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MIT lab recasting prosthetics via 3-D printing

Team employs computers, sensors to enhance contact points with limbs

Chris Ahern placed his limb into a measuring device in an MIT laboratory to map its anatomy.
Chris Ahern placed his limb into a measuring device in an MIT laboratory to map its anatomy. (BETTINA MAKALINTAL FOR THE BOSTON GLOBE)

Chris Ahern, who lost his foot and much of his leg below the knee after a motorcycle accident in 2006, places the remaining part of the limb into an elaborate, one-of-a-kind machine in a cluttered MIT laboratory. The device, dubbed FitSocket, is essentially a ring of 14 extendable indenters, each of which can press down on an object in their center.

Arthur Petron, a PhD candidate at MIT’s biomechatronics group, enters a command into a computer console and the indenters slowly descend onto the surface of Ahern’s leg with a high-pitched whine. They gently hold the leg in place and record how much force it takes to push in the tissue at each point.

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“Try to hold as still as possible,” Petron says as the indenters slowly press down into Ahern’s flesh and then retract.

Petron repeats this process up to 100 times, testing different points on Ahern’s leg and gathering data that will meticulously map the end of the limb. It records the surface’s shape, but also which areas are softer or harder due to bones, muscles, nerves and other tissues under the skin: properties that vary widely between different amputees due to different types of injuries, surgeries, congenital conditions, and normal variations.

FitSocket could be of use to researchers in many fields who study the mechanics of the human body, but its most immediate application could help amputees like Ahern. Petron and his collaborators at MIT believe it holds the key to creating next-generation prosthetic sockets.

Arthur Petron (left), an MIT PhD candidate, used a computer to assist in the analysis.
Arthur Petron (left), an MIT PhD candidate, used a computer to assist in the analysis.(BETTINA MAKALINTAL FOR THE BOSTON GLOBE)

Currently, most prosthetic sockets are made artisanally, in a process that often takes months. A prosthetist takes a plaster cast of the end of the limb, then adjusts its shape by hand so that it has minimal contact with the hardest areas of the end of the limb, in order to avoid pain, before using carbon fiber to create the final socket. As a result, the softest areas bear most of the load, which still causes discomfort for many amputees.

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With data from FitSocket, Petron can use one of MIT’s sophisticated 3-D printers to produce a socket that is hardest where the end of the limb is softest and vice versa, modeled for the specific limb of a specific patient. These sockets, made from different hardnesses of resin, do what traditional sockets cannot: distribute the load of a prosthetic leg across the entire surface of the remaining limb.

“It is a good idea,” said Gary Martino, who runs the production department at Dorchester’s United Prosthetics and is familiar with Petron’s work on FitSocket. “Getting it to a practical state could be challenging.”

So far, Petron has used data from FitSocket to print custom sockets for six patients, who have tried them out under experimental conditions at MIT. The response from these early users has been extraordinarily positive, and one patient described the effect as like “walking on pillows,” he said.

The next steps for Petron’s research will be to create sockets for a larger trial of patients, and to extend the testing conditions to more rigorous activity like sports or dancing. Petron expects further research will uncover weaknesses in the current design, particularly the possibility that resin used to print the sockets may not be durable enough for long-term use. But he’s confident the concept of an individually fitted socket will hold up to scrutiny.

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“It’s a more comfortable socket,” Petron said. “We know that.”

Petron is likely to graduate from MIT this winter. His collaborators will continue the FitSocket research, Petron said, but he plans to take a break from academia. He’s looking into jobs that will allow him to apply his FitSocket research to consumer-oriented technology outside the field of prosthetics, perhaps by designing customized shoes, backpacks, or seats for high-end vehicles.

He said the motivation behind his work at FitSocket is far broader than prosthetics. It’s about the science of supporting or attaching objects to the human body, which he believes is poised to be revolutionized by technologies like FitSocket and 3-D printing.

“We’re treating the body as a mechanical thing, because it is,” he said. “I want to understand the biomechanical properties of the tissue.”

For amputees like Ahern, though, the concept of FitSocket holds the promise of less pain and greater mobility.

“Every single socket I’ve had, I’ve always had to work out the kinks,” Ahern said as he put on the prosthetic leg he wore to his appointment at MIT. “I’ve had this socket for a year and a half, and I’m still having problems with it.”


Jon Christian can be reached at jonathan.a.christian@gmail.com.