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Artificial Hip a Story of Setbacks, Triumph

Pioneer had to overcome man-made side effects in early days

MONDAY, Jan. 3, 2005 (HealthDayNews) -- Medicine is the story of countless triumphs and countless disappointments, followed eventually by more triumphs.

Rarely, though, is one man so inextricably linked with these ups and downs to emerge victorious over one mysterious, man-made disease -- one that he helped beget in a groundbreaking effort to heal another ailment.

The tales of modern-day hip replacements, one of the major medical breakthroughs of our age, and Dr. William Harris, an orthopaedic surgeon at Massachusetts General Hospital, are so intertwined that they are figuratively joined at the hip.

"He's had a huge impact on the field of joint replacement," said Dr. Peter Slavin, president of Mass General. "There are lots of great clinicians in the world, but it's rare to have one of those physicians also be so inquisitive and looking for ways to improve the procedures that he's performing every day. That's indeed what Bill did."

"He first described the biology of this process, then looked at what was causing it, and then attacked the material to say, 'How can we make material better to lessen this problem?'" added Dr. Harry E. Rubash, chief of orthopaedics at Mass General, who did his fellowship under Harris. "He's made a remarkable contribution in developing the new low-wear plastics [as joint prostheses]."

Hip replacements entered the world in the 1960s, with Harris as one of the practitioners. "It was an absolutely dramatic change in what we could do in the treatment of severe end-stage osteoarthritis of the hip," attested Harris, who was one of the pioneers in the field. Harris detailed his accomplishments in an article in the December issue of Clinical Orthopedics and Related Research.

The initial delight would eventually give way to disappointment. Within a few years, people who had undergone hip replacements were beginning to suffer a mysterious erosion of the bone next to the implant, a disease process called periprosthetic osteolysis. "This would prove to be the number one long-term problem associated with total hip replacement," Harris said. Periprosthetic osteolysis was a prelude to the eventual failure of the implant.

No one knew why this was happening, but several theories began to circulate. One leading hypothesis held that the cement used as grouting material was responsible. But when scientists found a way to fix the implant to the skeleton without the cement, the same thing happened.

"It was absolute anguish when I saw the first problem in a cement-less device," Harris recalled. "The whole industry had gone cement-less and you say, 'What do I do now?' And then gradually you're able to peel the layers off the onion and unravel the whole thing."

Because Harris had feet in the worlds both of practice and research, he was key in this unraveling process.

When reoperating on people with periprosthetic osteolysis, he had noticed a thick membrane surrounding the implant, a membrane that was usually thrown away. "It seemed to me that that membrane had to have the key," he said. He studied some salvaged membrane under a microscope and saw that the cells were macrophages, or immune-system cells that go after foreign substances. "The questions were, 'Why are they there, and what are they doing?'" Harris said.

Harris eventually unearthed a complex biologic cascade whereby the metal head of the implant rubbed against the polyethylene joint socket, sending off particles of polyethylene less than a micron in size. The body detected the particles and sent in scavenger cells called macrophages to destroy them. While the macrophages had little effect on the plastic, they kept multiplying and eventually started destroying the surrounding bone tissue. The implant would get looser and looser until it no longer worked.

"This is a unique disease, never seen before in the history of mankind," Harris said. "We were looking at something nobody had ever seen before. We hadn't the foggiest idea what it was, and it was bad. Millions of people were being helped, and now you were getting large numbers of people who were being harmed."

Harris also discovered the metal ball was rubbing down an area of the plastic so that it became oriented in the direction of the person's main movement (usually walking back and forth). The problem arose when a person crossed her legs or climbed stairs, movements that shaved off tiny particles from the weaker areas of the joint. Harris realized they needed to prevent the implant from orienting itself to one movement.

Along with polymer chemists at the Massachusetts Institute of Technology, Harris eventually developed a new material called cross-linked polyethylene. Traditional ultra-high-molecular-weight polyethylene is like a bowl of spaghetti with all the pieces intertwined. When radiation is introduced, the strands link to each other to form cross-linked polyethylene, which resists orienting in just one direction. This is now the standard compound used in joint replacements.

Millions of people have now had successful joint replacement, without the fear of loosening or failure in five or more years.

More information

For more on hip replacements, visit the National Institute of Arthritis and Musculoskeletal and Skin Diseases.

SOURCES: William Harris, M.D., D.Sc., chief emeritus, adult reconstructive unit, orthopaedic department, director emeritus, orthopedic biomechanics and biomaterials laboratory, Massachusetts General Hospital, and clinical professor, orthopaedic surgery, Harvard Medical School, Boston; Peter Slavin, M.D., president, Massachusetts General Hospital, Boston; Harry E. Rubash, M.D., chief, department of orthopaedics, Massachusetts General Hospital, and professor, orthopaedic surgery, Harvard Medical School, Boston; December 2004 Clinical Orthopedics and Related Research
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