I. Stagnant venous blood flow Stagnant blood flow, firstly white blood cells and then platelets, can collect in the peripheral layer of blood flow; platelets are deposited on the intima, which can form the core of thrombus formation. After the blood flow is slowed down, the cellular components of the blood can be parked in the vessel wall and finally form a thrombus. After lower extremity cast immobilization, lower extremity braking, blood flow is sluggish and there is a high incidence of lower extremity deep vein thrombosis, which is often detected after cast removal. Surgery is closely related to slow blood flow. Braking of the patient during surgery, supine and anesthesia to stretch the peripheral veins, prolonged postoperative bed rest, semi-sitting position, pillow lining under the knee and lateral position can slow down the blood flow in the deep veins of the lower extremities. In addition, the left iliac vein is anatomically spanned by the right iliac artery, and its distal blood return is relatively slow. One important reason why deep vein thrombosis is more likely to occur after surgery is slow blood flow. In the patients seen clinically, the left lower extremity is more common than the right lower extremity; the gastrocnemius sinus and the valve pocket are good sites; about 24% of the external iliac veins have valves, and their proximal side also has a higher incidence, these facts are enough to show that slow blood flow has an important position in thrombosis. Second, the venous wall injury Any enough to cause damage to the venous wall, are conducive to the formation of coagulase and small platelet aggregation, prompting thrombosis, common can be summarized as follows: 1, chemical injury through the superficial vein after the injection of irritating solution, easy to cause strips of thrombotic superficial phlebitis. 2, mechanical injury venous local contusion, lacerations or fracture fragment creation, can induce venous thrombosis. . Repeated puncture of veins or intravenous retention of plastic infusion catheters can often be complicated by thrombophlebitis. Fractures of the femoral neck that damage the common femoral vein and fractures of the pelvis that often damage the common iliac vein and its branches can be complicated by thrombosis. Surgery directly involving veins, such as deep vein valve repair, venous segment grafting or venous diversion, can also be complicated by thrombosis. 3, infectious injury septic thrombophlebitis can be caused by the foci of infection around the vein, typical examples such as septic mastoiditis can be complicated by transverse sinus thrombosis; infectious endometritis can cause septic thrombophlebitis of uterine vein, etc. Third, blood hypercoagulability is one of the basic factors of venous thrombosis because of the change of blood composition and hypercoagulability. There are many factors that cause blood to be in a hypercoagulable state, but the most common and most closely related to thrombosis are major surgeries. In addition to surgery, many factors can increase the degree of blood clotting. In severe dehydration, blood is concentrated and blood cells are relatively increased; platelets increase significantly on days 9-12 after splenectomy. All of these can increase the degree of blood coagulation. Gram-negative bacterial sepsis is often accompanied by endotoxic toxemia, which breeds local and systemic Schwarzkopf and shock, with blood in a hypercoagulable state. Various sites of cancer, contraceptives, are prone to thrombosis. Pregnant women are in a hypercoagulable and anticoagulable state due to an increase in platelets and coagulation factors in the blood, an increase in procoagulant components, and an increase in the inhibition of the fibrinolytic system from the placenta.