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Prosthetist Inventor Seeks Solutions
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." 
Table Of Contents - January 2005
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