The Sphenoid Bone: The Keystone of the Cranium

1. Introduction

The sphenoid bone is often called the "keystone" because it articulates with almost every other bone of the skull. It is butterfly-shaped and forms much of the middle cranial fossa and the floor of the orbit.

2. Main Components

• The Body: Central part containing the Sphenoid Sinuses.
• Sella Turcica: A Turkish saddle-shaped depression on the superior surface. The Hypophysial Fossa houses the Pituitary Gland (Hypophysis).
• Greater Wings: Form part of the middle cranial fossa and the lateral wall of the skull.
• Lesser Wings: Form the posterior part of the anterior cranial fossa. They contain the Optic Canal (for CN II and the ophthalmic artery).
• Pterygoid Processes: Extend inferiorly; they serve as attachment points for masticatory muscles.

3. Important Foramina (Openings)

The sphenoid bone is perforated by several critical openings for cranial nerves:

1. Superior Orbital Fissure: Transmits CN III, IV, VI, and V1 (ophthalmic nerve).
2. Foramen Rotundum: Transmits CN V2 (maxillary nerve).
3. Foramen Ovale: Transmits CN V3 (mandibular nerve).
4. Foramen Spinosum: Transmits the middle meningeal artery (damage leads to epidural hematoma).

4. Clinical Significance

• Pituitary Tumors: Can be accessed surgically through the sphenoid sinus (Transsphenoidal surgery).
• Pterion Fractures: The pterion is where the sphenoid, temporal, frontal, and parietal bones meet. A blow here can rupture the middle meningeal artery.

5. Review Questions

1. Which gland sits in the Sella Turcica?
2. Which cranial nerves pass through the Superior Orbital Fissure?
3. Why is the Foramen Spinosum clinically important?
4. Which part of the sphenoid bone forms the optic canal?