A neuroprostheses for grasping is a device designed to improve or restore the grasping, holding and reaching functions in individuals with stroke and SCI (Baker et al. 1993; Cornwall & Hausman 2004). The neuroprostheses applies functional electrical stimulation (FES) of the motor branches of the peripheral nerve in which paralyzed muscles are electrically stimulated to produce muscle contraction, replacing the electrical signals coming from the brain through the injured spinal cord (Shimada et al. 1996; Handa 1997; Hincapie et al. 2008). The FES uses bursts of short electrical pulses (pulse widths 0-250 mSec and pulse amplitude 0-150 mA) to generate muscle contraction by stimulating motoneurons or reflex pathways. The key element for achieving synergistic activity of muscles that results with reaching and grasping is the appropriate sequencing of bursts of electrical pulses. For continuous contraction of the arm and hand muscles (tetranization), a FES system has to deliver at least 16 stimulation pulses per second to elicit action potentials (AP) in the motor nerve, causing the corresponding muscles to contract. FES enables the patients with high SCI to reconstruct grasp movements such as palmar and lateral grasps of the upper extremity (Shimada et al. 2003). The palmar grasp is used to hold bigger and heavier objects such as cans and bottles and the lateral grasp is used to hold smaller and thinner objects such as keys, paper and CDs (Popovic et al. 2002). It has been reported to be useful in improving ADL functions (Popovic et al. 2001a; Shimada et al. 1996). FES is also applied to generate elbow extension by stimulating the triceps brachii in combination with voluntary biceps contraction used to augment reaching (Grill & Peckham 1998; Popovic et al. 1998). Elbow and shoulder FES systems have not been developed into practical clinical devices.
The motor nerves can be stimulated using surface (transcutaneous), percutananeous or implanted electrodes (Mortimer 1981). Transcutaneous stimulation is performed with self-adhesive or non-adhesive electrodes that are placed on the subject’s skin in the vicinity of the motor point of the muscle that needs to be stimulated (Baker et al. 1993; Mortimer 1981). Percutaneous and fully implanted electrodes are placed close to the entry point of the motor nerve to the muscle which should be stimulated, either epimysial or intramuscular (Cameron et al. 1997; Hoshimiya & Nanda 1989).
Individuals with C5-C7 complete SCI and with no major degree of motoneuron or nerve root damage of the stimulated muscles benefit the most from neuroprosthesis. The use of an implanted FES system can only be applied once the patient reaches stable neurological status, which usually occurs two or more years post SCI. The use of surface FES systems can be introduced during the early rehabilitation period, as the patient does not have to be neurologically stable.
Gorman et al. (1997) and Cornwall and Hausman (2004) have presented guidelines for patient selection for consideration of for an implantable neuroprosthesis. They are as follows:
- Anatomic: Stable tetraplegia with C5 or C6 motor level with international classification motor scores of 0, 1, 2 or an impairment scale level of A, B, C (AIS)
- Physiologic: Presence of adequate ROM in joints of the shoulder, arm, forearm, wrist and hand
- Medical: Free of overwhelming medical problems
- Psychosocial factors: Sufficient motivation to learn its use and use it
- Adequate caregiver and/or family support
- One year post injury, plateau of functional recovery.
- Need for sufficient vision to provide visual feedback during training and use sufficient cognitive ability
Contraindications to neuroprothesis use include cardiac disease, arrhythmias, pace makers, chronic systemic infections, diabetes, and immune disease