Ankle Articulation Vs. Shock Pylon "Summary of replies"

Ankle Articulation Vs. Shock Pylon "Summary of replies"

Mark Raabe

8/26/1999

Dear Colleagues,
Below is my original message followed by the responses received.
Comfort seems to indicate prescription criteria in most cases.
Thank you to all that responded, I hope this topic is researched further.
Cheers
Mark Raabe

Dear Colleagues,
A recent message posted by Mark Benveniste CP, raised the issue of ankle articulation and the prescription criteria for such componentry.
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, with comfort being the big C word. It worries me however that we have provided a vertical compression that can only be returned in one way - vertically. I know that during my studies in P&O, that an important Biomechanics principle was to minimise 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 normalised 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 now-a-days 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

REPLIES
_______________________________________________________________________________________________________________
Mark Raabe submitted a post about shock pylons.
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 pt experience, would be valuable to share with the whole
professional community
Mark Benveniste CP
With this renewed interest is 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
It is my opinion that there is only one foot that really mimics the
anatomical ankle in function, and that is 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 to 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.
Michael Link BOCP
Mark:
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 absorbtion with some degree of rotation is the only way to go especially for an aK like me.
Hope this opinion helps.
Ian Gregson
Mark:

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 experientially. I have a lot of experience
testing all kinds of feet, with and without vertical shock pylons.

As a long-time user of College Park feet in particular, 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, and 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 significan 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 varyies 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
- 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 betwen an articultating, multi-axial foot
and a pogo stick. A patient new to both of these should try the foot 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
Mr. Raabe,

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 too 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.
Regards,
Eddie White, CP
Mark, in regard to your topic and other inquiries by other list members I offer the following: I am 40 years old. In May of 98 I was involved in a train accident to 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 in all. My stump is 90% skin grafted. I use a Tech liner with the IceX pin lock, the TT pylon by Endolite and the Cirus 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. I know this doesn't answer all your questions or concerns, but I how this will be helpful.
Bradley