The hip joint is a ball and socket synovial joint. The socket area (acetabulum) is inside the pelvis. The ball part of this joint is the top of the thighbone (femur). It joins with the acetabulum to form the hip joint which allows motion and is designed for stability and weight-bearing. The hip is one of the most stable joints in the body but because it bears your body weight, it is more likely to develop arthritis because of the extra pressure.