Assessment of intracochlear electrode position and correlation with behavioural thresholds in CII and 90K cochlear implants
Roberto FilipoPatrizia ManciniValeria PanebiancoMarika ViccaroEdoardo CovelliV. VergariRoberto Passariello
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Conclusion. The image quality of 64-MDCT provided excellent definition of the fine osseous structures and individual electrode contacts. Evaluation of electrode distances revealed a more focused stimulation for the Helix contacts, with better optimization of pulse width and frequency of stimulation. Objectives. A multi-slice CT scan was performed postoperatively to evaluate electrode distance from the modiolus and variability of fitting parameters (M level) for two different types of cochlear implant electrode carriers, CII and 90K implants with 1J and Helix electrode carriers. Materials and methods. The electrode's position in different cochlear implant (CI) electrodes, Advanced Bionics 90K 1J and Helix, was assessed postoperatively in 20 adult patients by means of a 64-MDCT scanner. Axial, coronal, and oblique 0.3 mm multiplanar reconstructions (MPRs) were obtained and datasets were analyzed to assess the intracochlear position and distance from the surface of the electrodes to the bony edge of the modiolus. Patients’ fitting characteristics were gathered at the time the CT was performed and correlated to intracochlear measurements. Results. Determination of contact distances confirmed smaller average values for the Helix at the apex and medial segments. Helix electrodes were closer to the modiolus in all segments. Likewise, M level determination showed lower values for the Helix carrier, confirming a more focused stimulation and better optimization of pulse width and frequency of stimulation.Keywords:
Modiolus (cochlea)
Electrode array
To develop a new method for REZ-1 cochlear implant electrode array insertion.REZ-1 cochlear implant electrode array was implanted in 22 human temporal bone specimens. Cochlear diameters were measured from spiral CT scan before implantation. Cochlear views were taken before and after pullback technique. Modiolus-electrode distances were measured and compared. The diameters of sixty normal cochleae were measured.In 3 cases, 27 electrode rings were inserted into the cochlea, while in others, all 28 electrode rings were inserted into the cochlea. After pullback of the electrode array, No. 12 to No. 19 electrode rings were closer to the modiolus in 17 cases (paired t test, P<0.01). The cochlear diameters in the 17 cases were smaller than 9.50 mm, while in the other 5 cases, the cochlear diameters were at least 9.60 mm. The cochlear diameters in the 17 cases were 9.11 (0.57) mm, while the cochlear diameters in the 5 cases were 9.78 (0.28) mm (Mann-Whitney test, P<0.001). The diameters of normal cochleae were (9.04 +/- 0.45) mm, with 90% larger than 9.50 mm.In cases whose cochlear diameter is smaller than 9.50 mm, pullback technique can help some electrode rings be closer to the modiolus. Measurement of the cochlear diameter can help the surgeon to have a better choice in the selection of REZ-1 cochlear implantation methods.
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Modiolus (cochlea)
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Pitch perception for different modes of stimulation using the Cochlear multiple-electrode prosthesis
Numerical estimations of pitch were obtained from nine postlinguistically deafened adults using the 22-electrode cochlear implant manufactured by Cochlear Pty. Limited. A series of electrodes on the array were stimulated using three modes of stimulation: Bipolar (BP), common ground (CG), and monopolar (MONO). In BP stimulation, an electric current was passed between two electrodes separated by one electrode for eight patients and two electrodes for one patient. In CG stimulation, a single electrode was activated and the other electrodes on the array were connected together to serve as the return path for the current. In MONO stimulation, an electric current was passed between a single electrode and the most basal electrode on the array. Pitch estimations were generally consistent with the tonotopic organization of the cochlea. There was a marked reversal in pitch for electrodes in the middle of the array using CG stimulation for three patients. A reduced range of pitch using MONO stimulation was recorded for patients where the most basal electrode was internal to the cochlea. There were also individual differences in pitch estimations between the three modes of stimulation for most patients. The current levels required to elicit threshold (T) and comfortable listening (C) levels were, in general, higher for BP stimulation than for CG stimulation and were lowest for MONO stimulation. For CG stimulation, there was a tendency for T and C levels to be higher for electrodes in the middle of the array than at the basal or apical ends. For MONO stimulation, T and C levels uniformly increased in an apical to basal direction for the majority of patients. There was no consistent pattern in T and C levels for BP stimulation. The size of the range of usable hearing using CG stimulation tended to be similar to that using BP stimulation and was usually higher than that using MONO stimulation.
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Basal (medicine)
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Objective: Cochlear implants are used in patients with profound deafness due to degeneration of hair cells inside the cochlea. As improvement for commercially available CIs with straight electrode arrays, it is desired that the electrode array hugs toward the nerve cells located at the central axis of the cochlea in order to reduce the stimulation distance. Therefore, we present a hydrogel-based actuated electrode shaft, which should bend itself when exposed to saline solution (simulating the intracochlear liquid perilymph). Methods:In vitro tests with the electrode arrays were performed in a cochlea model. Different quantities and grain sizes of the hydrogel-building polymer (polyacrylamide) were used to study the self-bending effect. Furthermore, material interfaces between the components were evaluated with scanning electron microscopy, fluorescence microscopy, and an adapted tape test according to DIN EN ISO 2409. Results: Self-bending of the electrode array was observed in the cochlea model and a maximum number of 3.1 turns was achieved. No delamination between the components could be observed. Conclusion: After insertion, we expect that the electrode shaft moves into a patient individual perimodiolar position without losing its functionality because of a delamination of the components. Significance: A modiolar hugging electrode design is presented, which may improve hearing restauration with cochlear implants due to an easier insertion technique and a patient individualized hugging to the modiolus.
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In the cochlear implant system, the distance between spiral ganglia and the electrodes within the volume of the scala tympani cavity significantly affects the efficiency of the electrical stimulation in terms of the threshold current level and spatial selectivity. Because the spiral ganglia are situated inside the modiolus, the central axis of the cochlea, it is desirable that the electrode array hugs the modiolus to minimize the distance between the electrodes and the ganglia. In the present study, we propose a shape-memory-alloy-(SMA-) embedded intracochlear electrode which gives a straight electrode a curved modiolus-hugging shape using the restoration force of the SMA as triggered by resistive heating after insertion into the cochlea. An eight-channel ball-type electrode array is fabricated with an embedded titanium-nickel SMA backbone wire. It is demonstrated that the electrode array changes its shape in a transparent plastic human cochlear model. To verify the safe insertion of the electrode array into the human cochlea, the contact pressures during insertion at the electrode tip and the contact pressures over the electrode length after insertion were calculated using a 3D finite element analysis. The results indicate that the SMA-embedded electrode is functionally and mechanically feasible for clinical applications.
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Three-dimensional (3D) computational models of the inner ear have been utilised to assist in investigating the factors that influence cochlear implant (CI) outcomes. A volume conductor cochlear model with an implanted electrode array was reconstructed from X-ray microtomography (μCT) scans of a cadaveric human temporal bone. To mimic an in-vivo setting, the cochlea was embedded in a head model. The finite element (FE) method was used to analyse the electrical potential φ in the cochlear nerve as a result of CI stimulation. In order to study the influence of electrode array placement on the current spread within the cochlea and the modiolus, computer simulations with six electrode array placements were conducted. φ was evaluated at the tip of nerve fibres reconstructed within the cochlear nerve so as to predict the stimulation of a neuron population. It was found in most cases that a medial electrode array placement produced a narrower φ peak at the fibre tip than a lateral one, although the differences were small.
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The degree of overlap among cochlear nerve fibers stimulated by different electrodes results in electrode interaction, which has been shown to have a significantly deleterious effect on speech recognition performance in multi-electrode cochlear implant users. The Nucleus CI24R(CS) Contour array, which lies substantially closer to the modiolus than the CI24M straight array, is expected to exhibit narrower excitation patterns. The neural response telemetry (NRT) 3.0 software provides a method of measuring the spread of neural excitation by presenting the masker and probe pulses on different intra-cochlear electrode bands. Nine pairs of children, using Nucleus CI24M/CI24R(CS) cochlear implants with a similar etiology and duration of deafness, insertion depth, age of implantation and loudest acceptable presentation level (LAPL) in NRT sessions, participated in the study. Profiles of the spread of neural excitation stimulated at the LAPL at 3 probe locations were examined for each pair of the 2 types of electrode array. The spread of neural excitation with respect to array type and location revealed significant effects (p < 0.001; p = 0.002) and no interaction between array type and probe location (p = 0.559). The results demonstrated that the Contour array improved electrode discrimination, especially for the electrodes at the basal end of the cochlea. The findings have implications for future electrode array design and current implant mapping strategies.
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Hypothesis: Perimodiolar intracochlear electrodes with contacts facing towards the modiolus have limited current flow towards the outer wall of the cochlea and therefore, may reduce the occurence of facial nerve stimulation (FN) in cochlear implant subjects. Background: Facial nerve stimulation is a well-known complication in cochlear implant treatment especially in the group of subjects with otosclerosis. The possible explanation of this side effect is a change of the electrical properties of the otosclerotic bone leading to leakage current and resulting in facial nerve stimulation. Methods: Four CI subjects who had been implanted with a Nucleus Mini22 device with a Nucleus Straight electrode between 9 to 12 years ago suffered from severe FN stimulation. Electrode contacts had to be switched off so that they could only use 4, 11, 13, and 15 electrodes of their usual set of 22. The switch off resulted in deteriorating speech understanding over time. Therefore, all subjects were reimplanted with a Nucleus 24R device with a Contour electrode. Preoperatively, the threshold of FN stimulation was obtained on all electrodes subjectively. Intraoperatively, FN stimulation thresholds were measured objectively with both, the old and the new device and were compared. NRT and SRT thresholds were also obtained with the reimplanted device to assure effective electrical stimulation of the auditory nerve. Results: In all four cases the postoperative fitting demonstrated no FN stimulation on all electrodes up to maximum comfortable level. The insertion of the Contour electrode array was complete in three cases, in one case the array could only be inserted partially similarly to the situation before the reimplantation. Speech perception tests showed a significant improvement in all subjects with the new device. Conclusion: Electrodes with modiolar facing contacts and perimodiolar position like the Nucleus Contour electrode reduce the possibility of facial nerve stimulation significantly due to more focused electrical stimulation.
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Abstract: The stimulation electrodes of cochlear implants are covered by a lymph liquid with high electrical conductivity, which causes the transmitted information to be reduced by channel interaction. Even if this problem were solved, the stimulation regions may be discrete; therefore, there may be regions in which the electrode array cannot stimulate selectively. To solve these problems, we proposed a new auditory nerve stimulation method which we call the tripolar electrode stimulation method for cochlear implants. Our method stimulates using 3 adjacent electrodes selected from among the electrodes of the electrode array. The center electrode receives the currents emitted from the electrodes on both sides. We conducted animal experiments using this method. On the basis of the results we obtained, we concluded that our method may succeed in narrowing the stimulation region and continuously moving the stimulation site.
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Cochlear implant electrode arrays are designed with specific characteristics that allow for the preservation of intra-cochlear structures during the insertion process, as well as during explantation. Straight lateral wall (LW) electrode arrays and pre-curved modiolar hugging (MH) electrode arrays are the two types that are commercially available. Although there is a third type of electrode array called the mid-scala (MS), which is positioned in the middle of the scala tympani (ST), and is usually considered as an MH type of electrode. Different lengths of straight LW electrode arrays are currently available which allow for insertion across a range of different sized cochleae; however, due to manufacturing limitations, pre-curved MH electrodes are generally only available to cover the basal turn of the cochlea, while the spiral ganglion cells are distributed in the Rosenthal's canal that extends into 1.75 turns of the cochlea. Both straight LW and pre-curved MH electrodes can cause a certain degree of intra-cochlear trauma, but pre-curved MH electrodes tend to deviate into the scala vestibuli from the scala tympani more often than the straight LW electrodes, resulting in damage to the osseous spiral lamina/spiral ligament which could initiate new bone formation and eventually affect the cochlear implant users' hearing performance. Structural damage to the cochlea could also affect the vestibular function. With pre-curved MH electrodes, higher degrees of trauma are related to the fixed curling geometry of the electrode in relation to the variable coiling pattern of individual cochleae, the orientation of the electrode contacts in relation to the modiolus wall, and how effectively the stylet was handled by the surgeon during the procedure. Wire management, metal density, and the shore hardness of the silicone elastomer all contribute to the stiffness/flexibility of the electrode. It is important to acknowledge the impact of bringing the stimulating contacts closer to the modiolus wall with an MH electrode type in terms of the resultant damage to intra-cochlear structures. The presence of malformed cochleae should be identified and appropriate electrodes should be chosen for each specific cochlea, irrespective of the cochlear implant brand. In order to utilize drug therapy, the cochlea should be free from any trauma.
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