Part 2 TSB Modifications

Part 2 TSB Modifications

Chet X Burdette

2/27/2001

PART 2 of 2
ORIGINAL POST:
The following is more of an observation than a question. I have always
believed that a TSB socket should distribute pressure evenly to the
entire surface of the limb. I usually do a x-percent (percent of the
measured circumfurence) over the entire mold top to bottom. The
percentage I take off depends on several factors including tissue density
and thickness/style of liner.
       Anyway, it has always bothered me that OWW suggests you take off
an even
.5inch or 13mm of circumference from the entire mold. This in most cases
doesn't result in even pressure distribution since most limbs taper in
size from top to bottom. For example removing that 13mm from a level on
the mold with a circ. of 34cm results in a 3.8% reduction in circ. Now
lets say this patient's limb tapers to a circ. of 29cm further down.
Removing 13mm at this level results in a 4.5% reduction of circumference.
 So wouldn't this modification produce a socket that was tighter at the
bottom than the top? Is this their intended result or just a result of
oversimplified instructions?
Chet Burdette CPO
Mountain State Prosthetics LLC
Charleston, WV

RESPONSES PART2:
__________________________
I think OWW's intention is to make a uniform modification to the mold so
that the properties of the liner are maximized. Modifications for the
Tec
liner are similar and the intent is for the multidurometer liner to
flow in a
way that provides efficient socket pressures. In theory the result is to
produce a TSB socket that applies as much pressure to each area as that
particular area can tolerate, rather than equal pressure to all areas.
Again, this
is my understanding of the theories relating to multidurometer liners and
of
course would not apply to conventional sockets, i.e., hard sockets,
pelite,
etc. Jim Price, MEd, CPO
_________________________
My comments here are that these instructions are most likely written by
the liner manufactureres and not necessarily a Prosthetist. I can't say
that I actually calculate things out that precisely. I have found my own
groove of modifying that works for me. That is what experience is all
about. Do what works for you.
Things such as these are dependant on so many factors. Our group was
discussing the pressure of the ICEX machine in casting over the liners
the other day. One practitioner said he gets a 2ply fit with the
pressure guage at 90. Another uses 120 and I, personally, pump it up to
180. Yhis is considering Comfort gel liners only. (This did not work
for me with a Dermo liner!) I don't even know what the manufactureres
recommend. The tightness of the pump is not an isolated unit. The
tightness of one's impression wrap, the material used (fiberglass,
elastic or standard plaster), at what point in the hardening process
you apply the pressure, etc. Each and every practitioner does all these
things differently, so what works well for one can be a disaster for
another. Prosthetics is truly an ART and a science. Find your own
groove and don't be too anal with those instructions! :-)

Have a great day,
joan cestaro, cp
__________________________
Chet,
The OWW method of modification also implies that bone atrophies like
tissue.
Why would you want the same amount of pressure on a fib head as on the
anterior tib muscles. Neither the Alpha nor any other product has the
ability to cushion a fib head that is pushing through the socket wall.
Jeff
__________________________
Hi
I agree, as it also has implications related to Size of stump. Imagine a
bony stump with 13-cm distal circumference, the resultant 10% reduction
would prevent donning (or at least increased distal pressures within the
socket.)
To elaborate and diverge a little
further..................................
Gels, silicones and almost all liner materials along with soft tissues of
the stump behave like semi fluid, that is they have the same
characteristics under low and high pressure. (Water doesn't stiffen when
you pressurize it; deep-sea divers would be in trouble if it did).
Another
fact is that fluids and semi-fluids are relatively incompressible, under
any pressure they will remain at constant volume.
If we reduce a cast we are not stiffening the materials we are quite
literally pushing the tissues and the whole stump towards the opening.
An effect I call displacement. The procedure you outlined involves
taking the best cast we can by hand, and supplement our tissue
displacement by reducing and deforming the shape. After this process we
hope to end up with a reduced volume which will
displace the stump and the liner towards the opening. When the socket is
loaded it reaches equilibrium with little further tissue displacement (if
there are any reliefs or air gaps in the socket then the material will
attempt to fill them when under pressure).
I believe that the most uniform spread of pressures is resultant from
Pressure casting. During casting the pressures we apply displace the
tissues relative to the bony structures (equivalent to the reduction of a
hand cast) and ensure no air spaces in the socket.
When body weight is borne into any sockets an equilibrium point is
achieved. A good fitting as one that reaches equilibrium with little
vertical movement. Equilibrium in engineering terms when fluids are
involved is called Hydrostatics, Hydro = fluid Static = stationary. In
prosthetics with so many influencing variables it may be termed Quasi or
Semi-Hydrostatics.
To explain this further the weight is transferred from the body via the
Skeleton. The rigid socket applies the opposing force; any soft material
between these two rigid components will attempt to equalize the pressure
over the whole system. This will be limited by the flow of the
material.
To reduce the flow of the material the socket should be shaped with a
uniform pressure (Pressure casting).
Now when the skeleton attempts to progress into the socket, the tissues
are ideally positioned, and the socket shaped such that there would be
little
movement. This is aided by the partial plugging of the top of our socket
with the Tibial flare.
We can achieve this type of socket consistently with pressure casting, in
my opinion hand casting with rectification is a much less consistent
method of achieving this. I am aware that there are many other issues
inherent with socket design, Suspension, skeletal control, pressure
peaks, vascular equilibrium etc.
these all influence our socket, if you would like more please contact me
directly.
Thanks for listening.
Ian Fothergill,
CPO
Ossur Prosthetics
______________________
Chet,
I agree completely with your observation. To achieve a true total
surface
bearing socket, a global reduction of the model is needed. This can only
be achieved by taking the same percentage reduction at each level.
Reducing the circumferences by the same incremental amount (i.e. 1/2
inch) results in unequal pressure distribution within the different
levels of
the socket as you pointed out. There is also variances between patients
depending on their limb circumferences. 1/2 inch reduction on a limb
that has a 40cm MPT circumference does not equal the same 1/2 inch taken
off a
limb with a 25cm MPT circumference. A constant percentage reduction is
the only way to achieve consistent pressures within the socket and
between
different patients. These instructions are either a result of
oversimplification or a lack of understanding of these principles by OWW.
Frank Ikerd, CPO
RDA
Joplin, MO
___________________
That's all of the responses I received minus a few in which the author
requested I not publish their responses to the forum. I will continue to
compile and post responses on this topic on a weekly basis or until they
stop coming.
Thanks for the responses,
Chet Burdette CPO

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