The hip joint is a ball and socket, allowing a wide range of motion. The ball of the joint, the top of the thighbone (femoral head), moves within the hollow socket (acetabulum) of the pelvis. A layer of cartilage allows the ball to glide smoothly inside the socket.
In total hip replacement, the surgeon cuts away the ball part of the joint, replacing it with a ball attached to a stem that is wedged into a hollowed-out space in the thighbone. Damaged cartilage and bone are removed from the socket and a cup-like component is inserted into the socket.
Hip replacements may be cemented or uncemented. If cemented, the hip parts are held in place with a fast-curing "bone cement" made from a type of polymer. If uncemented, the joint components are specially made to either press into the bone for a tight fit (press-fit) or to allow new bone to grow into the porous surface of the implant, holding it in place (biological fixation).
"All You Need to Know About Joint Surgery," © 2002, Arthritis Foundation