Osteoarthritis. Artwork of a human knee joint with osteoarthritis. Osteoarthritis is a common joint disease in the elderly aggravated by mechanical stress. At upper right (oval) is the knee-cap (patella). Entering the frame from top left is the head of the femur (thigh bone). This meets the shin bone (tibia, lower right); the fibula bone is at lower left. Where the bones of the thigh and shin meet the cartilage is rough and flaky with broken pieces visible (small white). This degeneration of the cartilage in osteoarthritis at the joint surface causes much stiffness and pain represented by the red area at lower centre.