FRIDAY, March 12 (HealthDay News) -- Patients who get a total knee replacement are usually advised to avoid high-impact sports to preserve their new body part. But a new study suggests sport participation is not only safe -- it may even help people gain better knee function.

''Initially, we though high-impact sports were terrible for the prosthesis," said Dr. Sebastien Parratte, a research collaborator at the Mayo Clinic in Rochester, Minn., and an assistant professor at the Aix-Marseille University Center for Arthritis Surgery at Hospital Sainte-Marguerite in Marseille, France.

"Our eight-year results have shown it is not the case," he said.

He is the lead author of the study, scheduled for presentation Friday at the annual meeting of the American Academy of Orthopaedic Surgeons in New Orleans.

More than a half million total knee replacements are performed annually in the United States, according to the American Academy of Orthopaedic Surgeons. Parratte and his colleagues conducted the study knowing that patients routinely ignore their doctor's advice to take it easy after receiving a knee replacement. In fact, about one of six patients engage in high-impact activities post-implantation, experts say.

Parratte's team followed 535 patients in all. A total of 218 underwent knee replacement and then performed heavy manual labor or engaged in a non-recommended sport, such as high-impact aerobics, football, soccer, baseball, jogging or power lifting. The control group of 317 patients had knee replacement but did not engage in sports that were not recommended.

The researchers evaluated the patients clinically and with X-rays. About eight years after surgery, they found no significant radiological differences and no significant differences in the implant durability between groups.

In fact, the sport group had slightly higher knee function scores than the control group.

A first comparison found that the control group had a 20 percent higher need to repeat the operation because of mechanical failure of the knee (from wear, fracture or loosening) compared to the sport group. But when they took into account other health problems such as obesity or diabetes, the sport group had a 10 percent higher risk of mechanical failure compared to the control group, but the difference wasn't statistically significant.

''The control group was more likely to have high blood pressure, obesity and diabetes," Parratte added.

He said he doesn't know why the sport group's knees held up better.

Still, the study findings are no reason to tell patients with knee replacements to exercise in a high-impact way, said Dr. Christian Christensen, an orthopedic surgeon and head of adult reconstruction at the Lexington Clinic in Lexington, Ky.

"I think it's a good study and certainly a worthwhile one," he said. "Would it encourage me to tell my patients to play football? No way."

More research is needed to evaluate what's happening, he said. It's possible that the people with the best results may be the ones engaging in high-impact sports. "People with the knees that feel great, who have excellent results,'' are perhaps the ones who can engage in the high-impact sports without ill effect, he said.

Another possibility, he said, is that the follow-up may just not be long enough, that ill effects may show up later. Christensen said he'll continue to tell his knee-replacement patients to avoid high-impact sports. "Implants aren't meant to tolerate high-impact sports," he said.

Dr. Benjamin Bengs, another expert, called the new study findings promising. It shows these devices are long-lasting, can lead to lifelong pain relief and excellent functioning and activity in patients, said Bengs, an orthopedic surgeon at Santa Monica-University of California Medical Center and Orthopaedic Hospital.

But more time and study are needed before we completely release people to all high-impact activities, he said.

''One study is not enough to change the recommendations," Parratte agreed. He plans to study the topic further.

More information

To learn more about total knee replacement, visit the American Academy of Orthopaedic Surgeons.