Occipital Bone

The occipital bone is an anteriorly concave bone that forms the base of the cranium. The occipital condyles are paired kidney-shaped structures that form the base of the occipital bone and are the structural bases for the articulation of the skull with the cervical spine. This articulation is mainly formed by the atlanto-occipital joint, a paired synovial joint composed of the bilateral occipital condyles projecting inferiorly to articulate with the concave lateral masses of the atlas. The atlas, in turn, articulates with the axis anteriorly via the odontoid process, and laterally via the lateral masses, with associated synovial capsules at each articulation.¹

The Atlas

The atlas, or C1, is the first cervical vertebra and is shaped to allow articulation with the odontoid process of C2 and the skull. It is unique in that it has no vertebral body. In place of a vertebral body, the anterior portion of C1 is composed of an anterior tubercle, which forms the site of attachment for the longus colli and the anterior longitudinal ligament, and an anterior arch, which is roughly cylindrical and anteriorly convex. The anterior arches of C1 extend slightly laterally and posteriorly to join the lateral masses. The C1 lateral masses makeup the majority of the C1 surface area and articulate with the large occipital condyles. The posterior arches of C1 extend posteriorly and medially from the lateral masses to terminate in the short posterior tubercle. The posterior tubercle is analogous to the spinous processes of the other cervical vertebrae; however, its small size allows for greater range of motion between the skull and C1 during neck extension.

The Axis

The axis, or C2, is the unique second vertebra. It is widely described as a pivot joint because the skull and atlas rotate around C2 with significant freedom. The axis is a transitional vertebra and shares properties with the unique C1 vertebra and the relatively uniform vertebrae of the subaxial spine. The body of the axis gives rise to its most unique feature, the odontoid process, which is a peglike extension of bone that tapers superiorly and terminates in the midline just behind the anterior arch of C1. The tip of the odontoid process is perfectly situated for ligamentous connections with the atlas and the occipital condyles. The superior articular surface of C2 is rounded and flat, like its counterpart on C1; however, the inferior articular surface of C2 is similar to the rest of the subaxial spine. Unlike C1, the axis has a pedicle, or isthmus, and a true lamina.

Ligaments of the Craniocervical Junction

The bony anatomy of the skull base, occipital condyles, atlas, axis, and odontoid process is of obvious importance in understanding biomechanical stability, fracture patterns, and surgical planning. The anatomy of the ligamentous structures of the craniocervical junction and upper cervical spine is also of crucial importance in maintaining biomechanical stability of the region, where injury to ligamentous structures can dramatically alter management of bony fractures. The nuchal ligament runs dorsally over the occiput and upper cervical spine, from the inion to the spinous processes of the cervical vertebrae. The ligamentum flavum runs underneath the laminae and projects superiorly to the base of the occiput. The anterior longitudinal ligament (ALL) has a dense arrangement of fibers and projects from the anterior tubercle of the axis inferiorly along the ventral surface of each cervical vertebral body. The anterior atlanto-occipital membrane, the superior extension of the ALL, is superficial, more loosely arranged, and connects the basilar part of the occiput to the atlas. The posterior longitudinal ligament runs along the dorsal surface of the cervical vertebral bodies and projects superiorly as the tectorial membrane, attaching to the skull base. The alar ligaments (attaching to the odontoid process, occipital condyles, and atlas), apical ligament (attaching the odontoid process to the clivus), and transverse atlantal ligament (restricting the odontoid to the anterior arch of the atlas) play a key role in maintaining the anatomic relationship of the odontoid process, the atlas, and the foramen magnum. Given the significant range of flexion-extension at O-C1 and rotation at C1-2, and the critical importance of the underlying neurovascular structures, biomechanical instability of this region can present with severe disability and must be treated aggressively.

The Vertebral Artery

Knowledge of the vertebral artery (VA) anatomy at the craniocervical junction is extremely important for understanding the mechanisms and consequences of injury to this region. Injury to the vertebral artery in this region can occur as a result of blunt or penetrating trauma, C1 or C2 fractures (especially those traversing the foramen transversarium), and iatrogenic injuries caused by aggressive manipulation of the cervical spine, for example.

The paired VAs usually arise from each subclavian artery. They ascend posteriorly and superiorly between the longus colli and scalenus anterior and enter the foramen transversarium of the upper six cervical vertebrae. The foramen transversarium pierces the transverse processes of each of these cervical vertebrae. The VA travels superiorly through the lower five cervical vertebrae and then curves laterally and superiorly to enter the foramen transversarium of the atlas. The artery then curves anteriorly and superiorly to enter the foramen magnum, traveling along the lateral aspect of the medulla.

Overview

As with any trauma patient, priority in management begins with the primary and secondary survey including craniospinal immobilization, hemodynamic stabilization, and radiographic evaluation. High-resolution computed tomography (CT) scans are indicated for any patients with clinical suspicion for cervical spine injury, including patients with altered sensorium or clinical evidence of head injury. Once identified, traumatic injuries of the craniocervical junction are triaged based on the clinical evidence of neural injury, vascular injury, and/or mechanical instability, and supplemental imaging such as angiography and magnetic resonance should be utilized liberally when indicated.

Occipitocervical Instability

OC instability is one of the most dangerous conditions affecting the cervical spine. True OC instability is a clinical emergency. The pathophysiology of OC instability ranges from trauma to inflammatory/neoplastic conditions, but trauma is the most common reason for operative intervention.

Occipitoatlantal Dislocation

Among traumatic injuries to the cervical spine and causes for OC instability, occipitoatlantal dislocation (OAD) is one of the most severe types of injuries. It is a hyperflexion-distraction injury which results in the ligamentous disconnection of the skull from the cervical spine. OAD is often immediately fatal because of associated neurological and vascular injuries. The first clue in the diagnosis of OAD is the mechanism of injury. High-impact injuries should always arouse the suspicion of OAD. Plain radiographs of the cervical spine reveal prevertebral soft tissue swelling and an increase in the basion-dens interval, which should measure 12 mm or less. More definitive diagnosis is made with reconstructed CT images of the craniocervical junction. MRI and CT of the craniocervical junction are comparable in terms of identifying OAD, but MRI can often identify the specific ligaments injured.

OAD injuries can be identified into three broad categories. Type I injuries are characterized by anterior displacement of the occipital condyles on the C1 lateral masses; type II injuries are characterized by displacement of the occiput and C1 in the vertical plane; and finally, type III injuries are marked by posterior displacement of occipital condyles compared to C1.

Occipital Condyle Fractures

Occipital condyle fractures can be classified into three main types according to the Anderson and Montesano scheme.² These fractures are seen in 1% to 3% of cases of blunt trauma to the craniocervical region. Type 1 fractures usually result from axial loading injuries and are comminuted. Type II fractures are linear fractures that originate in the squama of the occipital bone and extend into the condyle. Type III fractures are avulsion fractures of the condyles; these fractures are most prone to instability and atlanto-occipital dislocation.

C1 Fractures and Transverse Ligament Injuries

Fractures of the atlas are usually defined in relation to the lateral mass and extent of arch involvement.³ They can involve any parts of the ring in isolation or in combination, ranging from single unilateral fractures to burst-type fractures involving all four aspects, which is known as a Jefferson fracture. Since isolated atlas fractures without ligamentous injury are stable and heal with simple immobilization, the clinical importance of fractures of the atlas is to understand the possible involvement of the transverse ligament, the vertebral artery, and other associated spinal fractures. The most commonly cited radiographic criteria indicating unstable disruption of the transverse ligament include the Rule of Spence⁴ (lateral displacement of C1 lateral masses over C2 greater than 6.9 mm) and the atlantodental interval being greater than 3 mm. However, when feasible, this author prefers MRI evaluation of all atlas fractures to assess for concomitant ligamentous injury. Transverse ligament disruption, as with other cases of atlantoaxial instability, is an indication for surgical fixation.

Nontraumatic disruption of the atlantoaxial ligaments can also lead to gross atlantoaxial instability. C1-2 rotatory subluxation is a rare condition usually seen after inflammatory and/or infectious conditions of the pharynx and tonsils in the pediatric population. In the elderly, rheumatoid arthritis (discussed later) can lead to atlantoaxial instability requiring surgical stabilization.