Now, a new analysis puts numbers on the risk.
About 1 in 100 patients who undergo knee replacement surgery and 1 in 200 patients who have hip replacement surgery will develop a blood clot called venous thromboembolism before leaving the hospital.
"Actual rates in hospitals might indeed be higher," says researcher Jean-Marie Januel, PhD, MPH, RN, senior researcher at the Institute of Social and Preventive Medicine at the University Hospital of Lausanne in Switzerland. The new figures were estimates after Januel and his team evaluated the results of 47 published studies on blood clot risks after joint replacement surgery.
"Actual rates in hospitals should be monitored and made available to patients," Januel tells WebMD.
The new report is published in the Journal of the American Medical Association.
The new estimates may not reflect the true risk of blood clots, according to two doctors familiar with the new analysis. That is because the blood clot risk persists beyond the study period, they tell WebMD.
Orthopaedic Surgery and Blood Clot Risk
Knee replacement surgery and hip replacement surgery are done to repair arthritic or injured joints. The number of these surgeries has increased in recent years. The surgeries are also being done on younger patients.
About 230,000 hip replacement surgeries were done in 2007, according to the CDC. More than 600,000 knee replacement surgeries were done in 2009, according to the Agency for Healthcare Research and Quality. Blood clots after the surgeries can occur in the deep veins of the legs or elsewhere. They often occur without symptoms. Some of these clots can travel to the lungs.
Doctors routinely prescribe anti-clotting drugs for the patients to help prevent blood clots from forming after the surgical procedure. However, they cannot prevent all clots.
Risk of Blood Clots After Hip, Knee Surgery
Januel's team researched the medical literature to find studies that assessed the risk of blood clots after knee replacement or hip replacement surgery. They focused on studies that looked at clots that occurred before leaving the hospital.
They evaluated the results of 47 studies. Most were clinical trials. Of the 47, 21 studies included patients having hip replacement, 20 included those having knee replacement, and six included both. The studies included nearly 45,000 patients.
One unexpected finding: the risk of deep vein clots was higher after knee replacement than hip, the researchers say. Other studies have found that the clots are more common after hip surgery when the period after the hospital stay is included.
Risk of Blood Clots ‘Extends Beyond the Hospital Stay’
The estimates may not be accurate for a number of reasons, says John A. Heit, MD, professor of medicine and a cardiologist at Mayo Clinic in Rochester, Minn. He wrote an editorial to accompany the study.
"They are missing a lot of people who have clots," he says.
Their estimates may be on target for just the hospitalization period, he says. However, because the risk for clots persists much longer, depending on the surgery, the estimates of overall clot risk may be off, he says.
Citing several published studies, Heit says the risk period for clots in the deep veins, for instance, can be up to 12 weeks after hip replacement and up to six weeks after knee surgery.
These long-term risks are the most important for patients to know about, he says.
About 80% of the patients in the studies analyzed were in clinical trials, he says, noting that those patients may differ in important ways from other patients.
"The real risk extends beyond the hospital stay," says Bert Thomas, MD, attending surgeon and chief of the joint replacement service at Santa Monica-UCLA Medical Center & Orthopaedic Hospital. He is also a professor and division chief of the joint replacement service at the University of California Los Angeles David Geffen School of Medicine.
"So we recommend preventive anti-clotting medicine continue for six weeks," Thomas tells WebMD. "Some doctors think it should go longer."
However, he says, the anti-clotting medicines themselves carry risks, such as bleeding.
Risk of Blood Clots: What Patients Can Do
Patients can take steps to reduce the blood clot risk, Heit and Thomas say. Among the ways:
- Stop smoking. "Smoking boosts the risk of getting blood clots," Thomas says.
- Lead an active lifestyle as much as possible.
- Lose weight before surgery if necessary.
Thomas reports consulting work for Smith & Nephew, which makes orthopaedic products, and Consensus Orthopedics. Heit reports serving on advisory boards for Daiichi Sankyo, GTC, and Ortho-McNeil-Janssen.