Afferent electrical nerve stimulation for sensory feedback in hand prostheses. Clinical and physiological aspects

Abstract

The development of externally energized hand prostheses has been very rapid during the past two decades. The prostheses' control systems have reached a high degree of sophistication. Nevertheless, motorized prostheses are difficult to control as they supply the amputees with much less feedback information about movement, position, and force than a conventional cable-operated device. This creates a demand for sensory feedback systems in externally energized hand prostheses. It is concluded from a review of current neurophysiological theories on kinesthesis that a properly designed myo-electric control system may yield significant amounts of feedback information through action of muscles at the myoelectric control sites. Therefore it seems reasonable to believe that the need for purely artificial sensory feedback systems can be reduced but not totally abolished by further development of control systems. Electrical stimulation of nerves in the amputation stump can be used as an artificial system to convey sensory feedback from hand prostheses. The method has several advantages consistent with the principle of prostheses self-containment, an important factor for patient acceptance of prostheses. In an experimental study performed on both normal subjects and on amputees, electrical stimulation was applied to nerves in the forearm through percutaneous, intraneural wire electrodes. The subjects' capacity to discriminate changes in stimulation intensity and frequency was studied both with intermittent stimulation and in a tracking task using continuous stimulation. Optimal stimulation parameters for sensory feedback purposes were defined. The interference between the nerve stimulation current and various myoelectric control systems was measured on human subjects and in vitro. Nerve stimulation was found to be readily accepted by the subjects. In the amputees the sensations were located to the phantom hand. The discrimination capacity was deemed to be sufficient for sensory feedback applications. In the tracking performances the delay of the response was considerable, but was diminished by training to an acceptable level. The interference between the electrical stimulation and the myoelectric control systems was shown to be insignificant in amputation stumps of ordinary lengths for the pick-up systems studied. This was also corroborated in theoretical calculations using the electrical field theory. From this study it is concluded that sensory feedback from externally energized hand prostheses primarily should be derived from the operation of the myoelectric control systems. Feedback information not obtainable in this way can be transmitted through electrical stimulation of the stump nerves. This method is safe, reliable, and consistent with the concept of prosthesis self-containment.