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Silicone as part of a cochlear implant electrode may be responsible for a foreign body response in the human.Clinical evidence of a foreign body response to a cochlear implant has been reported. In a previous study, particulate material found within the fibrous sheath and within macrophages surrounding a cochlear implant has been identified as being consistent with platinum. However, to date, there has been no histologic evidence of a role for silicone in this cellular immune response.A total of 44 temporal bone specimens from 36 patients were reviewed by light microscopy for evidence of presumed platinum and/or silicone foreign bodies in an extracellular or intracellular location. Identification of cell type involved in phagocytosis of foreign body material was accomplished using CD163 immunostaining. The identity and source of the foreign body material was confirmed using energy-dispersive X-ray spectroscopy and scanning electron microscopy.Evidence for both platinum and silicone was found in all 44 specimens. In three patients, anti-CD 163 immunostaining demonstrated phagocytized platinum and silicone foreign bodies. In five specimens, energy-dispersive X-ray spectroscopy demonstrated that the birefringent foreign bodies were consistent with silicone. Scanning electron microscopy of two electrodes removed from temporal bones demonstrated small cracks, fragmentation, and small circular defects in the silicone carrier.Histologic evidence of a foreign body response to the presence of platinum and silicone in a cochlear implant has been demonstrated and may be responsible for some reported delayed failures or extrusion.