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US researchers closer to developing more comfortable prosthetics

14 January 2016

Novel prosthetic limb eliminates the need for a socket - a critical component of a prosthesis that can cause pain and make the prosthesis feel cumbersome.

Johnny Matheny using his revolutionary prosthetic arm (photo: Johns Hopkins University Applied Physics Laboratory)

A pioneering surgical technique has allowed an amputee to attach his Modular Prosthetic Limb (MPL) directly to his residual limb, enabling a greater range of motion and comfort than previously possible. This is a first for the field of prosthetics, says Michael McLoughlin, chief engineer at the Johns Hopkins University Applied Physics Laboratory (APL) Research and Exploratory Development Department, which developed the MPL. “This accomplishment has eliminated one of the biggest gaps in prosthetic development: the socket,” he adds.

The socket — the part of the prosthesis that attaches to the body — is the most critical component of a prosthesis. If it doesn't fit correctly, the patient can experience pain, sores and blisters, and the prosthesis will feel heavy and cumbersome, said APL’s Courtney Moran, a clinical prosthetist who works closely with patients. Even with well-designed sockets, patients have reported problems with heat, sweating and chafing, Moran said.

“The sling does get a little uncomfortable after working with the MPL for a while,” said Johnny Matheny, one of several patients who have worked with the MPL over the years to help scientists, engineers and physicians fine-tune its capabilities and test its usability.

Matheny, whose left arm was amputated in 2008 because of cancer, is considered a pioneer of advanced arm prosthetics. He was the first patient at the Johns Hopkins Hospital to undergo targeted muscle reinnervation (TMR), a surgical procedure that reassigns nerves that once controlled the arm or hand, which can make it possible for upper-extremity amputees to better use and control an advanced prosthetic device. In addition to the MPL, he has also test-driven some of the most advanced, state-of-the-art prosthetic arms in the world.

When introduced to the idea of undergoing a surgery that would allow him to mount the prosthetic arm directly to his residual limb, he jumped at the chance. The procedure is called osseointegration: first, a threaded titanium implant called a fixture is inserted into the marrow space of the bone of the residual limb; over time, the fixture becomes part of the bone. Several weeks after the first surgery, a titanium extension known as an abutment is attached to the fixture and brought out through the soft tissues and skin. The prosthesis can then be directly attached to the abutment.

Richard McGough, chief of the Division of Musculoskeletal Oncology at the University of Pittsburgh Medical Center, performed the stage one surgery on Matheny in March 2015 and stage two in June 2015, making Matheny the first patient in the United States to receive the TMR and osseointegration. Four months later, Matheny travelled to APL and attached the MPL directly to his body for the first time.

When asked to describe the almost immediate transformation in his ability to control the MPL, Matheny comments: “Before, the only way I could put the prosthetic on was by this harness with suction and straps; but now, with osseointegration, the implant does away with all that. It’s all natural now. Nothing is holding me down. Before, I had limited range; I couldn’t reach over my head and behind my back. Now - boom - that limitation is gone.”

Over the course of three days, Matheny went through a series of exercises in APL’s prosthetics laboratory to determine the limits of his newfound control and range of motion. “What he’s been able to do without that socket interface has been unbelievable,” said Moran.

Moran and the team were planning to take it slow and easy, she said. “We were conservative in our expectations,” she explained, “because we wanted to be cautious about how much load we were putting on the implant, how much we were asking him to do and make sure that we had a number of safety considerations in place and somewhat temper expectations for him.”

"However, Matheny had done his research and was more than prepared," she says. “He had configured a weighted attachment for the implant that was about three pounds, and he had been doing exercises on his own with this weighted implant, which really put us at a position to do more right from the get go. What ultimately happened was that Johnny met all of my planned goals within two hours of arriving.”

He was also able to demonstrate individual finger control, simultaneous finger control, two degrees of freedom at the wrist, multiple grasps, and worked through simulated activities of daily living.

The challenge for the team is to get the technology out of the laboratory and into the hands of people who need it. "For all the incredible things that we see Johnny doing with the MPL, when we’re finished here in the lab he has to leave it here and go home. We really want to enable him to leave with that arm and to use it every day. And that’s what Johnny wants too,” says McLoughlin.

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