P. Hunter Peckham

An Externally Powered, Multichannel, Implantable Stimulator for Versatile Control of Paralyzed Muscle

Through the use of semicustom integrated circuit technology, an implantable muscle stimulator has been developed. The unit is small, lightweight, has low power consumption, and is intended for permanent usage. The stimulator circuitry is externally controlled and powered by a single encoded radio frequency carrier. Up to eight independently controlled stimulus output channels are provided, with output channel selection, stimulus pulse width, and stimulus pulse frequency under external control. A constant current biphasic stimulus pulse is used, in which the stimulus current amplitude can be preset by a single resistor value. The stimulator circuitry has been implemented in thick film hybrid form, and has undergone laboratory evaluation for 48 months.

Modulation of Muscle Force by Recruitment During Intramuscular Stimulation

The input¿output relationships for modulation of force by recruitment during intramuscular electrical stimulation were examined for cat sleus muscles and human finger and thumb muscles. Recruitment was modulated by varying either the pulsewidth or amplitude of a monophasic, rectangular, cathodal current pulse train. Force was a nonlinear function of either pulsewidth or amplitude, and the shape of the nonlinearity was the same regardless of which parameter was modulated. The charge per stimulus pulse was lowest if pulsewidth was modulated with a fixed, high amplitude stimulus. The shape of the nonlinear relationship between pulsewidth and force (recruitment characteristic) depended on stimulus amplitude, electrode location in the muscle and muscle length. In most applications the amplitude and location would be fixed, so force would be a two-dimensional nonlinear function of pulsewidth and muscle length. The results are discussed with respect to possible mechanisns of recruitment during intramuscular stimulation, and the implications of the nonlinearities on the proportional control of orthoses employing electrically stimulated muscles.

Implantable functional neuromuscular stimulation in the tetraplegic hand

Functional neuromuscular stimulation of the upper extremity provides manipulative capacity to persons with high level tetraplegia who have insufficient voluntary muscles available for tendon transfer surgery. We report an enhancement of the technique to include surgical implantation of a multichannel receiver-stimulator, sensory feedback stimulation, and tendon transfers. Tendon transfers were done with spastic, rather than voluntary motors employing standard surgical techniques. The system described has been operational for more than 1 1/2 years.

Restoration of key grip and release in the C6 tetraplegic patient through functional electrical stimulation

Electrical stimulation of selected paralyzed forearm and hand muscles in C6 spinal cord injury patients provides control of lateral pinch (key grip) and release. The movement augments tenodesis grip or the grip achieved through tendon transfer procedures. Chronically indwelling percutaneous coiled wire electrodes were implanted with hypodermic needles into thenar (adductor pollicis and/or opponens pollicis), finger flexor (flexor digitorum superficialis and profundus), and thumb extensor (extensor pollicis longus) muscles. The strength of contraction is controlled by changing the stimulus pulse width and frequency, which determine the number of active muscle fibers and their rate of activation, respectively. Finger flexor activation always precedes thumb adduction /opposition to provide a stable platform for lateral pinch; release is provided by stimulation of the thumb extensor. The patient controls the timing and the strength of the contraction from a single control signal. This control signal is a myoelectric signal (MES) from a muscle which retains voluntary function, e.g., sternocleidomastoid. Electrical activity from the muscle is processed (rectified and averaged) and used to activate the stimulator. Each command to control the muscle is proportional in time, i.e., once a lower threshold bound is exceeded with the MES, the command increases (or decreases) linearly until the MES falls below threshold again. A zero level MES maintains the stimulus; exceeding the upper bound reverses the direction of stimulus change or deactivates stimulation if the level is held long enough. Five subjects have been involved in the development of this system for periods of up to 2 years, and three are presently involved in its evaluation. The function the electrical stimulation provides has been beneficial in performing tonic tasks such as eating and writing.

An Implanted Upper-Extremity Neuroprosthesis. Follow-up of Five Patients*

An implanted neuroprosthesis supplying functional neuromuscular stimulation was used to provide grasp and release to tetraplegic individuals. This article describes the results, at a minimum of three years, for the first five patients to have operative implantation of an eight-channel stimulator-receiver. All of the patients had a clinically complete spinal cord injury with motor function remaining at the level of the fifth or sixth cervical nerve root. In addition to implantation of the stimulator system, each patient had augmentative operations on the hand to improve function. The procedures included tendon transfers, side-to-side tendon anastomoses, arthrodesis of the interphalangeal joint of the thumb, and rotational osteotomy of the radius. The neuroprosthesis provides two grasp patterns controlled by voluntary motion of the shoulder or wrist. Functional evaluations included measurement of pinch force, a grasp-release test, evaluation of the level of functional independence, and usage surveys. Pinch force ranged from eight to twenty-five newtons. All five patients demonstrated functional grasp patterns, had increased independence, and were able to use the neuroprosthesis at home on a regular basis. The implanted stimulator has proved to be safe and reliable, with seven years as the longest time in situ at the time of writing.

Development of a quantitative hand grasp and release test for patients with tetraplegia using a hand neuroprosthesis

We developed a quantitative grasp and release test for assessing a hand neuroprosthesis in C5 and C6 level tetraplegic patients. The objectives were (1) to determine if a patients hand performance with the neuroprosthesis exceeded a defined, clinically acceptable baseline, (2) to compare performance with and without the neuroprosthesis, (3) to measure the consistency of performance over time, and (4) to compare performance among patients. In the test, patients grasped, moved, and released one of six different objects as many times as possible in five 30-second trials for each object, with and without the neuroprosthesis. Unlike earlier tests, the objects and the task were chosen to span a range of difficulties appropriate for C5 and C6 tetraplegic patients using a hand neuroprosthesis. Data from five patients showed that performance with the neuroprosthesis was above the baseline; performance improved with the neuroprosthesis, although it was not generally consistent across sessions; and the neuroprosthesis helped C5 patients manipulate most objects and helped C6 patients primarily with more difficult objects.

Closed-Loop Control of Force During Electrical Stimulation of Muscle

The control of contractions elicited by electrical stimulation of muscle could be improved if there was a linear repeatable input-output relationship. The input is the command to the controlled stimulator and the output is the evoked contraction. Systems employing closed-loop force feedback to provide regulation of contractions were investigated in these studies. Force was modulated by both recruitment and temporal summation during intramuscular stimulation. Closed-loop systems with combined proportional and integral control were found to be stable and linear and to have good compensation for variations in muscle properties. A low proportional loop gain (approximately unity) was found necessary to prevent oscillation when modulating recruitment. Ratios of integral to proportional gain of about 10 gave the fastest response without compromising stability. The response time of the closed-loop system was as fast as or faster than the open-loop system.

The choice of pulse duration for chronic electrical stimulation via surface, nerve, and intramuscular electrodes

The peak current, peak voltage, charge transfer and energy dissipation necessary for equivalent stimulation were measured for several pulse durations in the range from 0.01 to 1.0 msec. The unidirectional, regulated current, rectangular waveform was studied for subcutaneous nerve and intramuscular stimulation in animals and for surface stimulation in humans. In addition, the unidirectional, regulated current, exponential waveform was studied in humans and was compared with the rectangular waveform. The question of the relationship between charge transfer and energy dissipation and possible tissue damage due to the electrochemical formation of toxic compounds or a temperature rise in the surrounding tissue was examined. The optimal pulse duration for reducing the possibility of tissue damage was concluded to be less than or equal to 0.01 msec for intramuscular stimulation in the test situation. No conclusion was made as to the optimal duration for nerve or surface stimulation. Excitation of muscle fibers was found to take place indirectly by was of muscle nerves during intramuscular stimulation. The exponential waveform required less charge transfer and energy dissipation than the rectangular waveform, but higher peak currents.

An Implanted Upper-Extremity Neuroprosthesis Using Myoelectric Control

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.

Tendon transfers and functional electrical stimulation for restoration of hand function in spinal cord injury

Spinal cord injury at the C5 and C6 level results in loss of hand function. Electrical stimulation of paralyzed muscles is one approach that has demonstrated significant capacity for restoring grasp and release function. One potential limitation of this approach is that key muscles for stimulation may have lower motor neuron damage, rendering the muscles unexcitable. We have used surgical modification of the biomechanics of the hand to overcome this limitation. Tendon transfer of paralyzed but lower motor neuron intact muscles can compensate for potential function lost owing to muscles with lower motor neuron damage. Such procedures have been performed to provide finger extension, thumb extension, finger flexion, and wrist extension. Additional surgical procedures have been performed to enhance the function provided with electrical stimulation. These are side-to-side synchronization of the finger flexor and extensor tendons, the flexor digitorium superficialis Zancolli-lasso procedure, and thumb interphalangeal joint arthrodesis. These procedures have been performed in 11 patients with C5 and C6 level spinal injuries and functional electrical stimulation neuroprostheses. In these patients, 41 different functional electrical stimulation-related procedures were performed and 38 gave the desired result after surgery. One procedure resulted in no increase or decrease in function or muscle output, and two procedures resulted in a decrease in muscle force or joint range of motion. The issues that must be considered in performing functional electrical stimulation-related tendon transfers are discussed.