Rheumatoid arthritis of the knee joint, artwork. This is a condition in which the body's immune system attacks the tissues of the joints. The plates of cartilage (menisci, purple) on the knee joint are partly worn away. The menisci allow the bones of the knee to slide over one another smoothly. Erosion of the menisci causes severe joint pain and immobility. The bones of the knee are the femur (thigh bone, upper right), the tibia (shin bone, lower left) and the patella (kneecap, on tibia at far left, peeled back here to reveal the inner structures). Below the tibia is the smaller fibula. In the background is a conceptual image of an antirheumatic drug molecule.