Osteoarthritis. Coloured Xray of the knee of a patient with osteoarthritis, seen from the side. This disease causes wasting of the cartilage which reduces friction between moving bones. The bones (mainly white) forming the knee joint are the femur and tibia (upper and lower frame respectively). The narrow fibula (lower left) and the patella (kneecap, purple, centre right) are also seen. The loss of the lubricating joint cartilage is seen as a narrowing of the space in the joint. This causes pain and stiffness. Painkilling drugs can treat the pain, and weight loss reduces pressure on the joints. In serious cases the knee joints may be replaced surgically.