Prosthetist Inventor Seeks Solutions

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By Miki Fairley

Stan Patterson, CP, of Prosthetic & Orthotic Associates, Orlando, Florida, loves a challengeto see a problem and invent a solution. In fact, besides his clinical practice, Patterson and two partners operate Conundrum Research Labs in Orlando, in which they develop not only prosthetic and orthotic products, but also have come up with product designs for areas as diverse as the fitness and automobile industries. Says Patterson, "We see a need and try to figure out what has to happen to meet that need."

One of the needs Patterson saw was for a prosthetic liner that would solve some common problems. What Patterson and his partners at Conundrum developed is the Evolution SP liner, now licensed to Freedom Innovations, Irvine, California.

Patterson appreciates the value of liners: "They really help to avoid skin problems and breakdowns. We used to think any skin abrasion was a socket problem. Although no liner can make every socket fit fine, a properly made liner can make a well-made socket fit and feel better."

However, he noticed a common problem early on: "We found that a lot of material we used for liners would discolor and start to reek. And anytime a material starts to smell and change color, it means that something is becoming embedded in it. When the amputee can't wash the smell out, that means protein--bacteria--is breaking down inside the liner."

Patterson is concerned about the lack of regulation of materials being used against patients' skin. "Unfortunately, there is no regulation about what is used; we basically can use anything. There's no true testing of skin reaction or absorbent rate characteristics of liners by independent groups."

Material Guys

He continues, "So we talked with the engineering group at the Cape [Canaveral]--my father used to be a tool and die maker out there--because we wanted to come up with a material for a socket interface that would be closest to human tissue."

And these weren't the only material characteristics they were looking for. They also wanted a material that didn't show any cold-flow characteristics, in which a residual limb would compress material and the material would thin out permanently, not returning to its original thickness. As Patterson explains, "We design a socket to fit a certain way with a liner, and when the liner thins out more and more, the limb drops farther into the socket, causing problems for the patient. So we were looking for a material that didn't have cold-flow characteristics and wouldn't absorb sweat and bacteria."

They found their answer: platinum-cured silicone. "There are two types of silicone curing: platinum and tin," Patterson explains. "Platinum-cured is what is utilized in breast augmentation, hearing aids, etc. It's a very inert, pure type of medical-grade silicone."

Another common problem Patterson wanted to solve was the frequent cracking and tearing of liners. The low durometer and chemical makeup of many liners was the cause, Patterson thought. "We noticed that a lot of liners were very soft--low durometer makes the liners very gooey and elastic, the initial thought process of manufacturers being that a few standard sizes would be able to fit a variety of patients making custom liners unnecessary. But in my opinion, that is not necessarily what would be the best fit for a patient."

Patterson and his partners decided that imitating the durometer of human skin, which is in the low 50s on a shore 00 scale, would be best, he explained. "Thus the material would support the human body and not thin out. It was not so soft that bony prominences would sink down into the socket and cause problems. Bob Gailey [Robert Gailey, PhD, PT, a noted physical therapist in amputee rehabilitation] once said that the perfect interface material would be both hard and soft. If it is too soft, the limb would sink as the amputee loses volume throughout the day. If it is too hard, amputees would feel pressure against the skin." The durometer chosen "has worked really well," Patterson added.

Harder Durometer Ends 'Milking'

Patterson's group also developed a pin liner version now licensed to Freedom Innovations. The design solves a common problem--that of pin liners "milking" the distal end of the residual limb. "We bumped the durometer up to 50," Patterson explains. "With that ratio, we don't get the milking action inside the socket, but the socket has to be fitted much more accurately relative to pin location, because it doesn't move around as much in the bottom to guide the pin into the hole."

Aiding Amputees with HO, Painful Neuromas

Patterson notes that custom liners have worked very well for amputee soldiers at Walter Reed Army Medical Center, Washington, DC, who are difficult to fit due to heterotopic ossification (the abnormal formation of true bone within extraskeletal soft tissues). Patterson pointed out custom liners very much benefit amputees with odd-shaped residual limbs: "A liner can be designed to accommodate these problems, making fitting the socket much easier. We can insert material so that forces don't come up from the fib head, but go around it. It's like having a fat pad to allow relief when there are bony prominences."