Use of micro-computed tomography for the characterisation of intracochlear fibrosis following cochlear implantation

Abstract

Cochlear implantation is widely established as an effective treatment option for those who are suffering from severe-to-profound sensorineural hearing loss. Nevertheless, a substantial proportion of cochlear implant recipients have been shown to experience some degree of residual hearing loss in the months following cochlear implantation. In general, it is understood that this delayed loss of residual hearing is the consequence of an inflammatory response provoked by the surgical insertion of cochlear implants, which progressively leads to the loss of auditory sensory cells as well as the formation of fibrous scar tissue inside implanted cochlea. However, the underlying mechanisms behind such pathological responses to cochlear implantation have not yet been completely elucidated. Fortunately, a novel imaging modality known as micro-computed tomography (μCT) has emerged in recent years as a suitable candidate to enable the non-destructive visualisation of fibrous tissue in the cochlea following cochlear implantation. Accordingly, the primary objective of the present study was to develop an effective methodology using μCT to examine the progressive formation of fibrous tissue in a guinea pig model of cochlear implant surgery. Ten guinea pigs were surgically implanted with a dummy cochlear implant for either two, four, or eight weeks, after which the implanted cochleae were collected for subsequent processing. To enable the visualisation of soft tissue structures in the cochlea, samples were stained using a contrast agent known as osmium tetroxide (OsO4). Subsequently, the implanted cochleae were scanned using μCT to generate a series of X-ray projection scans, which were then reconstructed into a dataset of high-resolution tomographic images. Using the optimised methodology, morphological details of the fibrous tissue that was identified in the implanted cochleae were characterised in three-dimensions. Despite the consistency of the surgical procedure, a considerable degree of variability was observed in the shape and size of the fibrous tissue at each time point. Nevertheless, the formation of fibrous tissue appeared to originate from regions where physical trauma was sustained during cochlear implantation such as the site of the cochleostomy as well as the points of contact between the cochlear implant and the inner walls of scala tympani. In the present study, a novel experimental approach involving the use of μCT is presented by which the underlying pathological mechanisms of post-implantation fibrosis can be investigated in a collective effort to develop strategies for the preservation of residual hearing following cochlear implantation.