2.1. Rheumatoid arthritis (RA)

Rheumatoid arthritis is a heterogeneous disease with a variable course ranging from benign non-deforming oligoarthritis to rapidly progressing destructive polyarthritis. In most western countries, the prevalence of RA is about 0.5-1.0% of the adult population, being 0.8% in Northern Finland (Hakala et al. 1993a) if the ARA 1987 criteria are applied (Arnett et al. 1988). In Finland, Kaipiainen-Seppänen (1996) reported the annual incidence of RA to be 39/100 000 of the adult population, while the corresponding figures for ankylosing spondylitis, psoriatic arthritis and juvenile RA were 7/100 000, 6/100 000 and 14/100 000, respectively.

The major abnormalities caused by RA appear in synovial articulations symmetrically on both sides of the body. The initial changes consist of synovial proliferation, fusiform soft tissue swelling, and periarticular osteoporosis. Somewhat later, the inflamed synovial tissue or pannus extends across the cartilaginous surface, leading to chondral erosions. Small osseous erosions then appear at the margins of the joints, and finally, diffuse loss of joint space develops. At more advanced stages, large marginal and central erosions and cysts appear. In advanced RA, fibrous ankylosis of the joint may be seen. The synovium of bursae and tendon sheaths is also affected (Resnick & Niwayama 1995). The proliferative and heterogeneous mass of rheumatoid synovium has a capacity to erode bone and destroy cartilage and tendons similarly to localized malignancies (Harris 1997).