Visualisation of the Cochlea Implant/Tissue Interface

Aims The cochlear implant is an electroneural implanted device which helps to restore communication in patients with profound auditory deafness. The implant, works on the concept of direct electrical stimulation of the spiral ganglion cells, within the cochlea. The cochlea is a snail-like coiled structure, apart of the inner-ear labyrinth; its role is to convert hydro-dynamic vibrations to electro-chemical signals that travel up the auditory nerves to the brain. The electrode array of the cochlear implant is inserted into either, the round window or through a cochleostomy surgically drilled anterior-inferior to this structure. Insertion of the electrode array is without visual guidance and this may sometimes lead to damage to the delicate receptor structures within. Such damage may affect performance or result in complications such as: bleeding, meningitis, tinnitus, vertigo and poor hearing results (Kempf et al. 1999). Development of optimal insertional procedures, including the ability to assess the precise location of the electrode array and changes and /or damages caused during insertion is of high priority in the field of cochlea implantation. Histological techniques have been the traditional approach towards assessment of post-operative positioning of the electrode array and structural displacement within the cochlea. Samples are dehydrated, embedded in resin, sectioned and stained. The main limitation to this technique is the lack of 3D configuration between sections due to 'information' loss from sectioning. The conventional approach of clinical CT scanners to visualise the implant within the cochlea has been limited, due to both low resolution (>300µm) and the metallic nature of the device. The low resolutions of clinical CT mean that delicate membranes of the inner ear are not visually resolved (Aschendorff, A., et al. 2004). With the development of microCT systems capable of sub micron (<20µm) resolution and contrast staining techniques, detailed visualisation of the inner labyrinthine structures along with membranes have been successful However when the sample contains metallic components (Platinum, Platinum alloys) within implanted electrode arrays, x-ray interaction causes scattering artefacts. Such scattering often masks the underlying nature of the tissue structures surrounding the implant, leading to imprecise analysis of the implant-tissue interface. To overcome this problem we are developing electrolytic processing to dissolve or partially dissolve the implant in-situ. Thereby minimising or completely removing most of the metal components which impede good visualization of the tissue-implant interface.