Rheumatoid arthritis is a multifaceted immunologic disease that is characterized by chronic symmetric polyarthritis. It primarily affects the synovium causing potentially severe secondary changes in the periarticular soft tissues, articular cartilage and subchondral bone.
The hallmark of rheumatoid arthritis involving the joint is pannus formation. The pannus (Latin: piece of cloth) is a layer of synovium and synovial stroma accompanied by inflammatory cells, granulation tissue and fibroblasts. The pannus grows over the cartilage and interferes with the diffusion of nutrients from the synovial fluid into the articular cartilage.
The inflamed synovium can also destroy periarticular structures such as joint capsules, tendons and ligaments -- or erode into bone at the synovium insertion site. When the bone is eroded, the marrow is replaced by fibrous tissue that may contain chronic inflammatory cells and thereby simulate an infectious osteomyelitis.
In active disease the fibrinous exudate may be rich in neutrophils, however, they usually do not infiltrate the synovium proper. When this does occur it may be difficult to exclude infection. In treated RA, the inflammatory infiltrate may be sparse and consist mainly of histiocytes.
About 1% of the population is afflicted by RA. Women are three times more likely to be affected (Kumar). The disease has an insidious course. Initially, the symptoms are nonspecific e.g. malaise, fatigue and myalgias. Small joints of the hands and feet are affected before larger ones (elbows, knees) and become swollen, warm and stiff. Eventually, destruction of joints, tendons and ligaments lead to deformities e.g. ulnar deviation.
20% of patients have cutaneous rheumatoid nodules, which tend to arise in areas of skin subject to pressure (e.g. elbow, sacrum). However, these nodules can also occur on organs such as the heart, lungs, spleen and liver (Rosai).
Kumar V, Abbas AK, Fausto N. Robbins and Cotran Pathologic Basis of Disease. 7th Ed. Philadelphia, PA: Elsevier; 2005: 1306.
Rosai, J. Rosai and Ackerman's Surgical Pathology. 9th Ed. Philadelphia, PA: Elsevier; 2004:2202-3.