Responses: casting for viscoelastic liner

Responses: casting for viscoelastic liner

Randall McFarland, CPO

ORIGINAL POST:
Chris Burdette's discussion about possible shortcomings of reducing a cast
for the OWW Alpha liner reminded me of something I've wondered about when
casting over a viscoelastic liner. It seems to me that two things occur when
you put a liner over the stump.
First, it seems that the snug liner over the fleshy stump would tend to
distort the natural contours of the stump towards a rounded shape.
Secondly, it seems that during the casting, the viscoelastic material itself
would flow away, stretching thinner over prominent, bony areas to further
deviate from the original shape.... and then we manually reduce the cast (as
Chris described) to as to put the viscoelastic liner under pressure to
utilize the flow characteristics.
    It seems that if you casted over a non-distorting spacer that was
slightly thinner than the intended viscoelastic liner (to give the desired
pressurization), you would more closely capture the true contour of the stump
with fewer necessary cast modifications. (As we know, the more accurate the
original cast and the less you have to modify, the better.) I wonder if the
correct number of plys of thick elastic stump socks (protected from the
plaster by Saran wrap) would work for a uniform liner. Has anyone
experimented and developed such a low tech system that consistently gives
the proper volume for the liner to work as intended?

Randy McFarland, CPO
Fullerton, CA

RESPONSES (separated by a blank line)

I simply cast over the residual limb, controlled with a cast sock or nylon.
Then I fit the diagnostic socket and modify accordingly. I then pull the
liner over the positive mold, apply light vacuum to ensure proper molding of
the liner over the positive, then wrap plaster over the liner. It has always
worked very well for me. I am sure that someone could argue this or that,
but I have never had a better fit.

When you apply a liner you also have elongation of the residual limb that
should be accounted for or you end up with a short socket.

I have tried using sock as you suggested and doesn't quite work. 1st
problem is how to contour for the shape of the umbrella which never seems to
be the same on different liners. 2nd problem is compression of soft /
redundant tissue distally. I have tried the Iceross pressure / vacuum
casting system to solve these problems but is cumbersome. I have tried
various balloon casting techniques, again not consistent and not easy to
use. I agree with the principle of not changing the shape of the leg
which allows
the gel liner to flow and accomplish the purpose of the design. I usually
have to modify all areas where a cavity would occur such as in the
pretibial areas, and sometimes popliteal areas. Hope someone discovers an
easy solution.

If you preshape the liner, I use the OWW Alpha, over a patient's cast then
recast
the limb with the now preshaped liner you do not experience the distortions
you
describe. It is an extra step to do the initial cast for the liner model. I
have found this also alleviates patient complaints of irritation over the
patella
and that the liner feels too tight. I cast for the liner model in about 30o
of
flexion. I have also found that the simplistic modifications espoused by OWW
do
not work as well as the old method of loading the weight-bearing areas. I do
not
relieve over any bony areas as I want total surface contact. I have been
able to
achieve good control with no apparent flow of the material.

Very interesting idea (i.e. to use a spacer instead of the liner for
casting)! I have never tried it, but it sounds useful. It'd be interesting
to know if
anyone has any experience with that.
In our practice, we use the OWW Alpha and the Luxury Liner a lot for BKs.
In order to get a better cast with less modifications to make, I use one
normal and one or two elastic plaster bandages for the cast, and I take care
to get an
exact impression of all the frontal (bony) aspects of the stump, massaging the
excess plaster towards the back of the stump. During cast rectification, I
emphasize the
frontal shape whilst I reduce the cast mainly on the dorsal side (i.e. where
in casting I
distorted the excess volume to). This gives consistent good results and works
very
well.

TEC uses just the system that you describe involving a 3mm non-stretch liner
to cast over. The one that I saw at the Jim Smith Sales seminar last
Wednesday looked pretty good. Call JIm Smith Sales in Missouri and they
will give you all of the details. Sorry, I don't have their number close by.

Yes a simple method on this thought has been tried and is being taught by
myself
in a C.E.C. course that we are giving around the country, in short we are
casting over a 3 mm skin under vacuum and have been getting consistent
results.
Steve Smith of Jim Smith Sales

Randy, take the class here in Minnesota at TEC. They have a vacuum casting
method that does just what you want. They cast over a thin urethane liner
under vacuum and go right to a clear check socket from that cast. It works
so well with minimal alterations to the check socket. The thicker TEC liner
fits into the undersized check socket to give it the appropriate tension.
You will be pleased with the outcomes. I have been in prosthetics since
1973 and it is hard to teach old timers new tricks but this works. Bryan
Finley CP Advanced Prosthetic Technology

TDC total delivery costs adding an extra step or doing a more expensive step
to reduce the total delivery cost. You need to cast over the liner that you
are going to use otherwise you are guessing, and guessing increases the cost
for that product. Casting over liner helps with volume, pin placement, soft
tissue compression contours. Using plaster reduces the image/reality ratio,
because plaster has limited memory. With using fibercast you don't need
(Saran wrap) and you don't need to cleanup, saving on the most valuable
asset, your time. John Russell