Re: Vacuum assisted casting
Charles H. Pritham
Description
Collection
Title:
Re: Vacuum assisted casting
Creator:
Charles H. Pritham
Date:
6/25/2004
Text:
Dear Mr. Carstens,
You are of course welcome to your opinion and to theorize all you want.
However, do you have any experience with the technique? Can you argue pro or
con from the position of experience? The history of technical development is
replete with examples of things that shouldn't work but do. The developers
of the original socket vacuum forming technique employing Lexan
(polycarbonate) were informed by the experts that it couldn't be done.
Unfortunately they had already tried the method and proven the experts
wrong, shooting that fact all to hell. John Sabolich was assured by his
elders who knew better that he was wasting his time persisting in trying to
make Ivan Long's NSNA method work for him. It was John's obsessive ness that
brought the method to wide spread attention and led to the present day
ischial containment techniques.
If people are achieving desirable results with vacuum maybe the use of
atmospheric pressure to consolidate the mold and assure intimate contact
with the limb is all that is needed, if we accept your analysis. (I don't
dispute it.) Maybe the positive pressure created by the Ossurr style
technique is excessive. Maybe it just doesn't matter. Evidence to the
contrary, prosthetists are generally pragmatic people impatient with time
consuming, expensive protocols that do not yield practical results or make a
difference.
Over the course of thirty plus years in this profession, I have been
involved in a wide variety of roles. From this experience I have concluded
that new developments work in the hands of the developer, nothing works for
everybody, and it may work but not necessarily for the reasons we think it
does.
So, while I believe there is validity to your analysis, the fact that some
people continue to employ the technique and achieve desirable results leads
me to conclude that its not the entire story.
Your English is fine, by the way. Thank you for contributing to the
discussion and raising a pertinent issue. If we accept your argument about
the use of vacuum, by the way, then it wouldn't make any difference in
forming sheets thermoplastics if we use vacuum or not. Having seen more than
one sheet of plastic thrown away because a timely vacuum seal couldn't be
achieved, I am convinced that it does.
Charles H. Pritham, CPO, FAAOP
<Email Address Redacted>
-----Original Message-----
From: Orthotics and Prosthetics List [mailto:<Email Address Redacted>]On
Behalf Of Carstens
Sent: Friday, June 25, 2004 3:08 PM
To: <Email Address Redacted>
Subject: Re: [OANDP-L] Vacuum assisted casting
Dear listmembers,
I have been reading the discussion about vacuum casting and I can't help and
express my opinion about it to the list. Please prove me wrong but in my way
of viewing vacuum assisted casting I come to the conclusion that it is only
beneficial if the liner over which the cast is applied is either
compressible or if it has flow-properties as e.g. the TEC liner has (under
pressure in one direction it will flow to the direction with lesser
pressure, i.e., it will elongate when compressed radially). If the cast is
made over a regular silicone liner, for instance, vacuum casting is not
doing anything at all but compressing the layers of plaster of Paris (or
other cast material) and the liner. Why do I think this is so? Well, what do
we do when we apply vacuum over a cast using a flexible sealing layer? We
draw air out of the space between said flexible sealing layer and the
residuum (liner). This happens evenly all around. What pressure do we have
to compress the residual limb (and this is what we hope to achieve, don't
we?)? The only pressure that does compress the residual limb is the
atmospheric pressure and that has been there anyway to start with before we
applied any suction action! We have not created any additional pressure on
the residual limb by merely sucking air out of a gap between layers
sandwiched on the limb.
The casting systems out there wich are inflated (like e.g. the ICECAST or
the like) work differently for they do create additional pressure on the
residuum while the cast sets. Thus, those inflatable systems really do
achieve the desired effect of compressing the limb evenly all around while
casting.
I felt it might be worthwhile to point out what I think is a widespread
misconception. In case there are arguments to prove me wrong, I am looking
forward to learning about them and I am open to discuss the issue in more
detail.
Please excuse my english, but it is not my mother tongue.
Felix Carstens
Dipl.-Ing., MSE, CPO (D)
CO-MET GmbH
Germany
You are of course welcome to your opinion and to theorize all you want.
However, do you have any experience with the technique? Can you argue pro or
con from the position of experience? The history of technical development is
replete with examples of things that shouldn't work but do. The developers
of the original socket vacuum forming technique employing Lexan
(polycarbonate) were informed by the experts that it couldn't be done.
Unfortunately they had already tried the method and proven the experts
wrong, shooting that fact all to hell. John Sabolich was assured by his
elders who knew better that he was wasting his time persisting in trying to
make Ivan Long's NSNA method work for him. It was John's obsessive ness that
brought the method to wide spread attention and led to the present day
ischial containment techniques.
If people are achieving desirable results with vacuum maybe the use of
atmospheric pressure to consolidate the mold and assure intimate contact
with the limb is all that is needed, if we accept your analysis. (I don't
dispute it.) Maybe the positive pressure created by the Ossurr style
technique is excessive. Maybe it just doesn't matter. Evidence to the
contrary, prosthetists are generally pragmatic people impatient with time
consuming, expensive protocols that do not yield practical results or make a
difference.
Over the course of thirty plus years in this profession, I have been
involved in a wide variety of roles. From this experience I have concluded
that new developments work in the hands of the developer, nothing works for
everybody, and it may work but not necessarily for the reasons we think it
does.
So, while I believe there is validity to your analysis, the fact that some
people continue to employ the technique and achieve desirable results leads
me to conclude that its not the entire story.
Your English is fine, by the way. Thank you for contributing to the
discussion and raising a pertinent issue. If we accept your argument about
the use of vacuum, by the way, then it wouldn't make any difference in
forming sheets thermoplastics if we use vacuum or not. Having seen more than
one sheet of plastic thrown away because a timely vacuum seal couldn't be
achieved, I am convinced that it does.
Charles H. Pritham, CPO, FAAOP
<Email Address Redacted>
-----Original Message-----
From: Orthotics and Prosthetics List [mailto:<Email Address Redacted>]On
Behalf Of Carstens
Sent: Friday, June 25, 2004 3:08 PM
To: <Email Address Redacted>
Subject: Re: [OANDP-L] Vacuum assisted casting
Dear listmembers,
I have been reading the discussion about vacuum casting and I can't help and
express my opinion about it to the list. Please prove me wrong but in my way
of viewing vacuum assisted casting I come to the conclusion that it is only
beneficial if the liner over which the cast is applied is either
compressible or if it has flow-properties as e.g. the TEC liner has (under
pressure in one direction it will flow to the direction with lesser
pressure, i.e., it will elongate when compressed radially). If the cast is
made over a regular silicone liner, for instance, vacuum casting is not
doing anything at all but compressing the layers of plaster of Paris (or
other cast material) and the liner. Why do I think this is so? Well, what do
we do when we apply vacuum over a cast using a flexible sealing layer? We
draw air out of the space between said flexible sealing layer and the
residuum (liner). This happens evenly all around. What pressure do we have
to compress the residual limb (and this is what we hope to achieve, don't
we?)? The only pressure that does compress the residual limb is the
atmospheric pressure and that has been there anyway to start with before we
applied any suction action! We have not created any additional pressure on
the residual limb by merely sucking air out of a gap between layers
sandwiched on the limb.
The casting systems out there wich are inflated (like e.g. the ICECAST or
the like) work differently for they do create additional pressure on the
residuum while the cast sets. Thus, those inflatable systems really do
achieve the desired effect of compressing the limb evenly all around while
casting.
I felt it might be worthwhile to point out what I think is a widespread
misconception. In case there are arguments to prove me wrong, I am looking
forward to learning about them and I am open to discuss the issue in more
detail.
Please excuse my english, but it is not my mother tongue.
Felix Carstens
Dipl.-Ing., MSE, CPO (D)
CO-MET GmbH
Germany
Citation
Charles H. Pritham, “Re: Vacuum assisted casting,” Digital Resource Foundation for Orthotics and Prosthetics, accessed November 1, 2024, https://library.drfop.org/items/show/223318.
