Evaluation of the elution of dexamethasone from the silicone of the electrode array: Preliminary safety studies

Slow release of dexamethasone added in low amounts to cochlear implant silicone rods is able to reduce hearing loss secondary to insertion trauma. The effects of 10% dexamethasone added to silicone rods were evaluated in an animal model. Six guinea pigs were unilaterally implanted with tapered rods of eluting silicone and six controls with non-eluting ones. Implantation was performed through a 0.7-mm cochleostomy, followed by 3-mm deep rod insertion. Hearing threshold audiograms were acquired prior to implantation and during the next two weeks by recording compound action potentials with electrodes near the round window. The mean threshold shifts in ears with dexamethasone eluting rods two weeks after implantation were 2dB ± 2dB, while in ears with control rods they were 7dB ± 2dB. After two weeks bullae of each animal were filled with 10% horseradish peroxidase (HRP) immediately before sacrifice to verify cochleostomy sealing. Each bulla including the cochlea was removed, decalcified, embedded in paraffin and longitudinally cut into 5-μm thick sections. Sections were examined for HRP-positive particles inside the scala tympani at the cochleostomy site, macrophages, percentage of tissue growth in scala tympani and complete tissue sealing around cochleostomy. Also, rods samples were explanted and tested for bacterial contamination. Preliminary results show no bacterial contamination and no enhanced number of macrophages. A slight tissue growth was present in scala tympani which was, however not significantly different between the two groups. Apparently, dexamethasone eluting from silicone rods does not suppress cochleostomy sealing and reduces the residual hearing loss after implantation, supporting its use as slow-release as well as anti-inflammatory additive in cochlear implants