People having joint surgery had up to 31 times the risk of a heart attack shortly afterward - but still, no more than one in 200 had heart trouble.

Although it's not clear the hip and knee procedures, themselves, triggered the heart attacks, researchers said it's reasonable to think they would have some role.

"I'm not surprised that compared to someone who had not had surgery, there might be some increased risk," said Dr. William Hozack, an orthopedic surgeon at the Rothman Institute in Philadelphia, who wasn't involved in the new study.

"Surgery is a risk, because surgery and anesthesia that you need to have the surgery increase stress levels for patients," he told Reuters Health.

The findings come from hospital records for 95,000 people who had a hip or knee replaced in Denmark between 1998 and 2007. For each of those patients, the researchers found three people of the same age and gender who weren't having a joint replaced for comparison.

Over the six weeks after their surgeries, one in 200 patients who had a hip replaced and one in 500 who underwent knee replacement had a heart attack.

The researchers calculated that in the two weeks after a hip replacement, patients were close to 26 times more likely to be diagnosed with a heart attack than those who didn't have surgery. The risk was still five times higher in weeks three through six post-surgery.

After a knee replacement, patients had 31 times the chance of heart attack in the first two weeks - but their risk then returned to normal.

Arief Lalmohamed from Utrecht University in the Netherlands and his colleagues found the link between joint surgeries and heart problems was especially high for patients age 80 and older. On the other hand, those younger than 60 weren't at any higher risk of heart problems, according to findings published Monday in the Archives of Internal Medicine.

The results don't prove that getting hip or knee replacement surgery can cause a heart attack. And the researchers couldn't account for everything about patients that might affect both their risk of needing a joint replaced and having a heart attack - such as being overweight or obese, or having high blood pressure.

Hip and knee replacements are typically done to treat moderate or severe osteoarthritis, which is more common in heavy people.

According to the National Institutes of Health, close to 800,000 people in the United States have a hip or knee replaced each year. After 10 or 15 years, patients often need new artificial joints and undergo a second surgery.

Lalmohamed expects the procedures do have some direct impact on heart risk. Cutting into bones could promote clot formation in the bone marrow, especially in hip surgery, his team noted in the report.

Blood loss and oxygen deprivation can also happen during any major surgery, Lalmohamed told Reuters Health.

"These stressors are known to increase the risk of heart attack," he said in an email. "Furthermore, the perioperative period itself is a very stressful time for the patient."

People going in for a hip or knee replacement should discuss their heart history with their doctor, Lalmohamed said, so the doctor can determine whether they'll need extra careful monitoring during and shortly after surgery.

Hozack said that at his clinic, a cardiologist tends to have some role in checking all patients before joint replacement.

"If you're thinking of having hip or knee replacement surgery... it's always wise to have a good evaluation of your cardiac health," he said.

SOURCE: bit.ly/MbBLbb Archives of Internal Medicine, online July 23, 2012.

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