The hip joint is one of the body's most reliable structures, providing movement and support without pain or problems in most people for a lifetime. The hip's simple ball-and-socket arrangement — with the ball-shaped head of the thigh bone rotating inside a cup-shaped socket called the acetabulum — usually works well with amazingly little friction and little or no wear.
The well-fitting surfaces of the head of the thigh bone and the acetabulum, which face each other, are lined with a layer of cartilage, lubricated by a thin film of fluid. Friction inside a normal hip is less than one-10th that of ice gliding on ice.