Entheses and enthesitis: a histopathologic review and relevance to spondyloarthritides

There are two types of entheses: fibrous, by Sharpey's fibers in membranous bone, and fibrocartilaginous, on endochondral bone, with discontinuous cem ent lines at the interface between insertion and bone. The connection of hy aline cartilage to subchondral bone is a kind of fibrocartilaginous enthesi s. Fibrocartilages are structurally, chemically, and biomechanically interm ediate between tendon and cartilage. Enthesitis is not the sole pathologic feature of spondyloarthritides. Synovitis and subchondral bone marrow chang es outside the ligamentous insertions, and cartilage proliferation, are imp ortant too. In the subentheseal bone marrow and in the synovium, CD8+ T cel ls play a central role. Imaging of early changes is better achieved by ultr asonography and even better by magnetic resonance imaging than by radiograp hy. No single immunologic target can be identified. The G1 domain of aggrec an is the best candidate, but this does not apply to fibrous entheses. In t hese complex pathologic conditions, no single abnormality can thus far be d esignated as a unique hallmark. (C) 2001 Lippincott Williams & Wilkins, Inc .