The topmost illustration shows the cross section of a normal knee joint: a crescent-shaped disk held in place by ligaments (the meniscus) reduces friction during joint movement, while the membrane surrounding movable joints (the synovium) secretes a lubricating fluid.
Below that, the two illustrations on the left show the effect of rheumatoid arthritis, in which the immune system attacks the body's own tissues: the synovium becomes inflamed and thickened, and inflammation may later spread to the cartilage and bone.
In the two illustrations on the right, the effects of osteoarthritis are shown: the cartilage structure begins losing its elasticity and the synovium becomes inflamed. Inflammatory proteins and enzymes damage the cartilage further. As cartilage breaks down, small pieces break off to form loose bodies, explosing the underlying bone. The joint may then become enlarged and distorted.