Touch, self-touch and touch feedback in prosthetic arms I

It has been hailed as relevant innovation to provide prostheses with some force-feedback systems. Recent innovations apparently even include a "thought controlled" "bionic" arm by Otto Bock.

But from where I am standing, this is not necessary. If one reads some of their scientific articles, these guys seem to read research reports somewhat different from the way I do. If they read them at all, that is.

1. We stop wearing prosthetic arms because of missing sensation (misconception)

It is a common misconception that amputees stop wearing prostheses because of missing sensation, at least in a way that understands sensation as part of gripping, manual activity, getting a piece of bread and so on.

Amputees with above elbow amputations are not going to use their stump for too many daily grips. For a very few - but not for too many. With a below elbow amputation, I will use my stump to press stuff against surfaces or fix them. But with a shorter arm that is already of only limited use if at all, present sensation is great but not, ehm, critical. Conversely, too much sensitivity and pain due to minor or subtle trauma are real issues so if anything I will wear a protective sleeve, a compression sock or a woolen sock to keep it warm and out of harms way.

We stop wearing prosthetic arms because of a whole lot of other reasons though.

One is that technology fails, and due to the way industrial and technical providers regard themselves as the ultimate angels, failure then is attributed to faults of the amputee. After a couple of visits to such offices, blame is shifted, ultimately an excuse is born and the tragedy of prosthetic fitting is stopped and the poorly fitted arm is put in the attic, cellar or on eBay. This is such a regular and recurring event it must have stringent reasons why that hasn't popped up in scientific research so far.

Another reason is poor fitting. Sockets needs careful babysitting until they fit well. As that requires a high level of frustration tolerance and given their expectation of being regarded as the ultimate angels of orthopedic technicians, that client-provier-relationship also is doomed to fail. No real research about the way poor fits are "sold" to clients to make them shut up and stay home either. Just look at the failure rates and think whether the whole procedure would need some revision. I think that would be sorely needed.

Third reason is that the prosthetic arm - on average - is simply too heavy and cumbersome. There are three variety of arms available - myoelectric, body powered and cosmetic. Today, almost all companies and technicians will recommend and prefer myoelectric arms because they are by far the most profitable to sell. They are the heaviest, least balanced, least reliable and by far the most expensive. You would not, and I repeat, not want to wear one. Body powered arms or cosmetic arms are a lot better. Otto Bock does not even offer body powered arms through their Swiss partner "Dynortis" any longer - they are definitely moving out of that business. The problem is that prosthetic arms that are too heavy will cause damage to the wearer - elbow overuse, shoulder overuse, neck and back pain. Once these problems become chronic there is only one way to live - wearing no or only a light weight prosthetic. That means that the current industry is fast to dissatisfy some part of their users and contributes to long term damage of some othjer part.

That being a reality that is not happily heard, no wonder amputees will tell excuses that are hard to counter. One of the best excuses is "the prosthetic arm has no sensation".

Any feedback system will further increase the weight of a myoelectric arm and thus render it even more useless for prolonged use.

But there are no studies that prove that we stop wearing prosthetic arms because of missing sensation. Misquoted studies maybe - but that assumption is not founded anywhere.

2. We require a special machine to forward sensation to our arm stump (misconception)

If the socket is built tight and narrow, limited and graded feedback is available at all times. With my body powered arm, I can feel when I touch, push or pull an item. I feel feedback when typing, for example. Those are crude, rough, basic and restricted actions that I can provide with my prosthetic arm and only for these actions do I need feedback.

It would be really something to be able to find a bank note in a dark back pocket using a prosthetic hand. But with fingers not controlled in such a detailed way, there is absolutely no need for such detailed feedback systems. Until that type of control is reliably possible we will wait for a very long time.

Before that happens, maybe prosthetic researchers find back to their true duty - help amputees. There actually are studies that show that body powered arms provide a rather astonishing amount of feedback. Modern body powered arms use very stable light weight parts and are not at all the "hook on a stick" as some idiotic representatives or researchers portray it as.

3. We like getting lots of feedback from electric signals onto our stumps (misconception)

Researchers are often enthusiastic about their developments. As amputees may be hard to deal with, demanding, often disappointed by not only research but also available prosthetic products (not hard to understand given the situation in the last 50 years) and given the difficulty to understand amputees by lack of communication, ability to listen and feel, absent full biographies, descriptions and interviews, lack of studies and lack of biological models that really pertain to the issues at hand, tests with non-disabled subjects are conducted. Feedback electrodes or other manipulators then are placed on the arms of the volunteers and great success results.

A stump however is a tee-totally different type of extremity. My phantom pain management works by switching off perception. It works efficiently but I switch off cognitive processing of pain - at least of some of it. Still, minor stimulation of my nerves - particularly by cold or even minor trauma - can cause major episodes of pain. Also, my absent hand's cortical areas are shifted to the stump, arm, neck and face - and so these areas as well suffer from peculiar neurological function.

So I would do a lot to not get any feedback or stimulation onto my stump. The smallest irritation there will most likely cause big problems. And there are no studies showing it ain't so.

4. Feedback is feedback (misconception)

Self-touch is effective in reducing pain. Self-touch is effective in helping the brain learn to better deal with phantom sensations. Those are probably the other hard reasons why not wearing the prosthetic arm is popular as well.

You don't need to build extra electric parts for feedback when the real reasons to not wear a prosthetic arm are tee-totally different for the most part.

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Touch, self-touch and touch feedback in prosthetic arms I; published 31/10/2010, 03:30; URL: https://www.swisswuff.ch/tech/?p=362.

BibTeX: @MISC{schweitzer_wolf_1603663658, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Touch, self-touch and touch feedback in prosthetic arms I}}, month = {October},year = {2010}, url = {https://www.swisswuff.ch/tech/?p=362}}