Several methods have been developed to identify basilar invagination on cervical spine plain films. All the methods require recognition of certain basic, simple, and important landmarks on lateral films: the basion (ventral border of the foramen magnum), opisthion (dorsal border of the foramen magnum), hard palate, atlas, C2 pedicles, and the tip of the odontoid process (Figure 9-7). The following methods are quite simple and helpful. This is not a complete list of all the methods that are listed in the literature.
Chamberlain line: a line drawn from the posterior edge of the hard palate to the opisthion (Figure 9-8). Normally, the tip of the dens should not protrude more than 3 mm above this line.
McRae line: a line drawn from the basion to the opisthion (across the foramen magnum) (Figure 9-8). Normally, the tip of the odontoid should be below this line. Wackenheim line: a line drawn along the superior surface of the clivus (Figure 9-9). Normally, the tip of the odontoid should not be posterior to this projected line. McGregor line: a line drawn from the postero-superior aspect of the hard palate to the most caudal point of the occipital curve (Figure 9-10). Normally, the tip of the odontoid should not protrude more than 4.5 mm above this line. Clark stations: the odontoid process is divided into three equal parts in the sagittal plane (Figure 9-10). Normally the anterior ring of the atlas should be level with the upper third. If it is level with the middle or lower third basilar invagination exists.
Unfortunately, none of these methods in isolation has been found to be totally reliable. None reaches sensitivity above 90% or specificity over 80%. The methods with the least false negative results were found to be the Wackenheim line and the Clark stations.
When basilar invagination is suspected MRI studies should be obtained. Special attention should be paid to the brain stem, spinal cord, and the cervico-medullary angle. The latter normally ranges between 135 and 175 degrees. Cranial settling and peridental pannus formation may decrease this angle and correlate well with the presence of myelopathy and neurological changes.
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