Illustration of a prosthetic hip joint. Here, the hip bone (upper left) and femur of the leg (lower right) articulate on an artificial metal ball. A metal shaft on the ball is sunk into the shaft of the femur bone to anchor it. In this way, the weakened leg bone rides on a strengthened metal joint. This type of surgery is most common in older people whose joints are stiff and painful as a result of osteoarthritis. It may also be needed if rheumatoid arthritis has spread to the hip joint, making walking difficult, or if the top end of the femur is badly fractured. Hip replacement is usually successful. However, the cement securing metal to bone sometimes loosens.