Chronic response of the rat sciatic nerve to the flat interface nerve electrode.

The chronic effects of a reshaping nerve electrode, the flat interface nerve electrode (FINE), on sciatic nerve physiology, histology, and blood-nerve barrier (BNB) are presented. The FINE electrode applies a small force to a nerve to reshape the nerve and fascicles into elongated ovals. This increases the interface between the nerve and electrode for selective stimulation and recording of peripheral nerve activity. The hypothesis of this study is that a small force applied noncircumferentially to a nerve can chronically reshape the nerve without effecting nerve physiology, histology, or the blood-nerve barrier permeability. Three FINE electrode designs were implanted on rat sciatic nerves to examine the nerve's response to small, moderate, and high reshaping forces. The chronic reshaping, physiology, and histology of the nerve were examined at 1, 7, and 28 days postimplant. All FINEs significantly reshape both the nerve and the fascicles compared to controls. FINEs that applied high forces caused a neurapraxia type injury characterized by changes in the animal's footprint, nerve histology, and the BNB permeability. The physiological changes were greatest at 7 days and fully recover to normal by 14 days postimplant. The moderate force FINE did not result in changes in the footprint or BNB permeability. Only a minor decrease in axon density without accompanying evidence of axon demyelination or regeneration was observe for the moderate force. The small force FINE does not cause any change in nerve physiology, histology, or BNB permeability compared to the sham treatment. An electrode that applies a small force that results in an estimated intrafascicular pressure of less than 30 mm Hg can reshape the nerve without significant changes in the nerve physiology or histology. These results support the conclusion that a small force chronically applied to the nerve reshapes the nerve without injury.