Sphenoid Bone

1. Overview

The sphenoid bone is a complex, butterfly-shaped bone situated at the base of the skull. It forms a central part of the cranial floor and articulates with nearly every other cranial bone. Because of its central location and articulations, it is often referred to as the "keystone of the cranial floor." The sphenoid bone plays critical roles in protecting neurovascular structures and supporting the brain.

2. Location

The sphenoid bone is located in the middle cranial fossa:

• Anteriorly: Articulates with the frontal and ethmoid bones.

• Posteriorly: Articulates with the occipital and temporal bones.

• Laterally: Contributes to the floor and side walls of the skull and forms part of the orbit.

• Medially: Its body lies on the midline and contains the sphenoidal sinus.

3. Structure

The sphenoid bone is a single, irregular, unpaired bone with multiple anatomical components:

• Body: Central portion housing the sphenoidal sinuses and the sella turcica, which holds the pituitary gland.

• Greater wings: Extend laterally to form part of the floor of the middle cranial fossa and the lateral wall of the orbit.

• Lesser wings: Smaller, triangular projections that form part of the floor of the anterior cranial fossa and the superior orbital fissure.

• Pterygoid processes: Project downward from the junction of the body and greater wings, providing attachment points for muscles of mastication.

• Foramina and canals:

  • Optic canal: Transmits the optic nerve (CN II) and ophthalmic artery.

  • Superior orbital fissure: Transmits cranial nerves III, IV, V1, and VI, and the superior ophthalmic vein.

  • Foramen rotundum, ovale, and spinosum: Transmit branches of the trigeminal nerve and middle meningeal artery.

4. Function

The sphenoid bone has structural and functional roles critical to skull integrity and neurovascular protection:

• Forms part of the skull base: Contributes to both the anterior and middle cranial fossae.

• Protects vital structures: Surrounds and supports the pituitary gland within the sella turcica.

• Supports muscle attachment: Pterygoid processes serve as origins for muscles involved in chewing and speech.

• Allows passage of neurovascular structures: Multiple foramina allow transmission of cranial nerves and arteries to and from the brain.

5. Physiological role(s)

Beyond its skeletal function, the sphenoid bone participates in various physiological processes:

• Endocrine support: Houses the pituitary gland, a major endocrine organ, in the sella turcica.

• Sinus ventilation: Contains paired sphenoidal sinuses that open into the nasal cavity, contributing to humidification and voice resonance.

• Vision: Forms the optic canal and part of the orbit, guiding and protecting the optic nerve and associated vessels.

• Cranial nerve routing: Forms bony channels for multiple cranial nerves crucial to eye movement, facial sensation, and more.

6. Clinical Significance

The sphenoid bone's central location and relationships make it clinically important:

• Pituitary tumors:

  • Because the pituitary gland sits in the sella turcica, tumors can expand and impinge on the optic chiasm, leading to visual disturbances.

• Sphenoid sinusitis:

  • Infection or inflammation of the sphenoidal sinus can cause deep-seated headaches and may spread to nearby structures like the cavernous sinus.

• Trauma:

  • Fractures can damage multiple cranial nerves due to the bone’s foramina-rich structure.

• Surgical access route:

  • The transsphenoidal approach is a common surgical route to access pituitary tumors.

• Cavernous sinus syndrome:

  • Pathologies near the sphenoid bone can affect the cavernous sinus and its contained cranial nerves, leading to ophthalmoplegia and facial numbness.