More than a million people are walking, biking and running with a little piece of Mass General history in their bodies right now, and that number is only expected to grow. This month marks the 10 year anniversary of the first clinical use of highly cross-linked polyethylene, a material developed at the Massachusetts General Hospital's Harris Orthopaedics Biomechanics and Biomaterials Laboratory. The material is now used in knee and hip replacements around the world, helping people of all ages remain active and pain free.
"You cannot live in Boulder without being very active," says Ralph "Bud" Sorenson, a Mass General patient who splits his time between Boulder, CO and Cambridge, MA. At 75 years old, he is on his bicycle for at least an hour every day. "There are a lot of canyons in Boulder where you can ride up and get great elevation training."
Sorenson is one of those one million people with a hip implant that contains highly-cross linked polyethylene (HCLP). He had led an active lifestyle outside his professional career in education and business, jogging and playing tennis through mild hip discomfort for nearly a decade before deciding to get a hip replacement. Dennis Burke, MD, a surgeon in Mass General's Orthopaedics Department, performed the procedure in 2004 without complications.
"I've been pain free ever since," says Sorenson. "In fact, two years ago, I traveled with my biking friends to ride all the high altitude passes in the French Alps-section of the Tour de France, and just this past year we completed the sections in the Pyrenees. I am a very happy user of this new hip."
Previous joint replacements had been made with a smooth polyethylene that helped the metal parts glide naturally. After about five to ten years, consistent wear created tiny particles of that material that circulated in the joint causing bone deterioration around the implant. That phenomenon was first recognized in the 1970s by William Harris, MD, Alan Gerry Clinical Professor of Orthopedic Surgery at Mass General and Harvard Medical School.
In 1996, Orhun Muratoglu, PhD, and Harris – now the co-director and emeritus director, respectively, of Mass General's Harris Biomaterials and Biomechanics Lab – helped develop the highly cross-linked polyethylene used in Sorenson's joint replacement. The Muratoglu and Harris team heated and irradiated the traditional polyethylene that had been used in hip and knee replacements for years, creating a new material that did not wear down as quickly. They also melted the material after irradiation to improve its oxidation resistance.
The technology was licensed to two device manufacturers and replacement hips with the newly improved material were first implanted into patients in December of 1998. HCLP was first used in knee replacements in 2002.
Muratoglu has proposed that he could keep the wear durability of the new implants while making it even stronger – addressing an issue known as fatigue – by stabilizing any free-radicals that might be trapped in the material without melting. He achieved this new goal by diffusing Vitamin E – a known anti-oxidant – into the HCLP. Muratoglu jokes there is the equivalent of an avocado's worth of Vitamin E in each joint implant. The resulting material is durable enough to withstand wear and oxidation, with additional long-lasting fatigue strength for the 21st century patients who often challenge their replaced joints by high impact activities. In the past year, the FDA approved the use of the vitamin-E stabilized second generation cross-linked polyethylene in the hip and the knee. Already over four thousand people have received these hip implants worldwide, and that number is expected to grow exponentially over the next three years.
"Joint implants are meant to keep patients active," says Muratoglu. "Early implants – before the highly cross-linked polyethylene – had poor performance when implanted in young patients during the second or third decade after the replacement, so patients were told to wait, to live with the pain. But with these new materials, we're setting the age of the patient lower so the marathoners as well as baby-boomers are able to stay vitally active."
Researchers in Muratoglu's lab are setting their sights even higher. Currently under development is the third generation biomaterial for joint implants that combines the best attributes of each previous generation, with the wear resistance of highly cross-linked material on the surface of the implant, and strength of diffused Vitamin E in the middle. Muratoglu estimates that this new material could be implanted in patients as early as 2010.
While not all patients with joint implants are climbing to new heights in the French Alps like Sorenson, he is among the one million patients enjoying a better quality of life thanks to technologies developed within the Harris Orthopaedic Biomechanics and Biomaterials Laboratory at Massachusetts General Hospital. Researchers like Muratoglu continue to push the envelope to find better biomaterials for joint implants to improve the lives of even more of their patients, as well as future patients around the world.