Michael Tykocinski

Cochleostomy site: Implications for electrode placement and hearing preservation

Conclusions. With recent increased interest in minimizing intracochlear trauma and preserving residual hearing during cochlear implantation, increased attention must be paid to the cochleostomy site. The results of this paper demonstrate that the cochleostomy must be made inferior, rather than anterior, to the round window to ensure scala tympani insertion and to decrease the likelihood of insertion-induced intracochlear damage during electrode insertion. Objective. To describe the complex anatomy of the hook region of the cochlea, specifically in relation to the optimal placement of the cochleostomy for cochlear implant electrode insertion to potentially achieve hearing preservation. The authors believe that previous industry recommendations and described surgical techniques have resulted in cochleostomies being placed in anatomical positions that possibly result in electrode insertions that damage the basilar membrane and/or other cochlear structures. Material and methods. The results of a number of temporal bone studies were reviewed with attention being paid to the anatomical relationship of the basilar membrane and spiral ligament to the round window membrane. For different cochleostomy sites the potential for damage to intracochlear structures, particularly the basilar membrane and organ of Corti, was assessed. Results. The review of electrode insertion studies into human temporal bones, as well as a post-mortem anatomical study of implanted temporal bones, showed an increased risk of scala vestibuli insertions and insertion-induced damage to intracochlear structures when the cochleostomy was performed more anterior to the round window. These results were endorsed by studies detailing the anatomy of the hook region of the cochlea.

Threshold, comfortable level and impedance changes as a function of electrode-modiolar distance.

Objective The study investigated the hypothesis that threshold and comfortable levels recorded from cochlear implant patients would reduce, and dynamic range increase, as distance of the electrode from the modiolar wall (radial distance) decreases. Two groups of cochlear implant patients participated; one group using the Nucleus® 24 Contour™ electrode array, and one group using the Nucleus standard straight (banded) array. The Nucleus 24 Contour array has been shown in temporal bone studies to lie closer to the modiolus than the banded array. The relationship of electrode impedance and radial distance is also investigated. Design The study, conducted at three centers, evaluated 21 patients using the Contour array, and 36 patients using the banded array. For each patient, threshold, comfortable levels and dynamic range were measured at four time points. Common ground electrode impedance was recorded clinically from each patient, at time intervals up to 12 wk. An estimate of the radial distance of the electrode from the modiolus was made by analysis of Cochlear view x-rays. Results Threshold and comfortable levels were significantly lower for the Nucleus 24 Contour array than for the banded array. However, dynamic range measurements did not show the predicted increase. In a majority of subjects, a significant correlation was found between the estimated radial distance of the electrode from the modiolus and the measured threshold and comfortable levels. This trend was not observed for dynamic range. The analysis indicates that other factors than radial distance are involved in the resultant psychophysical levels. Clinical impedance measures (common ground) were found to be significantly higher for the Contour array. However, the electrodes on the Contour array are half-rings, which are approximately only half the geometric size of the full rings as electrodes of the standard array. When the geometric electrode area in the two array designs are normalized, the trends in the electrode impedance behavior are similar. Conclusions The results support the hypothesis that the relationship between the radial distance of the electrode and the psychophysical measures are influenced by patterns of fibrous tissue growth and individual patient differences, such as etiology and neural survival. Impedance measures for the Nucleus 24 Contour electrode array were higher than the banded electrode array, but this is primarily due to the reduction in electrode surface area. The different outcomes in impedance over time suggest differences in the relative contributions of the components of impedance with the two arrays.

Comparison of Round Window and Cochleostomy Approaches with a Prototype Hearing Preservation Electrode

Introduction: Preservation of residual hearing in cochlear implant recipients has been demonstrated to be possible and provides the potential benefit of combined electric and acoustic auditory stimulation. A prototype 16-mm multichannel array has been designed to facilitate placement of 22 electrodes without damage to intracochlear structures. The electrode array is suitable for insertion via the round window membrane (RWM) or a small cochleostomy. Aim: To evaluate the insertion trajectory and the presence of trauma to intracochlear structures with the prototype electrode inserted by either the RWM or a scala tympani cochleostomy. Materials and Methods: Eighteen fresh frozen human temporal bones were prepared for cochlear implantation using a standard transmastoid facial recess technique. Twelve electrodes were implanted at the University of Melbourne and 6 at the Medizinische Hochschule Hannover. In Melbourne fluoroscopy was used to monitor the insertions. Twelve prototype electrodes were inserted via the RWM. A further 6 electrodes were inserted via a small scala tympani cochleostomy. The cochleostomy was sited inferior to the RWM to avoid trauma to the basilar membrane and spiral ligament. Specimens were embedded and fixed with acrylic resin and the cochleae then examined histologically at 200-µm intervals using a grinding and polishing technique. Results: Full insertion of the electrode was achieved without significant resistance in all RWM and cochleostomy specimens. In two RWM specimens fold-over of the electrode tip occurred, and in one specimen the electrode penetrated the spiral ligament to lie in an ‘endosteal ‘position. In one cochleostomy specimen the electrode was rotated within the cochlea to face laterally rather than towards the modiolus. The final electrode position differed for the two groups, with the electrodes inserted via the RWM lying in a more perimodiolar position along the first part of the basal turn. The average depth of insertion was 240° for the RWM electrodes and 255° for the cochleostomy electrodes. Histologic examination showed no damage in any specimen to the modiolus, osseous spiral lamina or basilar membrane. Conclusions: A prototype hearing preservation electrode array was inserted by either a RWM or a scala tympani cochleostomy without evidence of significant intracochlear trauma.

The contour electrode array: safety study and initial patient trials of a new perimodiolar design.

Objective The aim of these studies was to investigate the insertion properties and safety of a new intracochlear perimodiolar electrode array design (Contour). Background An electrode array positioned close to the neural elements could be expected to reduce stimulation thresholds and might potentially reduce channel interaction. Methods Two sequential studies were conducted. In study 1, the Contour electrode array was inserted in 12 human temporal bones. After cochlear surface preparation, the position of the array was noted and the basilar membrane was examined for insertion damage. On the basis of the outcome of this temporal bone study, study 2 investigated the Contour array, mounted on a Nucleus CI-24 M device and implanted in three adult patients. Results Study 1 showed that in 10 temporal bones, the Contour array was positioned close to the modiolus, and the basilar membrane was intact. In the two remaining bones, the arrays had pierced the basilar membrane and were positioned in the scala vestibuli apical to the penetration. Statistical analysis showed an equivalent probability of insertion-induced damage of the two array designs. In study 2, image analysis indicated that the Contour electrodes were positioned closer to the modiolus than the standard Nucleus straight array. Lower T and C levels, but higher impedance values, were recorded from electrodes close to the modiolus. Initial speech perception data showed that all patients gained useful open-set speech perception, two patients achieving scores of 100% on sentence material 3 months postoperatively. Conclusions The temporal bone studies showed the Contour electrode array to be generally positioned closer to the modiolus than the standard Nucleus straight array, and to have an equivalent probability of causing insertion-induced damage.

Comparison of electrode position in the human cochlea using various perimodiolar electrode arrays

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 of implanted 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 atraumatic to implant.

Validation of a Networked Virtual Reality Simulation of Temporal Bone Surgery

Objectives: To assess the content validity and concurrent validity of a haptically (force feedback) rendered, virtual reality simulation of temporal bone surgery.

Surgical implications of perimodiolar cochlear implant electrode design: avoiding intracochlear damage and scala vestibuli insertion

Abstract Objective To review the mechanisms and nature of intracochlear damage associated with cochlear implant electrode array insertion, in particular, the various perimodiolar electrode designs. Make recommendations regarding surgical techniques for the Nucleus Contour electrode to ensure correct position and minimal insertion trauma. Background The potential advantages of increased modiolar proximity of intracochlear multichannel electrode arrays are a reduction in stimulation thresholds, an increase in dynamic range and more localized neural excitation. This may improve speech perception and reduce power consumption. These advantages may be negated if increased intracochlear damage results from the method used to position the electrodes close to the modiolus. Method A review of the University of Melbourne Department of Otolaryngology experience with temporal bone safety studies using the Nucleus standard straight electrode array and a variety of perimodiolar electrode array designs; comparison with temporal bone insertion studies from other centres and postmortem histopathology studies reported in the literature. Review of our initial clinical experience using the Nucleus Contour electrode array. Results The nature of intracochlear damage resulting from electrode insertion trauma ranges from minor, localized, spiral ligament tear to diffuse organ of Corti disruption and osseous spiral lamina fracture. The type of damage depends on the mechanical characteristics of the electrode array, the stiffness, curvature and size of the electrode in relation to the scala, and the surgical technique. The narrow, flexible, straight arrays are the least traumatic. Pre-curved or stiffer arrays are associated with an incidence of basilar membrane perforation. The cochleostomy must be correctly sited in relation to the round window to ensure scala tympani insertion. A cochleostomy anterior to the round window rather than inferior may lead to scala media or scala vestibuli insertion. Conclusion Proximity of electrodes to the modiolus can be achieved without intracochlear damage provided the electrode array is a free fit within the scala, of appropriate size and shape, and accurate scala tympani insertion is performed.

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates

While recent studies have suggested that electrical stimulation of the auditory nerve at high stimulus rates (e.g., 1000 pulses/s) may lead to an improved detection of the fine temporal components in speech among cochlear implant patients, neurophysiological studies have indicated that such stimulation could place metabolic stress on the auditory nerve, which may lead to neural degeneration. To examine this issue we recorded the electrically evoked auditory brainstem response (EABR) of guinea pigs following acute bipolar intracochlear electrical stimulation using charge-balanced biphasic current pulses at stimulus rates varying from 100 to 1000 pulses/s and stimulus intensities ranging from 0.16 to 1.0 microC/phase. Charge density was held constant (approximately 75 microC cm-2 geom/phase) in those experiments. To monitor the recovery in excitability of the auditory nerve following this acute stimulation. EABR thresholds, wave I and III amplitudes and their latencies were determined for periods of up to 12 h following the acute stimulation. Higher stimulus rates and, to a lesser extent, higher intensities led to greater decrements in the post-stimulus EABR amplitude and prolonged the recovery period. While continuous stimulation at 100 pulses/s induced no decrement in the EABR, stimulation at 200 and 400 pulses/s produced an increasingly significant post-stimulus reduction of the EABR amplitude, which showed only partial recovery during the monitoring period. No EABR response could be evoked immediately following stimulation at 1000 pulses/s, using a probe intensity 16-19 dB below the stimulus intensity. However, partial EABR recovery was observed for wave III following stimulation at the lowest stimulus intensity (0.16 microC/phase). These stimulus-induced reductions in the EABR amplitude were also reflected in increased thresholds and latencies. Providing stimulus rate and intensity were held constant, stimulation at different charge densities (37.7, 75.5 and 150.7 microC cm-2 geom/phase) had no influence on the post-stimulus EABR recovery. Significantly, the introduction of a 50% duty cycle into the stimulus pulse train resulted in a more rapid and complete post-stimulus recovery of the EABR compared to continuous stimulation. These data suggest that stimulus rate is a major contributor to the observed reduction in excitability of the electrically stimulated auditory nerve. This reduction may be a result of an activity-induced depletion of neural energy resources required to maintain homeostasis. The present findings have implications for the design of safe speech-processing strategies for use in multichannel cochlear implants.

Chronic electrical stimulation of the auditory nerve using high surface area (HiQ) platinum electrodes

High surface area cochlear implant electrodes with much smaller geometric surface areas than current designs might be used in the future to increase the number of stimulating electrodes along the carrier. Potential problems with an increase in charge density for a common stimulus resulting from decreasing the geometric surface area would be reduced by the enlarged real surface area of such electrodes. Electrochemically modified (HiQ) platinum (Pt) electrodes, with a real surface area approximately 75 times greater than the current standard Pt electrodes of the same geometric size, had shown in vitro a low polarization (Z(pol)) and electrode impedance (Z(e)), as well as a low residual direct current (DC). In this study we examined the chronic performance of HiQ electrodes in cats, which were bilaterally implanted with a two-channel HiQ or standard Pt scala tympani electrode array and unilaterally stimulated for periods of up to 2390 h. Stimuli consisted of 50 micros/phase charge-balanced biphasic current pulses presented at 2000 pulses/s/channel with a 50% duty cycle. Electrode impedance (Z(e)), access resistance (R(a)) and polarization impedance (Z(pol)) were calculated from current and voltage measurements obtained periodically throughout the implantation period. Immediately following implantation HiQ electrodes showed a significantly smaller Z(pol), resulting in a reduced Z(e) (P<0.0001) compared to standard electrodes, while there was no significant difference between R(a) of both electrode designs (P=0.91). Subsequently, Z(e) generally increased mainly due to a rise in R(a), which dominated Z(e) and obliterated the effect of a lower Z(pol) on Z(e) in HiQ electrodes. Peak R(a) levels correlated closely (r=0.85) with the amount of intracochlear fibrous tissue found adjacent to the array. Following explantation of the array, voltage waveforms for both electrode designs recorded in saline were again very similar to those recorded immediately after implantation. Mean DC levels were consistently lower for HiQ electrodes compared with standard electrodes (22.45 nA vs 134.7 nA). Histopathological examination of corresponding cochlear sections comparing the stimulated test side with the unstimulated control side showed no significant difference (P>0.05) for either animals implanted with HiQ electrodes (n=6) or standard electrodes (n=2). Nor were there any significant differences between the spiral ganglion cell density of the basal turn implanted with HiQ or standard electrodes for both the stimulated test (P=0.31) and the unstimulated control side (P=0.84). Although these findings are based on a small group of animals implanted with standard electrodes (n=2), and those negative statistical results could potentially be due to the small sample size, similar spiral ganglion cell survival was found in a previous study of a larger group of animals using standard electrodes stimulated with the same stimulus paradigm as in the present study [Xu et al. (1997) Hear. Res. 105, 1-29]. Our data indicate that while some initial advantages of HiQ electrodes are lost during chronic implantation due to intracochlear fibrous tissue growth, low DC levels and the high surface area appear to be maintained, suggesting that HiQ electrodes may have important clinical applications.

Measurement and analysis of access resistance and polarization impedance in cochlear implant recipients.

Background: Impedance measurements are commonly performed at the end of cochlear implant surgery, not only to confirm that all electrodes are working but also to monitor the impedances of the newly implanted electrodes. The current method of testing allows the determination of only the overall electrode impedance but not its components, access resistance and polarization impedance. To determine whether any longitudinal change in the electrode impedance is caused by a change in the endocochlear environment or rather caused by a change in the surface quality of the electrode, it is necessary to extract access resistance and polarization impedance. Methods: We applied an impedance model that enabled us to calculate access resistance and polarization impedance after measurement of electrode impedance at three points along the voltage waveform. Results: The results show that the value of the components of electrode impedance varied with time after surgery: access resistance increased slowly over time, whereas polarization impedance increased up to Week 2 but decreased after commencement of electrical stimulation at that stage. These results are consistent with the hypothesis that a layer of fibrous tissue forms around the electrode within the cochlear canal, resulting in a slow increase of access resistance, whereas a layer of proteins forms on the surface of the electrode in the early phase after implantation. Electrical stimulation appears to disperse this surface layer, thereby reducing both the polarization impedance and electrode impedance. Conclusion: The method presented enables the extraction of more detailed information about the longitudinal changes in the intracochlear environment after cochlear implantation.