T Hanekom | PubMed Health

PubMed2005

Modelling encapsulation tissue around cochlear implant electrodes.

The objective of the study was to explore the effect of electrode encapsulation by fibrous scar tissue on electrical potential distributions and auditory nerve fibre excitation patterns. A finite element model in combination with an auditory nerve fibre model was used to predict changes in threshold currents and auditory nerve fibre excitation patterns. The model showed that electrical potentials at the target nerve fibres and the electrode contacts changed in the presence of encapsulation tissue. This led to changes in threshold currents and spread of excitation. The effect of electrode encapsulation on threshold currents and spread of excitation depended on the thickness of the perilymph layer separating the fibrous tissue encapsulation and the electrode array, nerve fibre survival status, electrode geometry and configuration, and array location. Model results suggested that arrays located close to the modiolus were most sensitive to threshold changes caused by electrode encapsulation (changes were between -0.26 and 2.41 dB), whereas encapsulation of an electrode array had less effect on threshold currents when the array was located in a lateral position in the scala tympani (changes were between -0.64 and 1.5 dB). For medially located arrays, changes in the spread of excitation varied between an increase of 0.21 mm and a decrease of 0.33 mm along the length of the basilar membrane, and an increase of 0.18 mm and a decrease of 0.66 mm along the length of the basilar membrane were calculated for laterally located arrays.

Cicatrix↗

PubMed2001

Three-dimensional spiraling finite element model of the electrically stimulated cochlea.

OBJECTIVE: The objective of the article is to provide an accurate model of the human cochlea with which potential distributions and thus neural excitation patterns around cochlear implant electrodes can be determined. Improvements on previous models of the implanted cochlea are that this model 1) includes the spiral nature of the cochlea as well as many other anatomical details (and it is a model of the human cochlear rather than the guinea pig cochlea), and 2) facilitates modeling of different electrode geometries, array locations and electrode separations without changing the structure of the model. DESIGN: A three-dimensional spiraling finite element model of the human cochlea was created. The model incorporates the effect of neighboring canals and conduction along the fluid-filled canals of the cochlea. Potential distributions are used as inputs to a nerve fiber model to investigate auditory nerve excitation patterns around intracochlear electrode arrays. RESULTS: Potential distributions around intracochlear electrodes generated with the finite element model are presented. The effects of electrode separation, electrode geometry and array location on excitation threshold, excitation spread and ectopic excitation (i.e., excitation of nerve fibers at an undesirable location) are demonstrated. CONCLUSIONS: The following conclusions should be considered preliminary, as their accuracy depends on the exactness of the underlying model. The spiraling geometry of the cochlea causes asymmetry in potential distributions. The location of electrodes along the length of the basilar membrane has a stronger influence on the site of excitation than the polarity of the leading phase of the stimulus. Array location is the primary parameter that controls excitation spread. Threshold currents and the effect of ongoing loss of peripheral dendrites on threshold currents can be limited by placing arrays close to the modiolus. Point electrode geometries are recommended above banded electrode geometries only when the array can be placed close to the modiolus. There is a tradeoff between array location and the degree of ectopic stimulation caused by a specific array location. Bimodal excitation patterns exist at comfortable stimulus intensities for longitudinal bipolar electrode configurations. It is shown that an electrode configuration with an electrode separation of approximately half that of the bipolar electrode separation of the Nucleus electrode can be used instead of radial and offset radial electrode configurations to create unimodal excitation patterns. The stimulation resolution of cochlear implant electrode arrays can potentially be improved by increasing the number of electrode contacts in an array.

Cochlea↗

PubMed2000

[The value of basic research as applied to cochlear implants].

This article discusses the value of basic research as applied to cochlear implants. The article is aimed at clinicians and audiologists who are working in the field of cochlear implants or who are interested in this field. The article also gives a more general introduction to modelling for researchers in the clinical environment. It provides an entry point to cochlear implant research and reviews the application of basic research to new developments in cochlear implants. It is shown what has been achieved so far and which problems still exist. The role of multidisciplinary research teams to solve these problems is discussed. Experimental research and modelling co-operate to solve problems and make new discoveries. The importance of modelling as a tool for basic research is emphasized.

Cochlear Implantation↗