How is subaxial subluxation in rheumatoid arthritis (RA) confirmed radiographically?

Updated: Nov 01, 2018
  • Author: Michele Calleja, MD, FRCR, MRCP; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Answer

Answer

The diameter of the spinal canal is actually a better predictor of the development of paralysis than the degree of subluxation of one vertebral body on another. The normal sagittal diameter from C3 to C7 is 14-23 mm. A spinal canal sagittal diameter of at least 14 mm is critical at all levels in the cervical spine, as this is the minimum space required for the cord, CSF, and dura.

The cervical height index (CHI) is a method for assessing subaxial subluxation that takes into account the contribution of subluxations at multiple levels, together with loss of disk height and bony collapse. The CHI is calculated by first measuring the distance from the center of the sclerotic ring of C2 (as used in the Ranawat method) to the tip of the spinous process of C2. This distance is then divided into the distance from the center of the C2 sclerotic ring to the midpoint of the inferior border of C7 vertebral body. A CHI of less than 2 has a sensitivity and specificity approaching 100% in predicting neurologic compromise.

(See the image below.)

Lateral radiograph of the cervical spine shows how 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.

These plain radiographic measurements, along with dynamic flexion-extension views provide a reliable and efficient means of assessing spinal involvement of rheumatoid disease and targeting at-risk patients, as well as selecting those who warrant further radiologic studies with a view to surgical stabilization.


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