Graeme Clark Collection
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ItemInvestigation of curved intracochlear electrode arrays [Abstract]( 1992)It has been demonstrated that the Melbourne/Cochlear multi-channel cochlear implant is safe and effective for use in profoundly-totally deaf patients. Recent studies have highlighted the importance of deaf insertion and placing the electrodes closer to the spiral ganglion neurons. In order to improve the electrode insertion depth and proximity to the modiolus, we have investigated curved electrode arrays. Prototypes of such arrays and their accessory inserter have been made. Trial insertions were performed on skeletonized cochleae of human temporal bones. The preliminary results showed that, when compared with conventional straight electrode arrays, the curved arrays could be inserted deeper and located closer to the modiolus. These findings indicate that the curved --.~ electrodes currently under investigation should result in a reduction in stimulus threshold and improve pitch perception and may also result in the use of more channels of stimulation.
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ItemElectrical stimulation of the auditory nerve in deaf kittens: effects on the spiral ganglion [Abstract]( 1992)Cochlear pathology following the administration of ototoxic drugs results in a widespread and rapid loss of sensory hair cells followed by a gradual degeneration of auditory nerve fibres and their cell bodies, the spiral ganglion. Recently, two studies have described increased spiral ganglion cell survival in the cochleas of deafened animals following chronic electrical stimulation of the auditory nerve (Hartshorn et al., 1991; Leake et al., ]991). If electrical stimulation is shown to have a trophic effect on degenerating auditory nerve fibres, these findings will significantly influence the preoperative management of cochlear implant patients. The aim of the present study was to corroborate these earlier reports and to evaluate the general tissue response of deafened cochleae in young animals following chronic electrical stimulation.
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ItemThe origin of electrophonic activity evoked by electrical stimulation of the cochlea( 1992)Electrophonic activation of auditory nerve fibres via electrical stimulation is only observed in cochleas with residual hair cells. While the generation of neural activity associated with this phenomenon is thought to ultimately occur at the inner hair cell synapse (1) it is unclear whether hair cells are activated directly by the electrical stimulus or mechanically via a travelling wave propagating along the basilar membrane. Support for the travelling wave hypothesis has recently come from a masking study using evoked potentials (2). To provide verification of these results, we measured the latency of the electrophonic activity recorded in single ventral cochlear nucleus (VCN) units of known characteristic frequency (CF). Stimulating electrodes were placed on, or just inside the round window of normal hearing anaesthetized cats (n=6). The response of single VCN units were recorded extracellularly and units exhibiting "primary like" activity (3) were analysed. Each unit's CF to acoustic stimulation and response properties to biphasic electric pulses were determined. Electrophonic activity was identified by its long latency (> 2.5 ms) and poor synchronization compared with the response evoked by direct electrical stimulation. Electrophonic activity was observed in 20 units -approximately 25% of the units isolated. These responses were more commonly recorded from cochleas in which the round window had not been opened. The latency of the electrophonic response varied inversely with CF, implying that the response is generated at the basilar membrane and results in a mechanical travelling wave. Finally, no electrophonic activity was observed in units with CFs greater than 10 kHz. Our data would predict that the latency of electrophonic activity in these units -if present -would be very short. Presumably its absence is a result of refractoriness within auditory nerve fibres following activity evoked by direct electrical stimulation.
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ItemMultichannel cochlear implants in children: an overview of experimental and clinical results at the University of Melbourne [Opening Lecture]( 1992)During the last decade there has been great progress in the clinical management of profound, postlinguistically deafened adults through the use of multichannel cochlear implants. The device developed by The University of Melbourne in association with Cochlear Pty Ltd, electrically stimulates selective regions of the auditory nerve using an array of 22 platinum (Pt) electrodes located in the scala tympani. Its development followed basic experimental studies and the development and evaluation of a prototype device in the 1970's. Following safety studies and a successful clinical trial, the Melbourne/Cochlear multichannel implant was approved for use in adults by the United States Food and Drug Administration (FDA) in 1985. More than 3000 patients throughout the world have since been implanted with this device, many being able to understand a significant amount of unfamiliar, connected speech without lipreading Following miniaturization of the implant, it became suitable for use with children. In 1990, after additional biological safety and clinical investigations, the FDA approved the use of the Melbourne/Cochlear multichannel implant for profoundly deaf children above the age of two years. And in 1991, the device received the medical device implantation approval certificate from the Japanese Government. The present paper presents an overview of our recent biological safety studies and clinical experience in children, and discusses the likely future development of these devices.
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ItemAn antibacterial seal and fixation device for cochlear implants in young children [Abstract]( 1992)Concerns associated with cochlear implantation in young children include intracochlear spread of infection along the electrode array during otitis media, and electrode extraction caused by skull growth post-implantation. New biomaterials were used to seal and secure the electrode at its entry point into the cochlea. Hydroxyapatite was deposited around the outside of an electrode cuff and it bonded well to the surrounding bone of the otic capsule. The electrode cuff accommodated variable insertion depths with the help of a new, silicone based hydroscopic polymer. Preliminary results, including experimental testing of the device in an animal model of pneumococcal otitis media, indicate protection of the implanted cochlea against the spread of infection. Electrode leadwire displacement is prevented by fixation of the array at its entry point This concept may play an important role in the development of a safe cochlear implant design for children under two years of age, who are expected to benefit most from early auditory rehabilitation.
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ItemCochlear implantation in children: the risk of pneumococcal otitis media [Abstract]( 1992)Pneumococcal otitis media is most frequent in young children and is a matter of concern in cochlear implantation. In the course of the 'implantation surgery the physiological barrier between the middle ear and inner ear is broken down by incising the round window membrane or by fenestration of the cochlear wall. It is feared that the insertion of an electrode array into the scala tympani could provide a pathway for microorganisms and toxins to enter the cochlea, resulting in labyrinthitis. To assess the actual risk of, secondary inner ear infection post implantation we developed a cat animal model of otitis media. In addition we examined the, effectiveness of different sealing strategies compared to the alternative of leaving the electrode entry point unprotected. For sealing of the cochlea fibrous tissue or gelatine foam was wrapped around the electrode in the round window niche. 22 kittens (44 ears) were used for this study 32, ears were implanted at 2 months of age, and all 44 ears were inoculated after 2 months with a broth of, streptococcus pneumoniae and the animals sacrificed one week later. The bullae of the animals were swabbed and the cochleas processed and examined under light microscopy. Histological analysis of the cochleas showed ,the� highest incidence (45%) of labyrinthine spread of infection in the unimplanted control group. Suppurative or serous labyrinthitis was found in only one third (33.3%) of the implanted and unsealed cochleas. In contrast only one of 16 sealed cochleas, (6.2 %) showed labyrinthine signs of acute inflammation. Experimental pneumococcal otitis media could be reliably established in all animals and proved to be a valuable animal model for the testing of the intracochlear spread of infection. We conclude that a cochlear implant electrode inserted via the round window does not increase the risk of tympanogenic abyrinthitis. Our results indicate that grafting of the electrode entry point results in significant protection of the inner ear against labyrinthine spread of ototis media along the leadwire of the cochlear implant.
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ItemCochlear implantation in young children: studies on head growth, leadwire design and electrode fixation in the monkey model [Abstract]( 1992)For the safety of cochlear implantation in children under two, the implant assembly must not adversely effect the tissue of compromise head growth. Furthermore, growth changes and tissue responses should not impair functioning of the device. Dummy receiver-stimulators, interconnect plugs and leadwire-lengthening systems have been implanted for periods of 40 months in the young monkey to most effectively model the implantation of the young human child. The results show that implanting a receiver-stimulator package has no effect on skull growth or brain tissue under the package. The system for fixing the electrode at the fossa includes proved effective. There was marked osteoneogenesis in the mastoid cavity and this also resulted in fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young children.
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ItemCochlear implantation in young children: long-term effects of implantation on the skull and underlying central nervous system tissues in a primate model [Abstract]( 1992)Recent independent studies reporting results obtained by profoundly deaf children implanted with the Melbourne 22-channel cochlear implant have provided further impetus for assessing the feasibility of implanting children under two. Studies in appropriate animal models must first establish the safety of this procedure.
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ItemCochlear implantation in young children: long-term effects of implantation on normal hair cells and spiral ganglion cells in the monkey model [Abstract]( 1992)Recent independent results obtained by profoundly deaf children implanted with the Melbourne 22-channel cochlear implant (1) have provided further impetus. for examining thefeasibility of implanting children under two and children with profound deafness. Safety st1,ldies, in appropriate animal models, must first establish the safety of this procedure.