Lateral Radiograph

Four contour lines can be used to evaluate vertebral alignment on a lateral radiograph of the cervical spine ( Fig. 9.5 ). Each of the lines should form a smooth, gentle curve with no steps or discontinuities. Approximately 20% of children have a physiologic anterior offset of C2 relative to C3 through ~ 7 years of age (“pseudosubluxation”), but good spinolaminar alignment should still be preserved.

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  Anterior vertebral line: formed by the anterior borders of the vertebral bodies. On radiographs it can be traced from the anterior arch of the atlas through C7. Physiologic notches or bevels in the anterior vertebral margins (apophyseal region) may create slight irregularities.

  
  
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  Posterior vertebral line: formed by the posterior borders of the vertebral bodies, marks the anterior boundary of the spinal canal. It can be traced from the base of the atlas through C7. An electronic display with windowing options can often show the posterior margin of the dens as an upward prolongation of the line.

  
  
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  Spinolaminar line: formed by the junction of the C1 through C7 spinous processes and laminae, marks the posterior boundary of the spinal canal. The posterior arch of C1 may be a few millimeters anterior to the spinolaminar line as a normal variant. The line extends upward to the posterior rim of the foramen magnum.

  
  
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  Posterior spinous line: formed by the tips of the C2 through C7 spinous processes. The distance between the C2 and C3 spinous processes is physiologically smaller than in the other segments.

Thiebaut and Wackenheim described the basilar line, also called the Wackenheim clivus line, which represents a downward extension of the posterior surface of the clivus. This line should be tangent to the posterior cortex of the tip of the dens or should cross the dens at the junction of the anterior and middle thirds of the tip. Deviations from this alignment are evidence of atlanto-occipital dislocation. The normal variability of the clivus slope makes this technique very unreliable, however.

A practical alternative for the diagnosis of atlanto-occipital dislocation is to measure the basion–dens interval and basion–axial interval ( Fig. 9.6 ). Harris in 1994 evaluated these intervals on lateral radiographs of 37 patients with known atlanto-occipital dislocation and a control group of 400 adults with no occipitovertebral abnormalities. The basion–dens interval is determined by measuring the distance between the basion (caudal tip of the clivus) and the highest point on the dens. The basion–axial interval is measured between the basion and a line along the posterior surface of the dens. For both measurements, an interval > 12 mm is considered evidence of atlanto-occipital dislocation. This is a valid cutoff value for the basion–axial interval even on flexion and extension views. In rare cases the clivus may be physiologically up to 4 mm behind the line along the posterior cortex of the dens. Because the dens is not fully ossified until about age 12 years, it is recommended that the basion–dens interval be measured only in patients aged 14 years or older. By contrast, the basion–axial interval can be measured in younger children.