Functional electrical stimulation (FES) has been used experimentally to provide grasp and release for individuals with a spinal cord injury at the cervical level for many years.9,19-24 The objectives of these systems are to reduce the need of individuals to rely on assistance from others, reduce the need for adaptive equipment, reduce the need to wear braces or other orthotic devices, reduce the time it takes to perform tasks, and enable individuals to perform tasks more normally. Neuroprostheses using FES make use of the individual's own paralyzed musculature to provide the power for grasp and the individual's voluntary musculature to control the grasp. Stimulation is applied to the muscles of the forearm and hand via implanted electrodes to provide grasp and release. Grasp movement is controlled using voluntary movements generated by the quadriplegic individual. A controller/stimulator is responsible for converting the command signal obtained from the individual into the appropriate stimulus levels for each electrode so that the desired grasp pattern is achieved. Typically, subjects use the neuroprosthesis for such tasks as eating, personal hygiene, writing, and office tasks. These systems have moved from the experimental process and are now available clinically. Users have demonstrated improved independence and daily home use of the neuroprosthesis.11,25,26
The first generation implanted neuroprosthesis has been developed at Case Western Reserve University/Cleveland Veterans Affairs Medical Center.9-11,27 This system, shown in Figure 2.1, consists of an eight-channel implant receiver-stimulator, eight epimysial electrodes, leads, and connectors.27 Electrodes are surgically placed on the muscles of the forearm and hand. A radio frequency inductive link provides the communication and power to the implant receiver-stimulator. The external components of the neuroprosthesis are an external control unit, a transmitting coil, and a shoulder control unit.28 The external control unit performs the signal processing of the control inputs and generates the output signal (modulated radio frequency) to the implant receiver-stimulator. The radio frequency transmitting coil is taped to the individual's chest directly over the implant receiver-stimulator in order to make the inductive communication link. The shoulder control unit consists of a joystick mounted to the chest and a logic switch.
2.3.1 Candidate Selection
The most common upper extremity FES application has been to provide grasp and release for individuals with C5 and C6 motor level complete spinal cord injury. Damage at the fifth and sixth cervical vertebrae is the most common cervical level injury, resulting in reduced shoulder and elbow function, with complete loss of movement of the fingers and thumb. Elbow flexion is typically strong and can be near normal, but elbow extension is lost. Motor function at the C6 level includes wrist extension and forearm pronation. For these individuals, FES provides finger extension and flexion and thumb extension, flexion, and abduction. For injuries at the C4 level, control of elbow flexion must be provided by stimulation of the biceps and/or brachialis, or by a mechanical or surgical means. FES has been applied to a limited extent to these individuals, but there are no clinically deployed systems for this population to date.2930 For individuals with strong C6 or C7 level function, there are other surgical options such as tendon transfers to provide function, and FES is usually not indicated at the present time.4531
The primary physiological characteristic necessary for the application of FES for motor control is the presence of intact lower motor neurons innervating the muscles to be stimulated. This is because the low levels of electrical stimulation used in FES result in excitation of intact motor neurons rather than direct excitation of muscle fibers.1 When a spinal cord injury occurs, there may be damage to the anterior horn cells and/or spinal root evulsion at the injury site extending one or more levels in the cord. The extent of lower motor neuron damage varies from person to person and is determined by the precise nature of the injury and the location of the relevant motoneuron pools. Extensive damage to the motoneuron pool of the entire upper extremity is rare. Peckham and Keith10 found that between 80 and 100% of the muscles necessary for grasp had sufficient innervation intact to generate functional levels of force. In many cases, other paralyzed muscles can be used to substitute for the function that is not available, although this may require surgical intervention.32
Neurological stability is necessary for implanted systems; therefore, implant surgery is not typically performed until at least one year after injury. Joint contractures must be corrected, or grasp functions may be limited. Spasticity must be under control. Neither age nor time post-injury appear to be major factors when considering neuroprosthetic applications.
2.3.2 Operating Principles of Upper Extremity Neuroprostheses
The neuroprosthesis works in the following manner: the user depresses a switch on his or her chest which activates the system, and the user's hand opens in the lateral pinch. Graded elevation of the user's contralateral shoulder results in graded grasp closure. A quick jerk of the shoulder "locks" the hand so that it remains closed at the desired degree of closure until another quick jerk of the shoulder releases the lock command. Depressing the chest switch briefly causes the system to switch to the palmar grasp. Depressing the switch for three seconds turns the system off.
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