Answer
Studies suggest that the posterior atlantodental interval (PADI) is a better method of assessing AAS because the PADI directly measures the spinal canal and therefore shows how much is narrowed by the subluxation. The PADI is the distance between the posterior surface of the odontoid and the anterior margin of the posterior ring of the atlas. At all cervical spinal levels, the cord requires a minimum canal width of 10 mm; CSF, 2 mm; and dura, 2 mm. Therefore, a minimum PADI of 14 mm is required to avoid cord compression. The normal spinal canal measures 17-29 mm at C1.
Boden et al investigated the predictive value of the PADI and found that a value of less that 14 mm on plain radiographs had a 97% ability to detect patients with neurologic deficit. [12] Also, neurologic recovery from surgery was unlikely if the PADI fell beneath 10 mm. Moreover, complete motor recovery occurred if the surgery was performed while the PADI was greater than 14 mm.
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Lateral view of the cervical spine in a patient with rheumatoid arthritis shows erosion of the odontoid process.
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Plain lateral radiograph of the normal cervical spine taken in extension shows measurement of anterior atlantodental interval (yellow line) and posterior atlantodental interval (red line).
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Lateral flexion view of the cervical spine shows atlantoaxial subluxation.
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T1-weighted sagittal MRI of the cervical spine shows basilar invagination with cranial migration of an eroded odontoid peg. There is minimal pannus. The tip of the peg indents the medulla, and there is narrowing of the foramen magnum due to the presence of the peg. Inflammatory fusion of several cervical vertebral bodies is shown.
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Sagittal T2-weighted MRI of the cervical spine in the same case as in Image above. The compromised foramen magnum is easily appreciated, and there is increased signal intensity within the upper cord; this is consistent with compressive myelomalacia. Further narrowing of the canal is seen at multiple levels.
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Lateral radiograph of the same patient as in the two images above. Midcervical vertebral-body fusions are shown. The eroded peg is difficult to visualize, but inferior subluxation of the anterior arch of C1 is shown.
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Lateral radiograph of a normal cervical spine shows the McGregor line. The odontoid tip should not protrude more than 4.5 mm above the line, which is drawn from the posterior edge of the hard palate to the most caudal point of the occiput.
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Normal lateral magnified radiograph of the cervical spine shows the Ranawat method of detection of cranial settling. This method is used to measure the distance from the center of the pedicles (sclerotic ring) of C2 to a line drawn connecting the midpoints of the anterior and posterior arches of C1. (Normal values are 15 mm or greater for males and 13 mm or greater for females.)
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Lateral radiograph of the cervical spine shows how the cervical height index (CHI) is calculated. The distance from the center of the sclerotic ring of C2 to the tip of the spinous process of C2 (dotted line) is measured. This is then divided into the distance from the center of the sclerotic ring of C2 to the mid-point of the inferior border of the body of C7. A CHI of less than 2 mm is a sensitive predictor of neurologic deficit.
