Nanomechanical mapping reveals localized stiffening of the basilar membrane after cochlear implantation

Cochlear implantation leads to many structural changes within the cochlea which can impair residual hearing. In patients with preserved low-frequency hearing, a delayed hearing loss can occur weeks-to-years post-implantation. We explore whether stiffening of the basilar membrane (BM) may be a contributory factor in an animal model. Our objective is to map changes in morphology and Young's modulus of basal and apical areas of the BM after cochlear implantation, using quantitative nanomechanical atomic force microscopy (QNM-AFM) after cochlear implant surgery. Cochlear implantation was undertaken in the guinea pig, and the BM was harvested at four time-points: 1 day, 14 days, 28 days and 84 days post-implantation for QNM-AFM analysis. Auditory brainstem response thresholds were determined prior to implantation and termination. BM tissue showed altered morphology and a progressive increase in Young's modulus, mainly in the apex, over time after implantation. BM tissue from the cochlear base demonstrated areas of extreme stiffness which are likely due to micro-calcification on the BM. In conclusion, stiffening of the BM after cochlear implantation occurs over time, even at sites far apical to a cochlear implant.