A Lesson in Physics

Though I have written about
interface designs for many years, and lectured about them for
almost two decades, I have never been as passionate about the topic as I am
today, for the simple fact that I believe I have finally discovered what
I’ve been looking for since I started my career: an interface that
radically alters the prosthetic landscape for wearers. And not just for upper
limb, for which I am known, but for lower limb applications as well, including

Rather than go on at length about the patent-pending high-fidelity
interface, which I have mentioned in previous articles, it is probably of
greater value to discuss what in my opinion is the problem with traditional
interface design, and in so doing, I believe you will come to the same
conclusion as I did years before when the idea hit me like a bolt of lightning.

Beneath the surface

For years I have worked on improving interface designs in the arena of
upper limb, and while I think I’ve been largely successful, the XFrame and
ACCI have been recognized as evolutionary, not revolutionary improvements.
While these designs have been accepted by many in the clinical community as
positive alternatives to the standard shoulder
disarticulation/interscapulothoracic and radial sockets respectively, for which
I am grateful, it must of course be acknowledged that without the efforts of
those that came before me, I never would have had a baseline to appreciate and
a target to shoot for. So first and foremost, a big thank you to the prosthetic
pioneers of yesteryear.

  Alley suggests applying Newton’s First Law of Motion to socket and interface design.
  Alley suggests applying
Newton’s First Law of Motion to socket and interface design.
  © 2010 iStockphoto.com/Duncan
P Walker

In the simplest of terms, I believe we have been approaching prosthetic
care with too much emphasis on the limb as a whole, and not enough attention on
the skeleton lying beneath. I chose my words carefully, so please read them
again. Of course it is imperative that we pay attention to the surface contours
of the limb, as well as the skin condition and the underlying soft tissue. This
fact is unassailable. And although we often discuss stabilization of the bone
that is underlying all this soft tissue, we have really only paid it a passing
glance, choosing to stabilize the long bone by primarily addressing the
proximal portions of the interface. Such examples can be found in the likes of
ischial containment sockets and all the variations that look at control above
the zero line, or ischial level to affect some measure of stability on the
femur. But a couple of channels in a hydrostatically supported limb or a little
compression just proximal to the distal end does not a stable femur make. The
same goes for radial and humeral level interface designs, where little
attention is paid to what typically amounts to a majority of the limb that lies
distal to the cubital fold or axilla respectively.

Fundamental law

So why have we spent so much time dealing with the small percentage of
the limb that makes up the proximal segment? The only level where considerable
time appears to have been spent is the tibial level, which is why I have
restricted my efforts until just recently to the knee-disarticulation, femoral,
radial and humeral levels. This attention in tibial sockets is really out of
necessity, as the anatomy demands more care with regard to the surface
proximity of the medial aspect of the tibia, bony prominences, the peroneal
nerve, and the origin and insertion of musculature. With the other levels, we
have been able to get away with far more than we should have after all these
years thanks to many factors, not the least of which is the forgiving nature of
our patients’ limbs and of our patients in general.

I have made some bold statements, so allow me to explain using a
fundamental law, namely Sir Isaac Newton’s First Law of Motion, which

“An object at rest will remain at rest unless acted on by an
unbalanced force. An object in motion continues in motion with the same speed
and in the same direction unless acted upon by an unbalanced force.”

This First Law of Motion is often called “the law of inertia.”
Inertia equals the resistance an object has to change in its state of motion.

Traditional socket

So what happens inside the traditional socket? Keeping the above in mind
and understanding the anatomy of the limb we can begin to understand the
problem with the simple encapsulation model of the limb we have used for far
too long. Let us first all agree that associated musculature contracts pull on
the underlying bone to initiate movement. The shaft of the bone then swings
through an arc inside the surrounding soft tissue until enough resistance is
created by the compression of the surrounding soft tissue against the interface
wall such that the prosthesis eventually responds to forces acting against it
and prosthetic motion occurs. If we look at Newton’s first law, both the
skeletal anatomy and the prosthesis to which the interface is attached are
exemplifying the law of inertia. The intrinsic bone moving within the limb is a
prime example of an object in motion and the subsequent resistance of that
object in motion to alter its state.

The prosthesis at rest is an example of an object exhibiting zero motion
and its natural resistance to altering its state. The skeleton will remain in
motion unless acted on by an unbalanced force, in this case the density of
overlying soft tissue. The soft tissue immediately imparts a resistive force to
skeletal motion but is simply insufficient to completely prevent motion until
such time it is compacted so greatly against the interface wall that it
essentially approaches the density of the interface itself.

The prosthesis at rest also follows the law of inertia as it will remain
in its motionless state (considered zero motion) until such time a force great
enough to alter its state is present. Unfortunately all too often this force is
the distal end of the intrinsic bone crushing up against the wall of the
interface, having traveled “far and wide” to finally come to rest at
the interfacial boundary. While this distance might not seem to be significant
taken out of context, within the interface and in terms of lost energy and
range of motion with regard to the wearer it can represent an abyss a mile

Take a look around

Take a look around your prosthetic facility and make a note of the
distal 90% or so of each femoral, radial, humeral and – to a lesser degree
– tibial model, positive or negative, plaster, foam, thermoplastic or
carbon fiber. What do you see? You see a tube. Granted the tube has some
attention to detail at the proximal end and an “ode to stability”
here and there along its length to some miniscule degree. And yes there’s
room for muscle hypertrophy, vascular and neural network reliefs sprinkled
about, as well as some fancy frame cutouts that sure look cool. However, if we
are honest with ourselves, I think you will all agree we have more or less been
putting our patients and clients in tubes, and within that tube a bone moves
back and forth far more than it should, bottom line. Yes, we talk about energy
storing feet and pylons, we talk about ultralight components, suspension that
is rock solid and proximal trim lines that are all in the name of preservation
of energy and range of motion, yet we never quantified the tremendous loss of
both that occur deep down within the interface.

My clinical experience has shown the true depth of this loss and what it
means to capture intrinsic bone motion, not absorb it. I have seen a woman with
a four-inch humerus launch skyward a 6.5 pound research platform without
discomfort and hold it above her head and even out to her side at 90 degrees
for what seemed like minutes, not seconds. She was at the Amputee Coalition of
America Annual Meeting, demonstrating the DEKA Arm prototype live to everyone,
many of whom simply could not believe what they were seeing. There are many
more cases of results like this but rather than go on about them I simply ask
you to take a long look around at what we have done all these years, think
about the good Sir Isaac Newton and then ask yourself whether or not we have
done right by him.

Randall Alley, BSc, CP, FAAOP, CFT

Randall Alley, BSc, CP, FAAOP, CFT is
chief executive officer of biodesigns inc. He is chair of the CAD/CAM Society
of the American Academy of Orthotists and Prosthetists, an international
consultant and lecturer, and a member of the O&P Business News
Practitioner Advisory Council. Alley can be e-mailed at

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