According to the classical theory of thrombosis, the three factors that form thrombosis are slow blood flow, hypercoagulable state and vascular injury, as follows: 1. Vascular injury usually occurs in surgery or trauma, which causes the simultaneous initiation of endogenous and exogenous coagulation, so that thrombosis occurs at the injury site. 2. Hypercoagulable state, which often occurs when the body is suffering from immune diseases, such as rheumatoid or lupus erythematosus, and sometimes in tumor patients. 3. 3.Slow blood flow, which is common in the elderly, inactive patients or postoperative patients, where blood clots occur due to reduced muscle activity and slow blood flow caused by bed rest.