Coloured computed tomography (CT) scan of the pelvis of a 55 year old patient showing osteoarthritis of the right hip (left). The ball at the top of the femur (thigh bone) fits into the socket of the hip. Osteoarthritis results in the loss of cartilage between the joint. The healing process leads to the growth of bone in place of the cartilage, causing pain, stiffness and loss of mobility. Treatment is with the surgical replacement of the joint.