Atlanto-axial subluxation. This is one of the potential complications of rheumatoid arthritis, and is important to recognize as it can sometimes result in permanent neurological deficit. The key finding on a lateral radiograph is an increased distance between the anterior arch of C1 and the dens of C2. This distance should be no more than 3 mm in an adult. When this condition is suspected, lateral c-spine radiographs are performed with the patient’s neck extended and flexed, to compare the atlanto-axial distance in the two positions – it should not change, however in the example shown here it increases from around 1 mm on the extended view, left, to 8 mm on the flexed view (arrow).