Anatomy
The hip joint is a ball and socket joint: head and socket fit together precisely. The hip socket is part of the pelvis. The hip head is the ball of the joint and is part of the femur. Both the socket ('acetabulum') and head are covered with cartilage that acts as the joint's sliding layer. In combination with the joint (lubricating) fluid, this ensures that the hip joint moves almost without friction. At the edge of the hip socket is a cartilage ring ("labrum"), which increases the bearing surface of the hip socket but mainly functions as a sealing ring to keep the synovial fluid in the joint. Around the hip joint is a very strong joint capsule. This ensures that the head remains in the socket. On the inside of the capsule is a thin layer of mucous membrane that produces the synovial fluid and lubricates the joint. Surrounding the hip joint are many muscles.