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.