Sequential alterations of periarticular structures in antigen-induced arthritis in mice. Histological observations on fibrous capsule, ligaments, bone and muscles, using whole joint sections.

The influence of antigen-induced joint inflammation on periarticular structures was studied in mice on histological sections of whole knee joints prepared at various intervals after arthritis induction. The inflammatory process in the synovial membrane and fluid was associated with alterations in periarticular tissues from the very beginning of inflammation. Muscles nearby the joint became intensely infiltrated by (polymorphonuclear) leucocytes by apparent spreading of the infiltrate out of the synovial tissue into the surrounding muscles. An early destructive effect was found in the bony structures adjacent to the joint leading to erosive changes. The fibrous capsule showed fibroblast proliferation resulting in thickening of the capsule, whereas proliferative changes in the periosteum ultimately resulted in new bone formation. Changes in the fibrous capsule as well as in the periosteum were most pronounced at the sites of severe inflammation and at the sites of attachment of the capsule to the bone, suggesting a role of both the inflammatory process and biomechanical forces in the expression of tissue proliferation. These data indicate that arthritis may profoundly influence periarticular tissues. Whether this results in inflammatory, in proliferative or in destructive changes seems to depend on the structure and the location of the involved tissue.