The involvement of the sacroiliac joints in rheumatoid arthritis is neither as common nor severe as in ankylosing spondylitis. The sacroiliac joints are affected more typically in patients with longstanding disease. The involvement may be bilateral or unilateral, and symmetry is not a regular feature. A strong proclivity toward iliac involvement has been described, but isolated sacral involvement appears not rare (Fig. 10.37). X-ray tomography shows para-articular osteopenia in the sacrum that is delimited by the pencil-lined cortex laterally and faint sclerosis medially. In contrast to the findings in rheumatoid arthritis of other weight-bearing joints, the narrowing of the joint space in sacroiliac rheumatoid arthritis is often not so prominent even in the presence of para-articular cystic change (Fig. 10.38A). Using conventional tomography, Elhabali et al. (1979) noted marked ankylosis and subchondral erosions in 38% of 72 patients with rheumatoid arthritis of the sacroiliac joint.
Planar 99mTc-MDP bone scintigraphy demonstrates simply homogenous uptake that completely obliterates joints (Fig. 10.38B). Pinhole scintigraphy, however, shows such uptake to be localized in the synovial compartment of the joint and cysts when present (Fig. 10.38C). It is interesting to note that rheumatoid arthritis in the ankylotic stage accumulates tracer not so intensely (Fig. 10.39) compared to that in the florid stage (Fig. 10.38C).
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