Sphenoid bone

The sphenoid bone is a complex, butterfly-shaped bone located at the base of the skull. It plays a central role in cranial architecture by articulating with multiple cranial and facial bones. Its unique structure also houses important neurovascular passages, making it critical in both anatomy and clinical practice.

Anatomy of the Sphenoid Bone

General Description

The sphenoid bone is classified as an irregular cranial bone and has a central body with paired greater and lesser wings extending laterally. Its structure provides support to the cranial base and contributes to the walls of the orbit, nasal cavity, and middle cranial fossa.

Body of the Sphenoid

• Sella Turcica: A saddle-shaped depression on the superior surface that houses the pituitary gland in the pituitary fossa.
• Sphenoidal Sinuses: Paired air-filled cavities within the body that communicate with the nasal cavity and contribute to resonance and lightening of the skull.
• Clinoid Processes: Anterior, middle, and posterior clinoid processes serve as attachment points for dura mater and support the sella turcica.

Greater Wings

The greater wings project laterally from the body, forming part of the floor of the middle cranial fossa and the posterior wall of the orbit. They contain foramina for key neurovascular structures and provide attachment points for muscles and ligaments.

Lesser Wings

• Thin, triangular plates extending from the anterior body of the sphenoid.
• Form the posterior boundary of the anterior cranial fossa.
• Contain the optic canal, allowing passage of the optic nerve and ophthalmic artery.

Pterygoid Processes

• Medial and Lateral Pterygoid Plates: Extend inferiorly from the junction of the body and greater wings, providing attachment for the pterygoid muscles involved in mastication.
• Pterygoid Fossa and Hamulus: Serve as origin and pulley points for muscles and ligaments of the soft palate and pharynx.

Articulations

The sphenoid bone articulates with multiple cranial and facial bones, contributing to the structural integrity of the skull. These articulations include:

• Frontal Bone: At the anterior cranial fossa.
• Parietal Bones: At the greater wings, forming part of the middle cranial fossa.
• Temporal Bones: Along the lateral aspects, contributing to the cranial base.
• Occipital Bone: Posterior articulation at the base of the skull.
• Ethmoid Bone: Along the anterior body near the orbital plates.
• Zygomatic Bones: Lateral articulation via the greater wings, forming part of the orbit.

These articulations provide stability, transmit forces, and form key cranial and orbital boundaries.

Foramina and Neurovascular Structures

• Optic Canal: Transmits the optic nerve (cranial nerve II) and ophthalmic artery from the cranial cavity to the orbit.
• Superior Orbital Fissure: Passage for cranial nerves III, IV, V1, and VI, as well as ophthalmic veins.
• Foramen Rotundum: Transmits the maxillary nerve (V2) from the middle cranial fossa to the pterygopalatine fossa.
• Foramen Ovale: Provides a route for the mandibular nerve (V3), accessory meningeal artery, and lesser petrosal nerve.
• Foramen Spinosum: Allows passage of the middle meningeal artery, vein, and meningeal branch of V3.
• Foramen Lacerum: Filled with cartilage in life, allows passage of the internal carotid artery through its upper portion.

The foramina of the sphenoid bone are critical landmarks for neurosurgical procedures and for understanding the pathways of cranial nerves and vessels.

Development and Ossification

The sphenoid bone develops from both membranous and cartilaginous origins. The body, greater wings, lesser wings, and pterygoid processes each arise from separate ossification centers. These centers begin to ossify during the eighth to tenth week of fetal life and gradually fuse by adolescence. Proper ossification is essential for cranial stability and the formation of neurovascular passages.

Clinical Significance

Fractures

• Fractures of the sphenoid bone are often associated with high-impact cranial trauma.
• Can involve the cranial base and result in injury to cranial nerves passing through the sphenoid foramina.
• May lead to cerebrospinal fluid leaks if the body or sinus walls are disrupted.

Pathologies

• Sphenoid Sinusitis: Inflammation or infection of the sphenoidal sinuses can cause headache, vision changes, and cranial nerve involvement.
• Tumors: Both benign and malignant lesions can arise in the sphenoid bone or sinus, potentially compressing the optic nerve or pituitary gland.
• Pituitary Fossa Lesions: Tumors in the sella turcica, such as pituitary adenomas, may affect hormonal function and adjacent cranial nerves.

Surgical Considerations

• Transsphenoidal approaches allow access to the pituitary gland with minimal disruption of surrounding structures.
• Understanding sphenoid anatomy is critical to avoid damage to the optic nerve, internal carotid artery, and cranial nerves traversing the foramina.

Radiological Anatomy

• CT Imaging: Provides detailed visualization of bony structures, sinus pneumatization, and fractures of the sphenoid bone.
• MRI: Useful for assessing soft tissue structures, pituitary lesions, and the relationship of neurovascular structures to the sphenoid bone.
• Landmarks for Neurosurgical Procedures: Identification of the sella turcica, optic canal, and foramen ovale is essential for safe surgical approaches.

Variations and Anomalies

• Sphenoid Sinus Pneumatization Patterns: Vary among individuals and can influence susceptibility to sinus disease or surgical planning.
• Accessory Foramina: Additional foramina may be present, potentially transmitting aberrant neurovascular structures.
• Congenital Abnormalities: Rare malformations may affect cranial base stability, foramina size, or pituitary fossa development.

References

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