Deep vein thrombosis occurs in the veins of the legs, thigh, and pelvis. It can lead to a life-threatening condition called pulmonary embolism and is a common problem world¬wide. The incidence of venous thromboembolism in the United States is approximately 600,000 cases annually. Approximately 30% of patients undergoing major surgery develop deep vein thrombosis and some cases may go undetected. High-risk procedures such as implantation of knee or hip prosthesis or other orthopedic surgery on these joints have an incidence of deep vein thrombosis of approximately 50–60% with more dangerous proximal versus below-the-knee distal deep vein thrombosis.
B. Location
Thrombosis of veins that lie deep in the calf or thigh muscle are more dangerous than clots occurring in superficial veins just under the skin, because deep veins are much larger and communicate more directly with the lungs and the heart. A thrombus that is lodged in the femoral or iliac veins may break off and travel into the bloodstream, be carried to the right heart and pumped into the lungs. This traveling clot is called an embolus, thus the term pulmonary embolism. Because pulmonary embolism occurs in patients with thrombosis of the femoral and iliac veins, this condition is considered life-threatening. Thrombi in veins below the knee close to the ankle may not extend above the knee and this type of clot rarely embolizes. The incidence of postphlebitic syndrome, however, in these so-called benign clots is approximately 35%. Postphlebitic syndrome causes swelling of the legs and aching that may persist for years. It is important to make the diagnosis of thrombi above the knee. Although thrombi occurring in veins between the knee and the feet are not as dangerous as those above the knee, they do cause bothersome swelling and pain. Unfortunately the diagnosis of below-the-knee thrombosis is often missed by ultra-sonography.