The hip joint is one of the body's largest weight-bearing joints and is the point where the thigh bone (femur) and the pelvis (acetabulum) join. It is a ball and socket joint in which the head of the femur is the ball and the pelvic acetabulum forms the socket. The joint surface is covered by a smooth articular cartilage that cushions and enables smooth movements of the joint. Many injuries and diseases to the hip can cause pain and disability and may require hip replacement surgery. Hip replacement can involve a partial replacement where only those parts of the bone that are damaged are replaced with a prosthesis, or total replacement where the articulating regions of both the femur and acetabulum are replaced with prostheses.