Hip replacement surgery. Close-up of the opened leg of a patient during hip replacement surgery. The prosthetic joint can be seen at centre. The metal ball fits into a plastic cup, allowing the same range of movement as the natural joint. The ball is attached to a peg which fits inside the thigh bone (femur). The cup fits in a cavity made in the pelvis. Hip replacements are performed on people with osteoarthritis or rheumatoid arthritis or due to injury. The prosthesis gives the patient renewed independence and mobility.