Hip and knee replacement surgery is somewhat unique among medical innovations—it worked from the beginning.The first total joint replacement (also known as arthroplasty) was developed by John Charnley, a doctor and researcher in England. Charnley’s replacement hip consisted of a metal stem with a plastic cap, affixed to the person’s real bones with cement.
"This device was immediately, highly successful," said Dr. Thomas Miller, an orthopedic surgeon in Bemidji, Minn. "By that I mean it relieved pain and allowed people to get back to their daily routines." Fifteen years later, 80-90 percent of those joints still functioned, a standard which has become a benchmark for total joint success. Knee replacements were not initially effective, but now their success rates have almost caught up to artificial hips.
Arthroplasty’s high success rate has tremendous consequences for millions of Americans. Physicians perform about 266,000 total knee replacements and 160,000 total hip replacements every year, most of them on Medicare patients over 65. Total knee replacements save approximately $50,000 per patient in lifetime health care costs, because patients with new joints generally need less custodial care. That’s a total savings of more than $13 billion. New developments could help patients even more, and save another one billion dollars a year.
Despite Dr. Charnley’s high success rates, there were some problems with his design. Most importantly, Charnley did not use anyone under 50 or over 200 pounds in his sample. When patients were highly active or weighed over 200 pounds (whether or not they are overweight), the artificial joint did not hold up.
In the last 30 years, doctors have been working to overcome that difficulty and others that emerged as Charnley’s joints aged inside their hosts.
For example, researchers have tried to improve the cement that affixed the joint to the bone, since the glue would sometimes come loose and cause pain. They discovered a technique that makes cement obsolete: bone can attach itself to a rough metal surface by growing into it, much like bone heals after a fracture. That technique now appears superior to glue, and Dr. Miller said he uses it on all but his most elderly patients.
"We can’t leave well enough alone so we’re always trying to improve on things," Dr. Miller said. Improvements in the plastic, metal and cement have increased the success rates for highly active people and those who weigh over 200 pounds; now they are almost identical to those for smaller, elderly people.
Another issue in hip replacement is "stress shielding." In many patients, the upper femur (thigh bone) becomes weak because the replacement joint shields it from stress. "We really as of today have not come up with a design that clearly achieves that goal and still produces good overall outcomes," Dr. Miller said.
Unlike hip replacement, the first knee replacements had "astronomical" failure rates, Dr. Miller said. The knee rotates in more directions than the hip, and the artificial joint needs to be more complicated. Since the first knee replacements in the 70s, the joint has been redesigned to mimic the real joint, and now success rates are similar to those for hips.
The future holds improvements in plastic for knee replacements, and hopefully solutions to stress shielding for hips. Physicians may eventually perform surgery by looking at a computer screen, instead of directly at the joint, somewhat like laporoscopy or endoscopy. But. Dr. Miller warns against getting too excited about new developments in their early stages. "It’s foolish to jump on the bandwagon of somebody’s great idea before it’s been proven," he said. Because joint replacements are so successful already, most of them lasting 15-20 years, it takes that long to see if an advancement really has the power to improve success rates.
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