San Diego ? 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.

A huge industry says they don’t have to. TV ads show people water skiing with new hips. Ads tout “the athletic knee,” “the custom knee,” “the male knee,” “the female knee.” Tennis great Billie Jean King, 67, is promoting the “30-year” Smith & Nephew knees she got last year.

“I wanted to make sure whatever they put in me was going to last,” she said. “I’m not trying to win Wimbledon anymore. I’m trying to get my exercise in,” play a little tennis on the clay courts in Central Park, and walk to a movie or a restaurant. “If I’d known what I know now, I would have had it 10 years ago.”

Joint replacements have enabled millions of people like King to lead better lives, and surgeons are increasingly comfortable offering them to younger people.

But here’s the rub: No one really knows how well these implants will perform in the active baby boomers getting them now. Most studies were done in older folks whose expectations were to be able to go watch a grandchild’s soccer game — not play the sport themselves, as one researcher put it.

Even the studies presented at a recent orthopedics conference that found knee replacements are lasting 20 years come with the caveat that this is in older people who were not stressing their new joints by running marathons, skiing or playing tennis.

Besides the usual risks of surgery — infection, blood clots, anesthesia problems — replacing joints in younger people increases the odds they’ll need future operations when these wear out, specialists say.

“We think very carefully about patients under 50” and talk many of them out of replacing joints, said Dr. William Robb, orthopedics chief at NorthShore University HealthSystem in suburban Chicago.

But many don’t want to wait, even if they’re not much beyond that:

• Karen Guffey, a 55-year-old retired civilian police worker in San Diego, plans to have a hip replaced in September. “I can’t exercise the way I want to. I have to go slow, which is really aggravating. I want to go full force,” she said. “I’m not worried about how I’m going to feel when I’m 75. I want to feel good now.”

• Karen Cornwall, a Havertown, Pa., nurse who played a slew of sports since childhood, had both knees replaced last year when she was 54. “I just felt like I was too young and too active to be in pain all the time,” she explained.

• Bill McMullen, a former Marine and construction worker from suburban Philadelphia, had seven knee repair surgeries before finally getting a knee replacement at age 55 a decade ago. He took up weightlifting to spare his knees but damaged a shoulder and had it replaced two years ago. “People ask me if I’m happy and I say, ‘If you have pain, go and get it done,”‘ he said of joint replacement. “It was the best thing for me. I have no pain.”

People are urged to exercise because it’s so important for health, but there are “too many wannabes” who overdo it by trying to imitate elite athletes, said Dr. Norman Schachar, a surgeon and assistant dean at the University of Calgary in Alberta, Canada.

“They think if they’ve got a sore knee they’re entitled to having it replaced,” he said. “I think surgeons are overdoing it too, to try to meet that expectation.”