Design and technical construction of a tactile display for sensory feedback in a hand prosthesis system [article review]
I just read the recent article titled 'Design and technical construction of a tactile display for sensory feedback in a hand prosthesis system' by Christian Antfolk, Christian Balkenius, Göran Lundborg, Birgitta Rosén and Fredrik Sebelius of the Department of Electrical Measurements, Lund University, Lund University Cognitive Science and Department of Hand Surgery of the Malmö University Hospita, all in Sweden.
Abstract [footnotes and highlights by me]:
Background - The users of today's commercial prosthetic hands are not given any conscious sensory feedback [a]. To overcome this deficiency in prosthetic hands we have recently proposed a sensory feedback system utilising a "tactile display" on the remaining amputation residual limb acting as man-machine interface [b]. Our system uses the recorded pressure in a hand prosthesis and feeds back this pressure onto the forearm skin. Here we describe the design and technical solution of the sensory feedback system aimed at hand prostheses for trans-radial/humeral amputees. Critical parameters for the sensory feedback system were investigated. Methods - A sensory feedback system consisting of five actuators, control electronics and a test application running on a computer has been designed and built. Firstly, we investigate which force levels were applied to the forearm skin of the user while operating the sensory feedback system. Secondly, we study if the proposed system could be used together with a myoelectric control system. The displacement of the skin caused by the sensory feedback system would generate artefacts in the recorded myoelectric signals. Accordingly, EMG recordings were performed and an analysis of the these are included. The sensory feedback system was also preliminarily evaluated in a laboratory setting on two healthy non-amputated test subjects with a computer generating the stimuli, with regards to spatial resolution and force discrimination. Results - We showed that the sensory feedback system generated approximately proportional force to the angle of control. The system can be used together with a myoelectric system as the artefacts, generated by the actuators, were easily removed using a simple filter. Furthermore, the application of the system on two test subjects showed that they were able to discriminate tactile sensation with regards to spatial resolution and level of force. Conclusions - The results of these initial experiments in non-amputees indicate that the proposed tactile display, in its simple form, can be used to relocate tactile input from an artificial hand to the forearm and that the system can coexist with a myoelectric control systems. The proposed system may be a valuable addition to users of myoelectric prosthesis providing conscious sensory feedback during manipulation of objects[c].
[a] False assumption. I wear a hard prosthetic arm and do get sensory feedback. Check your sources, there are studies highlighting that advantage of body powered arms.
[b] As far as I can see they never showed this on amputees. They did show it on non-amputees only. this is a critical difference (see below).
[c] They showed function only on non-disabled subjects.
Tactile feedback to a body part seems like an attractive option indeed. However, upper extremity prosthetics seems like an extremely difficult field for non-disabled researchers as intricate problems, strain, wear and tear, reliability, cost and other factors all present a far different reality than appear to be apparent in the laboratory.
A stump is different from a sound limb in a range of ways. One reason why sockets are so much better than other options to fix the prosthetic arm to a stump is the protection it offers. Secondly, there are trigger points that can trigger phantom sensations or pain . These are located on the stump, in the vicinity of the stump but also elsewhere in the body. Neurologically, not only cortical remapping but also thalamic and subthalamic connections are responsible for that. Last but not the least, non-triggered but ever present phantom sensations and pain may just be suppressed. Directing attention to stimuli on a stump may then lift the suppression of pre-existing phantom sensation and pain and thus cause distress. So I would be extremely cautious to not overinterprete initially promising results regarding feedback systems on non-disabled volunteers. Personally, I wear a prosthetic arm to keep the congested stump tissue compressed and well ciruclated, and also to keep any such irritation or touch away from my stump. I get a lot of phantom sensation/pain problems from minor triggers, and irritation or skin feedback even on my upper arm is not tolerable. I would definitely even pay extra to not have any such feedback system on my stump or upper arm.
A second issue is additional battery weight. A real problem for upper limb amputees is chronic shoulder strain and rotatory cuff problems as early as five years after amputation, for many after 10-15 years after amputation. It can affect both sides if one arm is intact and the other one disabled. A far as I can see, no current academic research into upper extremity prosthetics mentions or respects that. Prosthetic weight and design play a paramount role. My current body powered arm weighs 570 to 750 grams (depending on the terminal device), a light myoelectric arm I have weighs around 1400 grams, a regular myoelectric arm may be as heavy as 1800 grams or more. Center of gravity has a big impact to coordination and muscle strain as well. While I cannot use intricate finger like motion to adapt to any task, I have a range of terminal devices with useful geometries that allow me to do many repetitive tasks with a symmetric balanced posture and avoid shoulder, back or neck strain. Prosthetic weight must be minimal and additional battery power and weight should be extremely well justified.
Thirdly, I cannot say that crude force feedback is absent from my prosthetic arm. I wear steel hard components, a Ramax milled ball lock wrist and an Ossur pin lock and I can blindly type (5+1 fingers), find light switches without looking and can confirm scientific studies that showed body powered arms to actually provide feedback. My stump is extremely sensitive and I feel minor bumps, mild touches on both socket and terminal device. I am not sure whether current feedback research is aware of the fact that good prosthetic function using body powered high tech does provide at least as much feedback as is needed to go through the day. What will be interesting will be to see really critical tasks being done that now require visual checks (e.g., pulling a bank note out of trousers pocket without looking, taking one but not two pieces of toast bread out of bread bag, feeling when an object starts to minimally deform under too much pressure from the prosthetic device).
 Ramachandran VS, Rogers-Ramachandran DR (2000) Phantom Limbs and Neural Plasticity. Arch Neurol 57:317