More On Noise

More On Noise

5/26/2000

As many of you recall, I have written on noise and prevention of noise in
prostheses, particularly focusing on the propensity of endo components to
make noise.

Endo component noise has been a long battle for me. I suppose when I
focused so intently on noise abatement, I became all the more sensitive
to it. The tinks and pops of endo components are traditionally
particularly irritating.

Most of what I have worn over the years has been either titanium or
aluminum based. Every system I have worn in these materials in whatever
brand has eventually started to get noisy. I have not worn any 34mm
stuff because I am only 170#, albeit active. I have also, being a
longtime off-road motorcycle competitor, had an aversion to steel
components. Steel was regarded as not high zoot enough. But I have
had a learning experience I wish to pass on.

I was recently testing a foot on one of our testers where I was applying
a very high cyclic load. We had just broken a pylon in half. I was
searching for something else to continue the test with, and dug up an
Otto Bock 2R3 pylon. The 2R3 consists of an stainless steel tube
adapter, and a 2mm-wall tube, adhesively bonded together. I threw it in
and restarted the test. Immediately the bond line at the top of the
clamp started to show a line where it was separating. BUT, after the
initial separation, the pylon hung right in there, even though the tube
was relatively thin. Hmmm.

Mike Link, our esteemed prosthetist, had pointed out that he had great
luck with these pylon. So I tossed my steel aversion aside and put one
on. Eh voila! Only a rare and very occasional tinking noise! Ahhh.
Silence!

You may note that on this tube assembly you will find etched INOX. This
is short for inoxidable, which is french for not oxidize or
stainless steel. The tube clamp is steel, more particularly of the
stainless variety. The tune clamp is pretty strongly magnetic, so could
be a ferritic stainless, maybe austinitic. Anyway, why is this relevant
to the real world? How will this material affect noise abatement?

My theory on endo component noise is based on micro movements that take
place between material interfaces, like between the set screws and
pyramid adapter, and between tube and tube clamp adapter. The flexing of
the system in reaction to bending moments and torsion produce the micro
movements, generating noise once the interfaces have degraded by galling,
polishing, etc. Flexing of components is strongly related to the
material they are made from, as various materials have various
stiffnesses. The measure of stiffness is called the modulus of
elasticity. In the U.S., our units for that are millions of pounds per
square inch, or Mpsi. Here are the stiffness values of the most common
materials used for endo components:

- steel 27 to 29 Mpsi, depending on the alloy
- titanium 15 to 18 Mpsi, depending on the alloy
- aluminum 10 Mpsi

Now what this says is that aluminum is 1/3 as stiff as steel, and
titanium is 1/2 as stiff as steel. Thus more of aluminum or titanium is
required to achieve an equivalent stiffness with respect to load. This
is why steel motorcycle and bicycle frames take up much less space than
aluminum frames.

As for tubes, I prefer aluminum to anything else. Aluminum is light with
good compressive strength, which is important to maintain the tightness
of the clamp. I have tried composite materials. While exhibiting good
tensile properties, the carbon composite tubes have not worked well for
me because they do not hold up under clamp pressures, thus the tube clamp
will rotate under the normal ground reaction torques. I have preferred
thick aluminum tubes in the past, specifically Ortho Enterprises 3mm wall
Gold Award Series. But, even with 2mm-wall tube, the Otto Bock pylon
with stainless steel tube adapter is doing very well, which leads me to
believe that stiffness of the tube and tube clamp adapters is more a
factor in noise abatement than tube stiffness.

The specific setup I have on me now is the 2R3 pylon, and an OWW tube
clamp adapter. Sorry, I don't know the model or part number of the OWW
part. It's their heavy duty version, several years old, in nonmagnetic
(probably austenitic) stainless steel with the clamp and gap that is on
and angle to the set screws. I should also try the Otto Bock stainless
steel tube clamp adapter, to see if I can generate the typical noises. I
don't think the OWW tube clamp adapter design is necessarily the reason
for no noise. My guess is that the material is the primary reason. I
say that because the bore diameter is a bit big on the tube clamp
adapter, relative to the aluminum tube, which I have found in the past
increases noise propensity. This one is on the sloppy side and still is
quiet. As a general rule, I believe proper fit between the tube and tube
clamp adapter, with clamp screw loose, is when you can hang the tube
clamp adapter on the tube and not have gravity pull it off. Yet is also
is not to tight making it hard to get on. Oh, and remember not to over
tighten the set screws - another source of noise.

Anyway, the above setup has been 98% quiet, a huge improvement. So the
technical moral of this story is that if you are experiencing tinking and
popping noises with heavier or more active patients, consider good old
steel as an alternative. The conclusion for me is, steel is better for
may activity level, and the slight increase in weight is worth it. That
does not mean I'd use steel foot or socket adapters. Nope, no need to.
Titanium is more appropriate there.

I'm not supposed to be having the noise problems I do, because I am
essentially a skinny guy, at 170 pounds. The again, I have bent the
tubes before.... Many prosthetists have said to me they never have this
problem. Because of my experiences, I suspect the problem is more common
than assumed, and often gets misdiagnosed to being noise problems with
ancillary components, like socket interfaces and feet.

Bear in mind the above is based on a sample size of one, and on the
observations and experiences of a few people, with myself as guinea pig.
That's not good statistics or good science. It is rather, highly
empirical. But, I thought I'd pass it on to you fine and practical
fellows for consideration.

Chris Johnson
Engineering Director
College Park Industries, Inc