Virtual cochlear implantation for personalized rehabilitation of profound hearing loss

The role of pressure and friction forces in automated insertion of cochlear implants

Objectives

Despite the success of cochlear implant (CI) surgery for hearing restoration, reducing CI electrode insertion forces is an ongoing challenge with the goal to further reduce post-implantation hearing loss. While research in this field shows that both friction and quasistatic pressure forces occur during CI insertion, there is a lack of studies distinguishing between these origins. The present study was conducted to analyze the contribution of both force phenomena during automated CI insertion.

Methods

Five MED-EL FLEX28 CI electrode arrays were inserted into both a regular and uncoiled version of the same average scala tympani (ST). Both ST models had a pressure release hole at the apical end, which was kept open or closed to quantify pressure forces. ST models were filled with different sodium dodecyl sulfate (SDS) lubricants (1, 5, and 10% SDS, water). The viscosity of lubricants was determined using a rheometer. Insertions were conducted with velocities ranging from v= 0.125 mm/s to 2.0 mm/s.

Results

Viscosity of SDS lubricants at 20°C was 1.28, 1.96, and 2.51 mPas for 1, 5, and 10% SDS, respectively, which lies within the values reported for human perilymph. In the uncoiled ST model, forces remained within the noise floor (maximum: 0.049 × 10-3 N ± 1.5 × 10-3 N), indicating minimal contribution from quasistatic pressure. Conversely, forces using the regular, coiled ST model were at least an order of magnitude larger (minimum: Fmax = 28.95 × 10-3 N, v = 1 mm/s, 10% SDS), confirming that friction forces are the main contributor to total insertion forces. An N-way ANOVA revealed that both lubricant viscosity and insertion speed significantly reduce insertion forces (p < 0.001).

Conclusion

For the first time, this study demonstrates that at realistic perilymph viscosities, quasistatic pressure forces minimally affect the total insertion force profile during insertion. Mixed friction is the main determinant, and significantly decreases with increaseing insertion speeds. This suggests that in clinical settings with similar ST geometries and surgical preparation, quasistatic pressure plays a subordinate role. Moreover, the findings indicate that managing the hydrodynamics of the cochlear environment, possibly through pre-surgical preparation or the use of specific lubricants, could effectively reduce insertion forces.

A full-head model to investigate intra and extracochlear electric fields in cochlear implant stimulation

Objective.Despite the widespread use and technical improvement of cochlear implant (CI) devices over past decades, further research into the bioelectric bases of CI stimulation is still needed. Various stimulation modes implemented by different CI manufacturers coexist, but their true clinical benefit remains unclear, probably due to the high inter-subject variability reported, which makes the prediction of CI outcomes and the optimal fitting of stimulation parameters challenging. A highly detailed full-head model that includes a cochlea and an electrode array is developed in this study to emulate intracochlear voltages and extracochlear current pathways through the head in CI stimulation.Approach.Simulations based on the finite element method were conducted under monopolar, bipolar, tripolar (TP), and partial TP modes, as well as for apical, medial, and basal electrodes. Variables simulated included: intracochlear voltages, electric field (EF) decay, electric potentials at the scalp and extracochlear currents through the head. To better understand CI side effects such as facial nerve stimulation, caused by spurious current leakage out from the cochlea, special emphasis is given to the analysis of the EF over the facial nerve.Main results.The model reasonably predicts EF magnitudes and trends previously reported in CI users. New relevant extracochlear current pathways through the head and brain tissues have been identified. Simulated results also show differences in the magnitude and distribution of the EF through different segments of the facial nerve upon different stimulation modes and electrodes, dependent on nerve and bone tissue conductivities.Significance.Full-head models prove useful tools to model intra and extracochlear EFs in CI stimulation. Our findings could prove useful in the design of future experimental studies to contrast FNS mechanisms upon stimulation of different electrodes and CI modes. The full-head model developed is freely available for the CI community for further research and use.

Correlation of Scalar Cochlear Volume and Hearing Preservation in Cochlear Implant Recipients with Residual Hearing

OBJECTIVE

Preservation of residual hearing is one of the main goals in cochlear implantation. There are many factors that can influence hearing preservation after cochlear implantation. The purpose of the present study was to develop an algorithm for validated preoperative cochlear volume analysis and to elucidate the role of cochlear volume in preservation of residual hearing preservation after atraumatic cochlear implantation.

STUDY DESIGN

Retrospective analysis.

SETTING

Tertiary referral center.

PATIENTS

A total of 166 cochlear implant recipients were analyzed. All patients were implanted with either a MED-EL (Innsbruck, Austria) FLEXSOFT (n = 3), FLEX28 (n = 72), FLEX26 (n = 1), FLEX24 (n = 41), FLEX20 (n = 38), or FLEX16 (n = 11, custom made device) electrode array through a round window approach. Main outcome measures: Cochlear volume as assessed after manual segmentation of cochlear cross-sections in cone beam computed tomography, and preservation of residual hearing 6 months after implantation were analyzed. The association between residual hearing preservation and cochlear volume was then assessed statistically.

RESULTS

Rapid and valid cochlear volume analysis was possible using the individual cross-sections and a newly developed and validated algorithm. Cochlear volume had the tendency to be larger in patients with hearing preservation than in those with hearing loss. Significant correlations with hearing preservation could be observed for the basal width and length of the basal turn.

CONCLUSIONS

Preservation of residual hearing after cochlear implantation may depend on cochlear volume but appears to be influenced more strongly by other cochlear dimensions.

Intracochlear Recording of Electrocochleography During and After Cochlear Implant Insertion Dependent on the Location in the Cochlea

To preserve residual hearing during cochlear implant (CI) surgery it is desirable to use intraoperative monitoring of inner ear function (cochlear monitoring). A promising method is electrocochleography (ECochG). Within this project the relations between intracochlear ECochG recordings, position of the recording contact in the cochlea with respect to anatomy and frequency and preservation of residual hearing were investigated. The aim was to better understand the changes in ECochG signals and whether these are due to the electrode position in the cochlea or to trauma generated during insertion. During and after insertion of hearing preservation electrodes, intraoperative ECochG recordings were performed using the CI electrode (MED-EL). During insertion, the recordings were performed at discrete insertion steps on electrode contact 1. After insertion as well as postoperatively the recordings were performed at different electrode contacts. The electrode location in the cochlea during insertion was estimated by mathematical models using preoperative clinical imaging, the postoperative location was measured using postoperative clinical imaging. The recordings were analyzed from six adult CI recipients. In the four patients with good residual hearing in the low frequencies the signal amplitude rose with largest amplitudes being recorded closest to the generators of the stimulation frequency, while in both cases with severe pantonal hearing losses the amplitude initially rose and then dropped. This might be due to various reasons as discussed in the following. Our results indicate that this approach can provide valuable information for the interpretation of intracochlearly recorded ECochG signals.

Optimization of pharmacological interventions in the guinea pig animal model-a new approach to calculate the perilymph volume of the scala tympani

Objective

The guinea pig serves as a well-established animal model for inner ear research, offering valuable insights into the anatomy, physiology, and therapeutic interventions of the auditory system. However, the heterogeneity of results observed in both in-vivo experiments and clinical studies poses challenges in understanding and optimizing pharmacotherapy outcomes. This heterogeneity may be due to individual differences in the size of the guinea pig cochlea and thus in the volume of the scala tympani (ST), which can lead to different drug concentrations in the ST, a fact that has been largely overlooked thus far. To address this issue, we aimed to develop an approach for calculating the individual volume of perilymph within the ST before and after cochlear implant insertion.

Method

In this study, high-resolution μCT images of a total of n = 42 guinea pig temporal bones were used to determine the volume of the ST. We compared fresh, frozen, and fixed tissues from both colored and albino strains to evaluate the potential influence of tissue condition and strain on the results.

Results

Our findings demonstrate a variability in mean ST volume with a relative standard deviation (RSD) of 14.7%, comparable to studies conducted with humans (range RSD: 5 to 20%). This indicates that the guinea pig cochlea exhibits similar variability to that of the human cochlea. Consequently, it is crucial to consider this variability when designing and conducting studies utilizing the guinea pig as an animal model. Furthermore, we successfully developed a tool capable of estimating ST volume without the need for manual segmentation, employing two geometric parameters, basal diameter (A) and width (B) of the cochlea, corresponding to the cochlear footprint. The tool is available for free download and use on our website.

Conclusion

This novel approach provides researchers with a valuable tool to calculate individual ST volume in guinea pigs, enabling more precise dosing strategies and optimization of drug concentrations for pharmacotherapy studies. Moreover, our study underscores the importance of acknowledging and accounting for inter-individual variability in animal models to enhance the translational relevance and applicability of research outcomes in the field of inner ear investigations.

The Dependency of Cochlear Lateral Wall Measurements on Observer and Imaging Type

HYPOTHESIS

Assessment techniques for the cochlear spatial lateral wall are associated with inter-rater variability, but derived clinical recommendations nonetheless offer value for individualized electrode selection.

BACKGROUND

Anatomical variations influence the location of cochlear implant electrodes inside the cochlea. Preoperative planning allows individualization of the electrode based on characterization of the bony lateral wall.

METHODS

The study used publicly available digitized temporal bones based on microslicing and computed tomography. Four experienced observers assessed the lateral wall applying manual tracing, linear regression scaling and elliptic-circular approximation methods in all modalities. Radial and height differences were computed in 90-degree steps from the round window center to the apex. Total length, total angular length, and tonotopic frequencies were computed for each reconstruction.

RESULTS

Differences were found most pronounced between assessment methods in vertical direction across observers and imaging modalities. One of the five anatomies was consistently found to be of shorter cochlear duct length with estimation techniques yielding more conservative results compared with manual tracings.

CONCLUSIONS

Assessment techniques for the bony lateral wall yield method, observer, and image modality related deviations. Automation of the anatomical characterization may offer potential in minimizing inaccuracies. Nonetheless, observers were consistently able to detect a smaller inner ear demonstrating the ability of current methods to contribute to an optimized choice of electrodes based on individual patient anatomy.