ethmoidal recess.
The sphenoidal sinus is a paired paranasal cavity located within the body of the sphenoid.
The septum of the sphenoidal sinuses is a bony partition separating the two sphenoidal sinuses.
The inferior surface of the sphenoid body presents in the middle line a triangular spine called the sphenoidal rostrum, which fits into a deep fissure between the alae of the vomer.
Greater wings of sphenoid
The greater wings of the sphenoid are the larger of the two lateral, wing-like extensions of the sphenoid body. The greater wings curve upwards, laterally and backwards to form a prominent concavity or fossa in the middle of the cranial floor.
Each greater wing of the sphenoid has five surfaces: cerebral, two parts of the lateral surface - temporal and infratemporal, maxillary, and orbital surfaces.
The cerebral surface of the greater wing of the sphenoid forms the anterior part of the middle cranial fossa, and this surface has the following features:
- foramen rotundum,
- foramen ovale,
- foramen spinosum,
- impressions of cerebral gyri,
- arterial grooves.
The foramen rotundum is a circular hole in the greater wing of the sphenoid bone connecting the middle cranial fossa to the pterygopalatine fossa. It is a passage for the maxillary branch of the trigeminal nerve (CN V2).
The foramen ovale is an opening in the greater wing of the sphenoid connecting the middle cranial fossa to the external surface of the skull and transmitting several vessels and nerves through the skull, including the mandibular nerve (CN V3), the motor root of the trigeminal nerve (CN V), and the lesser petrosal nerve.
The foramen spinosum is a round, small opening in the greater wing of the sphenoid located posterolateral to the foramen ovale and connecting the middle cranial fossa to the external surface of the skull. It transmits the middle meningeal artery and vein, and the meningeal branch of the mandibular nerve (CN V3).
The impressions of cerebral gyri (also called the digital impressions) are flat indentations on the inner surface of the skull corresponding to the cerebral gyri which produce them.
The arterial grooves are located on the inner surface of the skull produced by pressure from arteries, primarily the middle meningeal artery and its branches.
The lateral surface of the greater wing of the sphenoid is convex and divided by a transverse infratemporal crest into a temporal and infratemporal surface. The infratemporal crest is a bony ridge between the upper vertical temporal surface and the horizontally-oriented lower infratemporal surface of the greater wing of the sphenoid bone. The lateral surface is pierced by the foramen ovale and foramen spinosum, and features the downward projecting sphenoidal spine.
The temporal surface of the greater wing of the sphenoid is the attachment site for the temporalis muscle.
The infratemporal surface of the greater wing of the sphenoid is directed downwards and together with the infratemporal crest is the attachment site of the upper fibers of the lateral pterygoid muscle.
The maxillary surface of the greater wing of the sphenoid faces anteriorly, forms the posterior border of the pterygopalatine fossa, and is pierced by the foramen rotundum.
The orbital surface of the greater wing of the sphenoid bone faces anteromedially, and forms the posterior part of the lateral wall of the orbit.
The upper edge of the orbital surface of the greater wing articulates with the orbital plate of the frontal bone, while the lateral margin articulates with the zygomatic bone. The inferior margin of the orbital surface forms the posterolateral margin of the inferior orbital fissure, while its medial margin forms the inferolateral edge of the superior orbital fissure.
Lesser wings of sphenoid
The lesser wings of the sphenoid are the smaller of two lateral wing-like extensions of the sphenoid body. The lesser wings are flattened, triangular in shape, and located above and anterior to the greater wings.
Each lesser wing of the sphenoid has two surfaces (cerebral and orbital), and features the following structures:
- optic canal,
- anterior clinoid process.
The optic canal is a passage in the sphenoid bone connecting the orbit to the middle cranial fossa. The optic canal transmits the optic nerve (CN II) and the ophthalmic artery.
The anterior clinoid process is a cone-like process of the sphenoid on either side of the anterior part of the hypophysial fossa.
The lesser wing of the sphenoid also contributes to forming the superior orbital fissure, which is the gap between the greater and lesser wings of the sphenoid bone, bordered medially by the body of the sphenoid, and closed at its anterior extremity by the frontal bone.
The pterygoid process is a paired posteroinferior projection of the sphenoid bone each bifurcating into a medial and a lateral plate. Each pterygoid process also features:
- pterygoid fossa,
- pterygoid canal.
The lateral plate of the pterygoid process is the lateral division of the pterygoid process where the pterygoid muscles attach, the lateral pterygoid muscle to the lateral aspect, and the medial pterygoid muscle - to the medial aspect of the lateral plate.
The medial plate of the pterygoid process is the medial division of the pterygoid process. The superior pharyngeal constrictor muscle attaches to the inferior end of this plate.
The pterygoid fossa is a space between the lateral and medial plates of each pterygoid process of the sphenoid bone. The pterygoid fossa is the attachment site of the medial pterygoid muscle.
The pterygoid canal is a bony passage that extends anteriorly in the base of the pterygoid process of the sphenoid bone. The pterygoid canal transmits the greater and deep petrosal nerves from the middle cranial fossa to the pterygoid ganglion in the pterygopalatine fossa.