Thrombus mechanization, in the absence of new thrombus occurrence, usually takes about two weeks, and usually not more than three weeks. After a thrombus occurs, the vessel-thrombus junction will gradually form mechanized fibrous tissue, and once this mechanization is formed, it allows the thrombus that has occurred to adhere tightly to the vessel wall so that it cannot be dislodged easily, and the risk of pulmonary embolism is reduced, and the patient can usually get down to the ground and begin activities after two weeks. At this point the D-dimer will generally be close to normal and it will be safer to go down to the ground if the D-dimer can be referenced. In the case of a new thrombus with a high D-dimer, it is relatively unsafe to go down to the ground because the thrombus will continue to mechanize.