Sphenoid Bone
Os Sphenoidale
location_on Central base of the skull, articulating with all other cranial bones
The sphenoid is a butterfly-shaped bone that spans the entire width of the middle cranial fossa and articulates with every other cranial bone, acting as a keystone of the skull. It contains the sella turcica, a saddle-shaped depression housing the pituitary gland, and the sphenoid sinuses. The bone has a central body, paired greater and lesser wings, and pterygoid processes.
Key Anatomical Features
- Sella turcica houses the pituitary gland in the hypophyseal fossa
- Greater wings form part of the temporal fossa and middle cranial fossa floor
- Lesser wings form the posterior boundary of the anterior cranial fossa
- Superior orbital fissure between greater and lesser wings transmits cranial nerves III, IV, V1, and VI
- Pterygoid processes project inferiorly and provide attachment for muscles of mastication
- Optic canals transmit the optic nerves and ophthalmic arteries
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Lateral pterygoid | Lateral surface of lateral pterygoid plate and greater wing | Protrudes and depresses the mandible, opens the mouth |
| Medial pterygoid | Medial surface of lateral pterygoid plate | Elevates the mandible |
| Temporalis | Greater wing of sphenoid | Elevates and retracts the mandible |
| Tensor veli palatini | Scaphoid fossa of pterygoid process | Tenses the soft palate and opens the auditory tube |
| Superior pharyngeal constrictor | Medial pterygoid plate | Constricts the upper pharynx during swallowing |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Spheno-occipital synchondrosis | Cartilaginous synchondrosis | Occipital bone |
| Sphenofrontal suture | Fibrous suture | Frontal bone |
| Sphenosquamosal suture | Fibrous suture | Temporal bone |
Common Pathologies
Pituitary adenoma
Tumors of the pituitary gland expand within the sella turcica, potentially compressing the optic chiasm above and causing bitemporal hemianopia (tunnel vision).
Sphenoid wing meningioma
Meningioma arising from the greater or lesser wing can cause progressive visual loss, proptosis, and seizures depending on location.
Sphenoid sinusitis
Infection of the sphenoid sinus is less common but dangerous due to proximity to the cavernous sinus, internal carotid artery, and optic nerve.
Cavernous sinus thrombosis
Thrombosis in the venous sinuses lateral to the sella turcica can cause cranial nerve palsies (III, IV, V1, V2, VI) and is a life-threatening emergency.
Clinical Relevance
The sphenoid bone is the surgical approach route for transsphenoidal pituitary surgery, accessing the sella turcica through the nasal cavity and sphenoid sinus. The superior orbital fissure syndrome, involving palsies of CN III, IV, V1, and VI, localizes pathology to this area of the sphenoid. The pterion, where the sphenoid meets the parietal, frontal, and temporal bones, overlies the middle meningeal artery.
Development and Ossification
The sphenoid ossifies from about 14 centers, more than any other bone. The presphenoid and postsphenoid fuse around the 8th fetal month. The spheno-occipital synchondrosis is one of the last growth plates to close, typically between ages 18 and 25.
Did You Know?
- The sphenoid is called the keystone of the cranium because it articulates with every other cranial bone
- Sphenoid means wedge-shaped in Greek, though it more closely resembles a butterfly or bat with outstretched wings
- The sella turcica means Turkish saddle in Latin due to its resemblance to a horse saddle
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