The mechanism of collapse after femoral head necrosis is very complex, but it can be simply considered as the result of the joint action of necrosis repair and force. After the necrosis of femoral head, the repair reaction is bound to occur in the living bone area around the dead bone, i.e., a large number of capillary proliferation, various cells needed for repair are active, and the dead bone is resorbed, etc. With the continuous repair, lamellar granulation tissue will be formed around the dead bone, leading to the decline of the mechanical properties of the femoral head, and when the mechanical properties decline enough, or the femoral head is overstressed, the femoral head may collapse. If the necrosis is completely repaired, a completely normal joint can be obtained and artificial joint replacement can be avoided. After collapse occurs, the lesion is no longer confined to the femoral head, but affects the cephalo-malleolar relationship and articular cartilage, and then the nature of the disease changes qualitatively, affecting not only the pain, but also the function of the hip joint and the repair of the necrosis. The goals of treatment are also more complex and varied, involving not only the repair of necrosis, but also the reconstruction of the cephalo-malleolar relationship and the repair of articular cartilage. In the case of a collapsed femoral head, even though the necrosis is repaired, the anatomical shape of the femoral head cannot be regained, and therefore the development of osteoarthritis is inevitable over time. Therefore, it can be said that collapse is the most important factor affecting the prognosis of femoral head necrosis and suggesting that treatment to preserve the joint has become very difficult.