We're becoming a nation of bum knees, worn-out hips and sore shoulders, and it's not just the Medicare set. Baby boomer bones and joints also are taking a pounding, spawning a boom in operations to fix them.

Knee replacement surgeries have doubled over the last decade and more than tripled in the 45-to-64 age group, new research shows. Hips are trending that way, too.

And here's a surprise: It's not all due to obesity. Ironically, trying to stay fit and avoid extra pounds is taking a toll on a generation that expects bad joints can be swapped out like old tires on a car.

"Boomeritis" or "fix-me-itis" is what Dr. Nicholas DiNubile, a suburban Philadelphia surgeon, calls it.

"It's this mindset of 'fix me at any cost, turn back the clock,'" said DiNubile, an adviser to several pro athletic groups and a spokesman for the American Academy of Orthopaedic Surgeons. "The boomers are the first generation trying to stay active in droves on an aging frame" and are less willing to use a cane or put up with pain or stiffness as their grandparents did, he said.