It consists of the femoral head opposite the acetabulum and is a pestle joint. The acetabulum is only covered with articular cartilage on the lunar surface, and the acetabular fossa is filled with fat, also known as the Haversian gland, which can be squeezed out or sucked in with the increase or decrease of intra-articular pressure in order to maintain the balance of intra-articular pressure. At the edge of the acetabulum there is an attachment of the glenoid rim. The depth of the articular fossa is deepened. The transverse acetabular ligament is framed across the acetabular notch and encloses a hole with the notch, through which nerves and blood vessels pass. The joint capsule is thick and tough, with the upper end attached to the circumference of the acetabulum and the transverse acetabular ligament, and the lower end attached to the inter-rotor line in front and to the medial aspect of the inter-rotor ridge in the back (about 1 cm from the inter-rotor ridge), so that the back of the femoral neck is partly outside the joint capsule, while the front of the neck is completely enclosed in the capsule. Therefore, when the femoral neck is fractured, there is a distinction between intracapsular, extracapsular or mixed fractures depending on the location of the fracture. The hip joint is reinforced by ligaments, mainly the anterior iliofemoral ligament, which is long and tough, attached above to the lower part of the anterior inferior iliac spine in a herringbone shape and down to the intertrochanteric line of the femur. The iliofemoral ligament limits excessive posterior extension of the thigh and is important for maintaining upright posture. In addition, the lower part of the joint capsule is reinforced by the pubic capsule ligament, which can limit excessive thigh abduction and rotation. The posterior part of the joint capsule is reinforced by the sciatic capsule ligament, which has the effect of limiting thigh rotation. The fibrous layer of the joint capsule is thickened in a circular pattern and surrounds the middle of the femoral neck, which is called the vascular band and can restrain the femoral head from prolapsing outward. The femoral head ligament is a flat fiber bundle in the joint cavity, mainly starting from the transverse acetabular ligament and ending at the femoral head concavity. The ligament is covered with synovial membrane and has blood vessels passing through it. It is generally believed that this ligament does not limit the motion of the hip joint. The hip joint is a multi-axis joint, capable of flexion, extension, extension, rotation and circular motion. However, since the femoral head is deeply embedded in the acetabulum, and the acetabulum has a deepened glenoid rim that encircles nearly 2/3 of the femoral head, the difference in area between the joint head and the joint socket is very small, so the range of motion is small. In addition, the joint capsule is thick and the ligaments that limit the range of motion of the joint are tough and strong, therefore, compared with the shoulder joint, the stability of the joint is large. Flexibility is poor. This structural feature is a reflection of human walking upright and the transmission of gravity through the hip joint. When the hip joint is flexed, internally retracted and internally rotated, the femoral head is mostly detached from the acetabulum to the lower posterior part of the joint capsule, at this time, if external force acts on the knee joint from the front and then passes along the femur to the femoral head, posterior dislocation of the hip joint is likely to occur.