Answer
Only half of patients with radiographic evidence of atlantoaxial subluxation (AAS) are actually symptomatic. The role of plain radiography is to establish whether there are risk factors for cord compression. AAS is defined as an anterior atlantodental interval (AADI) greater than 2.5 mm in adults. This distance is measured as the interosseous distance between the posterior aspect of the arch of the atlas and the anterior aspect of the odontoid process. The point of measurement of the joint is a subject of debate; the inferior point is the most popular.
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).
There may be a slight variation in normal measurements between men and women. More importantly, it should be noted that an atlantodental interval of less than 2.5 mm, which changes considerably on flexion and extension, may also be abnormal. An AADI of 3-6 mm indicates early instability and implies transverse ligament damage. [11] An AADI greater than 6 mm indicates that the alar ligaments are also damaged. Some authors consider an AADI greater than 9 mm to be an indication for surgical stabilization.
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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.
