The sphenoid bone in the skull is one of the eight bones that constitute the neurocranium. This cranial bone also called the ‘Os sphenoidale,’ takes up the majority of the cranial floor’s central section. Thus the main section medically known as ‘cranial fossa’ is occupied by the sphenoid.
The etymology can be traced back to the Latin expression, ‘spheroidal’ (combination of the words ‘sphen’ and ‘eidos.)’ ‘Sphen’ means ‘wedge’ and ‘eidos’ means form, and hence ‘Os sphenoidale’ signifies wedge-shaped bone.
Sphenoid Bone Shape & Location
It lies on the cranium floor, positioned just ahead of the temporal bones forming the occipital bone’s base. The bone is a single unit instead of being paired. As its naming indicates, it is lodged centrally in the skull crammed together with rest of the seven bones. It constitutes the cranium or skull.
The shape of this bone wedged in the cranium closely resembles a bat with extended wings. It is because of the peculiarity that makes it the human skeleton’s most complicated bone. Its sophisticated form is also indicative of the vital role it plays in the development of the cranium’s fundamental anatomical components.
A detailed examination of the anatomy reveals five distinct sections:
- The body of Sphenoid also referred to as the Median Body or ‘corpus oasis sphenoidale’
- Two great wings, one on each side of the body of the sphenoid
- Two small wings or lesser wings on either end of the frontal section
- Two Pterygoid processes starting from where the great arms and the body meet and pointing downwards
- Two sphenoidal conchae located at the front and the lower end of the central body
1. Body of Sphenoid or Median Body (Corpus Ossis Sphenoidalis)
It is the central section and is shaped somewhat like a cuboid. The body comprises a total of six surfaces, namely:
- Right lateral
- Left lateral
- Anterior (front)
- Posterior (back)
- Superior (upper)
- Inferior (lower) surfaces
The Sphenoid body has a depressed or hollowed out interior which houses a pair of sizable cavities known as ‘sphenoidal air sinuses’ demarcated by a septum. The body contains the ‘sella turcica’ (meaning Turkish saddle in Latin) which is a cavernous hollow shaped like the saddle. The pituitary gland (hypophysis) lies in the seat of the sella turcica. It is the deepest section and clinically known as the hypophyseal fossa. Following are the salient features of each surface of the body of sphenoid:
Cerebral or Superior Surface
- The superior surface presents a conspicuous triangular protuberance extending within the pair of small wings. It is termed as ‘ethmoidal spine’ serving as a bridge connecting the cerebral surface with the ethmoidal cribriform
- There is a smoothened surface just at the back of the superior section. It is somewhat elevated in the middle with furrows on each side for sheltering the brain’s olfactory lobes. This flat surface is bordered by a strip which also is the ridgeline for the chiasmatic groove (optic groove), a narrow and slanting channel at the front.
- The optic chiasma is just at the back of the optical groove, slightly above it. The chiasmatic groove opens into the optic foramen on either end through which the ophthalmic artery. The optical nerve passes and terminates in the orbital cavity. Also lying at the back of chiasmatic groove is an elevation parallel to the plane known as Tuberculum
- Going down posteriorly from the Tuberculum sellae one comes across a saddle-style depression – the sella turcica, whose base (hypophyseal fossa) contains the pituitary gland. The frontal borderline of the sella turcica culminates in a tubercle or protuberance on the left and right ends termed as a middle clinoid process
- The posterior margin of the Turkish saddle (sella turcica) is taken up by the dorsum sellae, a bony plate that is square. It is located at the top situated at the preeminences on either end of the posterior clinoid processes.
- The posterior clinoid processes make the sella turcica deeper, thereby helping in articulation with the tentorium cerebelli. A sphenoidal notch or indentation on both ends of the dorsum sellae serves as the transit for the abducent nerve.
- Right below the abducent nerve groove lays the petrosal process. It links with the pinnacle of the temporal bone’s petrous section. The petrosal process also serves as the median border of the foramen lacerum.
- At the rear of dorsum sellae is the clivus, a shallow concavity that slants backward. It aligns with the furrow of the occipital bone’s basal section. The superior section of a pons, i.e., a group of nerve fibers that connect the cerebellum with the midbrain-is supported by the clivus.
Inferior or Lower Surface
- The sphenoidal rostrum, a triangular projection running downwards through the middle is contiguous with the anterior surface’s sphenoidal crest or ridge. It lies in the deep crevice in between the alae of the Volmer (a plate of bone). It is quadrangular in shape forming the posteroinferior part of the nasal septum.
- On the rostrum’s right and left side are two triangle-shaped plates of bones called the Sphenoidal concha that forms a joint with alae of Volmer.
- Also lying on both end of the rostrum are triangulated bony projections called the vaginal processes that point downward. These processes run through the middle of every medial pterygoid plate. The vaginal process contributes towards the creation of palatovaginal and vomerovaginal
Anterior or frontal surface
- In its median plane lies the sphenoidal crest which is somewhat elevated and joins with the ethmoid bone’s upright plate. This crest also constitutes a section of the nasal septum.
- On either side of the sphenoidal crest is a foramen or opening that leads to the sphenoidal air sinuses.
- The sphenoidal sinuses are large, uneven, and cavernous openings segregated by an osseous septum. These sinuses differ in size and shape and are further subdivided by bony plates. Often, they go up to the base of the occipital bone, coinciding with the foramen magnum in the skull through which the spinal cord passes.
- The sphenoidal air sinuses begin forming in the prenatal phase, become prominent enough when the individual is just 6-7 years old. Sphenoidal concha, a narrow and arched plate of bone partly covers the sinuses at the front and lower ends.
- The anterior surface at the sides has a jagged texture and links with ethmoid’s lamina papyracea. The upper and lower margins of the front surface unite with the frontal bone’s orbital plate and palatine bone’s orbital process respectively.
Posterior surface or backside
- The surface is quadrilateral in shape
- The plane stays connected to the base of the occipital bone all through the infancy and teenage years via a cartilage plate. Ossification of the surface starts when one is 18 years old, and the process is complete by the 25th
Lateral surface (s)
- Lateral surfaces linked to both ends of the great wings, project laterally and also with the pterygoid processes or medial pterygoid plates.
- Atop the great wings’ articulations with the lateral surface, there is a sulcus or depression called the carotid groove that houses the internal carotid artery.
- A bony ridge known as lingula lies on the space between the great wing and the body at the rear of the carotid sulcus.
2. Greater wings (Alae Magnae)
The two great wings or greater wings, (medically referred to as ali-sphenoids or ala major) are robust bony projections or outgrowths, originating from the body of sphenoid’s lateral ends.
- These bone projections curve towards the back, sides, and upwards.
- Each projection at its posterior end juts as a triangulated process that sits in the slant formed by the temporal bone’s petrous section and the squama.
- The tips of both the greater wings projecting in a downward direction are termed spina angularis.
Greater wings surfaces
A section of the cranium’s middle fossa is the cerebral or superior surface. It is remarkably inward curving in shape forming depressions for holding the complex folds of the brain’s temporal lobes. In the front and middle part, is the foramen rotundum, a circular opening through which passes the maxillary nerve. Right at the backside of this circular opening is another opening-foramen ovale-that acts as a passage for the accessory meningeal artery, mandibular nerve, and occasionally the lesser petrosal nerve.
A small orifice, the foramen Vesalii, in the middle of the foramen ovale and facing the basilar part of the Pterygoid process serves as a channel for a vein (originating from cavernous sinus). A dual-channeled canal, posterior to the superior surface and close to the spinal cord is the foramen spinosum via which the mandibular nerve branch and the medial meningeal vessels pass
The orbital surface shaped like a quadrilateral with a smooth texture is forward facing and is the backside of the cranial orbit’s lateral wall. Its superior saw-like border joins at the front with the orbital plate. Its lower border with a curvaceous edge becomes the rim for the inferior orbital fissure. Medially, it serves as the lower border for the superior orbital fissure, and from its middle emerges a small protuberance that connects with the Rectus lateralis oculi’s lower end.
At its upper part, towards the margin, there is a tiny nick which enables passage of branching of the lacrimal artery. The notched lateral margin forms a joint with the zygomatic bone. Just beneath the median part of the superior or upper orbital fissure, lies a ridged surface which the foramen rotundum punctures.
Convex in shape, the lateral surface is sectioned into two halves by the infratemporal crest, a sloping ridge. The upper half is the temporal section and a component of the temporal fossa as well as attaches with the Temporalis. The infratemporal or lower part is concave and comparatively smaller, and joins with the infratemporal fossa. Both the infratemporal crest and the infratemporal fossa together link with the Pterygoid externus
The apertures ‘foramen spinosum’ and ‘foramen ovale’ perforate the infratemporal crest at its inferior part. On its backside is the spina angularis that has a ribbed surface in its middle for receiving the chorda tympani nerve
Great wing margin
The margin of the great wings is that circumferential section which extends from the middle up to the spinal cord. The middle section is the frontal border of the foramen lacerum opening and also forms the posterior boundary for the pterygoid canal opening for the transmission of (pterygoid canal) artery and nerve. The lateral section unites with the temporal bone’s petrous part to form a nearly immovable joint. On the inner surface of the cranium, just beneath the extremities of the two wings are the sulcus tubae which are trough-like grooves for lodging the auditory tube’s cartilaginous section.
3. Lesser wings (alae parvae)
The small wings or lesser wings are two conical, slender plates of bone, originating from the frontal and upper parts of the sphenoid body.
Superior and inferior surfaces
The upper surface is smooth and provides support to a section of the brain’s front lobe. The lower surface is the cranial orbit roof’s backside and also the superior orbital fissure’s upper limit. The superior orbital fissure serves as the passage for abducent, trochlear, and oculomotor nerves, trigeminal nerve’s ophthalmic section, ophthalmic vein, lacrimal arterial branch, and the medial meningeal artery’s orbital branch
The front boundary is notched for attachment with frontal bone. The posterior edge, rounded in texture extends into the brain’s lateral fissure, and at its middle borders the anterior clinoid process. The lesser wings are articulated with the body via upper and lower roots, and the optic foramen lying in between the roots transmits the ophthalmic artery and the optic nerve
4. Pterygoid processes
- The processusprocess pterygoidei or the Pterygoid processes, are two bony outgrowths starting from the intersection of the body and the greater wings, extending downwards.
- A Pterygoid canal stretches from posterior to anterior at the basilar region of every Pterygoid process
- Both bony processes comprises of medial and lateral plates and situated between these plates is a shallow depression called ‘Pterygoid fossa.’
- The lateral Pterygoid muscle articulated with the lateral plate promotes the lower jawbone’s (mandible) movement
- Connected to the medial plate are the ‘tensor veli palantini’ and ‘superior pharyngeal constrictor’ muscles that help in swallowing
- This medial plate also features a fishhook-styled projection; the Pterygoid hamulus-that also contributes towards swallowing
5. Sphenoidal Conchae
Located at the front and the lower end of the sphenoid body are two curved, slender plates of bone known as sphenoidal conchae. A large orifice lies in the front wall of each sphenoidal concha which serves as the passage for sphenoidal sinus that forms the nasal cavity. Both the bones join at the front and the sides with the ethmoid bone and the palantine respectively.
Sphenoid development & ossification
The sphenoid, during the prenatal stage, basically constitutes of two sections, the presphenoid and the postsphenoid. The presphenoid, lies just ahead of the tuberculum sellae, aligning with the lesser wings. The postsphenoid made up of the dorsum sella and the sella turcica, develops into the great wings and the pterygoid processes. Majority of the sphenoid bone becomes ossified when it is still in a cartilaginous state.
The sphenoid bone links with a total of 12 cranial bones (4 single and four paired). Since it supports these bones and keeps them in place, it is often called the keystone of the cranium floor. It is pretty similar to the foundation stone of a building’s arch.
- Articulations with single bones:
- Articulations with paired bones:
Functions of Sphenoid Bone
Owing to its articulation with numerous cranial bones and the various functions it performs, the sphenoid has a complex structure. The bone performs the following vital functions:
- Contributes hugely towards development of the cranium
- Many significant muscles that facilitate mastication attaches to it
- Home to several orifices (foramen) that transits head and neck blood vessels and nerves
The clinical significance is attributable to the fact it encloses the pituitary gland. So, while the accessing the pituitary body for treating a hypophyseal tumor or affliction by performing endoscopic transsphenoidal surgery (ETSS), it provides easy passage. Thus, the neurosurgeon while conducting an ETSS is saved from the trouble of resorting to craniotomy for accessing the pituitary gland which could have made the surgery more complicated.
Pathologies Related to Sphenoid
Injuries, wounds, and fractures involving the bat bone are usually rare because of its embedded location. Damages and mishaps associated with the bones of the skull’s forefront are far more common.
- Nevertheless, the butterfly-shaped bone is highly vulnerable to fracturing or splintering during a severe collision or impact and also in the event of a whiplash injury. A fractured sphenoid could cause impairment of vision and grave nerve damage.
- Sphenoid sinusitis is a prevalent issue entailing this neurocranium bone. This problem occurs when the mucous membranes lining the sinus cavities become inflamed and tender. Nerve irritation, nasal drainage, pain in the facial muscles and eye sockets (behind which lie the body of sphenoid), and headaches are usual symptoms.
- In its severest form, sphenoid sinusitis could lead to the formation of tumors and depressions in the sinuses. Hence necessitating a sphenoidotomy or surgical opening of the sphenoid sinus. After that, a sphenoidectomy is performed for removing the affected sphenoid section.