New insights into inflammation in osteoarthritis

At 82 percent of the entheses, the formation of a SEC was found. As expected, this occurred in entheses very close to joint cartilage, where the synovium was often part of the joint itself. However, a SEC was also detected in 47 percent of the sites separated from joint cartilage. For example, the SEC found at the Achilles tendon was formed with synovium that protruded from a cavity called a “bursa”, located a considerable distance from the ankle joint.

Joint insertions are sites of high mechanical stressing and the authors speculated that this could lead to damage within them, including their fibrocartilage This is exactly what the authors found. Degenerative changes—at least one and sometimes several—were detected on the soft tissue side of attachment sites. Most notably, cell clustering and/or fissuring was found in 76 percent of entheses. In 85 percent of SECs, the synovial component also showed evidence of mild inflammatory change. Finally, in 73 percent of the attachments, small numbers of inflammatory cells were present in the enthesis itself. Therefore the authors suggest that joint degeneration of fibrocartilage at insertions could trigger inflammation within SECs.

As Professors Benjamin and McGonagle note, one their most striking findings was the large number of attachment sites with evidence of changes in the entheses mirroring those typically seen in joint cartilage in OA—fibrocartilage cell clusters, cell hypertrophy, and fissuring among them. “Such changes at certain entheses could be directly relevant to older subjects with joint symptoms due to degenerative disease,” Professor McGonagle observes, “and some of the symptoms could be emanating from the SEC.”

Affirming the concept of a “synovio-etheseal complex” as widely applicable at many sites in the body, both right next to and removed from joint cartilage, this study also supports the idea that biomechanical factors related to the enthesis could play an important role in synovial inflammation in both degenerative and inflammatory arthritis.

Article: “Histopathologic Changes at ‘Synovio-Entheseal Complexes’ Suggesting a Novel Mechanism for Synovitis in Osteoarthritis and Spondylarthritis,” Michael Benjamin and Dennis McGonagle, Arthritis & Rheumatism, November 2007; (DOI: 10.1002/art.23078).