CHICAGO (AP) -- Just like baby boomers, older folks have seen a surge in knee replacements.

The findings are from a study of more than 3 million Medicare patients who got artificial knees over two decades. The number of initial knee-replacements done each year more than doubled during that time.

There were nearly 244,000 in 2010. The study authors say the increase is partly driven by a desire to stay active and by obesity, which takes a toll on joints. The aging population and rising numbers of Medicare enrollees also contributed.

The pace of growth slowed in more recent years -- possibly because increasing numbers of younger adults have also been getting artificial knees.

The study appears in Wednesday's Journal of the American Medical Association.