Which radiographic findings are characteristic of vertical subluxation (VS) in rheumatoid arthritis (RA)?

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

Regarding the McGregor method, vertical subluxation (VS) has been defined as the protrusion of the odontoid tip by more than 4.5 mm above the McGregor line. [13] This is drawn between the posterosuperior aspect of the hard palate and the most caudal aspect of the occiput. However, it is not the most accurate measurement in the rheumatoid spine because of odontoid erosion and often marked osteoporosis.

The Ranawat index is used to assess VS by detecting settling of C1 on C2. The measurement is made from the center of the pedicles of C2 to a line connecting the midpoint of the anterior and posterior arches of C1. Normal values are 15 mm or greater for men and 13 mm or greater for women. Anything less than this confirms VS.

(See the images below.)

Lateral radiograph of a normal cervical spine show 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.
Normal lateral magnified radiograph of the cervica 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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