Hip arthritis is deterioration of the cartilage of the hip joint. The hip is a ball-and-socket joint with the ball at the top of the thighbone (the femoral head). The ball is separated from the socket (the acetabulum) by cartilage. The cartilage acts as a slippery coating between the ball and the socket that allows the ball to glide and rotate smoothly when the leg moves. The labrum, a strong cartilage that lines the outer rim of the socket, provides stability.