The clivus, derived from the Latin word for “slope,” is a smooth, sloping segment of bone situated at the center of the skull base. It forms part of the floor of the posterior cranial fossa, which houses the cerebellum and the brainstem. Its primary role is to provide structural support for the lower brain and act as a barrier separating the brain from the nasal and throat cavities. The clivus connects the upper skull structures with the spine.
Anatomical Definition and Location
The clivus is a midline structure extending from the dorsum sellae (a bony prominence on the sphenoid bone) down to the anterior edge of the foramen magnum, the large opening where the spinal cord connects to the brain. Its sloped orientation accommodates the curvature of the brainstem.
This bony slope lies directly anterior to the brainstem. The upper portion provides a resting surface for the pons, and the inferior portion is associated with the medulla oblongata, which controls autonomic functions. The basilar artery, a major blood vessel supplying the brain, runs along the anterior surface of the pons, directly in front of the clivus.
The clivus is positioned just posterior to the sphenoid sinuses. This deep location makes it a gateway to several surrounding neurovascular structures, including the abducens nerve (Cranial Nerve VI), which controls lateral eye movement and tracks along the clivus.
Structural Components and Fusion
The clivus is formed by the union of two distinct skull bones. The superior portion is the body of the sphenoid bone (basisphenoid), and the larger, inferior portion is the basilar part of the occipital bone (basiocciput). These two segments are joined together by a cartilaginous joint called the spheno-occipital synchondrosis.
This joint is a temporary site of growth and is crucial for the longitudinal expansion of the skull base during childhood. This synchondrosis begins to fuse, or ossify, into solid bone during adolescence. The process of fusion typically starts in girls between 12 and 13 years of age and in boys between 14 and 15 years. By the age of 17 to 18 years, this cartilaginous plate is generally replaced by solid bone, creating a single, continuous structure. This fusion process is an important indicator of skeletal maturity in forensic and medical contexts.
Clinical Significance in Medicine
The location of the clivus near the brainstem and major blood vessels makes it a site of significant medical concern, particularly in trauma and oncology.
Clival Fractures
Clival fractures are rare but highly dangerous injuries, often resulting from high-impact blunt head trauma. These fractures are categorized based on their orientation, such as longitudinal, transverse, or oblique patterns. Transverse fractures are particularly perilous due to the high risk of injury to the brainstem or basilar artery, leading to a high mortality rate. Even in survivors, clival fractures frequently cause damage to adjacent cranial nerves, with deficits in the abducens nerve (CN VI) being commonly observed, resulting in impaired eye movement. The presence of a clival fracture alerts clinicians to potential vascular complications, such as vertebrobasilar occlusion.
Chordomas
The clivus is also the most common site for a rare, slow-growing malignant bone tumor known as a chordoma. These tumors originate from remnants of the notochord, the embryonic structure that precedes the spine. Clival chordomas are locally aggressive and require complex surgical removal, often followed by radiation therapy.
Surgical Approaches
For neurosurgeons, the clivus is a central landmark and a challenging barrier, as it sits deep within the skull base, surrounded by the basilar artery, cavernous sinus, and multiple cranial nerves. Modern surgical techniques often employ endoscopic endonasal approaches, accessing the clivus through the nasal cavity. This method is preferred because it allows surgeons to reach tumors or other lesions in the area without the need for extensive retraction of the brain, which can reduce complications.