Vertical Shock Pylons: "Pogo Sticks"

[RENARDWC at ctrvax.Vanderbilt.Edu]

>From OANDP-L, with permission of Mark Raabe:

Dear Wayne: Happy for you to do so. My original summary seemed to
indicate the comfort afforded to amputees to be experienced across the
board! However, in a subsequent posting from Joseph Carideo, he states
that a user indicated negative effects. As a prosthetist and someone
that likes square blocks to fit into square holes, I strongly believe
more research in this area should happen! Something that provides
comfort will always be accepted with open arms: however, the long-term
side effects, (if any at all?), should be ascertained for the good of
all involved. Thanks for your interest:

Mark Raabe
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Comfort seems to indicate prescription criteria in most cases.

A recent message posted by Mark Benveniste, CP, raised the issue of
ankle articulation and the prescription criteria for such components.
This topic is one that could be expanded upon to include a new range of
components, ie. "Vertical shock pylon", "T/T Pylon" etc. collectively. I
will refer to them as "Pogo Sticks".

As you may surmise from the description used above, my opinion of the
"Pogo Stick" range is one of scepticism. I would dearly like to
encourage a list discussion about this subject to gauge experience and
opinions other than my own, (my own being limited).

There is no doubt that in nearly all cases of a "Pogo Stick" upgrade,
the response is a positive one. It worries me, however, that we have
provided a vertical compression that can only be returned one way -
vertically.

I know that during my studies in P&O, an important Biomechanics
principle was to minimize the vertical displacement of the centre of
mass during gait. A vertical displacement followed by subsequent
vertical replacement that is provided by the "Pogo Sticks" is surely
forming a tangent away from these principles.

The more normalized gait pattern incorporating an articulated ankle
provides not only compression qualities, but at the same time dissipates
these via a further progression into the gait cycle: (plantarflexion).

This is achievable today with a range of feet: from the simple
single-axis with bumper design, to energy-storage systems present at the
heel component. It would be interesting to hear from anyone with
research into this area, ie. Centre of mass differentials with and
without "Pogo Sticks"; Energy expenditure comparisons; Long term effects
of vertical displacement on the spine? I look forward to reading your
responses on this topic. It is, I believe, the main reason for this list
- "Information exchange leading to professional development of the
Prosthetist/Orthotist"

Mark Raabe
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REPLIES
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That was to be my next post (Benefits of shock and torque absorbing
pylons- should we be using more of them?) so I am glad that others are
thinking that these are important concerns in modern prosthetics and
also look forward to hearing responses.

I am aware of limited studies on a few of the older shock pylons and
have asked people at Northwestern University if there were more studies
on the way. I have asked a similar question about ankle motion. Of
course, I would like to be aware of ongoing studies worldwide. These are
two subjects where research/and observed clinical results/individual
patient experience would be valuable to share with the whole
professional community.

Mark Benveniste CP
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With this renewed interest in shock-absorbing systems, does anyone know
why systems like the Winkley Slip Socket, dating from the Civil War,
stopped being used?

Is this renewed interest a case of old ideas with new materials? Is
anyone looked at updating this old technology? I heard that someone on
the West coast was making a hammock type socket.

Al Pike, CP
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It is my opinion that there is only one foot that really mimics the
anatomical ankle in function: the TruStep by College Park. Dr. Jim
Breakey has done some excellent research into the normal gait vs. an
amputee with a TruStep Foot. The use of active planter and dorsiflexion,
rotation and vertical displacement at the correct time during the gait
cycle, and how this is key in generating normal gait, and at the same
time eliminating unnecessary wear and tear on the amputee.

The vertical displacement is not the same as that received with the use
of a shock pylon that takes length away at the wrong time and does not
give it back until it's too late. Vertical displacement is only achieved
through the use of the two independent axis of the TruStep. The
displacement takes place during early stance and is returned prior to
swing through where it is needed.

ML BOCP
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I can't direct you to any professional studies - but from 25 years of
using prosthetic limbs ranging from post WWII Blatchford models to Otto
Bock modular types - I have to say the Flex Foot Re-Flex is by far the
greatest thing I have had on the end of my socket.

The energy return it gives, the added "spring" to the step, is
incredible. Comfort is increased due to the shock absorbing capability -
I tried going to a shock-free set up and it was terrible. Some form of
shock absorption with some degree of rotation is the only way to go
especially for an AK like me. Hope this opinion helps.

IG
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Here are some thoughts on vertical shock and torque absorbing units
("pogo stick") from my limited perspective.

I am the Engineering Director for College Park Industries, Inc. in
Michigan, USA (see www.college-park.com). Your thoughts on the issue of
"pogo sticks" accord with what I have found experimentally. I have
experience testing all kinds of feet, with and without vertical shock
pylons.

As a long-time user of College Park feet I can attest to the benefits of
vertical displacement analogous to the human body minimizing deviations
in center of mass. In street talk, "it just plain hurts less".

I recently tested a new foot that does not have vertical deflection or
transverse rotation. I walked, played frisbee, and volley ball on it. It
was a pretty good design, but the lack of any foot-flat vertical
deflection or transverse plane rotation was very noticeable and a
definite negative.

I have one of my legs currently set up with a vertical shock and torque
absorbing pylon, the Century 22 Total Shock. I like this unit for its
simplicity and superior bearing overlap.

I find a "pogo stick" with significant vertical excursion does detract
from gait symmetry. I can walk with close to no noticeable gait
deviation. One would be hard pressed to see deviation even with the
"pogo stick" installed. Nevertheless, sighting on a spot on a wall and
walking toward it, I can see my center of mass varies more on the
prosthetic side with one of these units installed. However, there is a
perspective that must not be overlooked: the comfort issue you have
identified.

A purist may say that gait mechanics are it, just as the purist
bicyclist may say suspension is not needed and only adds weight. The
whole story includes, however, that the amputated limb is not ideal,
just as the ground is not flat everywhere.

So I see several primary uses for "pogo sticks":

- Amelioration of ground reaction forces to protect a compromised
  residuum [that word again]

- Adding vertical deflection to feet that do not have it built in

- Adding back transverse plane rotation missing through loss of joints

- Exaggeration of motion for special purposes (e.g., golf swing)

Indeed, there are times where the extra rotation and vertical movement
detract from sound gait. I prefer heavy sports activity with the college
Park TruStep foot alone: for example. I also do not think adding a "pogo
stick" to make up for lack of vertical deflection in a foot is the best
tack. It is almost impossible to have friction-free motion in these
units.

They all will inevitably display "slip-stick motion". In other words,
they get sticky and cease to move when needed. This effect does not
occur in the 3-bone, 2-axis system employed in our foot.

One thing to note is the interplay between an articulating, multi-axial
foot, and a "pogo stick". A patient new to both of these should try the
foot for a month without the unit: then add it. Trying both at the same
time, for the first time, can introduce too many variables during the
alignment, adjustment, and acclimation period.

Hope this patient/engineer perspective helps.

Chris Johnson
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I would also like too see research on the effects of the "pogo sticks"
as you have so affectionately named them. I too have been somewhat
skeptical of the benefit of these devices in the past. My experience has
led me to feel otherwise. While I am not a "pogo stick" spokesperson, I
see them as generally beneficial, particularly with the transfemoral
amputee.

I think your concerns about vertical displacement should be reexamined.
During normal human locomotion, the joints and muscles of the sound
knee, hip, and ankle work as "shock absorbers" with controlled knee
flexion (Quadriceps) contributing a large component of this. The knee
goes from full extension to between 15 and 20 degrees of flexion from
heel strike to foot flat, effectively preventing the rise of the center
of gravity.

This controlled knee flexion is not present in the prosthesis of a
transfemoral amputee that I am aware of. (Perhaps it can be argued that
the Bock 3R60 and other knees with stance flexion features can
approximate this, but that is a separate matter.)  In practice, I find
that few transtibial amputees actually ambulate with a normal range of
controlled knee flexion on the amputated side.

Controlled knee flexion in the sound limb occurs at the same time as
weight shift and vertical loading. This correlates with the "shortening"
action of the shock absorber during weight shift and vertical loading of
the prosthesis. I believe studies will show that there is less vertical
displacement (or vaulting) on the amputated side and therefore more
symmetry of gait in the transfemoral amputee (and perhaps the
transtibial as well) when utilizing a vertical shock absorbing pylon.

In my experience I have been able to "lengthen" the prosthesis by at
least 60% of the maximum compression (~3/8 inch) and find it is easier
to obtain a level pelvis standing with equal weight on each limb.

I have long been a believer in the benefits of the torsion absorber in
the transfemoral prosthesis and find some of the new devices as light as
older torque absorbers with the added "benefit" of shock absorption.

I have only fitted one such device: a Flex Foot VSP on a transtibial
amputee, but can attest to its benefits for the highly active patient or
recreational "runner."

Until there is research (gait analysis) and documentation, we can argue
their theoretical effects and benefits. In the meantime, I think you
will find it hard to convince those using these devices that they are
more functional, comfortable, or better off without them.

Eddie White, CP
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I am 40 years old. In May of 98 I was involved in a train accident that
took my left leg just below the knee. I work for a railroad in Maine.
The injury left me with only 3 inches of bone and muscle and tissue
below that, giving me about 5 inches total. My stump is 90% skin
grafted.

I use a Tech liner with the IceX pinlock, the TT pylon by Endolite, and
the Cirrus foot. The combination of these components provides maximum
shock absorption to protect the grafted skin, and so far it has worked
very well. I am very active. I walk very comfortably, exercise on a
Stairmaster, and a stationary bike. I also play golf, among other
things.

The TT pylon also features a swiveling action which works well,
especially when golfing. With such a short stump, the liner extends over
my knee, restricting some movement, but I am very happy with this
set-up. I am fortunate to be at the hands of two excellent prosthetist
in Bill Velicky and Molly Pitcher.

Bradley
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I read the postings regarding vertical shock pylons recently and had a
thought as I read through the different responses. Has anyone done any
studies on what effect these vertical shock pylons have on the way
prosthetic knees function? I would like to see some data if anyone has
done such a study.

I have used two different vertical shock pylons. The first one, a TT
pylon, was used on a trans-tibial amputee in conjunction with a College
Park foot. The patient ultimately rejected the limb because, in his
words, "there is too much life in the limb". I was taken aback by his
comment since I felt this is what I was trying to achieve, but his
statement proved true when I switched him back to the Flex-Foot VSP. The
second amputee is a trans-femoral. He came out of a Flex-Foot VSP, and
was put into a Century XXII shock pylon and an Ultimate Knee. His
comment was "it feels real". Both amputees are excellent walkers and
very active individuals. One felt inhibited by too much liveliness and
the other was thankful for it! There's a saying in Italian "Tutti gusti,
son gusti" loosely translated it means "to each his own". I guess that's
how we have to look at all this exciting technology we have today - it's
not for everyone.

Keep up the interesting postings.

Sincerely,

Joe Carideo
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