During hip replacement surgery, the diseased head of the femur is removed. The hip socket (acetabulum) is prepared by using special instruments to make it the right size and shape for a new implant. The femoral head is replaced with an artificial ball on a stem that goes down into the hollow part of the thighbone and may be pressed into place or cemented, depending on the surgeon’s preference. A metallic shell with a cup-shaped liner is pressed into the prepared socket to provide a smooth gliding surface. The ball and socket are then placed together to complete the procedure.