The reaction of the cerebral cortex to chronically implanted plastic needles.

Plastic (araldite) needles were implanted in the cerebral cortex in order to evaluate changes produced by an indwelling and relatively inert foreign body. The implants were removed together with surrounding cortex after 4-723 days and the implant-CNS boundary analyzed by light and electron microscopy. Three border conditions were observed at the surface of the implant and were related to the degree of implantation trauma. Type I is characterized by little or no gliosis and synapses within 1-5mu of the implant; type II contains a pronounced zone of reactive astrocytes; and type III is typified by a zone of connective tissue near the implant surface. The type I border predominates, but all three conditions may exist around a single implant. The fact that the implant-CNS boundary is stable following an initial period of readjustment suggests that intracortical implants fabricated from inert materials would be well tolerated and could be used as part of neuro-prosthetic device. Multinucleated giant cells were observed at the boundary of some implants. Their cytology, intercellular relations, and similarity to foreign body giant cells are discussed.