Experienced user based advice for investors in the domain of prosthetic arms [technical guide for understanding the field]

Investors may need to understand what prosthetic arms really are, how they come about, and what is there to be considered, before investing their investment or investments, or money, in a prosthetic arm component manufacturer or research spin-off.

A current list of this type of products or hopeful startup device ideas is:

Current options and directions of development

Actual devices

The Motorica devices look really well made and promising, in anticipation of a more detailed review.

I would be very reluctant and cautious with the others, and for now, definitely stay away from Macu4 or Swiss Prosthetics, where relevant steps seem to be still not made / completed. I mean, it is not forbidden to "dream", but building good prosthetic arm devices for real life still may require more.


With a realistic estimate of at least around 85% rejection rate, the prosthetic arm industry so far appears to be one of the most unsuccessful industries that there are both in high-tech and medical technology. That number as such may be not too interesting, however: it may be more relevant to identify how many initial prosthetic arm clients a prosthetist and the associated component manufacturers can successfully satisfy. Also there, there are considerable problems.

Now, what usually keeps developments, market introductions, and user feedback, as well as improvement circles going, is a successful social setting - where users can really voice their needs, and where developers really can voice their limitations and inabilities. Such a really successful circle, where respect combines with a striving for technical proficiency, in the context of prosthetic arms, appears to be largely absent. To a great part, this explains why the current status with regard to marketing, improving, or successfully selling prosthetic arms is not a lot better than maybe sixty years ago.

Marketing problems and how these relate to people and design of products

Once you face a particular solution or offering, a new startup or product line idea, very carefully check the people behind these solutions or new company ideas.

  • If they have not at least successfully built and sold at least 10 prosthetic arms, then be extremely careful. Chances these people really know what they are talking about are probably slim. And to understand your problem better, the problem that you face: the people behind the catastrophic LN-4 device also probably did not mean bad [link] and yet, were you in the situation to ever need a prosthetic arm, I most warmly recommend you stay away from an LN-4 as far as ever possible.
    • Ask the people behind that new project specifically for their difficult and their upsetting, disturbed clients and let them explain how they dealt with these, and how so successfully. A good prosthetist will go the way with the client. It is not acceptable for a prosthetist to act on impulse or to reject these clients. Conversely, having an arm amputation can be most disturbing, and most painful, on several levels and in more than one domain. That an arm amputee may be upset or not satisfied can have a bunch of extremely hard reasons, and a bunch of really relevant causes. The only way out of encounters there is to understand these clients and their often justified problems, and address the problems. With that, building a prosthetic leg is quite different with respect to a number of relevant issues. The experiences I made, when the ark guy was on holidays, the leg guys never managed to even provide a really simple cable fix on my arm, easily. It is a domain in itself. A prosthetist that thinks there are cheap or fast solutions to a prosthetic arm probably never worked in the field.
    • Particularly those amputees with very short stumps or other issues for conventional sockets may be very hard to fit with a conventional socket. Targeting these people for new prosthetic arm ideas may be a bad idea, because if a lengthy tedious cumbersome step wise approach by a prosthetic technician does not work, how much less will a quick-and-dirty approach of some startup company with some buzzword ideas work? So suspension can be a real issue. 
    • Gripper shape build and validation of new shapes is tricky. You may need dozens of volunteers to test these for you. The current most successful gripper shapes may be found in the conventional Otto Bock hands, and in a number of split hook designs. The shape that a gripper has is absolute key for a number of reasons: first of all, grip angle and ability to provide stable geometry during the grip - secondly, what covers can I use.
      • Geometry: These shapes dictate how safe and reliable my grip turns out in any real given situation, based on attack angle or approach angle [link]. The grip geometry is most supportive if it is rigid and stable whereas the always different grasp angles of uncoordinated oval finger tip ends provide me with an angular nightmare [link]. Successful devices like the Becker hand [link] or Hosmer hooks [link] will be based on designs that are probably over fifty years old and have seen numerous adaptations, which is why these shapes are so extremely useful for everyday activities. You probably cannot make up for 70 years of hard troubleshooting by a unforgiving user collective, and if you think you can, show me the data on which that is based. TRS devices [link] are built by absolute super-pros, do not ever try at home what they are doing, their device performance is really insane. Also, their devices have been tested by a large number of people, as also I provided timely feedback and these (but not other! OH NO!) manufacturers are great in accepting critique and providing swift design fixes. Also, new developments such as the Toughware V2P, Toughware Retro or Toughware Equilux [link] that are based on old validated and very well established designs as the Trautman hook, can easily be trusted from day 1. But, new gripper shapes that provide new grip angles require testing, a lot of testing.
      • Covers: Also, the grip shape determines what materials I can use to make the surface softer, and as I have to replace the soft surfaces after a few days or weeks, depending on what I use these for, they better be really available and really cheap: if the gripper is shaped like a normal hand, I can buy standard work gloves in the hardware store for a few bucks; if that hand is ill-shaped like many prosthetic hands are, then I need custom gloves that may cost between 100 and 700 USD a piece, and that is a real show stopper. For Hosmer hooks, I can use simple silicone tubing, for soft cover. Why are soft covers so important? They may even drastically reduce the need to have very strong gripper closing forces - as these can be hard on the cable control or in the case of myoelectric arms, strong grip can mean a too heavy motor.  So if you want to sell new grippers and have not considered the future supplies of soft device covers, start thinking hard.
      • Your new startup team should know all that. Let them first explain exactly why they think that a Hosmer 5 split hook is "bad" in any way. They should know that without a very thorough understanding of what that device really does (and without using it in everyday life for a few weeks or months that is simply not possible in any easy way - then you may need analytic thought of way above average functionality), they may not be able to successfully describe any new good problem-solution space for such terminal devices at all.
  • The user happiness is key. A new user however may be a person that just had an arm amputation and thus be super edgy. It is an edginess of an accentuation and sharpness you may not have experienced yet. The socket fit may have to be good or perfect. We may damage your new device immediately because you have no idea what a willing user that wants to exert power via an arm, by lifting a sofa for example, is capable of. Weight load is one thing but jerked motion and maximal torque quite another. Once the device died or got damaged too much, the honeymoon is over, and user and manufacturer both have a real problem. As a prosthetic arm user, the prosthesis only will be used when it suits me really well, but, see, using it is the only way to become proficient at using it. For that, I will have to be able to use it for at least ?~4-12 weeks without interruption - but if the prosthesis dies, rips, tears, breaks, after ?~4-10 days, again and again, you as a manufacturer won't make it past ?~2-3 repair cycles, then the customer may very likely start to forget about it. That is why a number of good and well built current prosthetic device parts, such as by TRS Prosthetics, are not cheap: they are really well built so the user will not complain about damaged or broken parts. Other manufacturers, like Otto Bock or Ossur, sell their produce for really high prices that from my experience do not seem to be matched by a particular quality; maybe they can explain why that is, or should we drop the subject. Immediate and sustained user happiness is key to getting your product onto the arm of the user, and having it stay there.

History and what can be learned maybe

The market is largely absent. And it is warped.

  • Useless prosthetic arms or those that only provide minimal practical support, gizmo, gadget, cosmetic, show-off or presentation devices are a very short lived thing1. I can and take one of these off the very moment no one is looking any more, and that is a reality for such devices: they end up in the cupboard or attic fast. These may be serving my vanity needs and I may succumb to this occasionally. Mostly, this seems to happen only at first, maybe during the first years after amputation, of if I feel my body image needs a gadget real bad. After a few years, I may realize that the deeper satisfaction that comes from wearing such a device is minimal. You could ask yourself, whether generally, myoelectric prostheses2 (Otto Bock, Ossur, Touch Bionics, BeBionic) and a number of 3D-printed devices fit this category, or at least partly. Developers such as Touch Bionics that came out with the iLimb seemed to have had problems staying in business, there was discussion in the media about them having massive problems, and they were then sold to other companies. So, Touchbionics was sold to Ossur. With that, Ossur first of all closed down the social media presence of Touch Bionics. As the main selling point of the iLimb is a multi-grip control leveraged via a mobile phone app, Ossur now canceled out the Windows/PC software, the Android app, and made it impossible for us users using Windows and Android to access our prosthetic hands the way they were advertised[link]. None of the manufacturers seem to sell a lot of "bionic" hands. BeBionic was sold to Otto Bock. Overall, no one survives selling gizmo arms, it is a limited undertaking.
  • Real sport and real work prosthetic arms dominate for sustainable support. But that is not market either: the arm amputees that really want to lift weight, really go for sweaty workouts or bike rides like all the time, the ones that really work hard, like, as carpenter, farmer, et cetera, are only a small fraction of the arm amputees. We know what we need, and cheap new gadgets is really not what we buy. We cannot afford cheap stuff. I may calculate prosthetic arm costs over 5 years or so, and under full work load. If the added cost of all parts plus repairs is cheaper, then, then I have a better product. So I save cost by buying really sturdy stuff. Some is expensive, others is simply unobtainable because no one has it, so we also designed and built our own parts and components. As I have tried out a lot, I may know the one or other thing as well. There, true reliability is the real issue and if ever you are into mission critical reliability, you do not buy from anyone, and you do your own very hard testing. And with that, I am not your market, probably. Here, brands like TRS or Hosmer as well as Toughware and Becker mechanical hands seem to prevail. There were attempts to offer cheap remakes or cheap copies, but these did not appear to take off. So for real use, a prosthetic arm is a balancing tool and overuse prevention tool first, and a neat looking piece only as secondary priority. To be that, it must be very well balanced weight wise [link], and it must be able to survive very well the type of work and activity that does take a toll in terms of asymmetry and overuse:  heavy and bimanual and repetitive activities. And to work well for bimanual activities, as right below elbow amputee, fast real time control is quite important from my own view, which also is a domain where body-powered arms outperform all others.
  • Really super real looking cosmetic arms are super nice to have but we cannot ever afford these. These are a great niche product currently in the hands of master artists. A well crafted cosmetic prosthesis copies my other arm and may cost around up to some 30 000 to 40 000 USD, particularly if it is going to be really well crafted. On still pictures, or when I am stationary and not moving, then it may look stunning and a lot like my other hand / arm, and that alone can be worth the price. What remains unsolved to this day is to also build a prosthetic hand that looks natural when it moves.
  • You will probably almost never get honest feedback from most prosthetic users. Most of us  are dependent on the prosthetic provider, or on anyone giving us nice things, and we avoid confrontation. Some of us suffer from fully blown psychiatric diagnoses, such as Posttraumatic Stress Disorder (PTSD) or from depression. So there may be real reasons, why doctors still "prescribe" a prosthesis, and that in all likelihood and guestimation will not be because doctors understand too much about these prostheses - but they look at the rest of the picture and the whole picture. Conversely, you and your company startup generators may have to make up for lack of adequate critical feedback with own testing. If you, yourself, can wear the device/s over extended periods of time and use them for critical work yourself, as the manufacturer, then maybe you are on the right path. If that does not work out, hm.

A new product that could be successful will be a product that works, and the problems to be covered are the following:

  • I may wear a prosthetic hand if it makes me look good, and that possibly where other folks are that may look, inspect, and check. So for parties, for going out, for giving presentations or workshops. But the problem is, current myoelectric prostheses or gizmo gadget arms cannot deliver that impression. With the slightest stress and sweat, the electrodes stop working. With the grip patterns, holding an olive snack plate or wine glass reliably, so it never ever falls down, seems just not possible. Any malfunction is unacceptable even at the party level. Just imagine it is your laptop keyboard over which I hold a paper cup of water using your device, or it is your new expensive carpet over which I try to balance the oily snacks you served, using your device. If you do not believe it, carry exactly these tests our yourself, monitor heart rate and saliva cortisol level or whatnot. All myoelectric devices excel mostly in totally stress-free environments, where the user never sweats, and they are best used in forgiving environments, where all grasps are accepted to malfunction. For all error-critical tasks, which starts at party level use, body-powered or cosmetic prostheses, or wearing no prosthesis, is the way to go. Market wise, adding real cosmetic looks to body-powered technology is the way to go. Adding really comfortable suspension and super robust cable control parts also is the way to go. [link]
  • I will wear a prosthetic hand if it alleviates my asymmetry and prevents overuse. The asymmetry is strained particularly when I work heavy bimanual or repetitive tasks. So the prosthetic hand must sustain very heavy use - and no myoelectric socket or myoelectric hand really does that. These devices are far too delicate. For all real heavy-duty tasks, body-powered prostheses are the (real, only) way to go.
  • I may wear a prosthetic device possibly if it looks totally stunning and if it really offers great visual effects. The best thing to have is still a hand no one recognizes as prosthetic. So far, that is best offered by my cosmetic hand. The iLimb is funny to look at, but not stunning. The device that I got by far the most unsolicited comments for so far is the TRS Jaws. Far more than for any other device. People seem to absolutely love that. Go figure. But as long as prosthetic arms do not very successfully cover my disability, they are always going to be uncool one way or another.

Try-use-drop use cycles

The most overestimated aspect of investors and tryout-generators is that we always stay, remain, hopeful. That is not the case. The reality is dire and dark, and you won't sell any units if you do not embody this mantra:

  • Only a prosthetic arm that fits perfectly (i.e.: manual cast, professional prosthetic technician, perfect suspension technology, stable stump size, stump ready to bear socket, etc.) and one that provides a near-perfect error-free control (i.e., no object drops except 1 in a few hundred thousand grasps, etc.), and one where parts survive normal use (read: from manufacturer view, the wording is "extreme use"), has a slight chance I wear it tomorrow.
  • Tomorrow is a new day, and the prosthesis then may be damaged, fit less, or provide less perfect control.
  • When I do not wear the prosthesis, I still live.
  • While I live, I adapt. When I live and work and play without prosthesis on, I get better at that also, day by day, want it or not.
  • Easily, I spend three months and without even thinking, I wore the prosthesis only on 4 days of these three months. The percentage that the use experience compares then is for three months (~90 days)4 use days versus 86 non-use days, so I spent ~95,6% of  my time exercising my stump, and ~4,5% exercising the prosthesis.
  • I build my life orientation, hobbies, skills, alongside that everyday experience. That may happen by itself, mostly.
  • I become very proficient in the way I am, the way I live, mainly if not particularly also without prosthesis on.
  • If it is anything like your business idea to pull money out of my pocket for some stupid trinket, or gadget, it is my business idea to not buy a nonsensical product. Wise people wait for 2-3 years, then ask the first adopters that are heavy on the material, personally. That is how we do it.
  • After six months I may have worn the prosthesis for another 3 days and due to all sorts of problems I will not feel it matches the overall comfort and by then also acquired skill level.
  • Due to further developments along these lines, wearing a prosthesis becomes an increasingly outlandish thought.
  • After a few years or so, I clean out my stuff and find that device. By then it does not really fit anymore. I may try it out a few more times, feel really awkward with it and then sell it on eBay or toss it.
  • Later, someone asks me how that device was.

So if you cannot get the first bullet point/list point here right, really right, then your product is not so likely to see a lot more than a few days of use before it becomes obsolete [link].

Are all amputees dumb, stupid, and a glutton for punishment? Can you screw us over successfully? Do we buy anything and everything just because it has colors on it and LED blinks?

Probably not. And if we do, the stuff usually is discarded real fast.

If your product is well-intended but is not mature, while it still has some remaining real potential before it will be anywhere near useful, we will likely know immediately.

Many current commercial components, engineering-wise, should be regarded as early-stage developments, as beta versions at best. Did the Otto Bock Movo2Hook ever strike you as a particularly well developed device - and if yes, how little did you use it : ) [link] Did the iLimb ever strike you as particularly well developed device? [link]

The thing is, everyone and their grandmother may want to suck money from our pockets or exert some unacceptable type of power somehow, also in context of their new alpha or beta version devices - and if anyone knows that, we do, unless there is something else going on. We may not always tell you though, but do not mistake euphemisms for praise.

Take the strange case of one particular startup team:

  • You want the creation and build of your prosthesis to be a slow path, and you want to explore the aspects of it with due diligence. They seem to believe, that somehow, time is a problem, and thus, some process needs to be sped up or hurried even. Like, is it a new business idea that they "hurry"? Geez. From my view, that is really not the case at all - we all do have a bit of time to get a prosthetic arm set up, made, and built. A conventional prosthetic arm may be built as fast as a few days - but that is not always sensible. There is a process involved and that requires a careful approach. This is a father bull and son bull situation.
  • For most popular sports like swimming or running, no arm prostheses are needed at all. Swimming competitively as an amputee does not allow for prosthetic arms either. So, no need to do that. Even training on some exercise devices in gymnastics halls, may be done without prosthetic arm for the most part.
  • Bike arms may need to be actively controlled and very sturdy and very comfortable. A passive bicycle arm sure is nice for quick rides and short use, but I get back pains after some 20 or 30 minutes. Why? Because the passive arm locks to the handlebar and stays there, and to let go and stretch my back I need to stop and use my other hand to unlock the passive device. So to reposition my prosthetic arm on the handlebar, and to sit differently, particularly on a road bike with a drop bar, while riding, I do not want a passive prosthesis - I need an active prosthesis. When I bike, I sweat, and when I sweat, myoelectric control craps out, so I need a body-powered arm. That much is already clear to begin with, however. The most important start point there is the socket fit - and even for a pro prosthetic technician, avoiding friction rashes in any given user / client / patient may be a really hard problem to treat. So I would not let amateurs near my arm. Also, biking as such is critical in terms of speed, safety, accident risk. So I would really be careful about a really proper setup. I have posted a number of bike modifications and I have considerable riding experience, and these views are my own.
  • A climbing arm may be nice, but there, I would first buy one of the US-American climbing attachments, such as offered by TRS Prosthetics. Why? They are real professionals, and they understand that climbing first and foremost is an injury risk. So why 3D-print some plastic thing when I can have a metal climbing attachment?  Why not buy a really sturdy wrist connector? Why not get a really good socket fit?
  • For skiing poles, I tried and tried. The only one that works is the Advanced Skiing Pole adapter by TRS, see the demo here [link], and for that, you really need a body-powered arm, that is not a passive device.
  • For all these situations, where I do need a prosthetic arm for sports, the procedure is as follows:
    • (1) I need a socket that sits really tightly, and a type of control that really works under sweat - so I urgently need a body-powered arm, not a myoelectric arm, and I need the socket to be manually cast by a real professional prosthetic technician. Why? That is the *only* chance that I have to get a really good fit. Why do I need a good fit? So the socket does not wiggle or is too narrow. If it is too narrow, it is very painful. If it is too wide, it may slip or it may also cause friction rashes. Also, a socket is costly, and in Switzerland (and many other places), insurances do cover a body-powered arm as such. They do not cover fancy plastic devices of questionable provenance. So, a body-powered arm that then is built so it can also serve for a particular or several sports uses is the way to go.
    • (2) I need a wrist connector and a cable setup that really holds up. I made my experiences, hit me up if you have questions there - and invariably you will make your own experiences.
    • (3) The terminal devices need to be of superb quality - otherwise, no fun, injury risk, etc. so I rather they spend sufficient time making these than they are too fast, speedy and cheap. TRS Prosthetics has a great catalog of so many useful sports attachments that have been tested by many users already - no need to just copy their stuff to make cheap copies, no need for amateur approximations.

Also, there is no market. There just isn't. We get our basic setup so it conforms to insurance policies and works with minimal add-ons and adaptations - most functional, most reliable, most comfortable. Try it out, too.

Overview of sports thingies3:

Sport Passive arm Active arm
Any Passive arm needs to be well built, technician must be proficient. Myoelectric device dies with any sizeable amount of sweat.

Body powered arm needs to be well built, technician must be proficient.

Swimming Not needed / forbidden for competitions Not needed / forbidden for competitions
Biking Risky sport, control reliability is key. Passive devices that work are Mert Hand and Freelock. Stay away from experimental or new fragile or untested items. Safety first.

Passive grippers fix sitting position and for longer rides I find that to be a real problem.

Body powered arm alleviates all problems but only if it is well built. I find that both VO and VC grippers work well, but if you do not know what these are you probably should not be building stuff.
Gymnastics, athletics, exercises I use a TRS Dragon on my prosthetic arm. Works great. No active control needed from where I am standing.
Lifting body weights as in climbing You need superb suspension and a really superb wrist and device. Get the best you can. And then get insurance.
Golf I should probably try that sometime soon?
Tennis, table tennis Not needed. Not needed.


Medical products should comply with CE-marking legal requirements in Europe [link].

You may want to make sure, beyond some doubt, that what you try to understand as a product actually follows the requirements - and I did copy/repost and comment on some of the relevant content [link]. Check it out in all detail. This one may be really important.

Hardly any currently sold prosthetic arm component is, from my personal view, free of doubt in this regard, which is why I can not at all seriously recommend investing in so many commercial attempts. The risk you may run is as follows:

  • Product may be over-advertised - i.e., promises are made that are not kept: first of all, CE-marking requires promotional material and product descriptions to be accurate to the point. If products are over-advertised, consumers buy them with the wrong expectations. You will then experience furious customers if things go smooth, or even class-action lawsuits if things escalate.
  • Product limitations are not technically proofed and comprehensively listed, with error margins, in the documentations. If this is not done, consumers will invariably do their own testing and reporting and after that has taken place, you can probably forget trying to promote or advertise these products. Then, "product management by banana" (product matures at customer site) has gone full circle and you won't sell many anymore. Could have as well not invested, to begin with.
  • Product failure points and weaknesses are not openly communicated and declared. While CE-marking legally requires problematic aspects to be made available to any consumer, we do not buy prosthetic devices because they are perfect in each and every regard. Hell, no. We know they all suck in their own way. But if you do not make it explicitly clear that a certain prosthetic glove cover falls apart by itself after 3 months of storage, and the customer finds out, you do make yourself the laughing stock at least of the user community. Whereas if you put that into the "caveat" section of product documentation, you can tell any user to watch out. In a world of fragile overpriced items, visual awareness and great respect for the customer is the only way you can go, ever. Has not been tried so far, but I suggest that if you do go such a path, you can surely avoid a run-in with outspokenly and aggressively critical users such as me - and I can say for sure that I would have never made that many super negative experiences with prosthetic components had it not been for CE-non-compliant documentation and advertising. It is in your own deepest interest to avoid users that are hard on the devices. The best way to achieve that is by openly declaring that your device fails, breaks, dies, or comes undone, or also, that it is not tested for this or that application - or, better, list the ones you tested and warn the user that other applications were not tested.

If you tell me that a device is robust, then I will use my own definition of robust to use and test it. Simple as that.

And if you do not know my definition of "robust" (and mind you: a product being CE-marked does not mean the consumer has to clarify their definitions... no, the manufacturer has to do that) then you may as well call it quits right there.

So if you are interested in whether a manufacturer actually complies with CE requirements, check the details of the requirements and then investigate your situation. It is worthwhile to check that before investing any money. You do your own research, all I can do is raise relevant questions.

Referring to prosthetic hooks by using Peter Pan and Captain Hook metaphors

Both the subject of prosthetic arms and their technical intricacies and the subject of fairy tales with their symbolic aspects and gory realities, are difficult and multi-layered subjects.

And anyone can tell you that actual understanding of dynamic, non-linear, or otherwise twisted aspects requires mental capacities above average.

So, unreflected use of a complex pointer may be a dead give-away that you are dealing with someone that has no clue, hidden agendas, or both. In particular, if ever you run into people that tell you that prosthetic split-hooks are bad because of Captain Hook being an evil guy, please make sure you ever so thoroughly check these people and their arguments.

Because the Captain Hook metaphor is one of the most wicked and unfortunate pointers anyone could give you, ever [read this: link].

So if you start pretending that body-powered technology is outdated or has no potential, you clearly exhibit a very serious lack of technological understanding:

  • The technology that does have significant potential for further improvements along many directions is body-powered technology, whereas myoelectric technology has arrived at functional stand-still. This is detailed in this publication [link]:
  • Most notably, myoelectric technology and "bionic" hands suffer a range of inherent problems that are largely insurmountable.
  • Prosthetic split-hooks are more modern than hand shaped devices [link].

Problems of the technology that are inherent

The problem of selling a prosthetic arm is that, currently, out of the box or built with commercial conventional items, it does not really work. Sure, they are also expensive, but making them cheap does not make them really functional.

The prime issue is that they initially lack the functionality to a point where they become dispensable and unnecessary while causing a lot of problems and cost - particularly follow-up cost and efforts with used up time - as well as anger, frustration, or disappointment.

  • Myoelectric control and socket: A myoelectric arm will stop working once you sweat on your stump. That is it. It is an inherent severe limitation. The electrodes also won't work for a few other reasons, so they are not reliable. One sweats when running on a bus, when being nervous, or when feeling just a bit warm otherwise - so one sweats relatively fast even during a so-called sedentary everyday life. The electrodes risk causing skin burns if used with less sweating. These may take 4-6 weeks to heal or so, during which you cannot wear the prosthesis. With no sweating and just a day of typing at the office, the prosthesis causes my stump to run up blisters and abrasions that are so significant they take a week to heal, after which I am not inclined to repeat the experience. This applies to a sufficient number of people. With that, a realistic >85% of arm amputees are non-users. A further large portion of amputees that start using a myoelectric prosthesis drop out, as the systems are never reliable in any comfortable way, really. In over 40 years, research and development never attained any actually useful control error rate for myoelectric control. Any promises that "this is improved" anytime soon cannot be taken seriously, as that actually is technically impossible. If it were, don't you think we would know that by now? You can always try to look forward, but literature analysis of almost fifty years if research and development tell a totally different story. A more detailed technical analysis that describes in understandable terms what the intractable, unsolvable, inherent problems of myoelectric technology are, is found here [link].
  • Myoelectric hands: You can wear a prosthetic hook gripper on your myoelectric arm, but really, everyone wants a "bionic" hand. They are fragile, the gloves may tear up as fast as within 10 minutes or so, they are relatively heavy and if not, they are really weak. Their grip may really be not configured well to a degree, where gripping itself becomes awkward, cumbersome, difficult, hard, due to the ill devised geometry.
  • Failure aspects of myoelectric arms: A realistic downtime after wearing the "bionic" hand prosthesis for just 1 day, just to recover from the strain and skin damage, is 1-2 days to 6 weeks. A realistic time for the myoelectric prosthesis to break is some 10 minutes (glove: torn) until a few months (other parts). A realistic count of uncomfortable problematic control situations over a day is maybe around 20.
  • Body-powered arms: They are usually built wrongly, using a figure 9 harness with a thin strap, and, using the wrong type of cable housing. Their split hooks, particularly those of Otto Bock, have some fun features that may make them fail surprisingly fast. Their wrists fall apart surprisingly fast, too. While a body-powered prosthetic arm may not be that ridiculously expensive, you can only really improve them with substantial-good engineering. Large German and other companies in my experience do not sell parts that really hold up. We developed our own parts and have two patents. They are not really for sale, we patented these to protect us from larger companies so they do not sell our ideas back to us. More details here [link].
  • 3D-printed arms: their sockets and grippers are not useful for realistic everyday work tasks at all. The sockets that are of the usual 3D printed materials are likely to tear up an arm stump, over a day of hard garden work, quite painfully. The grippers are not expected to reliably operate dangerous or heavy machinery. The use of 3D-printing has been declared to be extremely experimental, so while a 3D-printed hand is relatively cheap, there is often an almost entire absence of true usefulness, so it is still overpriced no matter what. It is the experience of me that only a really comfortable and really robust arm will actually be of great use in everyday life, as a ground-rule, whereas a prosthesis built for light everyday activities is likely to fail even these on a regular basis [link].
  • A passive or cosmetic prosthetic arm is one of the more functional concepts from the view of durability and reliability. It is possible to mount that on the arm with a relatively comfortable suspension even though even there, I did experience a relatively severe friction rash after only 90 minutes of slow ballroom dancing, which took 2 days or so to heal.

Requirements for a successful prosthetic arm


  • The socket must not cause abrasions or blisters even if worn for heavy work with sweating for at least 1-2 weeks without break. Hardly any socket can be survived with an intact stump when wearing it even for just 1 day. This causes the amputee to experience severe down times - not being able to wear the prosthesis. After a year or two mostly not wearing the prosthesis, you as a user usually find out that it is better and more functional that way, anyway. That is the death sentence of the idea of "wearing a prosthetic arm". That death sentence may start with having to take long breaks to let the overused stump skin recover.
  • The socket is the main part of the prosthesis that has to both fit the stump perfectly well and transmit the control commands, either physically in body-powered arms, or electrically in myoelectric arms.
  • A myoelectric socket contains electrodes and with that, ridges. That alone is a problem.
  • A suspension that does not damage the stump under work constraints necessarily is layered with sophistication. It may contain a first tubular gauze layer, then a soft thick gel liner layer, and then only a carbon or epoxy socket. Other sockets are available but they are not unknown to cause real trouble for the skin - and do not survive long, as a prosthetic arm under continuous use.
  • A good socket compresses the soft tissues at the right spots and provides release in other regions of the stump. Thus, a really good fit is best achieved by manual casting, by a professional who knows what they are doing. Not so much by 3D-scanning of the uncompressed stump.


  • Control of a prosthetic arm must attain industrial levels of quality in grip performance [link].
  • An acceptable quality level will offer failure or error rates below three or four sigma, not just the usual academic "success rates" of 90-98% representing error rates of 2-10%. That spells out as 1 out of 10 broken coffee cups when unloading a dishwasher. Details of these considerations are found here [link].
  • Myoelectric prostheses have inherent serious control issues and thus one may have a hard time boosting their control error rates down to below some 0,1% which still is not great. Sweat, body or limb posture, electric interference, and skin issues are the most frequent problems there.

Grip geometry and forward reliability

  • Grip geometry must allow for realistically plannable grips [link]. Hands are less ideal than grippers or split hooks [link].
  • An ill-designed prosthesis that has unplannable grip geometries such as the iLimb may even be a liability to wear.


  • Load testing and failure testing has easily to be as rough as you can think. An elderly lady even asked a presenter of a Michelangelo Hand by Otto Bock whether one could do push-ups on that prosthetic hand, a while ago [link]. The man tried to dodge the question at first but the answer is clearly no. So, there.
  • Repairs: the prosthesis ideally is easy to fix, or better, can be repaired by the user. Prostheses that require costly remote repairs are likely to strand sooner or later.

Past actual experiences

Devices for developing countries

Devices for "bionic" fans

  • Current "bionic hands are either too heavy (Michelangelo) or far too fragile (BeBionic, iLimb). Status: unquestioned [i.e., link] in the community of users.
  • A few follow the fates of "brand ambassadors" for prosthetic hands, that are advertising representatives and that thus are getting their extremely expensive devices cheaper, but do not take their word for bare value - they just provide optimistic loyal sales talk. We all respect that. It may be really hard to smuggle too crappy parts past people that know what they see when they see it. Never fall for their assertions.

Devices and parts for users of body-powered technology

  • The TRS Prehensor, a body-powered device, is very, very popular among users. It has also won the Cybathlon. Status: unquestioned.
  • TRS Jaws: very well accepted by fans of "bionic" design, I get compliments for that device all the time.
  • The Becker Hand (body-powered as well) can make manual workers very, very happy. Status: so far, that product is not too well known. No reports of that hand not working. Becker hands have been reported to be too weak by people that failed to go ahead to actually increase their spring settings (the hand can be tuned to higher spring settings but you have to do it in order to achieve that... it will not tune itself all by itself).
  • Alpha liners and Molnlycke tubular gauze work like a charm in sweaty heavy environments. Status: confirmed over about 5 years of continuous heavy use, by myself, confirmed by others.

Word of mouth gets around fast usually and most of us users are quite utilitarian in our needs.

Recommendations to build good parts

Validation, testing

  • Test locally, with users that are close and that you know.
  • Test hard, with users that go to the limits.
  • Test yourself: wear these grippers, but do so diligently as if you were one of us, so
    • Wear it daily
    • Perform all real manual duties with these on, like cutting hedges, unloading dishwasher, loading dishwasher, changing bedsheets, putting stuff back from clean dishwasher, assembling IKEA furniture, assembling a 3D printed gripper or hand, swapping a wrist watch battery, prepare dinner, ride bike for a real trip, vacuum clean a building (not just 10 square inches), and so on. Remember, an amputee may be stuck with your device - so try out that.
    • If you have a user, that tests for you, best to perform side by side testing, compare your device against what the user is wearing otherwise, such as a particular hand or split hook device. This also may lead to a better understand how "old" [link] split hooks are better than many competing devices: they are very, very mature.
    • Document all, analyze problems, fix problems, try again.
  • Test so you have all material for a CE-conformant product description. You should consider to provide CE-compliant documentation (and have advertising follow that). So instead of saying "this is the most robust device on earth", just say "this craps out after such and such use" or "this is expected to perform as such and such but has not been field tested" [link].

Technical grippers

  • Read about angles of grips [link]
  • Read about surface covers of grips [link].
  • Particularly for technical grippers, also give upper limits of performance / failure in your documentation and advertising.
  • Particularly for grippers, assume that the user wants a higher or lower grip force, allow for parts / spring / force calibration or swap, allow the user to access grip force options at any level easily. Provide spare parts for variation, i.e., stronger and weaker springs, explain how the device grip force can be tuned or modified.
  • Particularly for grippers, provide the user with consumables or swap parts - so when the device relies on plastic parts, make sure the user gets their hand(s) on supplies there. If you do not plan on getting rich, make sure any 3D-model is uploaded somewhere where users can easily get to it, such as on Thingiverse or the likes.
  • Particularly for grippers that are assembled with terminal gripper portions or claws, that can be exchanged, make sure the users that want to build their own gripper parts can get easy access to the 3D shapes of your interface / mount parts. It is always a bitch if the user has to start by reverse engineering your work - sooner or later, you force them to become better at what you want to be the provider of.

Hand shapes

I will wear a hand rather than a gripper, prehensor or hook when I feel that a hand shape is relevant. So the hand has to perform as a gripper but more importantly, it has to look like a hand.
  • Hand shapes must look the part. By far the best prosthetic hand look, as that what it is, in my view, is conveyed by the Monestier hand [link]. If ever you want to test how the hand looks, consider my totally unforgiving Appearance Test [link].
  • Hands usually have relatively low grip forces, for a few reasons, so best to make sure your customers can get the widest variety of perfect gloves no matter what. Regardless of these, they may still provide great grip function particularly if the hand is covered with a glove. Prosthetic hands may need a cover just to protect the mechanism from fluid or solid elements (such as from detergent when cleaning or from pizza pieces when eating pizza), but more importantly, hand covers are there to provide soft deformation as surface property that may significantly improve the grip, and last but not the least, whenever I wear a hand and not a split hook, I may want to wear a "cosmetic" glove to make the prosthesis look more real or at least less unnatural than it already is. There, sturdy / robust / affordable / available always is better than fragile / expensive. So to get a newly designed prosthetic hand to fit into all work gloves sold in hardware stores, its dimensions must match the dimensions of these gloves - and you can try that out while making that hand. If you want to avoid the massive cost of offering custom gloves for your custom shaped hand, make your hand shape and dimensions fit whatever is already available. Regal Prosthesis in Hong Kong offer high quality and great looking silicone glove covers, to name one, also Fillauer / Centri (Sweden) has good gloves, I remember buying good quality PVC gloves there. But, you do your own research, see what you come up with.


The ideal control is body-powered control.

  • Be aware of your own bias and feedback of your users. If the control sucks, the user may not even tell you about just how disappointed they are. They will silently put the device away. Reality has it, that, even if we knew of "product ambassadors" of, say, large brand produced "bionic" prosthetic hand, of brand ambassador type people that appeared to always assert just how great the product was, on every TV show, that their associates would tell everyone else that these "product ambassadors" would not wear the device one bit for everyday life and work  - but we could not necessarily comment in more detail. But if we knew these existed, we would not necessarily make it a big thing, and so, there are realities you may not know about.
  • Really good body-powered control is not a given and cannot be bought from Otto Bock, Ossur or Fillauer, or anyone else. But it can be built. It requires two things: well engineered cable housing and well engineered harness / shoulder brace design. Check my path to solving these problems [link], then use that or do it better.
  • Conversely, reliable myoelectric control in all likelihood cannot be built.

Technologies to pursue, to build

  • Body-powered control
  • Ultrasound-based control - far better muscle / tendon differentiation in theory, not intimidated by sweat in theory, very finely grained control variations possible in theory


  1. I should emphasize this is a technical right below elbow amputee issues blog, not a blog about other types of disabilities.
  2. I should emphasize this is a technical right below elbow amputee issues blog, not a blog about other types of disabilities.
  3. I should emphasize this is a technical right below elbow amputee issues blog, not a blog about other types of disabilities.

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Experienced user based advice for investors in the domain of prosthetic arms [technical guide for understanding the field]; published 27/07/2018, 15:17; URL: https://www.swisswuff.ch/tech/?p=8702.

BibTeX: @MISC{schweitzer_wolf_1620293553, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Experienced user based advice for investors in the domain of prosthetic arms [technical guide for understanding the field]}}, month = {July},year = {2018}, url = {https://www.swisswuff.ch/tech/?p=8702}}