An implanted upper-extremity neuroprosthesis using myoelectric control

Abstract

Purpose: The purpose of this study was to evaluate the potential of a second-generation implantable neuroprosthesis that provides improved control of hand grasp and elbow extension for individuals with cervical level spinal cord injury. The key feature of this system is that users control their stimulated function through electromyographic (EMG) signals.

Methods: The second-generation neuroprosthesis consists of 12 stimulating electrodes, 2 EMG signal recording electrodes, an implanted stimulator-telemeter device, an external control unit, and a transmit/receive coil. The system was implanted in a single surgical procedure. Functional outcomes for each subject were evaluated in the domains of body functions and structures, activity performance, and societal participation.

Results: Three individuals with C5/C6 spinal cord injury received system implantation with subsequent prospective evaluation for a minimum of 2 years. All 3 subjects demonstrated that EMG signals can be recorded from voluntary muscles in the presence of electrical stimulation of nearby muscles. Significantly increased pinch force and grasp function was achieved for each subject. Functional evaluation demonstrated improvement in at least 5 activities of daily living using the Activities of Daily Living Abilities Test. Each subject was able to use the device at home. There were no system failures. Two of 6 EMG electrodes required surgical revision because of suboptimal location of the recording electrodes.

Conclusions: These results indicate that a neuroprosthesis with implanted myoelectric control is an effective method for restoring hand function in midcervical level spinal cord injury.

Figure 1

Drawing of the second-generation neuroprostheric system. The internal components include the IST-12, 12 stimulating electrodes (not all shown), and 2 recording electrodes. The external components include the control unit and the transmit/receive coil. This system provides grasp/release, forearm pronation, and elbow extension with myoelectric control for improved function in the tetraplegic spinal cord-injured individual, MHS, myoelectric signal.

Figure 2

This graph shows the relationship between the recorded ECRL EMG (thin line, left axis) and stimulus command (thick line, right axis) as a subject closes their hand (1–5 seconds), locks their hand (6–7 seconds), unlocks their hand (7–9 seconds), and opens their hand (9–10 seconds). The recorded EMG is filtered by an adaptive step-size filter to produce the proportional command signal used to control grasp opening (0% command) and grasp closing (100% command). Between the lower and upper thresholds, the input to the adaptive filter is proportional to the level of the EMG. With each successive sample in which the change in signal is in the same direction as the previous sample, the allowable step size is increased. When the direction changes, the step size returns to a minimum value. When the command is above the “lock threshold” for 2.5 seconds, the command is locked at the 100% command level. To return to active control, the user generates 2 EMG twitches with a 1-second interval between them. When the unlock signal is generated, the command gradually returns to complete active control over a period of 1 second in order to avoid abrupt changes in command level. MES, myoelecctic signal; A/D, analog to digital.

Figure 3

Grasp force results for all 3 subjects. Grasp force obtained before surgery is due to passive tenodesis function (no active thumb flexion). Changes in grasp force after surgery with the NP turned off are due to augmentative surgical procedures, such as tendon transfers, and changes in passive properties of the hand. The increased grasp force with the NP on is due to the force provided by AdP and FPL stimulation. AdP, adduct or pollicis.

Figure 4

Subject performing embroidery using the NP. A palmar grasp pattern, providing tip pinch between the thumb and index finger, is used to hold the needle.