The sphenoid bone is an unpaired bone situated deep within the human skull as a component of the cranium, which encloses and shields the brain. Its name comes from the Greek word “sphenoeides,” meaning wedge-shaped. The bone’s intricate shape often earns it the nickname “butterfly bone” or “wasp bone,” as this structure sits at the base of the skull, forming a platform that connects various cranial elements.
Location and Structure
The sphenoid bone is centrally located in the middle of the skull, positioned anterior to the temporal bones and the basilar part of the occipital bone. It acts as a keystone, articulating with most other cranial bones, including the frontal, parietal, ethmoid, and occipital bones, providing structural stability. This central placement means it contributes to the formation of the middle cranial fossa, the lateral walls of the skull, and the floor and sides of the orbits.
Its form is composed of several parts that give it a butterfly-like appearance. At its core is the body, a hollow, cubical central part that contains the sphenoidal sinuses. Extending laterally from the body are two pairs of wings: the larger greater wings and the smaller lesser wings. The greater wings sweep outwards to form part of the skull’s side walls, while the lesser wings are triangular projections that form a shelf at the front of the middle cranial fossa. Projecting downwards from the junction of the body and greater wings are the pterygoid processes.
Key Functions and Articulations
One of the sphenoid bone’s primary functions is to provide a rigid, supportive base for the skull, helping to define the shape of the cranial floor. Its extensive connections with surrounding bones, including the temporal, zygomatic, and palatine bones, reinforce the entire cranial structure. This network of articulations helps distribute forces across the skull.
Another function is serving as an anchor point for the muscles of mastication, or chewing. These muscles attach to the pterygoid processes, allowing for the complex movements of the lower jaw. The sphenoid bone also offers protection to the pituitary gland, which rests within a saddle-shaped depression in its body known as the sella turcica.
Important Passageways
The sphenoid bone is perforated by numerous openings, called foramina and fissures, that allow nerves and blood vessels to pass through. For instance, the optic canal, bordered by the lesser wing and the body of the sphenoid, allows the optic nerve and the ophthalmic artery to reach the eye.
A large gap between the greater and lesser wings, the superior orbital fissure, provides passage for multiple cranial nerves that control eye movements. Other openings are located in the greater wings. The foramen rotundum permits passage of the maxillary nerve, and the foramen ovale allows the mandibular nerve to pass through; both are branches of the trigeminal nerve. The foramen spinosum transmits the middle meningeal artery and a branch of the mandibular nerve.
Clinical Significance
The sphenoid bone’s protected location means fractures typically occur as part of severe head trauma, such as basilar skull fractures. These injuries are dangerous due to the proximity of nerves and blood vessels passing through its foramina, which can lead to neurological deficits or vision loss. Another clinical issue is sphenoid sinusitis, an infection of the air-filled sphenoid sinuses. This condition can cause fever and post-nasal drip and may lead to serious complications if untreated.
Due to its relationship with the pituitary gland, the sphenoid bone is part of a neurosurgical procedure called transsphenoidal surgery. Surgeons access pituitary gland tumors by creating a path through the nasal cavity and the sphenoid sinus. This minimally invasive approach avoids the need for a more extensive craniotomy.