Variations in microanatomy of the human modiolus require individualized cochlear implantation

Defining the ideal trajectory into the inner ear in image-guided cochlear implant surgery

The aim of robot-assisted cochlear implant surgery (RACIS) is to access the inner ear with minimal trauma. High-resolution imaging, empowered with a highly accurate navigation system can enable the planning of a direct keyhole drilling trajectory toward the inner ear. The time has come to (re)define the ideal trajectory into the inner ear with robot-assisted tools that can drill with the highest accuracy. The trajectories of past RACIS procedures were analysed to gain insight into how to calculate the most ideal trajectory and to determine which trajectory parameters influence the course of the procedure. Data-analysis was performed on three groups of previously performed RACIS-procedures. Group 1 included the RACIS-procedures with a round window (RW) approach. Group 2 included the RACIS-procedures with difficulty entering the inner ear. Group 3 included the converted cases to conventional CI surgery due to inadequate intra-operative safety margins. The RW diameter was significantly smaller and its orientation significantly less favourable in group 2 compared to group 1. A smaller surface size and an unfavourable orientation of the RW in relation to the drilling trajectory are thus associated with a more difficult electrode array insertion in RACIS. Both must be taken into account when planning a drilling trajectory into the inner ear.

Is the spread of excitation different between adults and children cochlear implants users?

PURPOSE

While cochlea is adult size at birth, etiologies and bone density may differ between children and adults. Differences in neural response thresholds (tNRT) and the spread of excitation (SOE) width may impact the use of artificial intelligence algorithms in speech processor fitting.

AIM

To identify whether neural response telemetry threshold and spread of excitation width are similar in adults and children.

METHODS

Retrospective cross-sectional study approved by the Ethical Board. Intraoperative tNRT and SOE recordings of consecutive cochlear implant surgeries in adults and children implanted with Cochlear devices (Cochlear™, Australia) were selected. SOE was recorded on electrode 11 (or adjacent, corresponding to the medial region of the cochlea) through the standard forward-masking technique in Custom Sound EP software, which provides SOE width in millimeters. Statistical comparison between adults and children was performed using the Mann-Whitney test (p ≤ 0.05).

RESULTS

Of 1282 recordings of intraoperative evaluations, 414 measurements were selected from children and adults. Despite the tNRT being similar between adults and children, SOE width was significantly different, with lower values in children with perimodiolar arrays. Besides, it was observed that there is a difference in the electrode where the SOE function peak occurred, more frequently shifted to electrode 12 in adults implanted. In straight arrays, there was no difference in any of the parameters analyzed on electrode 11.

CONCLUSION

Although eCAP thresholds are similar, SOE measurements differ between adults and children in perimodiolar electrodes.

Investigating Additional Cochlear Parameters: A follow-up systematic review and meta-analysis

Objectives

The movement towards personalization of cochlear implantation has continued to generate interest about variabilities in cochlear size. In a recent meta-analysis, Atalay et al. (2022) examined organ of corti length, cochlear lateral wall, and "A" value and found that most covariates, other than congenital sensorineural hearing loss, did not impact cochlear size via these measurements. However, no meta-analysis exists on how patient-specific variables could impact other cochlear size measurements, such as cochlear height (CH), and "B" value (defined as the distance between opposite lateral walls and perpendicular to "A" value). The purpose of this systematic review and meta-analysis is to examine how patient-specific variables impact additional cochlear size measurements to assist clinical decision-making.

Databases reviewed

A systematic review for cochlear size measurements using PRISMA methodology was performed using PubMed, CINAHL, and MEDLINE from database inception to October 1st, 2022.

Methods

Search terms used included English, cochlea, size, histology, anatomy, and human. Inclusion criteria were measurements for human cochlea, full-text articles, and articles in English. Primary measurements were "B" value and CH, as these measurements differ from the recent meta-analysis on this topic. Cochlear duct length (CDL) was also included. A random-effects continuous model for meta-analysis was performed. Measurements were stratified by gender (male/female) and disease type (sensorineural hearing loss (SNHL)/conductive hearing loss (CHL)).

Results

A total of 7 articles met final inclusion criteria from a total of 674 articles received on initial search, resulting in 2263 total human cochleae. There was a statistical difference between male CDL (n = 681 cochlea) compared to female CDL (n = 657) from four articles (p < 0.001; Cohen's d effect size (ES):0.421; 95% confidence intervals (CI): 0.171, 0.671). The frequency weighted mean for male CDL was 33.5 mm ± 1.8 mm and the frequency weighted mean for female CDL was 32.4 mm ± 1.5 mm with an unstandardized mean difference of 0.854 mm. There was no statistical difference between male "B" value (n = 329) and female "B" value (n = 349) for cochlea from two studies (p = 0.184; Cohen's d ES: 0.410; 95% CI: 0.194, 1.014). The frequency weighted mean for male "B" value was 6.5 mm ± 0.1 mm and the frequency weighted mean for female "B" value was 6.4 mm ± 0.1 mm with an unstandardized mean difference of 0.126 mm. There was no statistical difference between CH for SNHL (n = 282) and CHL (n = 275) from two studies (p = 0.486; ES: 0.085; 95% CI: 0.323, 0.153, F ig. 3). The frequency weighted mean for SNHL CH was 4.6 mm ± 0.8 mm and the frequency weighted mean for CHL CH was 4.3 mm ± 0.8 mm with an unstandardized mean difference of 0.032 mm.

Conclusion

Male CDL is statistically larger than female CDL. There is no statistically significant association between gender or hearing loss type and "B" value or CH. The effect size for all comparisons is small, indicating little practical significance between any existing differences. The results of this study provide two additional cochlear metrics and indicate similar findings to the study by Atalay and colleagues as patient-specific characteristics appear to have no statistically significantly impact on cochlear size.

Insertion trauma of a novel inner ear catheter for intracochlear drug delivery

Introduction

Even with recent research advances, effective delivery of a compound to its target cells inside the inner ear remains a challenging endeavor due to anatomical and physiological barriers. Direct intracochlear drug administration with an inner ear catheter (IEC) aims to overcome this obstacle and strives to provide a safe and efficient way for inner ear pharmacotherapy. The goal of this study was to histologically and audiologically evaluate the traumatic properties of a novel IEC for intracochlear drug delivery in a large animal model.

Methods

Seven inner ears of piglets that had undergone intracochlear fluorescein isothiocyanate dextran application via an IEC (n = 4) or round window membrane (RWM) puncture with a needle (n = 3) followed by sequential apical perilymph sampling were histologically analyzed. Additionally, obtained objective auditory compound action potential and cochlear microphonic measurements were compared. Cochlear cryosections were stained using hematoxylin and eosin, and preservation of inner ear structures was investigated. Moreover, one cochlea was methylmethacrylate-embedded and analyzed with the IEC in situ.

Results

Histological evaluation revealed an atraumatic insertion and subsequent compound application in a majority of IEC-inserted inner ears. Click cochlear compound action potential (CAP) shifts in the IEC groups reached a maximum of 5 dB (1.25 ± 2.5 dB) post administration and prior to perilymph sampling. In comparison, application by RWM puncture generated a maximum click CAP hearing threshold shift of 50 dB (23.3 ± 23.1 dB) coinciding with coagulated blood in the basal cochlear turn in one specimen of the latter group. Furthermore, in situ histology showed an atraumatic insertion of the IEC demonstrating preserved intracochlear structures.

Conclusion

The IEC appears to be a promising and efficient way for inner ear drug delivery. The similarities between the porcine and human inner ear enhance the clinical translation of our findings and increase confidence regarding the safe applicability of the IEC in human subjects.

On the interdependence of insertion forces, insertion speed, and lubrication: Aspects to consider when testing cochlear implant electrodes

OBJECTIVES

During the insertion of cochlear implant (CI) electrode arrays, forces occur which may cause trauma and poorer hearing outcomes. Unfortunately, research groups investigating factors influencing insertion forces come to contradicting results, especially regarding insertion speed. This study was conducted to investigate the origin of these contradicting results and to determine how different testing conditions influence experimental findings.

METHODS

Repeated, automated insertions with three different FLEX28 CI electrode arrays (MED-EL, Innsbruck, Austria) were performed into a newly developed, anatomically correct and 3D-printed mean scala tympani phantom. The testing protocol for each electrode included variations in insertion speed (v = 0.1-2.0 mm/s) and lubrication (90%, 50%, and 10% liquid soap), resulting in 51 insertions per electrode array and a total of 153 insertions.

RESULTS

The test setup and protocol allowed for repeatable insertions with only minimal change in the morphology of the insertion force profiles per testing condition. Strong but varying dependencies of the maximal insertion forces and work were found regarding both lubrication and speed: work-speed dependency is constant for the 10% lubricant, negative for the 50% lubricant and positive for the 90% lubricant.

CONCLUSION

Our results can explain part of the contradicting results found within previous studies by translating interrelations known from lubricated rubber friction to the field of CI electrode array insertion. We show that the main driver behind measured bulk forces are most likely the generated friction forces, which are strongly dependent on insertion speed and lubrication. The employed test setup allows for conducting repeatable and comparable insertion studies, which can be recapitulated by other centers due to the detailed explanation of the test setup as well as the developed and freely available insertion phantom. This study hence represents another important step toward standardizing CI array insertion testing.

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.

Virtual cochlear implantation for personalized rehabilitation of profound hearing loss

In cochlear implantation, current preoperative planning procedures allow for estimating how far a specific implant will reach into the inner ear of the patient, which is important to optimize hearing preservation and speech perception outcomes. Here we report on the development of a methodology that goes beyond current planning approaches: the proposed model does not only estimate specific outcome parameters but allows for entire, three-dimensional virtual implantations of patient-specific cochlear anatomies with different types of electrode arrays. The model was trained based on imaging datasets of 186 human cochleae, which contained 171 clinical computer tomographies (CTs) of actual cochlear implant patients as well as 15 high-resolution micro-CTs of cadaver cochleae to also reconstruct the refined intracochlear structures not visible in clinical imaging. The model was validated on an independent dataset of 141 preoperative and postoperative clinical CTs of cochlear implant recipients and outperformed all currently available planning approaches, not only in terms of accuracy but also regarding the amount of information that is available prior to the actual implantation.