FRIDAY, Feb. 10 (HealthDay News) -- More than 4 million Americans now live with an artificial knee, and increasing numbers of younger patients are undergoing knee replacement surgery, new research reveals.

Researchers at Brigham and Women's Hospital in Boston estimate that more than half of adults who are diagnosed with knee osteoarthritis will receive a total knee replacement in their lifetime.

Senior author Elena Losina, co-director of the hospital's Orthopaedic and Arthritis Center for Outcomes Research, said the country's aging population and high rates of obesity are only partly responsible for the rise in total knee replacements.

"We think that as more and more people began participating in active sports, they sustained injuries earlier in life, and therefore developed knee osteoarthritis earlier," Losina said. "And these active people are probably more willing to undergo surgery that will enable them to continue to be active."

Also, with improved success rates over the past 20 years, "surgeons are more comfortable offering it, and patients are more comfortable having it," Losina added.

A shorter postoperative hospital stay has also made the procedure more acceptable, she said. "Ten or 15 years ago, patients stayed in the hospital for a week," Losina said. "Now, they're usually discharged on the third day after surgery."

The number of new-knee procedures doubled over the last decade, reached more than 620,000 in 2009, and the researchers said younger patients -- those 45 to 64 -- accounted for a disproportionate amount of that growth. Their relatively young ages means many are at risk of revision surgery as well as potential long-term complications of surgery, the authors warned.

The researchers estimate that more than 4.2 million Americans currently have an intact total knee replacement, which represents 4.4 percent of the total population aged 50 and over. Prevalence is slightly higher in women versus men.

They further estimate that nearly 53 percent of men and 52 percent of women diagnosed with symptomatic knee [osteoarthritis] will receive a total knee replacement in their lifetimes. The risk of subsequent revision is nearly 15 percent for men and roughly 18 percent for women, the authors wrote.

In osteoarthritis of the knee, the cartilage wears down, causing changes in the adjacent bone, resulting in pain, swelling and stiffness.

The study is scheduled for presentation Friday at an American Academy of Orthopaedic Surgeons' meeting in San Francisco. The researchers' data included information from the U.S. Census, two national studies on people with knee arthritis, and a computer model on the history and management of knee arthritis.

One leading orthopedic surgeon, Dr. William J. Robb III, chairman of the department of orthopaedic surgery at NorthShore University Health System in Evanston, Ill., said the findings were useful in that they provided more details about the number of patients now living with artificial knees. But he questioned the estimate of patients who might require revision surgery.

"The study used historical failure and complication rates to predict future numbers of failures that might require revision surgery," Robb said. "The methodology used likely projects the 'worst-case scenario' for total numbers of failures, as implant materials have been improved and those improvements may decrease the overall revision rate."

The study was funded by the U.S. National Institutes of Health's National Institute of Arthritis and Musculoskeletal and Skin Diseases.

Data and conclusions presented at medical meetings should be viewed as preliminary until published in a peer-reviewed medical journal.

More information

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