Neural Prostheses

Background

Traumatic injury to peripheral nerves is a common clinical problem that results in a substantial functional loss and decreased quality of life due to permanently impaired sensory and motor functions and secondary problems, such as neuropathic pain. PNI results in partial or total loss of motor, sensory, and autonomic functions conveyed by the lesioned nerves to or from the denervated segments of the body. Significant morbidity is associated with peripheral nerve injuries, and the outcomes of peripheral nerve repair are poor, especially when delayed. The most severe and debilitating lesions are those involving the proximal nerve part and the brachial plexus, for which treatment is unreliable, affecting a great proportion of the young male population between 15 and 25 years of age. Due to the young age of patients at the time of injury and the lifelong burden their injuries present, the financial costs associated with nerve injury sum to nearly $7 billion in the United States alone (American Paralysis Association, 1997).

If the upper extremity is affected, the most frequently injured nerves are the proper and common digital nerves, followed by the median and ulnar nerves. The most severe injury is a complete nerve transection. After a nerve is severed, the distal portion undergoes Wallerian degeneration as a result of the protease activity and separation from metabolic resources of the nerve cell bodies.

Functional reinnervation does occur to a minimal extent, during which that axons extend until they reach their distal target; in humans, axon regeneration occurs at a rate of about 1 mm per day. As a consequence, however, distal muscles of the hand undergo irreversible atrophy before any possibility of reinnervation, and hand dexterity is permanently lost. Modern surgical options include nerve grafting of viable roots, nerve transfers, free-functioning muscle transfers, tendon transfers, and a combination of these techniques. However, with nerve lesions proximal to the central nervous system (CNS), functional recovery in distal limb function is almost never achieved

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