Test Electrode-Based Estimation of Achievable Insertion Depth in Cochlear Implantation

Summary

This study evaluates the use of a test insertion electrode with colored depth markings to assess the electrode insertion depth test before cochlear implantation. Here, 10 patients underwent the procedure. The test helped select and improve surgical techniques, promoting full insertions and minimizing partial insertions during cochlear implant surgeries.

Abstract

Processing the optimal length of electrode arrays for cochlear implants (CIs) is vital for achieving maximum effectiveness, and the results tend to differ between the preoperative radiological estimations and the depth reached at the time of surgery. This study evaluates the feasibility of using a flexible insertion test electrode with colored depth markers to determine the practically achievable electrode insertion depth before CI electrode array placement. The study was conducted at a tertiary center and included patients with inner ear anomalies, reimplantation cases, and profound deafness with no residual hearing. A custom insertion test electrode, 31.5 mm long, was inserted into the scala tympani (ST) to assess cochlear lumen accessibility. Standard CI surgical procedures were followed, including impedance field telemetry testing and evoked compound action potential measurements. A total of 10 patients (11 ears), aged 1 to 29, met the inclusion criteria. The proposed test electrode enabled real-time determination of insertion depth, allowing the surgeon to tailor electrode lengths to the most suitable depth for implantation. This advancement mitigated incomplete insertion and improved preoperative planning. This research describes a new approach for setting the limits of electrode selection that minimizes the complications of electrode placement during cochlear implantation. The proposed insertion test electrode could help achieve better accuracy in surgery and, consequently, better outcomes for patients with cochlear implants.

Introduction

Cochlear implants (CIs) are one of the most effective interventions for restoring hearing in individuals with severe-to-profound sensorineural hearing loss (SNHL) who do not benefit from traditional hearing aids¹. A CI system combines external and internal components to convert environmental sounds into electrical impulses². The external system includes a microphone that captures sound and a sound processor that converts it into coded signals, which are wirelessly transmitted to the internal system². The internal system comprises a stimulator, implanted beneath the skin, that processes the signals and delivers them as electrical impulses through an electrode array inserted into the cochlea². Accurate placement of the electrode array within the scala tympani (ST) is essential for optimal auditory outcomes³. While deeper angular insertion of the electrode array beyond the basal turn has been associated with improved speech perception, achieving full insertion remains challenging^4,5,6,7.

Despite the availability of various CI electrode arrays, selecting the appropriate length is complex, as it depends on individual cochlear duct length (CDL) and anatomical variations. CDL estimation formulas have been proposed to guide electrode selection^8,9,10,11,12, but clinical validation remains limited. Radiological measurement techniques that estimate CDL commonly include computed tomography (CT) and/or magnetic resonance imaging (MRI). There are cases, however, when these estimates do not reflect the actual insertion depth achieved intraoperatively, which might result in over-insertion, partial insertion, or complete misplacement of the electrode array. In malformed cochleae, improper insertion may result in the electrode entering unintended structures, such as the vestibule, internal auditory canal, or semicircular canals, further complicating CI outcomes^13,14,15. Consequently, a more reliable and practical method for assessing electrode insertion depth is needed. Soft surgical techniques are followed during CI procedures to conserve the amount of residual hearing and reduce cochlear damage¹⁴. Most surgeons stop at a considerable resistance level during electrode insertion; however, full insertion within the anatomical boundaries is still challenging in some cases¹⁵. This challenge is particularly relevant in pediatric patients with post-meningitis cochlear ossification or cases with inner ear malformations, where the risk of incomplete insertion is higher¹⁶. Studies suggest that at least eight electrode channels must be positioned within the cochlea to optimize hearing outcomes, making partial insertion unsatisfactory for many surgeons and patients¹⁷.

To address these limitations, this study introduces an insertion test electrode designed to provide a real-time assessment of achievable insertion depth before implantation. Unlike conventional methods that rely solely on CDL estimation from radiological imaging, this test electrode allows surgeons to physically measure the cochlear lumen's accessibility using a flexible dummy electrode equipped with colored depth markers in patients with specific situations.

Despite the availability of specific insertion test electrodes, they are often limited by their designs, which rigidly fit specific electrode lengths and, therefore, require separate test electrodes for separate arrays¹⁸. The proposed insertion test electrode solves this issue with multiple depth markers, facilitating standardized measurement across different cochlear lengths. This method improves the surgical technique and the processes of electrode selection, enhances the likelihood of complete insertion, and reduces some of the complications that occur post-surgery, thus aiding in planning the surgery. The research aims to analyze the effectiveness of the proposed test electrode in the controlled insertion of the electrode and the cervical fastening of the implant. 

Protocol

This prospective study was conducted at a tertiary CI center from June 2022 onwards, with institutional review board approval (IRB: H-13-S-071) and adherence to relevant guidelines and regulations. Informed consent was obtained from all participants or their legal guardians.

1. Inclusion and exclusion criteria

1. Recruit based on the following inclusion criteria: inner ear anomalies, CI reimplantation surgery, post-meningitis status, expected electrode array fibrosis (confirmed by MRI), or complete profound sensorineural hearing loss (SNHL) with no detectable ABR wave V at 90 dB.
2. Exclude patients with any residual hearing.

2. Pre-operative cochlear size assessment

1. Estimate the cochlear duct length (CDL) using pre-operative computed tomography (CT) scans for patients with normal inner ear anatomy.
2. Review CT and magnetic resonance imaging (MRI) scans for all patients considered to confirm eligibility for cochlear implantation and verify that they meet the inclusion criteria. Ensure that imaging is interpreted by qualified personnel experienced in evaluating cochlear anatomy for CI candidacy.

3. Surgical procedure

1. Follow standard CI surgical procedures to access the mastoid and middle ear space using a posterior tympanotomy and an extended round window approach. This approach is favored for its direct, safe access to the cochlea, minimizing trauma risk.
2. Perform a cortical mastoidectomy to expose the middle ear structures. Perform posterior tympanotomy by creating a window through the facial recess to access the round window niche.
3. Identify and expose the round window membrane. Perform extended bone removal for better access. 
4. Insert the electrode array through the round window membrane into the scala tympani, minimizing trauma. Secure the electrode and close the site in layers.

4. Insertion test electrode

1. Use a custom-made insertion test electrode (31.5 mm long) with five distinct insertion depth markers provided by the manufacturer (Table of Materials). Place the colored insertion depth markers as distinct rings at fixed distances from the electrode tip, each corresponding to a specific insertion depth. Determine these positions by MED-EL based on typical cochlear anatomy, with the round window used as the main anatomical reference point for alignment during surgery. This ensures accurate and consistent electrode insertion.
2. Open the device from the standard CI sterile package under sterile conditions. Identify insertion depths using the colored rings on the electrode (see Figure 1). This test electrode with colored markers simulates five commercially available electrode array lengths. Avoid using separate test electrodes for each array length using this single, multi-length test tool.
   NOTE: Colored markers enhance visibility under the microscope during surgery, allowing the surgeon to assess insertion depth accurately and select the optimal electrode length for each cochlea.
3. Insert the test electrode carefully into the scala tympani (ST) at this stage. Use visual cues to guide the insertion into the ST through the round window with smooth advancement aided by colored markers. Postoperative imaging can confirm placement if necessary.
4. Advance the electrode slowly in the ears with normal anatomy until encountering the first significant resistance point. In incomplete partition type II (IP-II) cochleae cases, limit the insertion to the third marker from the tip (24 mm) to avoid over-insertion.
5. Observe the colored markers under the surgical microscope to assess the achieved insertion depth. Select the appropriate implant electrode length based on the observed depth from MED-EL's FLEX or FORM electrode families.
   NOTE: All electrodes have 12 stimulating channels. FLEX electrodes: 5 apical channels (single-side openings), seven basal channels (dual-side openings). FORM electrodes: All 12 channels have dual-side openings and a cork-shaped insertion stopper. Markers indicate insertion depth. Normally, insertion proceeds until the first resistance is felt, indicating ideal depth. In IP-II cases, insertion should stop at the third marker (24 mm) to prevent over-insertion. Marker positions guide the selection of electrode length for optimal fit and safety.

5. Intra-operative measurements

1. Once the real electrode array had been placed, measure the impedance field telemetry (IFT) to confirm the integrity and functionality of the device, in addition to the impedance values
2. Measure the evoked compound action potential (ECAP) thresholds to evaluate the auditory nerve's responsiveness. Determine the endpoint as the lowest stimulus level reliably producing an ECAP response, identified by characteristic negative (N1) and positive (P1) waveform peaks.
3. Confirm the device's operating capability and the auditory pathway's responsiveness. Record ECAPs intraoperatively by stimulating each electrode contact and recording auditory nerve responses through the implant's telemetry system. Clinical software delivered pulses and detected waveforms.
4. Determine the endpoint as the lowest stimulus level eliciting a measurable response. Ensure the ECAP measurements are performed by trained personnel to ensure accurate reading and proper interpretation.

Results

For this study, 10 patients with profound SNHL were enrolled, contributing 11 ears. The ages of the participants ranged from 9 months to 29 years. Normal anatomy (NA) in the inner ear was observed in seven ears, while Mondini dysplasia or incomplete partition (IP) type II was identified in four ears. Pre-operative estimates of the CDL were assessed using formulas^9,10,11 applicable only to cases with normal anatomy, such as the Escudé formula, the Alexiades formula, or the Erixon formula, as shown in Table 1. The insertion depth achieved by the insertion test electrode and the selected electrode arrays that reached full insertion are summarized in Table 2. Among the 11 ears, 90.91% received implants on the right side, and 9.09% were on the left.

In terms of electrode types, FORM 24 was used in 27.27% of the ears, FORM 19 in 27.27%, FLEX 26 in 18.18%, FLEX 28 in 18.18%, and the STANDARD electrode in 9.09% of the ears. Notably, four ears (from 3 patients) were diagnosed with IP type II inner ear malformations, resulting in an incidence rate of 36% within the study population. This should not be generalized to represent the prevalence of malformation in the region.

Figure 2 illustrates post-operative X-rays demonstrating the full insertion of chosen electrodes across various cochlear anatomies. Specifically, FORM 19 in an IP II cochlea (3R) covered an angular depth of 360°, while FORM 24 in another IP II cochlea (1R) covered 450°. In contrast, FLEX 28 in an NA cochlea (10R) achieved approximately 540° of angular coverage. Following the insertion of the electrode array, intraoperative recordings of ECAP thresholds confirmed auditory nerve responses, as illustrated in Figure 3.

These results demonstrate the practical effectiveness of the custom-made insertion test electrode with colored depth markers in cochlear implantation surgery. The technique enabled real-time assessment of achievable insertion depth, allowing the surgical team to select the most appropriate electrode array length for each patient's unique cochlear anatomy. The successful full insertion of the selected arrays in all cases, regardless of anatomical variation, highlights the adaptability and precision of this approach. The colored markers provided clear visual feedback under the surgical microscope, facilitating accurate placement and minimizing the risk of partial insertions or misplacements.

Moreover, the correlation between the insertion depths indicated by the colored markers and the angular coverage achieved, as confirmed by post-operative imaging, validates the reliability of this technique. Intraoperative ECAP threshold measurements further confirmed the functional integrity of the implants, indicating that accurate anatomical placement translated into effective auditory nerve stimulation. For outcome analysis, it is recommended to compare the achieved insertion depths with preoperative CDL estimates and post-operative imaging and to correlate these findings with intraoperative and post-operative functional measures such as ECAP thresholds. This comprehensive approach ensures both anatomical and physiological success, supporting the value of the test electrode in improving cochlear implant surgical planning and outcomes.

Figure 1: Illustration of the proposed insertion test electrode. This figure shows the insertion test electrode with colored depth markers designed to assess the achievable insertion depth before cochlear implant electrode placement. Please click here to view a larger version of this figure.

Figure 2: Post-operative X-ray images of electrode insertions. Radiographic images displaying full insertion of the selected electrode arrays in two different cochlear anatomies, highlighting variations in insertion depth. Please click here to view a larger version of this figure.

Figure 3: Intraoperative evoked compound action potential (ECAP) thresholds. Measurements of ECAP thresholds were recorded post-insertion to evaluate the auditory nerve's response and confirm electrode functionality. Please click here to view a larger version of this figure.

Studies	Equation
Escudé et al.9	CDL(LW) = 2.62 × A × loge (1+ (Ө/235))
Erixon et al.10	CDL(LW) = 3.08 × A + 12.44
Alexiades et al.11	CDL(OC) = 4.16 × A − 4
Koch et al.12	CDL(OC) = 4.16 × A − 5.05
Schurzig et al.13	CDLLW(θ)= pBTL(θ)/BTLLW ; CDLi(θ)= pBTL(θ)/BTLi
Khurayzi et al.14	CDLOC = (1.71*(1.18(A−1)+.9(B−1)−√0.72(A−1)(B−1)) + .018) + 1.58

Table 1: Comparison of different CDL estimation formulae.The table summarizes various cochlear duct length estimation methods, including their parameters and reported accuracy.

No	Age (years)	Anatomy identified	Estimated CDL (mm)	Insertion depth (mm)	Electrode selected and fully inserted
1R	4	IP II	-	24	FORM 24
2R	1	NA	36.1	24	FORM 24
3R	3	IP II	-	19	FORM 19
4R	0.75	NA	33.2	19	FORM 19
4L	0.75	NA	32.9	26	FLEX 26
5R	2	NA	33.5	28	FLEX 28
6R	1	IP II	-	19	FORM 19
7R	1	NA	32.3	26	FLEX 26
8R	29	IP II	-	24	FORM 24
9R	23	NA	34.65	31	STANDARD
10R	2	NA	35.6	28	FLEX 28

Table 2: Patient characteristics.The table provides demographic and clinical details of study participants, including age, cochlear anatomy, and surgical outcomes.

Discussion

To the best of our knowledge, this is the first prospective study to report the application of an insertion test electrode aimed at identifying the practically achievable electrode insertion depth in live patients, accounting for the operating surgeon's insertion capabilities. The goal of achieving full insertion of the chosen electrode array was accomplished using the proposed insertion depth device with colored markers. This innovative insertion test device is the first of its kind, featuring five different insertion depth markers in two colors. It was considerably more straightforward in this study to track the colored markers used in the patients to pinpoint the precise insertion depth under the surgical microscope than with the shiny platinum contact pads of the electrode array. This could aid in improving surgical accuracy and control while deciding the depths for electrode insertion during CI operations.

The color coding for the insertion test electrode was designed to maximize intraoperative visibility and ease of depth estimation. Five distinct depth markers were incorporated as colored rings along the shaft of the test electrode, with alternating colors (for example, blue and red) to differentiate each depth interval. Each colored ring corresponds to a specific distance from the electrode tip (e.g., 19 mm, 24 mm, 26 mm, 28 mm, and 31.5 mm), allowing the surgeon to quickly and reliably identify the achieved insertion depth under the surgical microscope. This systematic color arrangement was determined in collaboration with the manufacturer (MED-EL) based on typical cochlear anatomy and common electrode array lengths, ensuring both standardization and practical utility during surgery.

Preserving cochlear structure is crucial for the success of any CI surgery. The insertion of the test device before the actual implant electrode insertion was a significant consideration. Through detailed discussions and establishing specific inclusion criteria, we collaborated with MED-EL, a CI manufacturer recognized for its variable-length flexible electrodes, to develop a test electrode with insertion depth markers that mimicked the mechanical properties of an actual implant electrode. This collaboration instilled confidence in the ability to gently insert the device into the scala tympani (ST) and assess how far the electrode array could be placed into the cochlea, rather than solely relying on pre-operative cochlear duct length (CDL) assessments. However, this approach is not recommended for patients with functional low-frequency residual hearing, even though electrically evoked compound action potential (ECAP) measurements confirmed cochlear functionality following double insertion attempts.

While pre-operative CDL assessments theoretically assist in electrode selection and post-operative audio processor fitting, they do not ensure full insertion of the selected electrode in every case. There is encouraging literature regarding the accuracy of predicted insertion depths based on various mathematical models^19,20,21. However, validations through actual electrode insertions remain limited. This limitation led us to consider a practical approach whereby a test device was placed in patients with certain anatomical features to determine achievable insertion depths. This approach made it possible to carry out the entire insertion of the implanted electrode in all subjects. Notably, all CDL estimation formulas have been validated solely for cochleae with normal anatomy, characterized by 2.5 turns, and have not yet been tested for anatomical anomalies. Recent studies have proposed methods for estimating cochlear length that only consider 360° insertion depths in malformed cochleae, lacking formulas for calculating more resounding insertions of 450° or 540°, which is particularly relevant for cases of incomplete partition type II and enlarged vestibular aqueduct syndrome. Factors that may hinder full electrode insertion include the surgeon's proficiency in handling the device, constraints associated with surgical maneuverability, and anatomical variations in the basal turn²².

The goal remains to achieve maximum insertion of electrodes for all profoundly deaf patients, as maximizing the number of stimulating channels within the cochlea is preferable. Unfortunately, partial electrode insertion remains an underestimated challenge in the CI field, particularly with flexible, free-fitting electrodes from various CI brands. This issue, while not extensively documented in the literature, is frequently discussed among CI surgeons during conferences and workshops. Consequently, we opted for the practical method of using a dummy electrode before inserting the CI implant electrode. The hearing outcomes of our patient group will be followed up and reported after reaching a proper CI use period. The proper CI use period typically refers to a follow-up duration of at least 6 to 12 months after cochlear implant activation, which is widely accepted in clinical studies as the minimum period needed to assess stable hearing outcomes and device performance. This timeframe allows for adequate auditory rehabilitation, device programming (mapping), and adaptation by patient^23,24. This continued assessment will provide further insights into the efficacy of the insertion test electrode and its potential role in optimizing surgical outcomes.

It is important to note that the study's small sample size limits its ability to validate the utility of the colored insertion electrode. Additionally, the current findings should not be generalized to pre-curved electrode applications, as insertion and explantation may lead to significant intra-cochlear structural damage. Future studies with larger patient cohorts and diverse electrode designs are necessary to assess this technique's broader applicability and impact on CI outcomes.

To the best of our knowledge, this study represents the first instance of utilizing an insertion test electrode array to ascertain the practically achievable electrode insertion depth before the placement of the implant electrode array. The application of the insertion test electrode facilitated the successful full insertion of the selected electrode in patients with both normal cochlear anatomy and those with incomplete partition type II malformations. This approach is a valuable resource for CI centers facing challenges related to partial electrode insertion with free-fitting electrode types from manufacturers in specific cases. Moreover, these findings could inspire further research to refine CDL estimation techniques and establish improved methodologies for determining electrode insertion depths across varying anatomical conditions.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Cochlear Implants	MED-EL		FLEX 26, FLEX 28, FORM 19, FORM 24, STANDARD
Insertion Test Electrode	MED-EL		A custom-made insertion test electrode array of 31.5 mm in length, featuring five distinct insertion depth markers
Software			Any software used for imaging analysis or cochlear duct length (CDL) estimation.