The human skull, a complex structure safeguarding the brain, contains many intricate components. Among these is the sphenoid bone, a uniquely shaped structure positioned centrally at the skull’s base. This bone extends in various directions, forming several distinct projections, known as its processes. These extensions are important for skull stability and provide pathways for neurological and vascular structures.
Anatomy and Location
The sphenoid bone, an unpaired bone, occupies a central position in the cranium. It is situated anteriorly and contributes significantly to the middle cranial fossa, the lateral wall of the skull, and the floors and sides of both eye orbits. Its shape is often described as resembling a butterfly or a wasp with extended wings. This bone is composed of a central body, from which two sets of lateral extensions—the greater wings and lesser wings—and a pair of downward-projecting pterygoid processes emerge.
The pterygoid processes descend from the inferior surface of the sphenoid bone, near the junction of its body and greater wings. Each pterygoid process consists of two thin bony plates: a medial pterygoid plate and a lateral pterygoid plate. The medial plate features a small, hook-shaped projection at its inferior tip, known as the pterygoid hamulus.
The greater wings extend laterally from the sphenoid body, forming a significant portion of the middle cranial fossa and part of the orbital wall. The lesser wings, triangular in shape, arise from the anterior aspect of the sphenoid body, separating the anterior cranial fossa from the middle cranial fossa.
The sphenoid body also contains the sella turcica, a saddle-shaped depression partly enclosed by the anterior and posterior clinoid processes. The sphenoid bone articulates with twelve other bones, including the frontal, parietal, temporal, occipital, ethmoid, vomer, zygomatic, and palatine bones.
Role and Connections
The various processes of the sphenoid bone serve multiple functional roles. They primarily provide attachment points for muscles and ligaments, and form protective pathways for neurovascular structures. The pterygoid processes are origins for some muscles involved in mastication, specifically the medial and lateral pterygoid muscles. These muscles, by attaching to the pterygoid plates, facilitate movements of the mandible required for chewing and contribute to swallowing.
The greater and lesser wings of the sphenoid bone contribute to the structural integrity of the skull and eye orbits. The lesser wing, along with the sphenoid body, forms the medial border of the optic canal, a passageway for the optic nerve and ophthalmic artery to reach the eye. A slit-like gap between the greater and lesser wings, known as the superior orbital fissure, allows numerous nerves and blood vessels to pass into the bony orbit.
The greater wings also contain several foramina, such as the foramen rotundum, foramen ovale, and foramen spinosum. These transmit important cranial nerves like branches of the trigeminal nerve and blood vessels, including the middle meningeal artery, to and from the cranial cavity. The sella turcica, situated within the sphenoid body and partly surrounded by the clinoid processes, cradles the pituitary gland.
Clinical Significance
The sphenoid bone and its processes are important in medical contexts due to their complex anatomy and proximity to numerous delicate structures. Fractures involving the sphenoid bone often result from trauma to the orbit or the base of the skull. These fractures can lead to a range of neurological complications, given the sphenoid’s close association with many nerves and blood vessels. Symptoms may include loss of vision, ocular damage, or various cranial nerve palsies, depending on the injury’s location and extent.
The sphenoidal sinuses, located within the sphenoid body, can become inflamed, a condition known as sphenoid sinusitis. Due to their deep and central location, inflammation here can cause significant pain and potentially affect adjacent structures. The sphenoid bone is also a common route for certain surgical procedures. Trans-sphenoidal surgery, for instance, is a technique used to access the pituitary gland, which resides in the sella turcica of the sphenoid bone. This surgical approach involves passing instruments through the nasal cavity and sphenoidal sinuses, allowing for the removal of pituitary tumors or access to other nearby lesions.