Spend a little time where seniors hang out and there's a good chance you'll hear about somebody getting a new knee — maybe two.

Some figures pulled from Medicare data analyzed in the latest JAMA, the Journal of the American Medical Association, help explain why.

There are about 600,000 knee replacements a year now, at a cost of around $15,000 a piece. All told, the tab for all that orthopedic work is about $9 billion a year, the JAMA study says.

What's going on with the Medicare set?

First off, there are a lot more older people with worn-out knees these days. In 1991, the knee-replacement researchers point out, there were about 30 million people with traditional Medicare coverage. By 2010, the figure was 39 million.

But the artificial knees have gotten better, and the procedures to implant them have improved too. So the annual rate of implants has gone way up, doubling, in fact, to 62 per 10,000 Medicare beneficiaries in 2010 from 31 per 10,000 in 1991.

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