Graeme Clark Collection
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ItemComparison of electrode position in the human cochlea using various perimodiolar electrode arrays( 2000)Objective: This study was conducted to evaluate the insertion properties and intracochlear trajectories of three perimodiolar electrode array designs and to compare these designs with the standard Cochlear /Melbourne array. Background: Advantages to be expected of a perimodiolar electrode array include both a reduction in stimulus thresholds and an increase in dynamic range, resulting in a more localized stimulation pattern of the spiral ganglion cells, reduced power consumption, and, therefore, longer speech processor battery life. Methods: The test arrays were implanted into human temporal bones. Image analysis was performed on a radiograph taken after the insertion. The cochleas were then histologically processed with the electrode array in situ, and the resulting sections were subsequently assessed for position of the electrode array as well as insertion-related intracochlear damage. Results: All perimodiolar electrode arrays were inserted deeper and showed trajectories that were generally closer to the modiolus compared with the standard electrode array. However, although the precurved array designs did not show significant insertion trauma, the method of insertion needed improvement. After insertion of the straight electrode array with positioner, signs of severe insertion trauma in the majority o fimplanted cochleas were found. Conclusions: Although it was possible to position the electrode arrays close to the modiolus, none of the three perimodiolar designs investigated fulfilled satisfactorily all three criteria of being easy, safe, and a traumatic to implant.
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ItemChronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study( 1997)A major factor associated with recent improvements in the clinical performance of cochlear implant patients has been the development of speech-processing strategies based on high stimulation rates. While these processing strategies show clear clinical advantage, we know little of their long-term safety implications. The present study was designed to evaluate the physiological and histopathological effects of long-term intracochlear electrical stimulation using these high rates. Thirteen normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods of up to 2100 h using either two pairs of bipolar or three monopolar stimulating electrodes. Stimuli consisted of short duration (25-50 µs/phase) charge-balanced biphasic current pulses presented at 1000 pulses per second (pps) per channel for monopolar stimulation, and 2000 pps/channel for bipolar stimulation. The electrodes were shorted between current pulses to minimize any residual direct current, and the pulse trains were presented using a 50% duty cycle (500 ms on; 500 ms oft) in order to simulate speech. Both acoustic (ABR) and electrical (EABR) auditory brainstem responses were recorded periodically during the chronic stimulation program, All cochleas showed an increase in the click-evoked ABR threshold following implant surgery; however, recovery to near-normal levels occurred in approximately half of the stimulated cochleas 1 month post-operatively. The use of frequency-specific stimuli indicated that the most extensive hearing loss generally occurred in the high-frequency basal region of the cochlea (12 and 24 kHz) adjacent to the stimulating electrode. However, thresholds at lower frequencies (2, 4 and 8 kHz), appeared at near-normal levels despite long-term electrode implantation and electrical stimulation. Our longitudinal EABR results showed a statistically significant increase in threshold in nearly 40% of the chronically stimulated electrodes evaluated; however, the gradient of the EABR input/output (I/O) function (evoked potential response amplitude versus stimulus current) generally remained quite stable throughout the chronic stimulation period. Histopathological examination of the cochleas showed no statistically significant difference in ganglion cell densities between cochleas using monopolar and bipolar electrode configurations (P = 0.67), and no evidence of cochlear damage caused by high-rate electrical stimulation when compared with control cochleas. Indeed, there was no statistically significant relationship between spiral ganglion cell density and electrical stimulation (P = 0.459), or between the extent of loss of inner (IHC, P = 0.86) or outer (OHC, P=0.30) hair cells and electrical stimulation. Spiral ganglion cell loss was, however, influenced by the degree of inflammation (P=0.016) and electrode insertion trauma. These histopathological findings were consistent with the physiological data. Finally, electrode impedance, measured at completion of the chronic stimulation program, showed close correlation with the degree of tissue response adjacent to the electrode array. These results indicated that chronic intracochlear electrical stimulation, using carefully controlled charge-balanced biphasic current pulses at stimulus rates of up to 2000 pps/channel, does not appear to adversely affect residual auditory nerve elements or the cochlea in general. This study provides an important basis for the safe application of improved speech-processing strategies based on high-rate electrical stimulation.
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ItemChronic electrical stimulation of the auditory nerve at high stimulus rates: preliminary results( 1994)The present preliminary report describes the electrophysiological response of the cochlea during long-term stimulation. The data indicate that electrical stimulation at a rate of 1000 pulses per second does not appear to adversely affect the implanted cochlea.
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ItemCochlear implantation in young children: histological studies on head growth, leadwire design, and electrode fixation in the monkey model( 1994)For safe cochlear implantation in children under 2 years of age, the implant assembly must not adversely affect adjacent tissues or compromise head growth. Furthermore, growth changes and tissue responses should not impair the function of the device. Dummy receiver-stimulators, interconnect plugs, and leadwire-lengthening systems were implanted for periods of 36 months in the young monkey to effectively model the implantation of the young child. The results show that implanting a receiver-stimulator package has no adverse effects on skull growth or the underlying central nervous system. The system for fixing the electrode at the fossa incudis proved effective. There was marked osteoneogenesis in the mastoid cavity, resulting in the fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young subjects.
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Item"Cochlear View" and its application in cochlear implant patients [Abstract]( 1994)Recent advances in multichannel intracochlear implantation have generated interests in correlating individual stimulating electrodes to pitch perception. An appropriate radiographic technique is required to precisely document the location of the implanted intracochlear electrode array. Anatomical studies, including the measurements of the temporal bone using high-resolution CT films and 3D reconstruction from the petrous bone sections, were conducted to define the spatial position of cochlea in the skull. Thus, a "Cochlear View" was designed and introduced for postoperative radiological evaluation of multichannel intracochlear implantation. In this paper, a detailed radiographic method and radiological interpretation of the "Cochlear View" are described. A plain radiograph of the "Cochlear View" was taken of 120 patients who had received the Nucleus multichannel implant. Studies have shown that a plain radiograph of the "Cochlear View" provides sufficient information to correctly evaluate the results of implantation, including the insertion depth and position of individual electrodes. It plays an important role in guiding the management of frequency mapping and acts as a useful reference for further research purposes.
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ItemPaediatric cochlear implantation: radiological and histopathological studies of skull growth in the monkey( 1993)The human skull undergoes significant growth within the first two years of life (Dahm et aI, 1992). Therefore, before children under two can be considered candidates for cochlear implantation, the effects of the surgical procedure on subsequent skull growth must be well understood. To evaluate the effects of implantation on skull growth four macaque monkeys were implanted with dummy cochlear implants at six months of age. To model the procedure in the very young child, the bed for the receiver-stimulator was drilled across a calvarial suture down to the underlying dura and an electrode array inserted into the scala tympani via a mastoidectomy and posterior,tympanotomy. Plain skull radiographs were perioqical1y taken to monitor skull growth for periods of up to three years following implantation. Their longitudinal measurements revealed no evidence of asymmetrical skull growth when compared with unimplanted control animals. Computer tomographic scans taken at post-mortem confirmed these findings. Finally, subsequent histopathological evaluation of the receiver-stimulator package bed indicated that it becomes obliterated by hard tissue, resulting in a localized flattening of the vault under the receiver-stimulator. However, this tissue exhibited histological evidence of sutures, indicating that the surgical procedure should not lead to premature sutural closure. In conclusion, the present experimental results suggest that long-term cochlear implantation in very young children will not lead to any significant skull deformity.
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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.