Biomedicine

Implanting Artificial Limbs in the Body

Surgically installing prostheses into bones works better than traditional methods, but it still presents a significant risk of infection.

Johnny Matheny, a former commercial baker from Redhouse, Virginia, lost his left arm to bone cancer in 2008. He now wears a hook-style prosthesis strapped onto his chest; he can laboriously open and close the hook and move the arm up and down by flexing certain muscles. But he is avidly awaiting new technology that he thinks will work much better: a surgically implanted device that attaches directly to bone, potentially enabling superior range of movement and more precise control.

Better connections: This is an x-ray of a woman who lost part of her arm in the 2005 London subway bombings. She uses an artificial limb that snaps onto a connector attached directly to the remaining bone. She now swims with the prosthesis every day.

The devices have been tested in people for more than a decade in Europe, but they carry significant risks. Because they require a connection that protrudes through the skin, infections are fairly common, often requiring secondary surgeries. Scientists in Europe and the U.S. are trying to develop ways to better integrate the device with the body—creating stronger connections between metal, bone, and flesh—in order to reduce this risk.

“We are very hopeful. The fact that folks who received the implants are ambulatory means that their quality of life is obviously much better than it was,” says Grant McGimpsey, director of the Bioengineering Institute at the Worcester Polytechnic Institute. “But we need to think about [infection risks] before implementing it in large numbers of people. We are looking for a prosthetic solution that will last 70 years.”

The prostheses currently available to amputees fit over the wearer’s stump. While they can vastly improve quality of life, allowing many people to walk, for example, they also have serious disadvantages. Walking can be quite painful, and friction between the stump and the socket of the prosthesis can lead to chronic sores and infection. “Overwhelmingly, the biggest reason people can’t walk after amputation is because they can’t wear a socket,” Richard McGough, an orthopedic surgeon at the University of Pittsburgh.

With so-called osseointegrated implants, which attach directly to bone, a cylindrical device is surgically inserted into the hollow of the remaining bone. The goal is to encourage the bone to grow into the metal, similar to what happens after joint replacement surgeries. The artificial limb itself attaches to a short connector that protrudes from the skin, eliminating some problems of socket prostheses.

To date, many of the implants of this type have been performed in Germany, under the guidance of Horst Aschoff, director of the department of Plastic, Hand, and Reconstructive surgery at the Sana Clinic, in Lubeck. His team has treated more than 50 patients over the last decade. Aschoff’s research shows that people with the lower-limb implants move more naturally than those with traditional prostheses, have a more symmetrical gait, and use less energy to perform the same movement.

But the procedure is still quite risky. “The biggest hurdle is fear of infection,” says McGough, who has collaborated with Aschoff. “There are not a lot of other systems in medicine where you deliberately have a hunk of metal sticking out of skin.” According to a survey of 40 of Aschoff’s patients who received implants between 2003 and 2009, about half had to undergo a second surgery to deal with infections or other complications. Five had their implants removed. However, 38 of the 40 said they would undergo the original surgery again.

Aschoff’s team has followed the model of a tooth. The group theorizes that a well-anchored implant—in which the bone has grown into the metal—will prevent bacteria from migrating into the bone and causing dangerous infections. (The bacteria in our mouths, for example, typically stay on the surfaces of our teeth, tongue, and gums.) Gordon Blunn, head of the Centre for Bio-Medical Engineering at University College London, has taken a somewhat different tack, getting inspiration from deer, whose antlers provide a natural model for a healthy interface between skin and bone.

As part of normal wound healing after surgery, the edges of the sliced sections of skin will try to knit together, growing downward along the connector pin of the prosthetic implant in search of another piece of skin. But that produces a pocket that can collect dirt and increase the chances of infection. Blunn’s team has focused its efforts on encouraging the skin to form a tight seal around the implant, thereby decreasing the risk of infection. Deer seem to do this via large pores in the bone just beneath the skin. These pores encourage soft tissue to adhere. Blunn and colleagues mimicked this process by adding a porous flange, implanted just below the skin, encouraging the optimal skin seal to form. Blunn is a scientific consultant for Stanmore Implants, which aims to commercialize the technology.

So far his team has surgically attached implants to four people, one of whom was a lower-limb amputee who climbed Kilimanjaro with his prosthetic leg in September. (One of Blunn’s most famous subjects is Oscar the cat, who received two prosthetic hind limbs after an accident involving a combine harvester a year ago. Two months after receiving the implants, Oscar could run.)

To Matheny’s disappointment, human testing has not yet begun in the U.S. That’s in large part due to the high risk of infection, but Matheny says that’s a chance he is willing to take. McGough, who is Matheny’s surgeon, is part of a team working with European scientists and other groups in the U.S. to garner approval from the U.S. Food and Drug Administration to bring this technology stateside.

“I think this will change everything for amputees,” says McGough. The surgeon has traveled to Germany to learn the procedure and has already given one patient an implant there. He hopes that Matheny will be next; researchers there have already built him a custom-designed device. For legal reasons, the surgery needs to take place in Germany. “And I don’t have the money to go there yet,” says Matheny.

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