Grandpa's Got a Brand New Hip. And Elbow. And . . .

Twenty-four years ago, Arnold Schwarzenegger starred in "The Terminator," a movie about a cyborg -- part man, part machine -- sent back in time from the year 2029. He was young and buff, and the movie became his shtick. Campaigning for George H.W. Bush in the 1988 presidential campaign four years later, he joked, "When it comes to the American future, Michael Dukakis will be the real Terminator."

Today, Schwarzenegger is 61, and the joke's on him. The cyborgs have arrived, and he's one of them. He has had a hip and two heart valves replaced, plus a femur repaired with screws, cables and a metal plate. "This is what happens when you are the Terminator," he quipped. "They switch body parts."

In real life, cyborgs aren't studs from the future. They're old folks. As we age, our parts wear out. That used to mean immobility or death. Today, we can replace them. The elder Bush had a hip switched out at 76, went skydiving on his 80th birthday and got the other hip switched out at 82. Barbara Bush has an artificial hip; Cindy McCain has an artificial knee.

Millions of Americans are walking around with artificial body parts. According to a U.S. government report, the annual tally of hip replacements increased from 290,000 in 1997 to 383,000 in 2005. Knee replacements increased from 328,000 to 555,000. Throw in another 50,000 to 55,000 shoulder replacements and other "arthroplasties," and you're looking at 1 million surgeries per year. "The demand for these procedures is projected to double in the next two decades," says the report. The average artificial knee recipient is 66; the average artificial hip recipient is 70.

And that's just joints. About 2 million people worldwide, including Vice President Cheney, have implants that monitor and regulate their heartbeats. Around 100,000 have cochlear implants, which restore hearing. Others have artificial hearts, heart valves, elbows or spinal disks. And, of course, teeth.

This is only the beginning. Artificial hands, eyes, lungs, pancreases and "bio-artificial" kidneys are well into development. The baby boom has aged into a bionics boom. The current global market for implantable defibrillators is around $6 billion. Collectively, three corporations that specialize in implantable medical devices -- Medtronic, Stryker and Zimmer -- already match the combined market value of General Motors, Ford, Sears, Dell and Sony.

The industry is booming because it combines two things. One is the innovation of high-tech engineering. The other is the world's most powerful desire: to live longer and more comfortably. The market for replacement body parts is like the market for car or computer parts, except that your body is paramount, and you can't get a new one.

The merger of medicine with engineering is wonderful. But it also brings new problems. The first is that our technological reach exceeds our economic grasp. Depending on how you count, hip and knee replacements cost $15,000 to $30,000 or more. Prosthetic limbs can run $3,000 to $50,000. Cochlear implants cost about $40,000; implantable heart defibrillators cost $25,000; implantable artificial hearts can go for $250,000. Medicare covers 60 percent of hip and knee replacements and 80 percent of prosthetic limbs. Can we afford all these devices? If not, are we consigning some people to disability or death?

Second, the devices don't always work. People with ceramic hips have complained of squeaking. Defibrillators have even been recalled due to bad wiring. You can fix a squeaky bike, but what about a squeaky hip? You can take a bum cellphone back to the store, but what about a heart implant? What do you do when the product you want to return is part of your body?

Even when implants work, they wear out, the way most household devices do. Hip replacements often have to be repeated. Heart pumps can give out. Last year, Cheney had surgery to switch out the power components in his defibrillator because, as a press aide dryly explained, "The device's battery has reached the level where elective replacement of the device is recommended."

Bionic parts also raise privacy questions. Wireless devices allow your doctor to monitor your health without so much as a phone call. Will they give your insurer the same access? And what about complete strangers? Earlier this year, researchers demonstrated that implanted defibrillators could be hacked.

Then there's the sticky question of turning off the machine. A life-support apparatus at your bedside is distinct from you. But what about an implant? If you want to die, and your defibrillator keeps shocking you back to life, can your spouse or doctor turn it off? Who has the right to terminate a Terminator?

Twelve years after "The Terminator" came out, another actor, Al Franken, offered a modest proposal: "Why not shoot the elderly into space?" That way, we could explore without worry. "I'm not saying we don't try to get them back," he joked. "We just don't make such a big deal about it." Today, another 12 years later, Franken, like Schwarzenegger, is an aging politician. And here's the punch line: We're not shooting the elderly into space. We're shooting them into time. We're testing cyborg gadgetry on them because they're the ones whose parts have worn out. If it works, they're exploring the future. And if it doesn't, no big deal.

human@slate.com

William Saletan is the national correspondent for Slate, the online magazine at www.slate.com.