Osteoarthritis of the knee. Side view X-ray of the knee of a 70 year old man showing signs of osteoarthritis. The spaces between the bones are far smaller than is normal, and the bones rub together as the joint moves, causing stiffness and pain. Osteoarthritis is a common disorder in the elderly, caused by wear and tear thinning the cartilage lining of the joints, exposing the bone beneath. The front of the knee, capped by the patella (kneecap), is at right. The femur (thigh bone) is at top, with the tibia (shin bone) at bottom. The thinner lower leg bone to the left of the tibia is the fibula. This joint is seen from the front in image M110/610. For images of the joint after a knee replacement, see M600/292 (front view) and M600/293 (side view).