Prosthesis breakthrough

NH firm develops new duplication process


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Chris Anderson, a mechanical engineer at Mentis Sciences in Manchester, demonstrates the old method of fitting carbon fiber on a prosthetic thigh mold, which compromises the strength of the material.

Courtesy photo

The process for duplicating prosthetic legs has essentially remained the same for over 30 years.

When transferring the alignment of the original prosthesis, doctors use a metal alignment fixture to take the measurements of the socket (the device where the limb is inserted), locking them down with handles.

“Once you put your prosthetic into the alignment fixture, it has to stay there until you’re completely done. You can’t move any of the fixtures, because if you move that, even a quarter-inch, you’re going to lose the alignment of that person’s leg,” said Chris Anderson, mechanical engineer at Mentis Sciences in Manchester.

The process takes between a week and two weeks, and during that time, some doctors use electrical tape to keep fixtures – or “jigs,” as they’re commonly called – in place, along with signs warning, “Do not touch.”

“If somebody comes by and bumps it, by the time a person puts that prosthetic on, their ankle could be twisted a little bit and now you have to start the process all over again, meaning you have to go back to the doctor’s, have them realign you and then get it duplicated,” Anderson explains.

In fact, each time a subsequent duplicate is needed, a trip to the doctor’s office is required, and the original prosthesis must be withheld for days.

“There’s a lot of soldiers going back to active duty now, so if they’re overseas and their prosthetic fails or breaks, or they lose it, and need a duplicate, they have to see a doctor to realign them, so they need to get shipped back to the States,” said Anderson.

And while soldiers receive multiple duplicates, civilians typically have just one prosthesis, which doesn’t fit properly over time, due to physical activity or water retention.

The digital vertical alignment jig, which Mentis Sciences patented in 2013, simplifies the prosthesis duplication process.

Beta testing

Mentis Sciences has a 21st century solution, said Anderson: the digital vertical alignment fixture. The jig operates the same, but takes digital readings that are stored in a database.

“If you need a duplicate, the same exact thing a year from now, you can pull up that patient’s data and realign the jig to what that digital reading was,” said Anderson.

All of the knobs are the same, but upgraded versions include a counterweight to assist in lifting heavy plaster and a ball-bearing system to easily roll the mechanisms, plus an easily detachable piece for below-the-knee prostheses.

A digital reading is more critical now that doctors have moved away from completing the duplication process in-house. Typically, doctors mail the jig to a fabrication facility, with the doctor blaming the facility for alignment problems, said Anderson.

With Mentis’ fixture, the doctor would only need to send the digital reading, allowing “multiple people to use one jig, because now you don’t have to lock this down until you’re done,” said Anderson. It also allows the doctor “to verify what they sent out and what they got back from the central fabrication facility. And it also gives the repeatability so it can be duplicated at any point of time without having to see the patient.”

Mentis’ digital vertical alignment fixture is currently being beta-tested at Friddle’s Orthopedic Appliances, a South Carolina facility that transfers alignments for Walter Reed Medical Center, which will likely obtain its own digital vertical alignment this fall.

The South Carolina firm has two in its facility, which are used every day versus the conventional method. St. Petersburg College in Florida is seeking to obtain one, as well as another fabrication facility down south.

Temperamental alignment

Mentis wants to ensure the fixture can last for decades.

“Especially if you’re an AK, an above-the-knee amputee, a little bit off, by the time you translate it all the way down to your foot and you get to the bottom, then you’re close to being a lot off, so, if we’re not going to see the patient anymore, we need to ensure that what they had and what they get is going to be 100 percent identical” he said.

The alignment of the leg is temperamental because the plate that attaches to the prosthetic limb is not always perfectly straight, depending on the reaction of the nerves and muscles after an amputation, said Anderson. Thus the prosthesis assists to ensure the foot is flat on the floor.

The most current upgraded alignment fixture will head to Next Step Bionics Prosthetics in the Manchester Millyard.

Mentis also has created a new method to manufacture the socket that is attached to the amputated limb.

A plastic check socket, which can easily be heated up to adapt to a person’s amputated limb, creates the initial mold before the plaster is cast and then the permanent, carbon fiber composite socket is made.

The integrated manufacturing cell braids carbon fiber to the mold of an amputated leg, creating the socket that connects to the rest of the artificial limb.

Traditionally, the carbon is braided and then stretched over a mold of the socket, as Anderson put it, “kind of like a Chinese finger trap” to fit both the thigh-width and the small, sometimes two-inch-round, base that connects to the rest of the prosthesis.

“But what happens is it all bunches and gets all weird because you’re trying to take something that’s six inches around and trying to make it down to two inches around,” said Anderson as he demonstrated. “And what happens is that causes a very weak point right there because – as far as composites work – the angle of your fiber, what direction it goes gives you your strength. So when you’re pulling and twisting it all right here, and bunching it, you’re kind of changing the angles everywhere.”

If something’s going to break, it’s not going to break at the middle of your leg because it’s inside of it, said Anderson. The prosthesis is going to break near the bottom.

So in 2004, Mentis created the integrated manufacturing cell, which braids carbon fiber to the shape of the diameter of the plaster, making it tight all along the mold of the socket. Afterward, a resin is added to harden the socket.

While the knobs must be adjusted manually for the first copy, a digital database saves the measurements and speed controls.

Mentis attends three to four prosthetic shows each year and receives a lot of positive feedback on their devices, but the shift from traditional methods to high-tech can be slow, said Anderson.

“Because everything has been the same forever, it’s hard to get people to adapt to new things,” said Anderson. “We’re trying to benefit them – give them better tools to use.”

Liisa Rajala can be reached at lrajala@nhbr.com.

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