CHICAGO (September 25, 2012)—A study of more than 3 million Medicare patients who got artificial knees over two decades shows that the number of initial knee-replacements done each year more than doubled during that period.

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 appeared in Wednesday's Journal of the American Medical Association.