Overview
The sphenoid bone is an unpaired butterfly-shaped bone which comprises part of the anterior midline of the cranium, contributing to both the anterior cranial fossa and middle cranial fossa, lateral wall of the skull, and the floor and sides of both orbits.
Structure
Body
- Cuboidal shaped centre hollow mass containing the sphenoid sinuses
- Articulates with the ethmoid bone anteriorly
- Forms the medial border of the optic canal – which transmits the optic nerve (superiorly) and ophthalmic artery inferiorly.
The superior surface of the sphenoid body contains some important bony landmarks:
- Sella turcica – a saddle-shaped depression. It has three parts:
- Tuberculum sellae – forms the anterior wall of the sella turcica, and the posterior aspect of the chiasmatic groove.
- Hypophyseal fossa – the deepest part of the sella turcica, where the pituitary gland is located.
- Dorsum sellae – forms the posterior wall of the sella turcica.
- Chiasmatic groove – a sulcus formed by the optic chiasm (where the optic nerves partially cross).
- Anterior clinoid processes – arise from the sphenoidal lesser wings. Serves as attachment points for the tentorium cerebelli.
- Posterior clinoid processes – arises from the dorsum sellae. Serves as attachment points for the tentorium cerebelli.
Sphenoid Sinus
- Paired spaces formed within the body of the sphenoid bone, communicating with the roof of the nasal cavity via the sphenoethmoidal recess in its anterior wall. The two hemisinuses are separated by a septum which may or may not be in the midline. It usually lies anteroinferior to the sella.
Greater Wing
- Extends from the sphenoid body in a lateral, superior and posterior direction.
- Forms the floor of the middle cranial fossa, lateral wall of the skull and posterolateral wall of the bony orbit. Also forms
- Contains three foramina:
- Foramen rotundum – transmits the maxillary division of the trigeminal nerve
- Foramen ovale – transmits the mandibular division of the trigeminal nerve
- Foramen spinosum – transmits the middle meningeal artery, nerve and vein.
Lesser Wing
- Arises from the anterior aspect of the sphenoid body in a superolateral direction. It’s posterior bony edge separates the anterior cranial fossa from the middle cranial fossa.
- Forms the lateral border of the optic canal – which transmits the optic nerve (superiorly) and ophthalmic artery inferiorly.
- The medial border of the optic canal is formed by the body of the sphenoid.
- Forms the superior orbital fissure between the lesser and greater wings of the sphenoid – the superior orbital fissure. Numerous structures pass through here to reach the bony orbit.
Pterygoid Process
The two pairs of vertical bony processes project inferiorly from the junction between the sphenoid body and the greater wing:
- Medial pterygoid plate – supports the posterior opening of the nasal cavity.
- Lateral pterygoid plate – site of origin of the medial and lateral pterygoid muscles
Articulations
- Unpaired bones – Occipital, vomer, ethmoid and frontal bones.
- Paired bones – Temporal, parietal, zygomatic and palatine bones.
Muscular Attachments
The following muscles of mastication originate from the lateral pterygoid plate
- Medial pterygoid muscles – inserts on the the medial surface of the mandible (near its angle)
- Lateral pterygoid muscles – inserts on the condyle of mandible and capsule of temporomandibular joint
Anatomical Variants
Variations of the sphenoidal sinuses with respect to optic nerve relationship:
- Type 1: adjacent to sphenoid sinus
- Type 2: indenting the sinus
- Type 3: traversing the sinus
- Type 4: adjacent to posterior ethmoid sinus