Functional hearing and low frequency hearing preservation after cochlear implant surgery is achievable with FLEX electrode arrays: Real world evidence from the MEHS Registry

Uwe Baumann; Tobias Weissgerber; Andreas Radeloff; Ilona Anderson; Karin A Koinig; Magdalena Breu; Stefano Morettini; Vera Lohnherr; Joachim Müller; Daniel Polterauer-Neuling

doi:10.1371/journal.pone.0345295

. 2026 Apr 17;21(4):e0345295. doi: 10.1371/journal.pone.0345295

Functional hearing and low frequency hearing preservation after cochlear implant surgery is achievable with FLEX electrode arrays: Real world evidence from the MEHS Registry

Uwe Baumann ¹, Tobias Weissgerber ¹, Andreas Radeloff ², Ilona Anderson ³, Karin A Koinig ³, Magdalena Breu ³, Stefano Morettini ³, Vera Lohnherr ⁴, Joachim Müller ⁵, Daniel Polterauer-Neuling ^5,^*

Editor: Toru Miwa⁶

¹ENT/Audiological Acoustics, University Hospital, Goethe University Frankfurt, Frankfurt am Main, Hesse, Germany

²Division of Otorhinolaryngology and Research Center Neurosensory Science, Carl von Ossietzky University Oldenburg, Oldenburg, Lower Saxony, Germany

³Clinical Research Department, MED-EL Elektromedizinische Geräte GmbH, Innsbruck, Tyrol, Austria

⁴Hals-, Nasen- und Ohrenklinik (Kopfklinik) Im Neuenheimer Feld, University Hospital Heidelberg, Heidelberg, Baden-Württemberg, Germany

⁵Clinic and Polyclinic for Ear, Nose, and Throat Medicine, LMU Klinikum, Munich, Bavaria, Germany

⁶Teikyo University Hospital Mizonokuchi, JAPAN

^✉

* E-mail: daniel.polterauer@med.uni-muenchen.de

Competing Interests: Authors Ilona Anderson, Stefano Morettini, Karin Koinig, and Magdalena Breu are employed at MED-EL Elektromedizinische Geräte G.m.b.H. Uwe Baumann and Daniel Polterauer-Neuling have received travel support & research support from MED-EL GmbH (Austria). Polterauer-Neuling is part of MED-EL’s audiological scientific advisory board (MAUWIB). Other authors disclose no conflict of interest. Commercial affiliation does not alter our adherence to PLOS ONE policies on sharing data and materials.

Roles

Uwe Baumann: Formal analysis, Investigation, Writing – review & editing

Tobias Weissgerber: Data curation, Methodology, Project administration, Resources, Validation, Writing – review & editing

Andreas Radeloff: Investigation, Writing – review & editing

Ilona Anderson: Conceptualization, Investigation, Writing – review & editing

Karin A Koinig: Formal analysis, Validation, Writing – review & editing

Magdalena Breu: Data curation, Writing – review & editing

Stefano Morettini: Conceptualization, Investigation, Writing – review & editing

Vera Lohnherr: Conceptualization, Investigation, Writing – review & editing

Joachim Müller: Investigation, Writing – review & editing

Daniel Polterauer-Neuling: Conceptualization, Investigation, Project administration, Writing – review & editing

Toru Miwa: Editor

PMCID: PMC13089756 PMID: 41996338

Abstract

This study uses data from the MED-EL Hearing Solutions (MEHS) multicenter registry to examine the impact of a series of flexible lateral wall electrode arrays on postoperative low frequency hearing preservation (LFHP) of cochlear implant (CI) users. Participants were members of the MEHS registry who had received a MED-EL CI with a FLEX electrode array. LFHP was evaluated using 2 formulae: the Vienna Consensus (VC), which assesses hearing preservation at 250‒1000 Hz; and the Minimum Reporting Standards for Adult Cochlear American Academy of Otolaryngology—Head and Neck Surgery (AAO), which assesses hearing preservation at 125‒500 Hz. LFHP was assessed via air conduction preoperatively and between 6 and 36 months postoperatively. LFHP was achievable with all the FLEX arrays evaluated, regardless of length. With the VC, 60% of ears (n = 95) had complete or partial LFHP at 6‒12 months postoperatively and 68.4% (n = 19) had complete or partial LFHP at 24‒36 months postoperatively. With the AAO, 43.4% of ears (n = 83) had LFHP at 6‒12 months postoperatively and 84.2% (n = 19) had functional low frequency residual hearing at 24‒36 months postoperatively. In conclusion, implantation with FLEX electrode arrays can preserve functional low-frequency residual hearing up to three years after implantation. The MEHS multicenter registry appears to be a valuable tool for collecting large amounts of real-world data from CI users despite challenges in harvesting usable data.

Introduction

A goal in cochlear implant (CI) surgery is to preserve as much of the CI recipients’ preoperative residual hearing as possible because CI users with more residual hearing often enjoy greater benefit than those with little or no residual hearing [1,2] and may allow CI recipients to benefit from future technologies. Preserving residual hearing in the low frequencies is regarded as particularly important, as many CI candidates, especially candidates for electro acoustic stimulation, have some degree of residual hearing at these frequencies. To this end, concepts of “soft” (or atraumatic) CI surgery, first promoted by Lehnhardt [3], have become widely accepted, and surgical techniques and materials that preserve residual hearing and intracochlear structures are being continually developed and improved [e.g., 4]. Indeed, atraumaticity is currently a central focus in CIs, as evidenced by an ever-increasing number of articles on corticosteroid regimes, robotic surgery, surgical techniques, and electrode array designs capable of maximizing hearing preservation [e.g., 5–7]. While many studies have broadened our understanding of residual low-frequency hearing preservation (LFHP), most are limited by their comparatively low number of study participants, as is often the case in studies on CI users. Furthermore, data are difficult to compare between studies due to differences in inclusion/exclusion criteria and in their use of varying definitions of what constitutes successful LFHP.

Recently, some countries have established registries of CI recipients, which have enabled researchers to pool very large amounts of real-world data including participants from a variety of surgeons and hospitals during routine, clinical practice [e.g., 8,9]. Registries are an exciting development in the CI field because they have the potential to enable a greater understanding of issues and, therefore, better evidence-based practices.

The primary aim of this study is to determine LFHP rates in CI recipients who received a FLEX series (MED-EL, Innsbruck, Austria) electrode array. To assess this in a large cohort, independent of surgical technique, we used data from the MED-EL Hearing Solutions (MEHS) Registry. As this study is based on registry data, our objective was descriptive, not causal or explanatory. To provide a fuller view, two classification systems were used: the Vienna Consensus (VC) and the Minimum Reporting Standards for Adult Cochlear American Academy of Otolaryngology—Head and Neck Surgery (AAO) [10].

Materials and methods

Ethics committee approval

MEHS Registry data for this investigation were pooled from four clinics in Germany, all of whom received Ethics Committee consent for this study: LMU Klinikum (Ethics number: #17-227), University Hospital Frankfurt (Ethics number: #288/17), Evangelisches Krankenhaus Oldenburg (Ethics number: #2018−110), and Universitätsklinikum Heidelberg (Ethics number: #546/2018). The MEHS Registry procedures were performed in adherence with the standards set in the latest revision of the Declaration of Helsinki. The MEHS Registry was registered within the clinical trials database (ClinicalTrials.gov: NCT05668338). All CI users provided written informed consent before participating in all procedures.

Participants

Data extraction from the MEHS registry was accessed and completed for all clinics on 28/06/2022. The MEHS is organized as a prospective anonymized data collection from clinical routine datasets. Clinicians in their respective registry center and as part of their clinical routine had access to information that could identify individual participants during study. Only anonymized data were exported and analyzed.

The inclusion criterion for MEHS is all CI recipients at the participating clinics who consent to be included. For more information on the MEHS, see Baumann et al. [11]. The inclusion criteria for this study differed according to assessment method (VC and AAO). For both methods, individuals were included if they had a FLEX array and unaided pure tone average (PTA) measured via air conduction at ≤12 months preoperatively and at least one assessment 6–36 months postoperatively. For the VC, potential participants were excluded if they did not have preoperative low frequency residual hearing, i.e., had ≥ 111.67 dB HL (average maximum output levels of the audiometers) at 250, 500, and 1000 Hz. For the AAO, potential participants were excluded if their preoperative PTA was ≥ 80 dB HL at 125, 250, and 500 Hz.

FLEX series electrode arrays

First introduced in 2004, the FLEX series of lateral wall arrays are designed to minimize intra‑cochlear trauma during insertion. They are available in various lengths to accommodate different cochlear sizes. Except for the FLEXSOFT, which has a length of 31.5 mm, the numbers after “FLEX” in the title of each array (e.g., the “24” in FLEX24) refer to the length of the array in millimetres. The FLEX24 was formerly named the FLEXEAS. For a more detailed description of FLEX arrays, see Dhanasingh [12].

Vienna consensus (VC)

The VC was introduced in 2023 by an international group of surgeons who met at a surgical advisory board in Vienna on 10-Jan-2023. The VC focuses on the low-frequency part of the audiogram representing relative LFHP by calculating the average from the measurements obtained at 250, 500, and 1000 Hz; measurements exceeding the maximum audiometer test thresholds to the values given in Table IV in Skarzynski et al. [13] are truncated, to obtain a uniform maximum per frequency. To be assessed via VC, CI recipients must have some preoperative hearing at those frequencies <111.67 dB HL (average maximum output levels of the audiometers). Postoperative scores are classified into Complete LFHP (≤15 dB change), Partial LFHP (>15 to ≤30 dB change), and Complete low frequency hearing loss (LFHL, > 30 dB change). The physiological fluctuation influenced by the status of the CI recipient at testing (e.g., time of day at which the test was performed, hydration status, etc.) is ± 15 dB [14,15]. Therefore, a negative threshold difference of more than 15 dB was defined as some degree of postoperative hearing loss.

Minimum reporting standards for Adult Cochlear American Academy of Otolaryngology—Head and Neck Surgery (AAO)

A protocol for reporting postoperative LFHP was proposed by Adunka et al. [10] and endorsed by the Implantable Hearing Devices Committee and the Hearing Committee of the American Academy of Otolaryngology—Head and Neck Surgery, hence our referring to it here as the AAO. They advocate reporting residual hearing only for CI users with functionally relevant preoperative hearing, which they define as a PTA of <80 dB HL at 125, 250, and 500 Hz. Postoperatively, air conduction thresholds should be reported individually for each frequency from 125‒8000 Hz. If a CI user’s postoperative residual acoustic hearing (as defined by their PTA at 125, 250, and 500 Hz) has become poorer than 80 dB HL, it is considered no longer functionally relevant and, therefore, no longer necessary to report.

VC and AAO convergence

It is important to remember that the VC and AAO use different frequencies to assess different things and for different purposes. The VC categorizes relative hearing preservation (based on postoperative threshold shifts at 250‒1000 Hz). Its purpose is to enable standardized, comparable reporting across centers and studies, regardless of whether the absolute thresholds remain within a functionally usable range. In contrast, the AAO defines if postoperative residual hearing is functional benefit (defined as <80dB HL at 125‒500 Hz), especially for EAS use. As such, it a clinically oriented framework. Thus, as the VC and AAO were designed to answer different questions and use different frequencies, agreement or discordance between them is neither expected nor clinically meaningful. Nonetheless, should readers be interested in how many cases the outcomes coincided or diverged between metrics, we compared results on a per participant basis. Results were classified as either a match or a mismatch. A match meant a participant had both complete or partial LFHP on the VC and functional hearing on the AAO or had complete LFHL on the VC and no functional hearing with the AAO. Two types of mismatch were calculated: firstly, compete LFHL on the VC and functional hearing on the AAO and secondly, complete or partial LFHP on the VC and no functional hearing on the AAO.

Safety and device deficiencies

Safety was assessed by collecting clinical incidents. Within the context of the registry, incidents were monitored on-site and reported according to the incident reporting procedure for approved products. Device deficiencies related to the CI system were eligible for the report during the timeframe considered in the data analysis.

Statistics

Descriptive statistics were used to report participants’ demographic and baseline characteristics and to present the study results. Quantitative data were presented as mean with standard deviation (SD) and/or median with range (minimum and maximum); qualitative data were presented as absolute and relative frequencies. All statistical analyses were performed with Microsoft 365 Excel Version 2208 (Microsoft Corporation, Redmond, WA, USA).

Results

Participants

Analysis of the registry yielded 704 CI recipients (823 ears). Of these, 115 participants (122 ears) fulfilled the criteria for the VC and 86 people (96 ears) for the AAO. Over 80% of participants in each cohort received a FLEX28 or FLEXSOFT array. For the selection criteria for each cohort, see Fig 1 (VC) and Fig 2 (AAO); for demographics for each cohort, see Table 1.

Fig 1 — The number of people (not ears) excluded and reason for exclusion are given in light gray.

Fig 2 — The number of people (not ears) excluded and reason for exclusion are given in light gray. It should be noted that there are 47 FLEX28 users but data for 48 PTA assessments. This apparent discrepancy is because 1 participant was assessed before and after reimplantation.

Table 1. Demographic and baseline characteristics of each study cohort.

	VC n (%)	AAO n (%)
Participants	115	86
Ears with CI	122	96
Unilateral	108 (93.9)	76 (88.4)
Bilateral	7 (6.1)	10 (11.6)
Age (years)
Mean ±	59.3 ± 15.7	59.7 ± 13.6
Range	6.4‒85.1	30.6‒85.1
<20	3 (2.6)	0 (0)
20 to <40	12 (10.4)	9 (10.5)
40 to <60	43 (37.4)	35 (40.7)
60 to <80	49 (42.6)	37 (43.0)
≥80	8 (7.0)	5 (5.8)
Sex
Female	69 (60.0)	48 (55.8)
Male	46 (40.0)	38 (44.2)
Other	0 (0.0)	0 (0)
Ear implanted
Left (all)	65 (53.3)	47 (49.0)
Right (all)	57 (46.7)	49 (51.0)
Electrode array (per ear)
FLEX24	14 (11.5)	14 (14.6)
FLEX26	4 (3.3)	4 (4.2)
FLEX28	59 (48.4)	47 (49.0)
FLEXSOFT	45 (36.9)	31 (32.3)

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It should be noted that for the AAO, there are 47 FLEX28 users but data for 48 PTAs (see Table 1 and Fig 2). This apparent discrepancy is because 1 participant was assessed before and after reimplantation.

Hearing preservation/functional hearing preservation

For the mean pre- and post-operative hearing thresholds (125‒8k Hz) according to array type, see the (S1 Table). Whilst preoperative assessment could take place up to >12 before implantation, in most cases it was done earlier: 61.9% at 1‒10 days pre-op, 15.5% at 11 days to 3 months, 13% at 4‒6 months, 7.2% at 7‒8 months, and 2.1% at 9‒12 months.

VC.

Across all arrays, complete or partial LFHP was achieved in 60.0% of ears at 6–12 months, in 57.4% of ears at 12–24 months, and in 68.4% of ears at 24–36 months. For detailed results according to array and interval, see Table 2.

Table 2. Hearing preservation results per array and interval using the VC. n are per ear. LFHP = low-frequency hearing preservation; LFHL = low-frequency hearing loss.

	n	Complete LFHP ≤15 dB loss		Partial LFHP >15‒30 dB loss		Complete LFHL >30 dB loss
	n	n	%	n	%	n	%
ALL arrays
6 to < 12 months	95	31	32.6%	26	27.4%	38	40.0%
12 to < 24 months	54	15	27.8%	16	29.6%	23	42.6%
24 to < 36 months	19	3	15.8%	10	52.6%	6	31.6%
FLEX24
6 to < 12 months	13	6	46.2%	6	46.2%	1	7.7%
12 to < 24 months	8	3	37.5%	3	37.5%	2	25.0%
24 to < 36 months	7	1	14.3%	5	71.4%	1	14.3%
FLEX26
6 to <12 months	4	1	25.0%	0	0.0%	3	75.0%
12 to <24 months	2	1	50.0%	0	0.0%	1	50.0%
24 to <36 months	0	–	–	0	–	0	–
FLEX28
6 to < 12 months	43	12	27.9%	13	30.2%	18	41.9%
12 to < 24 months	26	7	26.9%	9	34.6%	10	38.5%
24 to < 36 months	11	1	9.1%	5	45.5%	5	45.5%
FLEXSOFT
6 to < 12 months	35	12	34.3%	7	20.0%	16	45.7%
12 to < 24 months	18	4	22.2%	4	22.2%	10	55.6%
24 to < 36 months	1	1	100.0%	0	–	0	–

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AAO.

Across all arrays, functional hearing was achieved from 43.4% (36/83) of cases at 6 to <12 months to 84.2% (16/19) at 24 to <36 months. For detailed results according to array and interval, see Table 3.

Table 3. Hearing preservation results per array and interval using the AAO. n are per ear.

Electrode array	N	Functional LFHP < 80 dB HL		No functional LFHP ≥ 80 dB HL
Electrode array	N	n	%	n	%
ALL
6 to < 12 months	83	36	43.4%	47	56.6%
12 to < 24 months	46	24	52.2%	22	47.8%
24 to < 36 months	19	16	84.2%	3	15.8%
FLEX24
6 to < 12 months	14	13	92.9%	1	7.1%
12 to < 24 months	10	9	90.0%	1	10.0%
24 to < 36 months	9	9	100.0%	0	–
FLEX26
6 to < 12 months	4	4	100.0%	0	–
12 to < 24 months	2	2	100.0%	0	–
24 to < 36 months	0	0	–	0	–
FLEX28
6 to < 12 months	40	16	40.0%	24	60.0%
12 to < 24 months	22	12	54.5%	10	45.5%
24 to < 36 months	10	7	70.0%	3	30.0%
FLEXSOFT
6 to < 12 months	25	3	12.0%	22	88.0%
12 to < 24 months	12	1	8.3%	11	91.7%
24 to < 36 months	0	0	–	0	–

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For the mean thresholds from 125 to 8000 Hz at each postoperative interval for each array, see Figs 3–6.

Fig 3 — Results are per ear (not per participant). Note, only 125‒500 Hz are relevant for the AAO hearing loss assessment. n = number at preoperative cases. Number of postoperative audiogram data varies over time (see Table 3).

Fig 6 — Results are per ear (not per participant). Note, only 125‒500 Hz are relevant for the AAO hearing loss assessment. n = number at preoperative cases. Number of postoperative audiogram data varies over time (see Table 3).

Fig 4 — Results are per ear (not per participant). Note, only 125‒500 Hz are relevant for the AAO hearing loss assessment. n = number at preoperative cases. Number of postoperative audiogram data varies over time (see Table 3).

VC and AAO convergence.

Results matched in 66.4% cases. Of the 33.6% of cases of mismatch, 14.9% were complete LFHL with the VC but functional hearing preservation with the AAO and 18.7% were complete or partial LFHP with VC but no functional hearing preservation with the AAO. There was no consistent pattern suggesting an overestimation of hearing preservation by either scale. More details can be found in S2 Table.

Safety and device deficiencies

There was 1 case of reimplantation; however, the cause was not recorded in the Registry. No device deficiencies related to the MED-EL CI system were reported within the timeframe considered in this data analysis. In addition, the data collected concerning the audiological background (PTA preoperatively tested in unaided conditions) shows clinical use in line with the indications as per electrode array variant.

Discussion

The primary objective of this data analysis was to determine the proportion of ears with LFHP up to 36 months after implantation with a FLEX series array. This objective was met: 704 CI recipients (823 ears) could be extracted and, after the application of inclusion/exclusion criteria, LFHP could be assessed in 115 people (122 ears) via the VC and 86 people (96 ears) via the AAO (see Figs 1 and 2, respectively). This indicates that LFHP is possible with FLEX arrays. These relatively large cohorts could be reached thanks to the MEHS Registry, a non-interventional, systematic collection of clinical data in which prospective data are collected from original clinical records.

Results show that (complete or partial) LFHP was achieved with FLEX arrays in 43.4‒60% of cases at up to 1 year after first fitting, 52.2‒57.4% of cases at up to 2 years after first fitting, and 68.4‒84.2% of cases at up to 3 years after first fitting. These results are without controlling for surgical technique, the choice of which is known to affect the likelihood of LFHP [16], or any other factor influencing hearing preservation. Thus, we conclude that LFHP is possible with all of the FLEX arrays.

The data from this descriptive dataset indicates that LFHP is achievable when shorter arrays are used. This is possible because shorter arrays may not reach the low‑frequency areas even when fully inserted and so cannot cause insertion trauma or an inflammatory response and fibrosis in these areas. The correlation between deeper insertion depth and poorer LFHP has been supported in multiple studies [e.g., 17–19], while other studies have disputed that deeper insertion depth negatively affects LFHP [20–22]. The present dataset does not enable us to address this question due to multiple factors. Firstly, shorter arrays are often chosen for candidates with more residual hearing, especially for EAS candidates. As is evident in Figs 3‒6, the FLEX24 and FLEX26 recipients in the present study (who may have been EAS recipients when this data point was collected) had more preoperative low-frequency residual hearing than FLEX28 and FLEXSOFT recipients and, therefore, could lose more hearing without suffering complete LFHL (see Figs 3‒6). It is very likely, although unproveable, that these individuals received partial, not complete, array insertion. This may have influenced results, especially on the AAO. Secondly, the present observational study did not factor in variables that are known, or hypothesized, to influence LFHP, e.g., surgical technique, cochlear duct length, completeness of array insertion, scala tympani volume, age, and sex [21–23]. The use of soft surgical techniques is regarded as especially important, although their use still does not guarantee complete LFHP [5,24]. It could be of substantial future benefit if these data points were recorded in the MEHS. Thirdly, several different surgeons performed the implantations, but it is not known how many. Finally, the sample sizes are small and uneven, especially at later intervals.

Another point of debate is comparing rates of LFHP achieved with perimodiolar arrays and lateral wall arrays. It is commonly thought that lateral wall arrays (e.g., the FLEX series) offer a better chance of LFHP than perimodiolar arrays [7,25,26]; however, studies showing similar rates of LFHP between array types have also been published [19,27]. The present study can add little to this debate because: a) all the arrays used were lateral wall arrays; and b) comparison with the results of previous studies is difficult due to the differences inherent in comparing registry data to study data, e.g., exclusion/inclusion criteria and ways of reporting what constitutes successful LFHP. On the latter point, several studies have used the AAO scale (or something similar). Results featuring the slim modiolar array vary substantially: Iso-Mustajärvi et al. [28] reported that at ≥12 months postoperatively, 82.4 (14/17) participants had LFHP. Jimenez et al. [29] reported that 66.7% (10/15) participants had LFHP at a mean 4.6 months postoperatively. In contrast, Woodson et al. [30] found that 56% (22/39) of participants retained LFHP at activation and Kay-Rivest et al. [31] found that only 34.7% (16/46) of participants had LFHP at 1-year postoperative.

Helbig et al. [32] assessed LFHP using a variety of FLEX arrays (of which about half of which were FLEX20 or FLEX24 arrays and another quarter of which were older MED-EL arrays) and found that LFHP could be achieved in approximately 71.6% (58/81) of ears at 1 year postoperatively. Helbig et al. [18] investigated the FLEX20, FLEX24, FLEX28, and FLEXSOFT, and while results appear to be impressive (see their Fig 5), they are not presented in such as way as to enable comparison. More recently, Moteki et al. [33] found 47.1% (8/17) of participants had LFHP at 6 months with the longer FLEX28 and FLEXSOFT arrays and Khan et al. [34] reported 78.4% (41/51) participants had LFHP at 1 year postoperatively (70.8% with manual insertion and 85.2% with robotic insertion) with FLEX20, FLEX24, and FLEX26 arrays. Results in the present study are worse than in Moteki et al. [33] but better than in Khan et al. [34], although, as mentioned previously, several factors make direct comparison problematic. Further, as per our study aims and data, while the results give a clear description of LFHP rates in real life in the included clinics within a certain timeframe, they are not of causal or explanatory value. To this end, multivariable modeling is theoretically possible; however, the current dataset and study design are not suited for robust causal inference, and future work should address this with prospective and hypothesis-driven analyses. In short, more research is necessary to determine the role that arrays and other factors play in LFHP.

Fig 5 — Results are per ear (not per participant). Note, only 125‒500 Hz are relevant for the AAO hearing loss assessment. n = number at preoperative cases. Number of postoperative audiogram data varies over time (see Table 3).

The present study has some limitations. As stated previously, there is much more data for the FLEX28 and FLEXSOFT arrays than for the shorter FLEX24 and FLEX26 arrays. This is because the two longer arrays are more commonly used in participating clinics. An additional limitation is that the LFHP rate is difficult to track across intervals because of missing data (i.e., incomplete data sets) and the lower n at later intervals. This is probably due to bias: users may have opted to go to outpatient clinics and/or hearing centers for routine maintenance and only scheduled multiple follow-up appointments at the clinic when there were more pronounced hearing problems. Furthermore, not every clinic follows the same procedures, i.e., entering pre- and postoperative PTA. This is a typical aspect in working with real-word data generated from multiple clinics. Lastly, as stated earlier, various factors that (may) influence LFHP have not been controlled for.

As a counterpoint to these limitations, the MEHS registry contains sufficient detail to capture the use of the device, exposures, and outcomes of interest in the appropriate population. Patient selection and enrollment criteria minimized bias and ensured a representative real-world population. Our data support the claim that using FLEX electrode arrays positively contributes to increased levels of LFHP, particularly since our data includes CI users with a wide range of preoperative hearing thresholds, 4 centers with different peri-operative routines, many surgeons, and arrays of various lengths. This reduces the bias introduced by “surgeon factors” and “patient factors” on the overall effect on LFHP outcomes. The MEHS registry enables large volumes of real-world data to be investigated, which there is a known need for in working out the relative impact of different factors on LFHP [16]. To that end, we echo Sladen et al. [16] that future studies may find it beneficial to report patient population, inclusion/exclusion criteria, CI surgery technique applied (including insertion route of array), steroid/drug administration and dosage, array type and length used, follow-up period / outcome measure timing, and prognostic factors for LFHP studied. Adding more detail to studies, and by extension to the MEHS Registry, would be optimal. Until then, the present data allow us to state only that LFHP is possible, and sometimes probable, with FLEX arrays.

The clinical relevance of our findings is that LFHP for up to 3 years postoperatively is probable in many cases using FLEX series arrays. As regards the future, there are exciting developments that may make LFHP consistently more achievable, e.g., corticosteroid-eluting arrays [35,36], robotic array insertion [34,37], and a better matching of array size to candidates’ morphologies.

Conclusions

Low-frequency residual hearing preservation is possible with FLEX electrode arrays of all lengths for up to at least 3 years after implantation, even without accounting for factors known to contribute to successful hearing preservation. The MED-EL Hearing Solutions (MEHS) multicenter registry enables research with large amounts of real-life data collected from a variety of surgeons and hospitals during routine, everyday practice. Adding more factors to the data collected in the MEHS registry, like surgical techniques and the recipients’ morphology, should increase its usefulness in assessing causality.

Supporting information

S1 Table. Recipients’ pre- and post-operative hearing thresholds per frequency.

a) FLEX24 recipients b) FLEX26 recipients, c) FLEX28 recipients, d) FLEXSOFT recipients.

(DOCX)

pone.0345295.s001.docx^{(31.6KB, docx)}

S2 Table. The number and percent of cases with matching or mismatching results with the VC and AAO classification systems.

(DOCX)

pone.0345295.s002.docx^{(16.4KB, docx)}

Acknowledgments

The authors would like to thank all the CI users who graciously agreed to contribute their data to the MEHS and Michael Todd (MED-EL) for writing and editing a version of this manuscript.

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

This study was funded by MED-EL Elektromedizinische Geräte G.m.b.H. There is no grant number. The funders assisted in study design, data collection and analysis, and preparation of the manuscript. Decision to publish was a joint decision between all authors.

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PLoS One. doi: 10.1371/journal.pone.0345295.r001

Decision Letter 0

Toru Miwa

26 Sep 2025

Dear Dr. Polterauer,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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We note that one or more of the authors are employed by a commercial company: MED-EL GmbH

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[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

Reviewer #2: Partly

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2. Has the statistical analysis been performed appropriately and rigorously? -->?>

Reviewer #1: Yes

Reviewer #2: No

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3. Have the authors made all data underlying the findings in their manuscript fully available??>

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.-->

Reviewer #1: Yes

Reviewer #2: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Thank you for the opportunity to review this manuscript and for the effort invested in conducting the study and preparing the submission. As the authors mentioned, similar papers have been published; however, the data in this study were obtained from four hospitals in Germany, which helps reduce bias related to both surgeon and patient factors. Overall, I think the manuscript to be well written and informative.

Minor Points

1. In the legend of Figure 2, please remove the word “right.”

2. In Table 1, please change “gender” to “sex.”

Reviewer #2: This multicenter registry study examines low-frequency hearing preservation (LFHP) after cochlear implantation with MED-EL FLEX arrays (FLEX24/26/28/Soft) using the Vienna Consensus (VC) and AAO metrics at 6–36 months after surgery.

The paper shows LFHP is achievable across arrays, with generally higher rates for shorter arrays and notable divergence between VC and AAO results. The topic is clinically important and valuable, and with more robust statistical analysis the manuscript would be very suitable for publication.

Before specific points below, I recommend conducting formal statistical tests to evaluate overall/time and between-array differences and using multivariable models (with existing variables only) to identify drivers of LFHP (e.g., array length, baseline LF PTA, bilateral status, center, age/sex). Therefore, I recommend Major Revision.

Major concerns

1. Bilateral vs unilateral patients

A non-trivial proportion of participants are bilateral (VC 6.1%; AAO 11.6%), raising concerns that baseline hearing severity, neuroplasticity, and ear-to-ear dependencies could yield different LFHP trajectories than in unilateral cases. Simple ear-level analyses may underestimate uncertainty if left/right ears in bilateral recipients are treated as independent. Provide a subgroup comparison of unilateral vs bilateral recipients for key endpoints (VC categories; AAO <80 dB HL) with appropriate standard errors (e.g., cluster-robust by patient). For bilateral recipients, pre-specify a consistent ear-handling rule—such as analyzing the first implanted ear only as the primary approach or using a within-patient average where justified—and include a sensitivity analysis using the alternative rule to demonstrate robustness.

2. Uneven follow-up and informative missingness

Later windows have fewer cases and potential return-bias, complicating interpretation of cross-sectional proportions at 6–12/12–24/24–36 months. Fit a repeated-measures mixed-effects model for threshold change (VC bands) using all available visits. For AAO (<80 dB HL), add a time-to-event analysis (e.g., Kaplan–Meier to “loss of functional LFHP”) based on existing timestamps/interval bins, incorporating center fixed effects or a patient-level frailty term where feasible. These steps clarify temporal dynamics under missingness without requiring new data collection.

3. Quantify and interpret VC vs AAO discordance

FLEXSOFT shows moderate “preservation” by VC but low “functional preservation” by AAO—a clinically meaningful divergence rooted in different frequency ranges/thresholds. Where both metrics are available at the same visit, present a cross-tabulation (VC category × AAO LFHP yes/no) with agreement/discordance statistics (e.g., Cohen’s κ, McNemar’s test). Add a brief decision aid outlining when each metric should guide clinical choices (e.g., EAS candidacy versus electric-only).

Minor concerns

• Sample imbalance across arrays: Clearly note that FLEX28/FLEXSOFT cases dominate, reflecting practice but complicating direct comparisons across lengths; flag this in figure captions and in the Limitations.

• Safety summary placement: Reiterate in the Results that there was one reimplantation (cause unknown) and no device deficiencies, so safety can be grasped at a glance.

• Uncertainty and significance in figures/tables: For all plotted means (e.g., Figures 3–6), add SEs or 95% CIs, specify which is shown, and annotate the sample size (n) at each time point and per array. Where hypothesis tests are reported, state the test/model, whether two-sided, exact p-values, and effect sizes with 95% CIs; note any multiplicity handling.

• Define visit windows and pre-operative timing: Restate that pre-op unaided PTA was measured ≤12 months before surgery and that post-op windows are 6–12/12–24/24–36 months to aid reproducibility.

• Per-ear reporting clarity: Add “per ear” to figure/table titles and direct readers to the bilateral-handling rule in Methods.

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Reviewer #1: Yes: Hidekane YoshimuraHidekane YoshimuraHidekane YoshimuraHidekane Yoshimura

Reviewer #2: No

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PLoS One. 2026 Apr 17;21(4):e0345295. doi: 10.1371/journal.pone.0345295.r002

Author response to Decision Letter 1

3 Nov 2025

EDITOR

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf

and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf.

The manuscript has been formatted to fit PLOS ONE.

2. Thank you for stating the following financial disclosure:

“Funding from MED-EL. There is no grant number.”

If this statement is not correct you must amend it as needed.

Please include this amended Role of Funder statement in your cover letter; we will change the online submission form on your behalf

The cover letter has been amended to include this information.

3. Thank you for stating the following in the Competing Interests section:

“Authors Ilona Anderson, Stefano Morettini, Karin Koinig, and Magdalena Breu are employed at MED-EL Elektromedizinische Geräte G.m.b.H.

Uwe Baumann and Daniel Polterauer have received travel support & research support from MED-EL GmbH (Austria).

Other authors disclose no conflict of interest.”

We note that one or more of the authors are employed by a commercial company: MED-EL GmbH

Please also include the following statement within your amended Funding Statement.

If your commercial affiliation did play a role in your study, please state and explain this role within your updated Funding Statement.

Please include both an updated Funding Statement and Competing Interests Statement in your cover letter. We will change the online submission form on your behalf.

The cover letter has been amended to include this information.

4. Thank you for stating the following in the Acknowledgments Section of your manuscript:

Please remove any funding-related text from the manuscript and let us know how you would like to update your Funding Statement. Currently, your Funding Statement reads as follows:

“Funding from MED-EL. There is no grant number.”

Please include your amended statements within your cover letter; we will change the online submission form on your behalf.

This has been deleted from the paper & the cover letter has been amended to include this information.

It is nice to see this, so thank you. However, in this case it is not applicable.

There are a few changes we would like to inform you of.

1. We have updated Figures 1 and 2 to include an arrow that links the Sorting info at the top to the final n at the bottom, as illustrated in the screenshots below. This is the only change.

2. We have added a reference (#11) to a recently accepted paper on this registry. This provides context but does not change content. All reference #s after 11 have been updated accordingly.

3. The corresponding author’s last name is now “Polterauer-Neuling” and not Polterauer“

4. The affiliation for 5 has been edited, but it is the same place.

5. Please note, the authors’ ORCIDs are:

a. Uwe Baumann: 0000-0002-1295-2661

b. Andreas Radeloff: 0000-0003-1881-4179

c. Ilona Anderson: 0000-0001-7518-6661

d. Karin A Koinig: 0000-0002-3659-4934

e. Magdalena Breu: 0009-0007-4480-2098

f. Stefano Morettini: 0009-0007-1568-4993

g. Vera Lohnherr: 0000-0002-5655-7671

h. Joachim Müller: (no ORCID)

i. Daniel Polterauer-Neuling: 0000-0001-5008-7595

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Thank you for the review & the kind words.

Minor Points

1. In the legend of Figure 2, please remove the word “right.”

Thank you, this has been corrected.

2. In Table 1, please change “gender” to “sex.”

Thank you, this has been corrected.

As stated in the manuscript, the primary aim of this data analyses was descriptive, that is to document the achievable rates of low-frequency hearing preservation (LFHP) across different MED-EL FLEX electrode arrays in a large, multicenter registry, and to compare outcomes according to two established hearing preservation definitions (Vienna Consensus and AAO). The analyses were not designed to explain longitudinal changes or to support causal inference regarding factors influencing LFHP. Accordingly, we did not conduct intra-individual longitudinal analyses, nor did we construct multivariable models for explanatory purposes.

Given that this is a registry study—with no obligation for CI users to return for follow-up visits, and with multiple outpatient facilities attended as alternative for follow up visits—we cannot ensure unbiased or continuous data collection. This would, however, be essential for robust intra-individual longitudinal analyses. Moreover, in the available registry data, variables such as electrode array length, baseline LF-PTA, and bilateral status are strongly interrelated. In addition, some key factors, such as course of deafness, hearing impairment duration, hearing aid usage prior to receiving an implant, or impedance measurements, were not included in this data set. This limits the ability to perform a multivariable model to isolate the independent effect of each variable, making robust causal or explanatory conclusions unreliable. Therefore, we believe that including such analyses would risk overstating the strength of any associations while missing out on some major factors affecting LFHP. We do not expect meaningful or clinically reliable correlations to emerge from such modeling.

Instead, the strength of our study is that it provides real-world evidence of LFHP across a spectrum of surgical practices and patient populations, while highlighting patterns of hearing preservation and differences between the Vienna Consensus and AAO definitions. We view this as a foundation on which future hypothesis-driven and longitudinal studies—designed explicitly to test causal relationships—can build.

That said, we took measures to strengthen the manuscript by:

1. Explicitly clarifying in the Introduction and Discussion that our objective was descriptive, not causal or explanatory: see lines: 83-84 and 285-290.

2. Reporting more detailed descriptive statistics in the appendix (e.g., group-level differences between arrays and over time) to aid interpretation while maintaining the descriptive scope: for see the appendix tables.

3. Highlighting in the Discussion that while multivariable modeling is theoretically possible, the current dataset and study design are not suited for robust causal inference, and that future work should address this with prospective and hypothesis-driven analyses: see lines: 291-295.

We hope this adequately addresses the reviewer’s concern while maintaining the integrity of our study design and objectives.

Major concerns

1. Bilateral vs unilateral patients

Our analytic intention, however, was not to model dependencies or to draw causal inferences about trajectories but to provide descriptive, ear-level summaries of LFHP outcomes across a broad, real-world registry. We chose ear-level as the unit of analysis because:

1. LFHP is an ear-specific phenomenon, influenced by surgical approach and array choice in that ear

2. The registry data are structured at the ear level, with no prespecified analytic framework for handling bilateral cases. By design we only know whether a patient is bilaterally implanted if CIs are included for both ears (thus, if the other ear had a non-MED-EL device or was implanted in a clinic that does not participate in the registry, we would not know that the person is a bilateral CI user),

3. From what we do know, the proportion of bilateral recipients is relatively modest (6.1% VC; 11.6% AAO)

We consider therefore our main findings as not meaningfully altered by bilateral status or analytic choice, while remaining aligned with the descriptive scope of our study.

2. Uneven follow-up and informative missingness

This is a registry study and, as such, looks at real world data to provide real world evidence. As a limiting consequence, participant numbers vary across follow-up intervals.

This reflects typical clinical practice: some patients do not come back, especially if they experience no issues. Other patients opt of follow ups at trained hearing aid centres instead of the participating clinics, as this can be more convenient in Germany.

3. Quantify and interpret VC vs AAO discordance

Thanks for noticing that FLEXSOFT illustrates differing results when applying the Vienna Consensus (VC) versus AAO metrics. However, we would like to clarify that this divergence is not clinically meaningful but rather an intrinsic feature of the two scales, which were developed to capture different aspects of hearing preservation (HP). The VC categorizes relative hearing preservation at 250, 500, and 1000 1500 Hz based on postoperative threshold shifts across a broad frequency range. Its purpose is to enable standardized, comparable reporting across centers and studies, irrespective of whether the absolute thresholds remain within a functionally usable range. The AAO metric, by contrast, is a clinically oriented framework that defines preservation as maintaining ≤80 dB HL at 125, 250, and 500 Hz—frequencies that are critical for electro-acoustic stimulation (EAS). It therefore directly addresses whether residual hearing remains of functional benefit.

Therefore, because the VC and AAO frameworks were designed to answer different questions, and because they assess different frequencies, agreement or discordance between them is neither expected nor clinically meaningful. A cross-tabulation or concordance analysis would risk suggesting comparability between two scales that are not intended to be interchangeable. Instead, their divergence simply reflects their different conceptual focus: VC quantifies relative preservation, whereas AAO defines functional preservation. (Note: This is now stated in the Methods and we have changed the 1st word of the paper’s title from “Functional” to “Low frequency”.)

This is also stressed by our results for the FLEXSOFT array—the longest in o

Attachment

Submitted filename: Response to reviewer comments.docx

pone.0345295.s005.docx^{(71.5KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0345295.r003

Decision Letter 1

Toru Miwa

19 Dec 2025

Dear Dr. Polterauer-Neuling,

Please submit your revised manuscript by Feb 02 2026 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

A letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
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Toru Miwa

Academic Editor

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Reviewer's Responses to Questions

Comments to the Author

Reviewer #2: All comments have been addressed

Reviewer #3: (No Response)

Reviewer #4: All comments have been addressed

Reviewer #5: All comments have been addressed

Reviewer #6: (No Response)

Reviewer #7: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions??>

Reviewer #2: Partly

Reviewer #3: Partly

Reviewer #4: Yes

Reviewer #5: Yes

Reviewer #6: Yes

Reviewer #7: Partly

**********

3. Has the statistical analysis been performed appropriately and rigorously? -->?>

Reviewer #2: No

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

Reviewer #6: Yes

Reviewer #7: N/A

**********

4. Have the authors made all data underlying the findings in their manuscript fully available??>

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

Reviewer #6: Yes

Reviewer #7: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

Reviewer #6: Yes

Reviewer #7: Yes

**********

Reviewer #2: Thank you for the opportunity to review the revised version of this manuscript entitled “Low frequency hearing preservation after cochlear implant surgery is achievable with each FLEX electrode array: Real-world evidence from the MEHS Registry.”

The authors have addressed the previous comments with clarity and appropriate modifications. The objective of the study as descriptive has been clearly restated, and additional context has been added to improve interpretation. The manuscript provides clinically meaningful real-world data, and the overall structure is sound.

However, there remain a few points that should be clarified further to ensure appropriate interpretation of the findings, particularly regarding the limitations of the descriptive approach given sample size imbalances. I recommend Minor Revision.

Major Points

1)Limitations of the descriptive approach due to subgroup size and imbalance

While the authors correctly emphasize that the study was designed as descriptive, this methodological approach is most informative when subgroup sample sizes are sufficiently large and balanced. In the present analysis, some subgroups—particularly FLEX26 and FLEXSOFT at later time intervals—include only a small number of cases.

Please clearly acknowledge in the Discussion that the interpretive strength of subgroup trends is limited due to small and uneven sample sizes, and that these results should be interpreted with caution.

2) Avoiding causal interpretation in observational statements

Reviewer #3: Dear

Thank You for manuscript . After previous review manuscript looks much better , but still there are several flaws. In introduction i can t see data or background and "fascinating" database . Register could be beneficial but better will be to to link that to essence of the study. Lenhardt "soft " technique had not so much connected with hearing preservation so that part of the introduction should be significantly adjusted to real history of that .

In results there is a lot of nice analysis but honestly there is significant decrease of the hearing thresholds so it should be the main issue why hearing was not preserved . Also after check there are other types of evaluation of that preservation and if applied VC it should be correlated with other more common and validated tools for that .

Reviewer #4: This is a well-written descriptive paper of hearing preservation defined by two classification systems using multiple different electrode array styles. The authors do well to temper their inferences and conclusions within the context of a descriptive database study, for which multiple factors which are thought to contribute to LFHP cannot be controlled in a prospective fashion, but can be appropriately described. This study adds valuable insights to the field, notably with the inclusion of hearing preservation rates within the FLEXSOFT group. One notable strength of the study is the inclusion of multiple surgeons and centers from which the data was drawn from. Results are appropriately compared to existing literature.

The Authors have provided a comprehensive response to reviewer critiques. Notably, I fully support the author's response to reviewer 2 critiques and agree with the authors in their approach to data analysis (mainly that this is a descriptive study, not meant to make statistically backed conclusions on factors influencing HP (such as electrode style)). Additionally, I agree with their approach to handling of individual ears in the setting of bilateral implantation. These efforts from the Authors appropriately temper their conclusions from the study within the bounds of its design.

Reviewer #5: (No Response)

Reviewer #6: PONE-D-25-39581_R1: Low frequency hearing preservation after cochlear implant surgery is achievable with each FLEX electrode array: Real world evidence from the MEHS Registry.

This paper presents the results of a registry study investigating the preservation of residual hearing with different cochlear implant (CI) electrodes, classified using two different methods. The strength of this approach lies in the large number of patients.

I was apparently brought in as an additional reviewer because one of the two original reviewers and the authors could not reach an agreement. Overall, the manuscript is well-written and presents relevant data. Nevertheless, I have a point of my own, which I believe needs to be added, before addressing the points of contention.

In my opinion, the statistics requested by Reviewer 2 make little sense. But what I find completely lacking is a contextualization of the results within the current literature, specifically the achieved residual hearing preservation rates for each electrode length. There are many studies where individual clinics investigate residual hearing preservation under more consistent conditions, naturally with significantly fewer patients each. Here, the rates from this large registry study need to be compared with the data from the literature.

Reviewer 2:

“Before specific points below, I recommend conducting formal statistical tests to evaluate overall/time and between-array differences and using multivariable models (with existing variables only) to identify drivers of LFHP (e.g., array length, baseline LF PTA, bilateral status, center, age/sex). Therefore, I recommend Major Revision.”

Author’s response:

“Given that this is a registry study—with no obligation for CI users to return for follow-up visits, and with multiple outpatient facilities attended as alternative for follow up visits—we cannot ensure unbiased or continuous data collection. This would, however, be essential for robust intraindividual longitudinal analyses. Moreover, in the available registry data, variables such as electrode array length, baseline LF-PTA, and bilateral status are strongly interrelated. In addition, some key factors, such as course of deafness, hearing impairment duration, hearing aid usage prior to receiving an implant, or impedance measurements, were not included in this data set. This limits the ability to perform a multivariable model to isolate the independent effect of each variable, making robust causal or explanatory conclusions unreliable. Therefore, we believe that including such analyses would risk overstating the strength of any associations while missing out on some major factors affecting LFHP. We do not expect meaningful or clinically reliable correlations to emerge from such modeling.”

My assessment:

I agree with the authors. It is very regrettable that the dataset is not more comprehensive and has many gaps. However, this is not a prospective study with precisely controlled conditions, but rather a retrospective analysis of existing data from various clinics, which, based on experience, can vary considerably in detail. For example, I am surprised that the preoperative audiogram is up to twelve months old in some cases, and this heavily reduces the significance of the preoperative residual hearing. According to extensive literature, the duration and progression of hearing loss, hearing aid use, and deafness also have a considerable influence on the preservation of residual hearing. If this data is simply not available, it makes no sense, in my view, to feign accuracy through extensive statistics.

Reviewer 2:

“A non-trivial proportion of participants are bilateral (VC 6.1%; AAO 11.6%), raising concerns that baseline hearing severity, neuroplasticity, and ear-to-ear dependencies could yield different LFHP trajectories than in unilateral cases. Simple ear-level analyses may underestimate uncertainty if left/right ears in bilateral recipients are treated as independent. Provide a subgroup comparison of unilateral vs bilateral recipients for key endpoints (VC categories; AAO <80 dB HL) with appropriate standard errors (e.g., cluster-robust by patient). For bilateral recipients, pre-specify a consistent ear-handling rule—such as analyzing the first implanted ear only as the primary approach or using a within-patient average where justified—and include a sensitivity analysis using the alternative rule to demonstrate robustness.

Author’s response:

“Our analytic intention, however, was not to model dependencies or to draw causal inferences about trajectories but to provide descriptive, ear-level summaries of LFHP outcomes across a broad, real-world registry. We chose ear-level as the unit of analysis because:

1. LFHP is an ear-specific phenomenon, influenced by surgical approach and array choice in that ear

2. The registry data are structured at the ear level, with no prespecified analytic framework

for handling bilateral cases. By design we only know whether a patient is bilaterally

implanted if CIs are included for both ears (thus, if the other ear had a non-MED-EL device

or was implanted in a clinic that does not participate in the registry, we would not know

that the person is a bilateral CI user),

3. From what we do know, the proportion of bilateral recipients is relatively modest (6.1%

VC; 11.6% AAO)

We consider therefore our main findings as not meaningfully altered by bilateral status or

analytic choice, while remaining aligned with the descriptive scope of our study.”

My assessment:

I agree with the authors, but I have a suggestion for further analysis, though probably only in the future when more bilateral datasets are available. The surgeon only manipulates the peripheral auditory system. Effects resulting from bilateral connections are not to be expected. The authors rightly argue that the necessary data is not available with the required clarity. This is also because it would be necessary to consider not only a cochlear implant (CI) on the contralateral side, but also residual hearing and/or hearing aid use. This data would be essential for any analysis.

Nevertheless, I find another effect very interesting. To investigate this, there are probably not yet enough datasets. If a CI is implanted on the second side, is the preservation of residual hearing comparable to that on the first side, or not? In other words, if the residual hearing on the first side is well preserved, does the patient then have a better chance on the second side? And if residual hearing is lost on the first side and no unusual occurrence is noted intraoperatively (which is naturally not recorded in the registry), does the patient then also have an increased risk on the second side? Because it may also be that factors such as predisposition or the nature of the underlying disease play a role, which have so far been recorded very little.

Reviewer 2:

“Later windows have fewer cases and potential return-bias, complicating interpretation of crosssectional proportions at 6–12/12–24/24–36 months. Fit a repeated-measures mixed-effects model for threshold change (VC bands) using all available visits. For AAO (<80 dB HL), add a timeto- event analysis (e.g., Kaplan–Meier to “loss of functional LFHP”) based on existing timestamps/interval bins, incorporating center fixed effects or a patient-level frailty term where feasible. These steps clarify temporal dynamics under missingness without requiring new data collection.

Author’s response:

“This is a registry study and, as such, looks at real world data to provide real world evidence. As a limiting consequence, participant numbers vary across follow-up intervals.

This reflects typical clinical practice: some patients do not come back, especially if they

experience no issues. Other patients opt of follow ups at trained hearing aid centres instead of the participating clinics, as this can be more convenient in Germany.”

My assessment:

I agree with the authors. In my opinion, the purpose of the study did not require these statistics; rather, these statistics would give a false impression of accuracy.

Reviewer 2:

“FLEXSOFT shows moderate “preservation” by VC but low “functional preservation” by AAO—a clinically meaningful divergence rooted in different frequency ranges/thresholds. Where both metrics are available at the same visit, present a cross-tabulation (VC category × AAO LFHP yes/no) with agreement/discordance statistics (e.g., Cohen’s κ, McNemar’s test). Add a brief decision aid outlining when each metric should guide clinical choices (e.g., EAS candidacy versus electric-only).“

Author’s response:

“Thanks for noticing that FLEXSOFT illustrates differing results when applying the Vienna

Consensus (VC) versus AAO metrics. However, we would like to clarify that this divergence is not clinically meaningful but rather an intrinsic feature of the two scales, which were developed to capture different aspects of hearing preservation (HP). The VC categorizes relative hearing preservation at 250, 500, and 1000 1500 Hz based on postoperative threshold shifts across a broad frequency range. Its purpose is to enable standardized, comparable reporting across centers and studies, irrespective of whether the absolute thresholds remain within a functionally usable range. The AAO metric, by contrast, is a clinically oriented framework that defines preservation as maintaining ≤80 dB HL at 125, 250, and 500 Hz—frequencies that are critical for electro-acoustic stimulation (EAS). It therefore directly addresses whether residual hearing remains of functional benefit.

This is also stressed by our results for the FLEXSOFT array—the longest in our cohort and

therefore potentially more traumatic than the shorter variants—it is expected that preservation may appear moderate under VC (reflecting limited threshold shift), while AAO indicates poor functional preservation if residual thresholds exceed 80 dB HL at low frequencies. This is not a contradiction but a natural consequence of applying two distinct scales which indeed capture different aspects of HP in a cohort of FLEXSOFT candidates who typically lack low-frequency residual hearing even pre-operatively.

In summary, the observed divergence between VC and AAO outcomes is not a clinically

meaningful inconsistency but an expected reflection of the complementary purposes of these two scales”

My assessment:

Here I find myself caught between reviewer and authors. Since both metrics (VC and AAO) have different objectives, statistics are of little use. However, I too would appreciate a descriptive comparison showing in which and how many cases the results of both metrics coincide and diverge; a table would suffice for this. The discussion should then include a paragraph that elaborates on the differences and provides an indication of which metric is better suited for which application.

Reviewer #7: I appreciate the opportunity to review the manuscript “Low frequency hearing preservation after cochlear implant surgery is achievable with each FLEX electrode array: Real world evidence from the MEHS Registry”. It is definitely still important to evaluate hearing preservation (HP) after CI surgery. The rationale for the study is certainly warranted and interesting aspect is to use real world evidence from the MEHS Registry. However, There are several major concerns that currently make the manuscript unsuitable for publication: 1) methodological unresolved dilemmas; 2) difficulties in interpreting between-electrode differences in hearing preservation (HP); 3) limited novelty of the study

1. Methodological concerns

A substantial number of previous studies have investigated hearing-preservation (HP) width using various approaches. At present, two systems are most commonly used: the HearRing Group Classification System (Skarzynski et al., 2013) and the American Academy of Otolaryngology (AAO) reporting system. The HearRing system, in particular, has been widely adopted and cited more than 200 times. It is therefore unclear why the authors chose to introduce yet another matrix for HP evaluation (Vienna Consensus- VC). By employing a new, non-standard scale, the opportunity to compare the findings with previously published results is significantly impaired. Furthermore, the manuscript does not reference other applications of the VC method, and it appears that VC has not yet been established as a validated assessment tool. As HP evaluation is described in the manuscript, there is a critical methodological dilemma regarding how to treat unmeasurable thresholds. The authors state:

“We truncated all measurements exceeding the maximum audiometer test thresholds to the values given in Table IV in Skarzynski et al., to obtain a uniform maximum per frequency.”

These maximum values are 105 dB, 110 dB and 120 dB for 250 Hz, 500 Hz and 1000 Hz respectively, resulting in a PTLF of 111.67 dB.

According to the authors:

“To be assessed via VC, CI recipients must have some preoperative hearing at those frequencies, i.e., ≤ 111.67 dB HL (average maximum output levels).”

However, this leads to conceptual inconsistencies. For example, if a patient has a preoperative threshold at the maximum output levels and postoperatively loses all measurable hearing (i.e., no auditory sensation), the VC method may still classify this case as complete preservation, because both values reside at or beyond the truncated ceiling. Similarly, a patient losing hearing at two frequencies but retaining ≤15 dB preservation at one frequency might also be misclassified as full preservation. Such outcomes generate bias and undermine interpretability. Therefore, I request clarification on how the authors handled unmeasurable thresholds (no hearing at all) within the VC framework. In addition, I strongly recommend applying the HearRing Classification System, which was explicitly designed to avoid this dilemma. The HearRing formula for qualitative HP classification:

Relative change = ((PTApost-PTApre)/(PTAmax-PTApre))

Where:

PTApost is pure tone average measured postoperatively; PTApre is pure tone average measured pre-operatively; PTAmax is the limits of the audiometer

This formula effectively resolves the bias introduced by unmeasurable thresholds. Because the classification scales results relative to the patient’s preoperative hearing, it prevents worse preoperative hearing from artificially inflating postoperative HP outcomes. The equation presents the relative change, as a percentage of hearing loss, Than, the relative change is converted to preservation by calculating 100% - relative change in %. Then, a percentage of hearing preserved can be converted into a categorical scale (complete, partial, minimal, or loss). This approach can also be applied specifically to the three low frequencies. I recommend recalculating HP rates using the HearRing Classification System.

2. Interpretation of electrode-related differences

The authors suggest that low-frequency hearing preservation (LFHP) appears more likely when shorter arrays are used because these arrays (e.g., FLEX24) may not reach apical low-frequency regions and therefore may not induce trauma or inflammation in these areas. However, this interpretation is not adequately justified given the methodology.

Several key confounding factors—surgical technique, patient age at implantation, onset and progression of hearing loss—were not controlled. Additionally, the preoperative thresholds differ significantly between electrode groups; notably, the FLEX24 group had better preoperative hearing. There is substantial evidence that preoperative thresholds predict postoperative HP outcomes (Lee et al., 2020; Wanna et al., 2017). Therefore, attributing differences solely to electrode length is not warranted without controlling for these variables.

3. Limited novelty

The conclusion that low-frequency residual hearing can be preserved with FLEX electrodes of all lengths is already well established in the literature. For example, the systematic review by Van de Heyning et al. (2022), “Systematic Literature Review of Hearing Preservation Rates in Cochlear Implantation Associated With Medium- and Longer-Length Flexible Lateral Wall Electrode Arrays”, comprehensively documents this. The present manuscript does not provide sufficient new insight beyond existing knowledge.

**********

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Reviewer #2: No

Reviewer #3: No

Reviewer #4: Yes: Alexander Dale ClaussenAlexander Dale ClaussenAlexander Dale ClaussenAlexander Dale Claussen

Reviewer #5: No

Reviewer #6: No

Reviewer #7: No

**********

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pone.0345295.s004.docx^{(19.7KB, docx)}

PLoS One. 2026 Apr 17;21(4):e0345295. doi: 10.1371/journal.pone.0345295.r004

Author response to Decision Letter 2

22 Jan 2026

For all reviewers:

Please note that we have slightly changed the title: 1) we added “functional” to account for the AAO results; 2) we deleted “each” because there exist FLEX arrays (e.g., FLEX20, FLEX34) that are not featured in this study, thus “each” is incorrect.

We have also made some edits to clarify that one scale is hearing preservation and the other is functional hearing preservation, e.g., line 198.

Major Points

1)Limitations of the descriptive approach due to subgroup size and imbalance

2) Avoiding causal interpretation in observational statements

Some wording in the Discussion still implies causality (e.g., privilege of shorter arrays in achieving better LFHP). Please rephrase such statements to indicate observational tendencies only (e.g., “a trend was observed that…” or “in this descriptive dataset, shorter arrays tended to show…”). Excellent point. Please see the response to the comment above, lines 34-6 (deletion) in the plain language summary, lines 212-3, and lines 318-324.

Reviewer #3: Dear

Thank You for manuscript . After previous review manuscript looks much better , but still there are several flaws. In introduction i can t see data or background and "fascinating" database We are not sure what this means. The place for data is the Results. The MEHS is covered in the Methods and Materials. Register could be beneficial but better will be to to link that to essence of the study. Thispaper is based on a registry, with a protocol, which makes it rigorous. Lenhardt "soft " technique had not so much connected with hearing preservation so that part of the introduction should be significantly adjusted to real history of that . As is well-known, soft surgery is a key component of HP surgery. Lenhardt’s paper is the essence of soft surgery, and many papers reference it.

In results there is a lot of nice analysis but honestly there is significant decrease of the hearing thresholds so it should be the main issue why hearing was not preserved . This is as to be expected in CI. As this was an unbiased study using real-world data from a registry where all patients were analysed, there was no control for surgical technique, or experience of surgeon BUT STILL hearing was preserved. Also after check there are other types of evaluation of that preservation and if applied VC it should be correlated with other more common and validated tools for that . AAO was also used, so two measurements, one looking at hearing preservation and the other looking at functional hearing preservation.

Reviewer #6: PONE-D-25-39581_R1: Low frequency hearing preservation after cochlear implant surgery is achievable with each FLEX electrode array: Real world evidence from the MEHS Registry.

We understand what you mean, however all other studies already have the inclusion criteria of participants with substantial pre-op residual hearing & the use of soft-surgery techniques to preserve the residual hearing. That is to say, the results they present show what is possible, not what actually goes in the real world. The present study is unbiased in that it uses real-world data from a registry where all patients were analysed: there was no control for surgical technique or experience of surgeon and thus cannot reasonably be compared to other studies. This would be like comparing apples to oranges.

Reviewer 2:

Author’s response:

My assessment:

Thank you and yes, real-world data shows the reality of clinical procedure. While some gaps in datasets of real-world evidence are unavoidable, we are working to improve the registry to fill-in these gaps where we can.

We looked at the data again and found that more than 75% of the pre-op assessments took place within 3 months of implantation (and 62% were within 10 days of implantation). We’ve added some sentences to this end (see lines 199-202). On the point, we’d like to point out that using older pre-op assessments would tend to underestimate the HP because while it’s quite possibly that candidates’ PTA is worse or equal at 10 days pre-op than it was at 11 months pre-op, but it’s very unlikely to have improved from 11m to 10d. Therefore, if a better PTA from an earlier assessment is used as the baseline for comparison with the post-operative PTA, this would lead to an underestimation of hearing preservation rather than an overestimation.

We agree that we would not want to extend the statistics further based on the limitations of pre-op information (please see our original answer to the reviewer from the last round: “some key factors, such as course of deafness, hearing impairment duration, hearing aid usage prior to receiving an implant, or impedance measurements, were not included in this data set”…). And, in the Discussion, we catalogue the potential casual factors for hearing preservation that we don’t know. We can’t present a nuanced picture here; however, the data do allow us to substantiate the claim that regardless of all the factors that may or may not influence HP, complete or partial HP was possible with FLEX electrode arrays.

Reviewer 2:

Author’s response:

1. LFHP is an ear-specific phenomenon, influenced by surgical approach and array choice in that ear

2. The registry data are structured at the ear level, with no prespecified analytic framework

for handling bilateral cases. By design we only know whether a patient is bilaterally

implanted if CIs are included for both ears (thus, if the other ear had a non-MED-EL device

or was implanted in a clinic that does not participate in the registry, we would not know

that the person is a bilateral CI user),

3. From what we do know, the proportion of bilateral recipients is relatively modest (6.1%

VC; 11.6% AAO)

We consider therefore our main findings as not meaningfully altered by bilateral status or

analytic choice, while remaining aligned with the descriptive scope of our study.”

My assessment:

Reviewer 2:

Author’s response:

“This is a registry study and, as such, looks at real world data to provide real world evidence. As a limiting consequence, participant numbers vary across follow-up intervals.

This reflects typical clinical practice: some patients do not come back, especially if they

experience no issues. Other patients opt of follow ups at trained hearing aid centres instead of the participating clinics, as this can be more convenient in Germany.”

My assessment:

I agree with the authors. In my opinion, the purpose of the study did not require these statistics; rather, these statistics would give a false impression of accuracy.

Thank you for your support on this.

Reviewer 2:

PLoS One. doi: 10.1371/journal.pone.0345295.r005

Decision Letter 2

Toru Miwa

4 Mar 2026

Functional hearing and low frequency hearing preservation after cochlear implant surgery is achievable with FLEX electrode arrays: Real world evidence from the MEHS Registry

PONE-D-25-39581R2

Dear Dr. Polterauer-Neuling,

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PLoS One. doi: 10.1371/journal.pone.0345295.r006

Acceptance letter

Toru Miwa

PONE-D-25-39581R2

PLOS One

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

S1 Table. Recipients’ pre- and post-operative hearing thresholds per frequency.

a) FLEX24 recipients b) FLEX26 recipients, c) FLEX28 recipients, d) FLEXSOFT recipients.

(DOCX)

pone.0345295.s001.docx^{(31.6KB, docx)}

S2 Table. The number and percent of cases with matching or mismatching results with the VC and AAO classification systems.

(DOCX)

pone.0345295.s002.docx^{(16.4KB, docx)}

Attachment

Submitted filename: Response to reviewer comments.docx

pone.0345295.s005.docx^{(71.5KB, docx)}

Attachment

Submitted filename: Review PONE-D-25-39581_R1.docx

pone.0345295.s004.docx^{(19.7KB, docx)}

Data Availability Statement

All relevant data are within the manuscript and its Supporting Information files.

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PERMALINK

Functional hearing and low frequency hearing preservation after cochlear implant surgery is achievable with FLEX electrode arrays: Real world evidence from the MEHS Registry

Uwe Baumann

Tobias Weissgerber

Andreas Radeloff

Ilona Anderson

Karin A Koinig

Magdalena Breu

Stefano Morettini

Vera Lohnherr

Joachim Müller

Daniel Polterauer-Neuling

Roles

Abstract

Introduction

Materials and methods

Ethics committee approval

Participants

FLEX series electrode arrays

Vienna consensus (VC)

Minimum reporting standards for Adult Cochlear American Academy of Otolaryngology—Head and Neck Surgery (AAO)

VC and AAO convergence

Safety and device deficiencies

Statistics

Results

Participants

Fig 1. CONSORT diagram for participants in the VC group.

Fig 2. CONSORT diagram for participants in the AAO group.

Table 1. Demographic and baseline characteristics of each study cohort.

Hearing preservation/functional hearing preservation

VC.

Table 2. Hearing preservation results per array and interval using the VC. n are per ear. LFHP = low-frequency hearing preservation; LFHL = low-frequency hearing loss.

AAO.

Table 3. Hearing preservation results per array and interval using the AAO. n are per ear.

Fig 3. Mean thresholds (dB HL) from 125 to 8000 Hz at each postoperative interval for the FLEX24 group.

Fig 6. Mean thresholds (dB HL) from 125 to 8000 Hz at each postoperative interval for the FLEXSOFT group.

Fig 4. Mean thresholds (dB HL) from 125 to 8000 Hz at each postoperative interval for the FLEX26 group.

VC and AAO convergence.

Safety and device deficiencies

Discussion

Fig 5. Mean thresholds (dB HL) from 125 to 8000 Hz at each postoperative interval for FLEX28 group.

Conclusions

Supporting information

Acknowledgments

Data Availability

Funding Statement

References

Decision Letter 0

Toru Miwa

Roles

Author response to Decision Letter 1

Decision Letter 1

Toru Miwa

Roles

Author response to Decision Letter 2

Decision Letter 2

Toru Miwa

Roles

Acceptance letter

Toru Miwa

Roles

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

PERMALINK

RESOURCES

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