Craniovertebral Height

The craniovertebral height is measured by a modification of the Klaus posterior fossa height index ( Fig 29.1 ).8,9 A line is drawn from the tuberculum sellae to the torcula. From the midpoint of this, a line is drawn that connects to the midpoint of the inferior surface of the body of C5. Care is taken to select the images that are in a neutral neck position. C5 is selected to assess the lower limit of the neck, as in a significant percentage of our cases, the investigations did not show the C6 and C7 vertebrae.

Cervical Height

The cervical height is measured by a line drawn from the tip of the odontoid process to the midpoint of the base of C5 ( Fig. 29.1 ).

Cervical Lordosis

Cervical lordosis is measured by a modification of the omega angle ( Fig. 29.2 ).8,9 The line of the hard palate is taken as a fixed line parameter, and a parallel line is drawn to it that passes from the center of the base of C3. The angle of the odontoid process is measured on this line. The base of C3 is selected instead of C2, as discussed by us elsewhere,8 because in several cases there are C2–C3 fusions.

Craniocervical Angle

On a lateral radiograph, a line is drawn along the clivus corresponding to the Wackenheim clival line ( Fig. 29.3 ).10 Another line is drawn along the posterior surface of C2–C3. The angle between these two lines is identified as the craniovertebral angle.7

Measurements are made on all three forms of investigation (plain radiographs, CT, and MRI) and are then averaged. Because stainless steel metal implants are used in the majority of our cases for fixation, postoperative MRI was not possible.

Neck size is assessed by two parameters. A line is drawn from the inion to the tip of the spinous process of C7. Another line is drawn from the angle of the mandible to the medial end of the clavicle. Both measurements are done in a neutral neck position.6

Clinical photographs help in assessing the alterations in the degree of torticollis.6

Table 29.2 lists the various abnormalities that were observed on imaging, and Tables 29.3, 29.4, and 29.5 show the various postoperative changes in the parameters assessed in our patients.6 Figures 29.4, 29.5, and 29.6 illustrate the various musculoskeletal changes associated with basilar invagination. On MRI, in addition to the features observed in Table 29.2, large and dilated subarachnoid spaces were observed anterior to the cord both above and below the point of maximum neural compression at the tip of the odontoid process in at least 66% of cases ( Figs. 29.4a and 29.5b ). Although there were spondylotic bone changes, neural compression by the osteophytes was not prominent because of the presence of “buffering” by enlarged subarachnoid spaces.6

Reduced neck size has been considered diagnostic of basilar invagination. In our series, partial or complete fusion of the vertebral bodies appeared to be directly related to long-standing reduction of the disk space height. This feature was apparent by the range of reduction of the disk space height in these cases. Spondylotic changes with formation of osteophytes indenting into the cervical subarachnoid space at multiple levels are observed in these patients. These “spondylotic” changes are disproportionately more common when related to patients’ ages. Misdiagnosis of cervical spondylotic disease and erroneous treatment for cervical osteophytes are possible in such cases.11 The spondylotic changes are more marked and prominent at the level of C2–C3, or in cases where there is C2–C3 fusion, at the level of C3–C4. Fusions are also more prominently observed both above (partial or complete assimilation of the atlas) and below (C2–C3 fusion) the point of maximum neural compression at the tip of the odontoid process. The anterior subarachnoid space is remarkably dilated both above and below the point of maximal neural compression by the odontoid process.