Title: A smart electrode housing to improve the control of upper limb myoelectric prostheses
Funder: Department of Health and Social Care

Abstract: In 2012, there were almost 400 referrals to UK disablement service centres after upper limb amputation [1]. Myoelectric prostheses are designed to restore appearance and some degree of function. They are controlled using EMG (myoelectric) signals generated on contraction of remaining musculature within the residual limb, acquired by socket-located electrodes. Despite considerable efforts to improve myoelectric prostheses, there remains significant evidence of dissatisfaction, with self-reported rejection rates between 23% and 35%. Even amongst upper limb absent individuals who have not rejected their prosthesis, a heavy reliance on the intact limb appears to be common. The reported reasons for rejection and/or limited prosthesis use include weight, comfort and, of most relevance to this bid, poor prosthesis control and limited functionality. A key problem faced by many myoelectric prosthesis users is the unreliability with which the signal is transduced by socket-located electrodes. The myoelectric signal is highly sensitive to the &ldquo;location&rdquo; (position and orientation) of the electrode with respect to the muscle fibres. Furthermore, maintaining good electrode-skin contact in the presence of the forces acting on the socket during daily life is extremely challenging. These forces change with the orientation and movement of the arm, as well as with interaction with objects. In the current design, the electrode moves with the socket, which often leads to socket motion artefacts, causing false activations of the prosthesis. To solve these problems we will develop a smart electrode housing that provides: a) partial mechanical decoupling of the electrode from the socket (to reduce false activations); b) contact pressure adjustment; and c) electrode location adjustment. We will also develop a smart signal monitoring system for estimating the likelihood of EMG socket motion artefacts and rejecting the control signal when the likelihood is high.

Lay abstract: The effect of upper limb absence can be very hard on the individual, both physically and psychologically. The most advanced artificial limbs are called myoelectric prostheses, controlled by muscles within the user s stump. These muscles create small electrical signals when they contract, which are measured from electrodes housed in the prosthetic socket, and used, for example, to open or close the hand. Most users find it very hard to control their prosthesis. Since the electrodes are fixed in position within the socket, they are likely to move when the socket moves with respect to the skin. If they do move, the measured control signal can be disrupted, sometimes leading to the hand either opening unexpectedly, or not opening when the user wants it to. Understandably, users find this frustrating and perhaps 1 in 3 people report stopping using their prosthesis altogether. Our recent work has also shown that even people who continue to wear their prosthesis rely heavily on their intact arm. Over the last 10 years, more sophisticated hands have come on to the market and impressive marketing videos suggest they are remarkably good replacements for the human hand. However, in reality users only have limited control of these prostheses, which falls short of allowing them to coordinate anything like the number of joints we effortlessly control with our anatomical intact hands. Despite considerable research to address this problem, the vast majority of studies have not progressed beyond the research phase, partly as a result of the electrode contact problem, and hence have yet to benefit patients. In this project, we will enhance the reliability of prosthetic hands with two approaches. First, we will design a mechanical housing for the electrodes which allows the user to adjust the contact with the skin and also uses mechanical decoupling (so the electrode is not dragged over the skin by the socket) to reduce the likelihood that any small movements of the socket will result in movement of the electrodes. Secondly, we will use information from movement sensors on the arm to give the controller (the software which takes the user s muscle signals and produces instructions to the prosthetic hand to move) a better chance of distinguishing real from unintended signals. In parallel to this, we will build on our work to monitor how these devices are used in everyday life, data which is likely to be of great value in the future in persuading NHS commissioners of the value of these new devices. We will integrate these developments in a new prosthetic hand, being developed by our industrial partner, COVVI. Finally, we will compare how well and how often people use the new COVVI hand with a clinical standard myoelectric hand.

Research activity codes: 5.3 MEDICAL DEVICES
Health categories: INJURIES AND ACCIDENTS;CONGENITAL DISORDERS