How should a normal dens look? The image below shows a midline sagittal image through a normal dens. Note the normally bright marrow signal in the dens, as well as the thin, discrete black line representing the cortical bone surrounding the marrow. Then look back at the large glob of pannus on the prior images and see how different they look from this one.
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.
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.