Using a bubble to alleviate post implantation hearing loss – a new model of the cochlea

Abstract

Title: post-implantation reduction of residual acoustic hearing due to round window stiffening, and its alleviation with a bubble inside the cochlea implant.

Aim: the aim is to improve the quality of life of patients with a cochlea implant, by explaining the cause of their impairment of acoustic hearing and proposing a remedy. Preservation of low frequency hearing allows access to low frequency speech cues, which have been found to enhance speech perception in noise, music perception and perception of supra-segmental features of speech. To achieve this, a suitable finite element model of the cochlea is to be developed, validated and made available to others to facilitate implementation of the proposed remedy.

Method: a cochlear implant is normally inserted through the round window, which becomes stiffened because the flexible area is reduced. This stiffening is often exacerbated by the formation of callous tissue on the round window around the implant. The function of the round window is to release the pressure in the cochlear fluid that is caused by the action of the stapes and the oval window; the pressure release allows movement of the almost incompressible fluid from the stapes to the round window, via the basilar membrane where the auditory nerve is stimulated. To replicate this effect of the implant on the acoustic velocity of the basilar membrane, a new finite element model has been developed to accommodate significantly different physiology for the upper and lower chambers.The model has also simulated the effect of including a bubble within the implant, to act as a pressure-release, and hence a remedy.

Results:  the new model has been validated by comparing the simulation results with the average measured acoustic hearing loss in 105 patients; the simulated and measured results agree within 5 decibels. When a cochlea with normal hearing is modelled, the frequency dependent locations of maximum vibration of the basilar membrane agree, again within 5 dB of those measured and recorded in the literature. The new model shows that the ideal position for the bubble is as close as possible to the round window; however, precise proximity may not be achievable in practice. The length, stiffness and position of the bubble can be input easily to the model, and so the model can be used to predict the performance resulting from practically possible implantation, where the bubble needs to be a small distance from the round window.

Conclusion: a two-chamber finite element model of the cochlea has been developed that can be used to predict acoustic hearing loss caused by implantation (other than by inadvertent damage of tissue). The model has been validated by comparison of its results with those of measurements of hearing loss. The model has also been used to show that such loss of hearing can be alleviated by manufacturing the implant so that it contains a small bubble, to be located about 1 mm from the round window. Preservation of low frequency acoustic hearing will be valuable to more patients, now that the NICE criteria have been relaxed in accordance with BCIG recommendations.

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