Impedance development after implantation of hybrid-L24 cochlear implant electrodes.

Shorter and thinner electrodes were developed for preserving residual hearing after cochlear implantation by minimising trauma. As trauma is regarded as one of the causes of fibrous tissue formation after implantation, and increase in impedance is considered to be connected to fibrous tissue formation, the aim of the current study was to evaluate impedance development after implantation of Hybrid-L electrodes. Impedance values were retrospectively collected from our clinical database and evaluated for all active contacts and basal, middle and apical contacts separately for up to 10 years. All 137 adult patients received a Hybrid-L electrode and had to be implanted for at least 1 year. On average impedances increased to 13 kOhm before first fitting and dropped to 5-7 kOhm under electrical stimulation with lower values measured on apical contacts. Mean values remained stable over years, but variability increased. Values before first fitting were independent of age at implantation whereas lower values were found later in patients of higher age at implantation. Despite smaller contacts, impedance values after start of electrical stimulation were comparable to published values of Contour electrodes. This might suggest less tissue growth with the Hybrid-L electrode array.