An overview of cochlear implant electrode array designs.

Cochlear implant electrode arrays are designed with specific characteristics that allow for the preservation of intra-cochlear structures during the insertion process, as well as during explantation. Straight lateral wall (LW) electrode arrays and pre-curved modiolar hugging (MH) electrode arrays are the two types that are commercially available. Although there is a third type of electrode array called the mid-scala (MS), which is positioned in the middle of the scala tympani (ST), and is usually considered as an MH type of electrode. Different lengths of straight LW electrode arrays are currently available which allow for insertion across a range of different sized cochleae; however, due to manufacturing limitations, pre-curved MH electrodes are generally only available to cover the basal turn of the cochlea, while the spiral ganglion cells are distributed in the Rosenthal's canal that extends into 1.75 turns of the cochlea. Both straight LW and pre-curved MH electrodes can cause a certain degree of intra-cochlear trauma, but pre-curved MH electrodes tend to deviate into the scala vestibuli from the scala tympani more often than the straight LW electrodes, resulting in damage to the osseous spiral lamina/spiral ligament which could initiate new bone formation and eventually affect the cochlear implant users' hearing performance. Structural damage to the cochlea could also affect the vestibular function. With pre-curved MH electrodes, higher degrees of trauma are related to the fixed curling geometry of the electrode in relation to the variable coiling pattern of individual cochleae, the orientation of the electrode contacts in relation to the modiolus wall, and how effectively the stylet was handled by the surgeon during the procedure. Wire management, metal density, and the shore hardness of the silicone elastomer all contribute to the stiffness/flexibility of the electrode. It is important to acknowledge the impact of bringing the stimulating contacts closer to the modiolus wall with an MH electrode type in terms of the resultant damage to intra-cochlear structures. The presence of malformed cochleae should be identified and appropriate electrodes should be chosen for each specific cochlea, irrespective of the cochlear implant brand. In order to utilize drug therapy, the cochlea should be free from any trauma.

Effects of a dexamethasone-releasing implant on cochleae: A functional, morphological and pharmacokinetic study.

AIM: This study evaluated the impact of a dexamethasone-releasing silicone implant on hearing function preservation, cochlear morphology and perilymph pharmacokinetics after cochlear implantation. METHODS: Guinea pigs were implanted unilaterally with silicone rods containing either 2% dexamethasone (DEXA group, n = 18) or no dexamethasone (control group, n = 17). Auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAEs) were measured preoperatively and over 6 months postoperatively. Cochlear histology using standard hematoxylin and eosin (H&E) staining and tumor necrosis factor (TNF)-alpha staining was performed 1 month postoperatively. Twenty-two guinea pigs were involved in the pharmacokinetic study, and real-time drug concentrations in perilymph were investigated using high-performance liquid chromatography (HPLC). The Mann-Whitney U test (1-tailed) was used for statistical analyses. RESULTS: ABR and DPOAE testing demonstrated decreased hearing function immediately postoperatively followed by a progressive hearing loss within the first day postoperatively. There was almost no observable hearing improvement in the control group from 1 week to 6 months postoperatively, but hearing levels in the DEXA group improved gradually from 1 week to 12 weeks. Hearing loss in the DEXA and control group was 5.0 ± 3.4 dB and 21.7 ± 5.3 dB, respectively at a 16-kHz stimulus frequency 6 months postoperatively. The difference in threshold shifts was present throughout all measured frequencies, and it was significant at 4-24 kHz. The morphological study revealed new fibrosis formation in the scala tympani, which encapsulated the implanted electrode. TNF-alpha positive staining in the cochleae of the DEXA group was less evident than the control group. The pharmacokinetic study revealed a peak perilymph concentration 30 min postoperatively and sustained dexamethasone release at least 1 week postoperatively. CONCLUSION: Cochlear implants that incorporate dexamethasone can release drug chronically in the inner ear and induce significant long-term recovery and preservation of auditory function after implantation.