Because of the unique anatomy of the occipital-atlantoaxial complex, applied forces result in characteristic injury patterns, as described below.
This is a four-part fracture of the ring of the atlas (C1) caused by axial loading (e.g., a fall on the vertex of the skull). If the combined lateral displacement of the lateral masses exceeds 7 mm, disruption of the transverse ligament has occurred, indicating a greater degree of instability4 (Fig. 94-2A). This injury must be differentiated on a lateral x-ray view from a stable fracture involving only the posterior arch of C1 caused by extension (see Fig. 94-2B).
A. AP tomogram confirming an unstable Jefferson's fracture with avulsion of the transverse ligament from the lateral mass of C1. B. Lateral view showing fracture of the posterior arch of C1 (arrow).
Atlantoaxial Instability (C1 and C2)
A gap greater than 4 mm between the anterior arch of the atlas and the odontoid is due to insufficiency of the transverse ligament.This condition may result from trauma or inflammatory erosion (e.g., rheumatoid arthritis).
This injury is a subluxation of variable degree, recognized by asymmetry of the gap between the lateral aspect of the odontoid and the lateral mass of C1 on each side and also a decreased joint space between the lateral masses of C1 and C2.
Fractures of the Odontoid
Shear forces in the sagittal plane cause these fractures. The level of the fracture is variable and has been classified by Anderson and D'Alonzo5 into three types (Fig. 94-3).
Fractured odontoid. (Used with permission from Anderson and D'Alonzo.5)
This injury is a traumatic spondylolisthesis of C2–C3. It is a bipedicle fracture of C2, usually from an extension force, with anterior displacement of the body of C2 on C3. There is a low incidence of neurologic injury.
These injuries are classified on the basis of the force applied to the neck, as described below.5
The applied force may be either compressive or distractive.
The force acting through the anterior column results in fracture of the vertebral body. The extent of the injury varies from stable minor wedging of the anterior vertebral body only, with no neurologic loss, to marked intrusion into the neural canal, with frequent severe neurologic loss. There is usually associated facet subluxation and instability (e.g., teardrop and quadrangular fractures).
The applied force begins posteriorly, involving the posterior ligamentous complex and, if severe, may involve all three vertebral column components. The extent of the injury varies from relatively stable subluxation through unilateral and bilateral facet dislocations to a complete vertebral body dislocation. The extent and frequency of neurologic injury usually are in proportion to the vertebral injury, but the initial displacement of the vertebrae may have been much more severe than that seen on x-ray.
The applied force may also be compressive or distractive.
This type of force results in fractures through the laminae, which may be sheared off. These fractures can be unilateral or bilateral, and at the extreme, there may be displacement of the vertebral body anteriorly and severe instability. However, since the laminae are sheared off, there may be little intrusion into the neural canal, so the extent of neurologic injury may be less severe than the vertebral injury (Fig. 94-4).
Compressive extension fracture dislocation of C6–C7. There is dislocation of the facets, but since the spinous process and lamina were sheared off (arrow), there was minimal neural canal encroachment, and the patient suffered only bilateral C7 root injury.
There is failure of the anterior column (anterior longitudinal ligament and disk) in tension, and therefore, the anterior column is usually stable in flexion. In young individuals, this condition may be missed easily clinically and radiologically and is associated with a low incidence of neurologic involvement. However, in the elderly with associated preexisting degenerative changes, there may be avulsion of bony spurs from the anterior vertebral body. The cervical spinal cord may be pinched between osteophytes on the posterior vertebral body and infolded ligamentum flavum posteriorly, typically resulting in incomplete injury (usually central cord syndrome; Fig. 94-5).
Distractive extension injury with severe preexisting osteoarthritis of the cervical spine. The disk space is opened anteriorly (arrow). Posterior osteophytes encroach on the neural canal, and there is a myelographic block (arrow).
Axial load causes centrifugal displacement and intrusion into the neural canal and subsequent serious neurologic injury.
This results in compression and fracturing of the lateral mass and also frequent contralateral ligament disruption. It is usually associated with nerve root injury on the compression side.
Since the underlying anatomy in the thoracic and lumbar spine is similar, the mechanistic classification proposed by Denis2 is helpful.
This is usually stable and only involves the anterior column, so there is a low incidence of neurologic injury.
This condition results from axial load forces and results in major injury to the centrum of the vertebral body and intrusion of bone into the neural canal (Fig. 94-6), often associated with lateral displacement of the pedicles and a vertical fracture of the laminae. There frequently is associated neurologic injury.
CT scan showing nearly total encroachment on the neural canal.
Seat-Belt Type (Chance Flexion-Distraction)
This injury is caused by a severe flexion force with the axis of rotation anterior to the vertebral body, resulting in failure of all three columns in tension. The pattern can involve various combinations of bony and ligamentous injury. There is minimal encroachment into the neural canal, and the most common neurologic injury involves the nerve root exiting beneath an involved pedicle and may be unilateral or bilateral.
These injuries usually are caused by a combination of forces (flexion, rotation, and shear) and result in translation of the vertebrae and severe neurologic loss.