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 protected]
—–Original Message—–
From: Orthotics and Prosthetics List [mailto:[email protected]]On Behalf Of Carstens
Sent: Friday, June 25, 2004 3:08 PM
To: [email protected]
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
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