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 cement lines at the interface between insertion and bone. The connection of hyaline cartilage to subchondral bone is a kind of fibrocartilaginous enthesis. Fibrocartilages are structurally, chemically, and biomechanically intermediate between tendon and cartilage. Enthesitis is not the sole pathologic feature of spondyloarthritides. Synovitis and subchondral bone marrow changes outside the ligamentous insertions, and cartilage proliferation, are important too. In the subentheseal bone marrow and in the synovium, CD8+ T cells play a central role. Imaging of early changes is better achieved by ultrasonography and even better by magnetic resonance imaging than by radiography. No single immunologic target can be identified. The G1 domain of aggrecan is the best candidate, but this does not apply to fibrous entheses. In these complex pathologic conditions, no single abnormality can thus far be designated as a unique hallmark.