People over age 60 who have surgery to replace a worn-out hip or knee have a significantly higher risk of having a heart attack after their surgeries, a large new study shows.

The study is published in the Archives of Internal Medicine. It reviewed the medical records of more than 95,000 adults in Denmark who had hip or knee replacement surgeries between 1998 and 2008.

Researchers found that one in 200 people who had a hip replacement had a heart attack within six weeks of surgery. The study also found that one in 500 people who had a knee replacement had a heart attack within six weeks of surgery.

Compared to adults of the same age and sex who didn't have joint replacement surgeries, people who had hip replacements were 25 times more likely to have heart attacks, within two weeks of their procedures. People who had knee replacements were about 30 times more likely to suffer heart attacks in that same time period.

Those risks remained even after researchers adjusted their data to account for a variety of things that can increase the risk of heart attacks, such as:

    age
    sex
    socioeconomic status
    use of certain medications known to increase the risk of heart attacks, such as non-steroidal anti-inflammatory drugs (NSAIDs)

To be sure, surgery is stressful. Previous studies have found that having surgery of any kind raises the risk of having a heart attack. That's especially the case for adults who are over age 60 with a history of heart disease.

Even thinking about surgery has been shown to increase the risk of a heart attack, notes Arthur W. Wallace, MD, PhD. Wallace is an anesthesiologist at the University of California, San Francisco, and the San Francisco Veterans Affairs Medical Center. He wrote an editorial on the study but was not involved in the research.

Slideshow: A Visual Guide to Knee Replacement
Study Details

The researchers checked to see if the increase in heart attacks they noticed in joint replacement patients might also be associated with any kind of elective surgery. They used hernia repairs for comparison. Joint replacement patients still had significantly elevated heart attack risks -- about 21 times higher than adults who had a hernia repairs within the same study period.

Previous studies have shown that joint replacements increase a patient's risk for dangerous blood clots that can lodge in the legs or lungs. For that reason, joint replacement patients are routinely prescribed blood thinners to prevent those kinds of events.

Researcher Arief Lalmohamed, PharmD, a pharmacist at the University of Utrecht, says the risks of heart attacks after surgery haven't been fully appreciated.

He isn't sure why joint replacement may increase the risk of heart attacks. But he thinks it may have something to do with a loss of "hemodynamic balance." That's the disruption of blood flow and blood oxygen during surgery. It can stress the heart.

"In high-risk patients, our study shows we need new strategies to prevent these heart attacks," Lalmohamed says in an email to WebMD.

Reducing the Risk of Heart Attacks After Joint Replacement

Matthew Hepinstall, MD, an orthopaedic surgeon at Lenox Hill Hospital in New York City, says the results of the study aren't really that surprising. He says doctors have long understood that major surgery can raise heart risks for vulnerable patients.

Hepinstall says the real benefit of the study is that it helps doctors recognize patients who may be at increased risk of a heart attack.

"This is elective surgery we're talking about," says Hepinstall, who was not involved in the research. "For the most part, these are people with arthritis choosing to have surgery for a quality-of-life issue."

That's why it's important to make sure that patients are healthy enough to have major surgery in the first place.

"We need to think very seriously about elective surgery in patients who are over age 80, with known heart disease or who have had a heart attack within the past year," Hepinstall says, since those were the patients at highest risk of heart attacks in the study.

Additionally, he says, the study helps to define a window for when joint replacement patients need to be most closely watched. That window is six weeks after surgery in hip replacement patients and up to two weeks after surgery for those who have their knees replaced.