Deep Vein Thrombosis of the Knee
What Is Deep Vein Thrombosis of the Knee?
Joint replacement surgery, especially in the lower extremities, is becoming more common.
It is estimated that Orthopaedic surgeons perform over 720,000 hip replacements and over 681,000 knee replacements a year. The vast majority of these surgeries are very successful, and patients go on to live fuller, more active lives without pain.
But no operation is without risks. One of the major risks facing patients who undergo surgery in the lower extremities is a complication called deep vein thrombosis, a form of venous thromboembolic disease.
What Causes Deep Vein Thrombosis of the Knee?
Deep vein thrombosis (DVT) refers to the formation of a thrombus (blood clot) within a deep vein, commonly in the thigh or calf.
What Are The Contributing Factors of Deep Vein Thrombosis of the Knee?
Although venous thromboembolic disease can develop after any major surgery, people who have surgery on the lower extremities are especially vulnerable.
Three factors contribute to formation of clots in veins:
1. Stasis, or stagnant blood flow through veins
This increases the contact time between blood and vein wall irregularities. It also prevents naturally occurring anticoagulants from mixing in the blood. Prolonged bed rest or immobility promotes stasis.
Coagulation is encouraged by the presence of tissue debris, collagen or fats in the veins. Orthopaedic surgery often releases these materials into the blood system. During hip replacement surgery, reaming and preparing the bone to receive the prosthesis can also release chemical substances that stimulate clot formation into the blood stream.
3. Damage to the vein walls
This can occur during surgery as the physician retracts soft tissues as part of the procedure. This can also break intercellular bridges and release substances that promote blood clotting.
Other factors that may contribute to the formation of thrombi in the veins include:
- Previous history of DVT or PE (Pulmonary Embolus-a blood clot in the lung)
- Metastatic malignancy
- Vein disease (such as varicose veins)
- Estrogen usage or current pregnancy
- Genetic factors
What Are The Symptoms of Deep Vein Thrombosis of the Knee?
After hip surgery, thrombi often form in the veins of the thigh. These clots are more likely to lead to PE. After knee surgery, most thrombi occur in the calf. While less likely to lead to PE, these clots are more difficult to detect.
Fewer than one third of patients with DVT present with the classic signs of calf discomfort, swelling, distended veins, or foot pain.
Diagnosing DVT is difficult. Current diagnostic techniques have both advantages and disadvantages. The most commonly used diagnostic test is a duplex or Doppler ultrasonography.
Venography uses a radiographic material injected into a vein on the top of the foot. The material mixes with blood and flows toward the heart. An X-ray of the leg and pelvis will then show the calf and thigh veins and reveal any blockages.
Although venography is very accurate and can detect blockages in both the thigh and the calf, it is also costly and cannot be repeated often. In addition, the injected material may actually contribute to the creation of thrombi. Therefore it is not often used.
Duplex ultrasonography can also be very accurate in identifying clogged veins. Projected sound waves bounce off structures in the leg and create images that reveal abnormalities. The addition of color Doppler imaging improves accuracy.
This test is noninvasive and painless, requires no radiation, can be repeated regularly, and can reveal other causes for symptoms. It also costs substantially less than venography. However, it is technically demanding and requires a skilled, experienced operator to obtain the most accurate results.
Ultrasonography is less sensitive in detecting thrombi in the calf and it has limited ability to directly image the deep veins of the pelvis.
Magnetic Resonance Imaging
Magnetic resonance imaging is particularly effective in diagnosing DVT in the pelvis, and as effective as venography in diagnosing DVT in the thigh. This technique is being increasingly used because it is noninvasive and allows simultaneous visualization of both legs.
However, an MRI is expensive, not always readily available, and cannot be used if the patient has certain implants, such as a pacemaker. In addition, the patient can experience claustrophobia.
The risk of developing DVT extends for at least three months after joint replacement surgery. The risk is greatest two to five days after surgery; a second peak development period occurs about 10 days after surgery, after most patients have been discharged from the hospital. In general, risk is greatest in the first 30 days after surgery.
Treatment is the same for both asymptomatic and symptomatic venous thromboembolisms. If the clot is located in the femoropoliteal vein of the thigh, treatment consists of bed rest and heparin therapy, followed by six months of warfarin. A clot in the calf veins does not normally require heparin treatment; outpatient warfarin treatment for six to 12 weeks is sufficient. These treatment regimens are designed to prevent the occurrence of a fatal pulmonary embolism and reduce the morbidity associated with DVT.