The pannus appears dark on these T1-weighted images and is best seen the image acquired in flexion. Compare this pathological dens with normal dens seen in the image below. Note the normally bright marrow signal in the dens, as well as the thin, discrete black line representing the cortical bone surrounding the marrow.
Also note how the predental space widened with flexion. Normally, the predental space is measured between the anterior margin of the dens and the posterior margin of the anterior arch of C-1, as shown in the magnified image below. This space should be no larger than 3 mm in adults and 4 -5 mm in children.
In the rheumatoid patient presented here, the predental space widened from 3 mm to 8 mm, as shown in the magnified image below.
60-70% of patients with RA develop cervical spine symptoms sometime in the course of their disease. A not infrequent (up to 25% of patients with RA) and potentially devastating complication of RA is atlantoaxial subluxation, as seen in these cases. The predental space between the posterior aspect of the anterior arch of the atlas and the anterior aspect of the dens does not normally exceed 3 mm. In this case, this distance changed from 3 mm to 8 mm between flexion and extension.
Erosion of the dens occurs in 14-35% of patients with RA and is a consequence of synovial inflammation in adjacent joints. This type of erosion is frequently associated with atlantoaxial subluxation as seen above. The pathogenesis of this type of atlantoaxial subluxation relates to the presence of transverse ligament laxity or rupture owing to the synovial inflammation and hyperemia of the adjacent articulations. Pathologic fractures of the weakened dens can be seen in patients after minimal trauma.
Vertical translocation of the dens is another finding that may be seen in patients with severe RA. As this finding progresses, the anterior arch of the atlas gradually assumes a position near the lower portion of the axis.
Subluxation of varying severity is observed at one or more subaxial levels frequently in patients with RA. Multilevel subluxations are typical and often produce a stepladder-like appearance on lateral radiographs. Joint space narrowing and superficial erosions are common in the apophyseal joints. Intervertebral disc space narrowing and subchondral osseous irregularity are also common. Erosions and destruction of one or more spinous processes are detected in approximately 10% of patients with RA.