The hip joint is a ball and socket synovial joint between the head of the femur and the acetabulum of the hip bone. The head of the femur is somewhat larger than half a sphere. The acetabulum is deepened by a ring of flbrocartilage (the acetabular labrum) which is attached to its edge and bridges the notch on the lower part of the acetabulum. The labrum is triangular in section with the base attached to the bone. The diameter of the free edge of the labrum is less than that of the acetabulum and extends beyond the equator of the head so that it grips the head to some extent. The articular surface of the femoral head has a depression in it to which the ligament of the head of the femur is attached. The other end of this ligament is attached to the edge of the notch. The articular area of the acetabulum is shaped like a horseshoe with the opening of the shoe inferiorly. The concavity of the shoe is non-articular and roughened and contains a fibrofatty pad.