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System May Better Detect Acute DVT

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System May Better Detect Acute DVT

January 04, 2001

By Tom Keppeler, Knee1 Staff

Receipt of a total knee replacement carries with it many risks: a loss of mobility, chance that the prosthetic will not last as long as promised, and other serious complications. However, for some, the surgery may have dire consequences due to a rare condition known as deep vein thrombosis, or DVT.

In DVT, blood in a patient's vein forms a clot, often in the calf or near the knee, slowing or stopping bloodflow past it. The condition is painful, and may deprive areas of the body, especially in the leg, of blood. However, the biggest risk with a thrombus (a singular clot) is that it may dislodge from the wall of the vein and travel throughout the bloodstream. This traveling clot, known as an embolus, may lodge itself in an organ (most often the lung, where it is known as a pulmonary embolism), and eventually destroy it by cutting off its blood supply.

Knee replacement recipients are at particularly high risk for DVT. For starters, surgical instruments and procedures can cause mechanical injury to the vein. In addition, some prosthetic knees are affixed within the leg with cement, which may disturb temperature within the joint. After surgery, knee replacement recipients remain off their feet and use their muscles little, which slows blood flow, increasing the risk of a clot forming. Other pre-existing risk factors include varicose veins, pregnancy, obesity, immobility, history of thromboembolism, and deficiency of certain "blood thinning" substances in the body. Low-risk patients—patients under 40 who have no risk factors and quick surgeries—run a 2 percent risk of developing DVT. High-risk patients, however, who are over 60 or who present risk factors for developing the condition, are at as much as a 10 percent risk of developing it, according to an article in Biomechanics Magazine.

To combat the risk of DVT, doctors prescribe prophylactic medications to many patients, including aspirin or anti-clotting agents such as Warfarin (Coumadin) or heparin. Despite the use of medications in high-risk patients and the seemingly low risk of other patients, however, thombii still occur, plaguing roughly 5 million patients every year, and artificial knee and hip recipients comprise the largest risk group, according to Diagnostic Imaging. As a result, early detection and treatment of DVT have received much attention.

Until recently, diagnostics for DVT fell into two categories. First, Doppler ultrasound imaging sends high-frequency sound waves into a patient's leg while a technician or doctor interprets findings on a screen, much like an ultrasound would be used to examine a pregnant woman's uterus. The alternative to Doppler (often used in addition to ultrasound imaging) is known as radiographic contrast venography, in which a dye is injected into the patient's foot and monitored through X-Rays. While both have been used for many years, each test has its shortcomings. While ultrasound is superior in detecting thrombi in the upper leg, it lags in detection of clots in the calf, where about 40 percent of thrombi occur, according to Wheeless' Textbook of Orthopedics. In addition, ultrasound results do not differentiate between fresh, or acute, clots and those that have formed over a longer length of time (chronic). Venography, according to Dr. Robert F. Carretta of the Sutter-Roseville Medical Center in Roseville, California, remains the gold standard for DVT detection, but also has its shortcomings. "[It] is not used routinely because of the time, expense, patient comfort, and safety and technical issues involved," Carretta writes in an article in Diagnostic Imaging.

However, a new technology may create a diagnostic link between the two. Technetium Tc99 Apcitide, which is marketed as "Acutect" by Londonderry, New Hampshire-based Diatide, Inc., is a radiographic pharmaceutical that targets acute clots. By its chemical makeup, it is designed to bind to chemical receptors found on activated red blood cells, or platelets. A doctor can use imaging equipment to look for any collection of the radioactive substance within the body, signalling a possible clot. Acutect both confirms its existence and helps the doctor decide more conclusively whether it is chronic or acute. In addition, Acutect can detect smaller clots than the other two methods and may be used on the calves effectively. Unlike the contrast medium administered in venography, which comes from a needle stuck in a foot vein, Acutect is administered through the arm, a less painful area to receive a shot than the foot. Studies suggest that about 80 percent of the FDA-approved substance leaves a patient's body within 24 hours, giving patients who undergo major knee surgery one less thing to worry about.

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