A Response to the “Open Letter”

When I logged in to our web content today, I was surprised to find a very long comment posted under our Open Letter blog post.  After reading through the content I got very excited and contacted the writer to enquire if I could re-post his comments as a full blog post. He gives good advice on how to use and mold the Orthoplast that Robohand use for making our mechanical devices and I think this will aid many of you that are trying this for the first time.

Crispin Miller, an independent biomedical design professional has this to say:

“I have some tips to offer about forming Orthoplast, the splint/socket material you shape by softening it in hot water. (This stuff:

– though I don’t know this particular vendor.)
I worked intensively with this material for several months, making specialized splints and mounts for biomechanics experiments I conducted for my master’s thesis on prosthesis design.

I found it to be an extremely adaptable material, but this same versatility means that it takes practice and attention to control what shape you get. This was underscored to me ten years later when I found myself on the other side of the counter — getting fitted with a splint after I’d hurt my hand — and the young man making it had not been adequately trained for forming the material. I had to go home and re-mold the splint myself, to get rid of just such a pressure point as therapists Esham, Poole, and Scheuber are warning us against.

Here are some things I’ve learned:

1. To form a splint around any part of the body other than perhaps the straight part of the forearm, you have to create some “compound curvatures” — requiring stretched areas in the material. Otherwise when you try to put it around areas which narrow and flare (such as the transition from forearm to wrist to hand), it will form big folds or wrinkles, departing from the natural shape of the body part underneath (think of how some flexible-but-not-stretchy leather would behave) — and these rumples will have an obvious risk of being pressure points. This was the trouble with the splint I had to fix.

To avoid these mis-fitting areas takes attention and practice. Perhaps highly skilled practitioners can do it by themselves (as a single person working), but in my experience a good job on something like a whole-arm splint also required having four hands on the work. One forgiving aspect is that you can reheat the material repeatedly, so as to proceed in multiple steps — on the first heating, softening the material and wrapping it on, you probably just learn what size to trim it to. Maybe also you can see where you’ll need to stretch it, and maybe get it suitably stretched on the next heating. And if you’re lucky, then that time you may still have time also to take the piece, once stretched, and get it suitably wrapped around the limb, before it cools and stiffens up. Or you may run out of time and need to throw it back in the hot water, which means you lose some of your shaping work (but maybe not too much of the stretch) — and then you try again, making sure to position it the same as before so that the trimmed edges and the stretched area(s) are still where you need them to be.

2. We found that the work was easier if you did some extra temperature manipulation of the material –

– the simplest part of this was that once you do have the piece formed the way you like it, and you’d like to go ahead and firm it up, you can sponge it with cold water and then you don’t have to keep sitting there holding it in place for so long. So we prepared both a hot-water dishpan and a cold-water dishpan, with big sponges in both, and if we needed to buy extra time on some part of the piece, we could keep it warm with the warm sponge, and once we wanted to solidify it we could cool it with the cold sponge. Typically we had three hands holding the material and the fourth hand applying the sponge. There’s a significant time lag you need to expect, for either heating or cooling, because the material takes time to change its temperature all the way through.

– for the initial stretching, it helps to have the material soaked in hotter water than you’d use for fitting it onto the wearer — BUT you cannot wrap it onto someone’s arm when it’s that hot! Do the stretching you need, then cool it off and check it against your own (scrubbed) skin, then wrap it on.

– One way we learned to speed this up was to sponge the wearer’s side of the hot material with a quick, uniform wipe of cold water and then wrap it on, while the rest of the material thickness was still nice and flexible — BUT this was with a fully able-bodied volunteer subject with no sensory or circulatory impairment, who could let us know if the splint was feeling too warm and should come off — so do think carefully about this approach, and test it on yourself first, to learn for example whether the interior of the material may be holding more heat than you think. If you’re working with someone who has any damage to skin, or to nerves, or to circulation, I wouldn’t use this trick.

– unless you have a thermostat-controlled water bath, never wrap any piece of heated Orthoplast onto someone without checking the material temperature against the back of your own hand first.

3. A technique I haven’t seen clinicians do, but that I’ve found invaluable, is to do finishing details by local heating afterward, with the splint off of the wearer. This lets you soften and fix small areas without losing the overall shape. Two examples of things this lets you do are:

– flare the edges, at places that would poke or dig when the limb is pressing against them;
– soften the appropriate spot on top of the wrist and poke a good outward bulge into the surface there, about an inch wide, to avoid pressure at the place where the ulna makes a lump there (a lump which varies as you twist the wrist, by the way!)

One way to do this local heating is with a workshop “heat gun” (a hair dryer on steroids), but do be careful and patient, because a heat gun can scorch the material if you try to soften it too quickly.

One extra trick with a heat gun is that you can WELD this material, face to face, if you want to apply a thickening patch or splice a bracket on, or something. One of the two pieces should be softened so it will mate tightly, and then you quickly heat both faces strongly but briefly with the heat gun and press them together.

A kinder-gentler way I found for forming finishing touches, though, was to use a small steam jet. You still have to take care not to scald yourself, but you won’t scorch the material. What I rigged up was a Pyrex flask of water on a lab hotplate, with a one-hole stopper and a bent copper tube (to send the jet sideways so I could see what I was doing). Less elegantly, you could use the spout of a pressure cooker with the pressure weight removed.

If any of this leaves you questions, I’ll try to respond –

best regards and good luck,
Crispin Miller, PhD mech. engr’g
Lincoln, Massachusetts”


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About Robohand

Humanitarian creating mechanical fingers and hands for the people.

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