Proefschrift_vd_Beek

improve the quality of the incoming signal. Although listening comfort in background noise improves with noise reduction algorithms [Buechner et al., 2010], not all studies could demonstrate a significant benefit in terms of speech intelligibility in noise [Dingemanse and Goedegebure, 2014]. Electrode-neural interface improvement and perimodiolar positioning In addition to improvements of the incoming signal, improvements of the electrode-neural interface are beneficial to patient perception. Theoretically, electrodes located near excitable neural elements increase the dynamic range and provide increased selectivity of stimulation. Data from Shepherd et al. indicating that electrodes closer themodiolus facilitated neural excitation in cats [Shepherd et al., 1993] encouraged the design of electrodes with a perimodiolar position. Computer modeling confirmed the potentially beneficial effect of the perimodiolar position [Frijns et al., 2001], and different perimodiolar electrodes have been developed. The Advanced Bionics HiFocus (Advanced Bionics Corp., Sylmar, CA, USA) electrode was intraoperatively forced into a perimodiolar position with a silastic positioner, whereas the Nucleus Contour (Cochlear Corp., Lane Cove, Australia) electrode was designed to be placed in a perimodiolar position via the removal of a stylet. Multiple experiments with cochlear specimens revealed a perimodiolar location of the inserted electrodes [Cords et al., 2000;Fayad et al., 2000;Richter et al., 2002;Roland, Jr. et al., 2000;Tykocinski et al., 2000]. Compared with the Nucleus Contour (Cochlear Corp., Lane Cove, Australia), for which the removal of the stylet places the array in a primarily apical perimodiolar position, the perimodiolar position of the HiFocus electrode array is primarily located in the basal turn [Balkany et al., 2002]. However, the positioner was withdrawn from the market after a number of implanted patients developed meningitis. Although the relationship between meningitis and the positioner could not be clearly established, it was assumed that forcefully inserting the positioner might damage cochlear structures and create a pathway for bacteria [Seyyedi et al., 2013]. The withdrawal of the positioner and the subsequent implantation of the same HiFocus electrode without the use of the positioner enabled researchers to study the effect of the intracochlear location of the electrode array on speech perception [van der Beek et al., 2005a] [Chapter 3] . CT data confirmed in vivo that the electrode array was primarily placed in the basal perimodiolar position. This position provided enhanced speech perception; however, because medial placement also entailed a greater insertion depth range, the effects of these two factors could not be separately analyzed. Furthermore, the insulating properties of the silastic positioner precluded a basal current drain, which may have influenced the effectiveness of the basal electrodes. Additionally, patients who were fitted without a positioner exhibited increased threshold levels for the electrodes at the basal end of the cochlea, a phenomenon that was not observed in patients whose arrays were placed with a positioner. Several cochlear implant manufacturers have developed perimodiolar electrodes. As previously described, a considerable amount of research has been performed using cadaveric temporal bones to determine whether the electrodes had a perimodiolar location after implantation [Fayad et al., 2000;Richter et al., 2002;Roland, Jr. et al., 2000;Tykocinski et al., 2000;Balkany et al., 2002].

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Furthermore, different studies have analyzed the effects of the intracochlear position on electrically evoked

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