To grip well and with ease (as already investigated earlier here), the gripper parameters usually are constrained to reside along that diagram:
With a solid slippery gripper, a complex, heavy mechanism requires at least 5 fingers to stabilize an object in 3D space (if not seven). See left side of the x-axis of that diagram above: the harder the grip surface (metal, wood, plastic), the more fingers are needed and thus the more complex the mechanism will be. Advantages of slippery smooth hard surfaces are the possibility to clean them thoroughly, and their longevity.
With increasing surface friction, less fingers are required. A simple prosthetic hook just has two "fingers" and optimally, both grippers are covered with silicone tubing. Grip is just as good as a heavy complex hard surface hand - but construction is a lot less complex and the weight can be kept a lot lower (or at same weight, the gripper is more sturdy). See right side of that diagram just above.
For grippers at that end of the diagram (far right), it is worthwhile to note that despite gripping beauty, soft deformable high friction surfaces wear down and require replacement. I have gone through tapes or gloves as rapidly as one set a week. Then, you need materials you do not have to order through special channels that cost a fortune but are straight forward.
Now, we are not free in our choices for everyday usage of a prosthetic. I recently learned of a ~ 7kg research prosthesis for robotic research of a multi-articulated prosthetic hand. The researchers could wear it for 20 minutes before their arms would start shaking.
Long term usage of a prosthetic arm forces a light design. There is no way anyone will wear a heavy arm.
As Michelangelo arm demos show, considerable weight also appears to accompany that new arm of Otto Bock's and the shoulder is pulled up tightly:
Shoulder, elbow and skin issues prevent long term satisfaction with heavy arms. Myoelectric arm issues are not a thing of the past. I shared my own seriously negative experiences with others. Two quotes from people knowing myo arms well:
Quote 1: "I have not yet seen any myo that can out perform a body powered unit unless there is no residual limb left to work with." (50 years of experience)
Quote 2: "And everything you describe is how it was for me. The elbow was pushed inward to fit tight over my elbow, to help hold the thing on...yeah right! The tiniest bit of sweat and it would slide down and pinch my elbow bones to the point of pain. (...) And so, besides the cost of the thing, that's why I don't wear anything anymore." (20 years of experience)
So, myo arms are not an option. Body powered arms are cool but harness issues remain a serious subject. So what else?
Overlooked by roboticists and prosthetists is the fact that at that end of the diagram, there are two further very useful solutions that are non-robotic - the stump, and, the stump after surgery. There is a surgical procedure called the Krukenberg procedure that turns a forearm stump into a two-finger pincer.
Even though forearm skin does not have the same 2-point discrimination that fingers have, the remaining sensitivity can help in a number of situations.
Self replacing deformable grip-optimized surface (skin) is there on the stump, and using that as 1 finger to press objects against a surface does not cause holding maneuvers to be that bad.
So far, using the stump is by far the easiest way for me to put a wristwatch on or take it off. It is by far the easiest way to type as despite the blunt rounded end, I can feel the key edges and type relatively precisely on an edgy keyboard such as an IBM type M keyboard.
Also, one employs 'manualization' not just of the whole body but of the immediate environment. With that, pure know-how and tricks will allow one to be largely self-sufficient without any prosthetic arm - which itself causes trouble, pain, skin damage, cost and time for me to keep up.
With the Krukenberg procedure, two pincers are made from the anatomical material contained in the forearm.
However, Swiss bioethicists nowadays talk Swiss health insurances out of approving this procedure, saying that it is an act of "human enhancement", whereas we know that maybe 10 years ago they still asked arm amputees to get it done. This is ethically and legally interesting, as it is an unprecedented combination of readiness to inflict further suffering (prosthetic arms do that), total disregard of the fact that a disabled body can usually not be enhanced, only maybe made a small bit less disfigured or disabled (but still), and a definitely vast ignorance of the fact that all other options (no Krukenberg surgery, ongoing usage of prosthetic arms) cost a lot of more money - so much more, that one seriously will question these people's state of mind.
Photos (C) Copyright Laury Dahners
Division of radius and ulna accomplished by the Krukenberg procedure. from "The Krukenberg Procedure in the Juvenile Amputee," by Alfred B. Swanson, M.D., Journal of Bone and Joint Surgery, Vol. 46-A, October 1964.
There is a presentation of two forearm pincers in action:
The skin is cut to obtain flaps to cover the pincers. Forearm muscles that help open/close the radius and ulna pincer are left in place. Also and to some degree, padding is left in place to allow for deformable pincer surfaces. To have enough forearm skin available to cover both pincers, all other forearm stump muscles are removed and thus not available any more for prosthetic control. That results in a slightly less bulky stump that then functions as pincer and that is covered with sensitive skin.
What results is a forearm gripper that yields far better acceptance rates with amputees than any prosthetic arm ever rated. Its technically deformable surface (skin) does not require particular efforts with frequent replacement but heals by itself. Practically all amputees that agreed to get the procedure done would get it again. That is definitely not the case for wearers of prosthetic arms.
A more rational point-by-point comparison of advantages and problems shows that indeed, a Krukenberg arm leads both in terms of performance and quality of life.
Of course it looks "different" - but what most people do not realize is that firstly, with a missing hand, one looks different no matter what, and secondly, quality of life with limb loss is determined by many other factors than just the presence of a rubber hand.
Krukenberg arms look "different" largely on still photos - in real life, even non-disabled people rated Krukenberg arms better than prosthetic arms when asked what they would rather see being used by an amputee. The thing is that seeing these pincers in action will convey a very natural and dynamic impression as they indeed are natural body parts - something no prosthetic arm can ever do. As that, they are far more sympathetic than some of the recent prosthetic anchors that mainly succeed in overloading shoulders and backs.
Full literature search result [PDFs, ~ 38 MB]
To allow for a faster update on the issues at hand, I put up a file archive with relevant PDF publications about the Krukenberg stump, long term results and complications, options for technique and so on.
Copyright (C) by various publishers.