PATIENTS going for knee and hip surgeries have to watch out for one possible complication: getting a blood clot that is potentially fatal. In North America now, it's mandatory for such patients to take a blood thinner after the surgery, says Dr Louis Kwong, an orthopaedic surgeon from Harbor-UCLA Medical Center in Torrance, California.

"These operations place patients at the very highest risk of blood clots and half of them will go on to develop deep vein thrombosis if they're not on blood thinners," says Dr Kwong, who was in Singapore recently to present his research in the field of thrombosis.

Unfortunately, even minimally invasive surgeries to the knee or hip don't reduce the risk of blood clotting, because the same amount of trauma is done to the bone, he explains.

"From the time we make an incision on the skin, we activate the clotting system in the patient. Then everything we do to the bone adds to the thrombosis risk. The marrow contents in the middle of the bone are potent activators of the clotting system," he says.

Where do blood clots come from? The body has something called the coagulation cascade, he explains, which is the body's mechanism to control bleeding. "Trauma to the tissue activates this system that leads the blood to clot."

Other causes of blood clotting is the well-known Economy Class Syndrome - when people are sedentary for long periods of time. Explaining that situation, Dr Kwong says that as we get older, the valves in the veins that pull blood in the lower extremities don't function as well. "The biggest jump occurs at the age of 40 . . . as a patient ages, the risk goes up," he adds.

But as orthopaedic procedures are getting more commonplace, with the number of hip fractures on the rise as the population ages, operations will give rise to thrombosis. "Seventy-five to 80 per cent of blood clots are silent. You won't see it unless you look for it . . . and usually the first evidence is when a person dies. Even then, without an autopsy, we won't know that it's caused by a blood clot."

Blood clotting after a hip or knee operation is the No 1 most preventable cause of death in a patient, highlights Dr Kwong. And it's very well researched now, which is why it's mandatory for patients to be on blood thinners after a hip or knee operation in the US.

"For operations to other limbs like arms, for example, we don't have enough research on blood-clotting after surgery," he says.

Patients are placed on blood thinners for only 10-35 days following a hip operation, as the body is hyper-coagulable for up to a month. For knee surgeries, patients are recommended to be on blood thinners for 7-10 days. So it's not medication for life, he adds.

Explaining blood thinners, Dr Kwong says that scientists now know that with time, regardless of the type of blood thinner taken, it increases the rate with which a blood clot is resorbed and its size shrinks. "This counters what was taught in medical school for 4-5 decades - that all anti-coagulants do is to prevent a blood clot from getting bigger.

But we know that it actually breaks up the clot now, and there's more data to show it's true." Advancements to blood thinners include Rivaroxaban, known as XareltoR, an oral tablet manufactured by Bayer Schering Pharma, which is easy to use as it doesn't require dose adjustments like heparin and Warfarin, and is very effective.

Blood clots when formed in the deep veins of the body is known as deep vein thrombosis (DVT) and occurs usually in the legs and is manifested by pain, swelling and oedema (fluid retention). When patients complain of these, an ultrasound test is given to look for the blood clot.

"But sometimes the blood clot can break free and go to the lungs. These people will then complain of chest pains or have difficulty breathing. That's called a pulmonary embolism and this has potentially fatal consequences 0.1 per cent of the time," says Dr Kwong. Once a person gets DVT, it also increases a person's risk for future DVTs.

This story was first published in The Business Times on Dec 6, 2008.