More Americans are getting total knee replacements than ever before, according to the first national estimate of the procedure's frequency. In 2009 alone, the number of surgeries topped 600,000, twice the number of  procedures  of the past decade.

The numbers, presented today at a meeting of the American Academy of Orthopaedic Surgeons, are higher than expected - 4.5 million Americans  currently live with  total knee replacement in at least one knee, which is nearly 5 percent of the population age 50 and older. More than 5 percent of women older than 50 have had the procedure, compared with 4 percent of older men.

Knee replacements become more common with age, when the pain and discomfort of arthritis become  more disabling for many people. According to the study, nearly 10 percent of Americans age 80 and older have had at least one knee replaced. But Dr. Elena Losina, the study's senior author, said the rising numbers of knee replacement surgeries are coming from people in their 50s and 60s.

"The demographics of this procedure are changing. More and more younger people are undergoing the procedure," Losina said.

As baby boomers age and the epidemic of obesity means many of them are heavier, the number of knees that wear out have  grown. But Losina said those factors account for only about a quarter of the number of knee replacement surgeries. Instead, a growing number of knee injuries in middle-aged adults can lead to the early development of arthritis.

"It's likely happening to people who are active and don't want to be bothered by knee pain," she said.

Advances in technology and improvements in surgical techniques have made the surgery more successful in recent years, although patients run the risk of infections and scar tissue development after the surgery. The procedure requires several days in the hospital, weeks of rehabilitation and costs about $40,000.

Currently, artificial knees, made out of metal and plastic, can last up to 20 years. But patients who get knees replaced at younger ages will probably need a revision surgery as they age and their new joint wears out.

"On average, we take 1 [million] to 2 million steps per year. That's a lot of back and forth on that hinge. And patients who are younger and active can put significant force on that," said Dr. Jason Koh, vice chairman of orthopedic surgery at the North Shore University Health System in Chicago. "But in terms of the value that you get from the surgery in terms of improvement in quality of life, this is one of the best ways we can use our health care dollars."

According to the AAOS, 90 percent of people who have a total knee replacement will experience a dramatic reduction in pain and discomfort.

Losina said it's likely that people who have a knee replacement at age 50 will need another one when they reach age 70 or 75.

"But the improvement in quality of life that we see with these procedures may way outweigh the potential for the need for another surgery down the road," she said.

Losina said scientists need to do more research on the long-term outcomes of the procedures.