Classification of inflammatory arthritis by enthesitis.

Imaging studies of early synovitis suggest that the first abnormality to appear in swollen joints associated with spondyloarthropathy is an enthesitis (inflammation at sites where ligaments, tendons, or joint capsules are attached to bone). We propose that the synovitis of spondyloarthropathy is secondary to liberation of proinflammatory mediators from the enthesis, whereas the synovitis of rheumatoid arthritis is primary. This suggestion allows a classification of arthritis as either primary synovial (rheumatoid-like) or entheseal (spondyloarthropathy-like) and allows differentiation of presentation of a polyarthritis with a good prognosis (spondyloarthropathy-like), from that with a bad prognosis (rheumatoid arthritis). Pathogenesis of spondyloarthropathy, in particular the part played by HLA-B27 and micro-organisms, should be assessed at the enthesis rather than in the synovium.