CHICAGO – Nearly 1 in 20 Americans older than 50 have artificial knees, or more than 4 million people, according to the first national estimate showing how common these replacement joints have become in an aging population.

Doctors know the number of knee replacement operations has surged in the past decade, especially in baby boomers. But until now, there was no good fix on the total number of people living with them.

The estimate is important because it shows that a big segment of the population might need future knee-related care, said Dr. Daniel Berry, president of the American Academy of Orthopedic Surgeons and chairman of orthopedic surgery at the Mayo Clinic in Rochester, Minn.

People with knee replacements sometimes develop knee infections or scar tissue that may require additional treatment. But also, even artificial knees wear out, so as the operations are increasingly done on younger people, many will live long enough to almost certainly need a second or even third knee replacement.

The new estimate comes in an analysis presented Friday at the academy’s annual meeting in San Francisco.

“These data are sobering because we didn’t know what an army of people we’ve created over the last decade,” said Elena Losina, lead author of the analysis and co-director of the Orthopedics and Arthritis Center for Outcomes Research at Harvard’s Brigham and Women’s Hospital.

Replacement joints can greatly improve quality of life for people with worn-out knees, but they’re not risk-free and it’s a major operation that people should not take lightly, she said.

Modern knee replacements in the United States date back to the 1970s. Since then, advances in materials and techniques, including imaging scans to create better-fitting joints, have made the implants more durable and lifelike, surgeons say.

Losina and colleagues came up with their estimate by analyzing national data on the number of knee replacements done from 1998-2009, U.S. census data, death statistics and national health surveys.

In 2009, more than 600,000 knee replacement operations were done nationwide. The study estimate includes people who had knee replacement operations that year and in previous years who are still living.

Overall, 4.5 million Americans are living with artificial knees. That includes an estimated 500,000 who have had at least two replacement operations on the same knee.

Knee replacements are most common in people older than 80 — 1 in 10 people in this age range have them, the study found. Though they’re less prevalent in people younger than that, there are still more than 500,000 Americans in their 50s with the artificial joints.