The hip joint is composed of the acetabulum and the femoral head and is a multiaxial ball and socket joint. The acetabulum is surrounded by an acetabular glenoid lip made of fibrocartilage to increase the depth of the acetabulum. The acetabular notch is closed by the transverse acetabular ligament, which enlarges the semilunar acetabular joint surface into a ring shape to hug the femoral head, and the joint socket is filled with fatty tissue. The joint capsule of the hip joint is tough and dense, attached upward to the circumference of the acetabulum and the transverse ligament, and downward to the femoral neck, reaching the intertrochanteric line in front and wrapping around the medial 2/3 of the femoral neck in the back, which can perform flexion and extension, extension and extension, internal rotation, external rotation, and circumferential motion in three axes. The head of the femur is hidden in the acetabular fossa, and the joint capsule is relatively tense and restricted by many ligaments.