Anterior dislocation of C1 on C2 in rheumatoid arthritis on CT

A sagittal reconstruction of a CT scan (A) shows a 7.5 mm separation between the posterior part of the arch of C1 (arrowhead) and the anterior aspect of the dens (arrow). Image B is a coronal reconstruction of a CT scan and shows a large erosion on the lateral aspect of the dens (arrowhead). Image C is an axial image and shows a large erosion (arrowhead) on the eccentrically positioned dens. Image D is an axial image using soft tissue windows through the same region as C, and shows extensive pannus formation around the dens (asterisks), which impinges on the CSF space (arrow) surrounding the spinal cord (arrowhead).