Histopathologic changes at "synovio-entheseal complexes" suggesting a novel mechanism for synovitis in osteoarthritis and spondylarthritis

Objective. To determine the extent to which different entheses form part of a "synovio-entheseal complex" (SEC) and whether such SECs are commonly associated with the presence of inflammatory cells and evidence of enthesis microdamage. Methods. Specimens from 49 cadaveric entheses were processed for histologic study, and all soft tissue components of the entheses or enthesis organs were examined. To exclude articular cartilage degeneration as a triggering factor for synovitis, the selected entheses included 17 that were not immediately adjacent to such cartilage. Results. An SEC was present at 82% of entheses. These included 47% of the attachments not adjacent to articular cartilage, where the synovium was that of bursae or tendon sheaths. One or more of a wide variety of degenerative changes were noted on the soft tissue side of every enthesis; the most common changes were cell clustering and/or fissuring (in 76% of entheses). Synovial villus formation or inflammatory cell infiltration was seen in 85% of entheses, and in 73% of attachments there were also inflammatory cells in the enthesis organ itself, The changes included synovial invasion (pannus formation) of the enthesis. Conclusion. Entheses are frequently juxtaposed to synovium, thus forming SECs. They are also often associated with both degenerative and inflammatory changes, and the latter may involve the immediately adjacent synovium. These findings suggest a novel mechanism by which synovitis could develop in both degenerative joint disease and spondylarthritis.