# Comparison of Slim Perimodiolar and Slim Straight Electrode Arrays in Pediatric Cochlear Implantation: A Radiologic, Electrophysiological, and Audiological Study

Maria-Pia Tuset, Emilie Bois, Nathalie Noel-Petroff, Natacha Teissier, Thierry van den Abbeele, Charlotte Benoit

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Maria-Pia Tuset, Emilie Bois, Nathalie Noel-Petroff, Natacha Teissier, Thierry van den Abbeele, et al.. Comparison of Slim Perimodiolar and Slim Straight Electrode Arrays in Pediatric Cochlear Implantation: A Radiologic, Electrophysiological, and Audiological Study. Otology and Neurotology, 2026, 47 (3), pp.447-455. 10.1097/mao.0000000000004784. hal-05515648

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# Comparison of Slim Perimodiolar and Slim Straight Electrode Arrays in Pediatric Cochlear Implantation: A Radiologic, Electrophysiological, and Audiological Study

Maria-Pia Tusetª, Emilie Boisª, Nathalie Noel-Petroffª, Natacha Teissierᵇ, Thierry Van Den Abbeeleᵇ,*, Charlotte Benoitᵇ

**Objective:** Electrode array design may influence auditory and language performance in pediatric cochlear implant (CI) recipients. Slim perimodiolar arrays, positioned closer to the modiolus, are hypothesized to provide more focused stimulation and lower current requirements. This study compares outcomes of slim perimodiolar (PM) and straight (ST) electrode arrays in children.

**Study design:** Retrospective cohort review.

**Setting:** Tertiary pediatric otolaryngology referral center.

**Patients:** Eighty-eight children underwent 122 implantations with slim PM (CI632/532, n = 65) or slim ST (CI422/522/622, n = 57) arrays between 2016 and 2023. For speech and language outcomes, 72 children were assessed globally at the patient level, including 68 sequential cases (first implanted side) and 4 simultaneous bilateral cases.

**Main outcome measures:** Auditory thresholds, impedances, T/C levels, and neural response telemetry (NRT) were analyzed postoperatively, at 6 and 12 months, speech and language performance (CAP and APCEI profile at 2 y). Angular depth of insertion (aDOI), basal insertion depth (BID), and modified intracochlear position index (mICPI) were derived from preoperative CT and postoperative radiographs.

**Results:** PM arrays achieved significantly closer modiolar positioning (mICPI: $0.72 \pm 0.07$ vs $0.98 \pm 0.06$, P < 0.0001) with no difference in aDOI and BID, lower impedances at 1 and 12 months (P < 0.0001), lower NRT levels at 6 months (P = 0.001), and reduced energy requirements. In patient-level analyses, children implanted ≤3 years with PM arrays had higher APCEI (P = 0.006) and CAP scores (P < 0.03) compared with ST arrays. In children implanted >3 years, electrode type was not associated with outcomes, while age at implantation was the main determinant of language scores.

**Conclusions:** Slim PM arrays provide closer modiolar proximity, lower impedances, reduced energy consumption, with more consistent early hearing outcomes and better speech/language scores if implanted before 3 years of age. These findings support early implantation with slim perimodiolar arrays to optimize pediatric auditory and language performance.

**Keywords:** Angular depth of insertion, Impedance, Pediatric cochlear implant, Perimodiolar electrode

## Introduction

Hearing loss affects ~0.25% of newborns in France each year, with over 40% of cases diagnosed as severe to profound sensorineural hearing loss, according to the French Health Authority (HAS).[1] Cochlear implantation (CI) has become the gold

standard for managing such cases in children. In France, more than 10,000 patients have received implants, nearly 50% of whom are children. Given the long-life expectancy of these young recipients, preserving cochlear structures and minimizing insertion trauma are very important considerations to optimize long-term hearing outcomes, maintain future therapeutic options, and optimizing energy efficiency.[2-4]

Cochlear implantation inherently induces trauma, inducing an inflammatory foreign-body response that leads to intracochlear fibrosis and potential new bone formation, particularly at the basal turn of the cochlea. These changes may alter apical basilar membrane vibration, impact low-frequency hearing[5-7], and degrade hearing performance over time. While these changes are difficult to quantify directly, electrode impedance serves as an objective marker of cochlear alterations related to hearing decline.[8-10] Increased fibrosis elevates impedance, requiring higher voltages for stimulation, which reduces the dynamic range and shortens battery life. Over the long term, fibrosis and ossification may complicate future surgical interventions, limit the efficacy of emerging inner ear therapies, and negatively affect speech recognition outcomes.[11]

Electrode design plays an important role in cochlear

<footnote>ªPediatric Otolaryngology Department, Robert Debré University Hospital, Assistance Publique - Hôpitaux de Paris and ᵇPediatric Otolaryngology Department, Robert Debré University Hospital, Assistance Publique - Hôpitaux de Paris, Institut Hospitalo-Universitaire Robert-Debré du Cerveau de l'Enfant, Paris, France; Université Paris Cité, INSERM NeuroDiderot, Paris, France</footnote>

<footnote>The authors disclose no conflicts of interest.</footnote>

<footnote>*Corresponding author. Address: Hôpital Universitaire Robert-Debré Oto-rhino-laryngologie pédiatrique, Hôpital Universitaire Robert-Debré: Hopital Universitaire Mere-enfant Robert-Debre, Paris, France, 48 Boulevard Sérurier, 75019 Paris. E-mail address: thierry.van-den-abbeele@aphp.fr (T. Van Den Abbeele).</footnote>

<footnote>© 2025, Otology & Neurotology, Inc.</footnote>

<footnote>Otology & Neurotology (2026) 47:447-455</footnote>

<footnote>Supplemental Digital Content is available for this article. Direct URL citations are provided in the HTML and PDF versions of this article on the journal's website, www.otology-neurotology.com.</footnote>

<footnote>http://dx.doi.org/10.1097/MAO.00000000000004784</footnote>
preservation and electrical stimulation efficiency.[10] Precurved perimodiolar (PM) electrodes are designed to position the intra-cochlear array closer to the modiolus, where auditory fibers converge. This placement allows more precise and selective stimulation, requiring lower current levels and potentially enhancing auditory performance and extended battery life. However, PM arrays have historically been associated with a higher risk of insertion trauma and translocations, which can compromise cochlear structure.[10] In contrast, straight (ST) electrodes follow the lateral wall of the cochlea, theoretically reducing the risk of tip fold-over and translocation. However, their positioning requires higher stimulation currents, which may decrease selectivity and increase power consumption.

Given the expanded indications for CIs in both adults and children, including implantation in patients with significant residual hearing, minimizing trauma is essential to reduce the inflammatory response triggered by implantation, preserve sensory structures, and optimize outcomes.[12-14] Following the introduction of the first PM electrode array by Cochlear in 1999, several models have been developed. More recently, the nucleus slim PM electrodes (CI532/CI632) were designed to improve atraumatic insertion while maintaining the benefits of PM positioning. However, most studies comparing PM and ST electrodes have focused on adults, making extrapolation to pediatric populations challenging due to the higher prevalence of inner ear anomalies in children.[15-17]

While hearing and structural preservation are critical in children, selecting the most appropriate electrode for cochlear implantation remains a clinical challenge. This study aimed to compare radiologic measures, electrophysiology parameters, and some speech and language outcomes in a pediatric cohort implanted with either lateral wall cochlear nucleus slim ST electrodes models (CI422/CI522/CI622) or nucleus slim PM electrodes (CI532/CI632). We hypothesized that electrode design would influence impedances and related measures, and that these differences, in interaction with age at implantation, could impact auditory and language performance.

# Materials and methods

## Subjects

Children meeting the HAS criteria for CI (2011) were included. These criteria encompass children younger than 5 years of age with bilateral severe-to-profound prelingual sensorineural hearing loss, children 5 years old or older who have developed or were developing oral communication, and children with auditory fluctuations. Audiometric criteria included an intelligibility score < 50% during free-field testing at 60 dB with both their hearing aids.[1]

A total of 88 children (122 ears) implanted at a single pediatric CI referral center in Paris, France (December 2016 to April 2023), were included. These patients received cochlear implants manufactured by Cochlear. ST electrode implantations were performed with the nucleus slim straight arrays (CI422/CI522/CI622), and PM implantations with the nucleus slim PM arrays (CI532/CI632). For speech and language outcomes, analyses were restricted to 72 unique patients: the first implanted side in 68 unilateral or sequential bilateral cases and 4 children with simultaneous bilateral implantation, analyzed at the patient level. Written informed consent for the use of clinical data was obtained

in accordance with the reference methodology of the National Commission for Data Protection and Liberties (CNIL n° 2236975). All patients were also included in the French National Cochlear Implant Registry (EPIIC).

## Cochlear implants used in this study

The nucleus slim perimodiolar CI532/CI632 is a precurved electrode with a tip diameter of 0.35×0.4 mm, a base diameter of 0.45×0.5 mm, and an active length of 14.4 mm. It features 22 half-banded platinum contacts and 3 white markers to control insertion depth. This array is inserted with a silicone sheath composed of 2 parts: a white handle and a highly flexible silicon insertion tube. Both parts are separated by a "stopper" (0.8 mm button) to prevent overinsertion of the electrode into the basal turn. The most recent CI632 model incorporates a reloadable sheath allowing multiple insertions.

The nucleus slim straight CI422/522/622 is a slim ST electrode with an active length 19.1 mm, a basal diameter of 0.6 mm, and an insertion depth ranging from 20 to 25 mm. At the apical end, it incorporates a Softip with a diameter of 0.3 mm to minimize insertion trauma. The difference between the CI400/500 and CI600 series is that CI600 series implants include implant coil plates on either side of a magnet pocket containing a removable magnet cassette. This design allows magnet removal and replacement from the distal end of the implant coil if required.

## Surgical procedure and electrode selection criteria

All surgeries were performed by 4 senior cochlear implant surgeons. From 2016 to 2018, only the slim ST electrode was available in France. After the slim PM array was introduced in France in early 2018, surgeons underwent dedicated training and progressively adopted its use. Electrode selection was not randomized and was typically determined during preoperative multidisciplinary meetings, with the slim PM arrays being the default. Intraoperative modification was possible in the presence of anatomic constraints, particularly in very young children (eg, narrow facial recess, inflammatory middle ear mucosa, or a prominent facial nerve). In accordance with current recommendations, slim ST arrays were preferred in cases with significant cochleo-vestibular malformations. The surgical procedure consisted of a standard antro-mastoidectomy with a facial recess approach, performed under an operating microscope (Leica F40) or, more recently, using a 3D exoscope (Vitom 3D, Storz). The electrode was inserted into the scala tympani either through a cochleostomy, performed with low-speed 0.8 to 1 mm curved cochleostomy burrs near the round window or through the round window itself after drilling the niche edges. A round window insertion was preferred when anatomic exposure allowed it. Cochleostomy approaches were used in cases of narrow facial recess, overhanging facial nerve, or poor round window visibility, which were more frequent in younger children. The ground electrode was placed under the temporal muscle. Correct intra-cochlear position of the electrode array was confirmed intraoperatively by electrophysiological testing, including impedance and NRT measurements, using the Cochlear Remote Control CR220 or CustomSound EP software. Intraoperative x-rays were performed in cases of suspected tip fold-over or insertion difficulties. These tests were carried out in the operating room after the electrode array insertion and during wound closure. Skull radiographs on the first postoperative day confirmed the cochlear position of the electrode array and the absence of tip fold-over.
## Outcome measures

### Impedances, neural response telemetry, and threshold/ comfortable levels

A single audiologist collected impedance, NRT thresholds, and threshold levels (T-levels) (minimum stimulation) and comfortable levels (C-levels) (maximum stimulation) during follow-up visits for hearing aid fittings. Impedance data were analyzed at 1 month post activation, once stable values were reached, and 12 months. Measurements were collected from 5 distinct electrodes (2, 7, 12, 17, and 22) using the common ground (CG) mode, chosen for its sensitivity to local electrode-tissue interface changes and its routine use in early postoperative monitoring. Although monopolar modes (MP1, MP2, MP1+2) were available, they were not included in the present analysis.

NRTs were analyzed at activation and 6 months, and T/C levels at activation, 6, and 12 months post implantation using Cochlear Custom Sound 5.1 software. T/C levels were adjusted using the Electrically evoked Compound Action Potential (ECAP) thresholds measured intraoperatively. Because pulse width (µs/ phase) varied across children and implant types (25 to 50 µs), delivered charge per phase was calculated for T and C levels in nanoCoulomb (nC), using the conversion provided by Cochlear: charge-per-phase (nC) = $100 \exp (CU/255) \times$ pulse width (µs) / 1000 nC.[18]

## Audiometry

Free-field postimplantation pure tone audiometry (PTA) testing was conducted in a soundproof hearing booth. Testing was conducted using a Madsen Conera audiometer (GN Otometrics, Denmark). Mean thresholds (dB HL) were calculated by averaging 250, 500, 1000, 2000, and 4000 Hz frequencies. PTA was performed at activation, 6 months, and 12 months post implantation.

## Evaluation of speech and language outcomes

Speech audiometry was performed by pediatric audiologists using Boorsma word lists, typically after most fitting sessions. Two performance scales were evaluated at 2 years post implantation. The Categorical Auditory Performance (CAP; see Appendix 1, Supplemental Digital Content 1, http://links.lww.com/MAO/C317) rates auditory perception from 0 (no awareness of environmental sounds) to 7 (uses the telephone with a familiar listener). CAP is widely used to monitor the progress of cochlear implant users and is applied in many pediatric implant centers worldwide.[19] The APCEI score (see Appendix 1, Supplemental Digital Content 1, http://links.lww.com/MAO/C317), developed in our department in 2006 and adopted by many francophone centers, evaluates audiophonatory performance in children.[20,21] It rates 5 domains—acceptation, perception, comprehension, expression, intelligibility—on a 0 to 5 scale, and also provides a global performance class from 0 (poor performers) to 5 (best performers). This score offers an age-appropriate audiophonologic profile and is suitable for postimplantation follow-up.

## Radiographic analysis

### Measurements of the angular depth of insertion and basal insertion depth

Skull radiographs taken postoperatively (day 1) were analyzed using a standardized method in our center to assess electrode

array position. The aDOI was independently measured by 2 physicians using the technique validated by Svrakic et al (2015).[22] The round window position was defined as the intersection of a line from the apex of the superior semicircular canal to the center of the vestibule and the electrode lead. The aDOI represents the angle formed by the most distal electrode relative to the 0-degree reference line drawn through the round window and the modiolus (defined by the center of the electrode spiral, Fig. 1). Following Beck et al[23], the BID was also measured, corresponding to the angle of the most basal electrode relative to the 0-degree reference line.

## Measurements of the modified Intracochlear Position Index

To measure the PM position of the electrodes, we used the method described by Lee et al (2020).[24] This measurement, called “mean Intracochlear Position Index” (mICPI), compares the distance from the modiolus to the lateral cochlear wall with the distance from the modiolus to the electrodes. It is calculated by dividing the sum of the 2 perpendicular diameters of the ellipse formed by the projected intracochlear array on skull radiographs by the sum of the 2 perpendicular external diameter of the cochlea measured on preoperative CT reconstructions (Fig. 1). A lower index indicates closer proximity of electrodes to the modiolus. All measurements were performed using the Carestream software.

## Statistical analysis

Analyses were performed with BiostatGV (INSERM) and Sigmastat 4.0 (Systat). Demographic characteristics presented in Table 1 for the 2 CI groups were compared using the $\chi^2$ test and Fisher exact tests for categorical data. Student *t* test or Mann-Whitney rank-sum test was used to analyze auditory and radiographic data between both groups. For multiple group comparisons, one-way ANOVA test or Holm-Sidak method (in case of non-normality and unequal variance) was used. Coefficients of variation and the Brown-Forsythe test were used to compare the variability observed within the 2 groups concerning auditory outcomes. For speech and language outcomes, analyses were restricted to 72 patients. Multivariable ordinary least squares (OLS) regression was performed for APCEI (global and subscores) and CAP. Covariates included electrode type (PM vs ST), age at implantation (continuous), surgical approach (RW vs cochleostomy), and imaging variables (presence of malformation, aDOI). Subgroup analyses were conducted by age strata (≤ 3 vs > 3 y). A *P*-value < 0.05 was considered statistically significant.

## Results

### Subjects

We included 88 children (122 ears) who underwent CI between December 2016 and April 2023. Among them, 44 received bilateral sequential and 5 bilateral simultaneous implantations. Forty-six patients were implanted with 65 slim PM electrodes (including 20 bilateral cases), while 47 patients received 57 slim ST electrodes (including 14 bilateral cases). Five patients received a slim PM electrode in one ear and a slim ST electrode in the other. Demographic characteristics were similar between groups (Table 1), except for age at implantation, which was significantly lower in the PM group (3.1 ± 2.9 vs 5.6 ± 4.9 y, *P* < 0.001). Surgical techniques included cochleostomy in 86 cases (70%) and
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Principles of radiographic measurements. Drawings (A and D) illustrate the formulas used to calculate the modified Intracochlear Position Index (mICPI), angular depth of insertion (aDoi), and basal insertion depth (BID) in Stenvers sections. Preoperative CT provides measurements of distances from the modiolus to the lateral walls, while postoperative x-rays measure the distances from the modiolus to the electrodes. In A, the red vertical line (VL) serves as a landmark, passing through the anterior (superior) semicircular canal, vestibule, and promontory near the round window niche. L1 represents the major cochlear diameter, parallel to VL. L2 represents the minor cochlear diameter, perpendicular to both L1 and VL. L3 and L4 correspond to the portions of these diameters limited by the intracochlear electrode (yellow). The mICPI, adapted from Lee et al (2020)[24], is derived from these 4 measurements. In D, the horizontal line (HL) is parallel to the basal turn of the cochlea and the initial portion of the electrode array (yellow). The aDoi is defined as the angle between: a first dotted line, which extends from the modiolus (Mod) to the intersection of VL and HL, and a second dotted line, which extends from Mod to the apical end of the intracochlear electrode array. The BID is the angle to the most basal electrode. B and E, Show the preoperative CT and postoperative x-ray of a child implanted with a ST electrode (mICPI ≈ 0.99, ADoi = 344 degrees, BID = 6 degrees). C and F, Depict the preoperative CT scan and postoperative x-ray of another child implanted with a PM electrode (mICPI = 0.71, indicating closer proximity to the modiolus, ADoi = 405 degrees, BID = 5 degrees). Adaptations are themselves works protected by copyright. So in order to publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.</figcaption></figure>

enlarged round window (RW) approach in 36 cases (30%), with the RW more frequently used for PM arrays (*P* < 0.01). In accordance with recommendations, 10 children with significant cochlear malformations were implanted with a ST electrode array; 2 were implanted with PM when no significant oozing was observed at opening of the cochlea.

## Radiographic measurements

The 3 radiographic techniques, aDOI, BID, and mICPI, provided complementary insights into electrode positioning. The mean aDOI and mean BID were (360 + 25.3 degrees) ± 36.6 degrees and 14.4 ± 14.8 degrees, respectively, in the PM group and (360 + 28.9 degrees) ± 53.7 degrees and 17.3 ± 21.6 degrees in the ST electrode group, with no statistically significant differences observed between the two groups. In contrast, mICPI values demonstrated significantly closer modiolar positioning of the PM electrodes (0.72 ± 0.07) compared with ST electrodes (0.98 ± 0.06, *P* < 0.0001, Fig. 1). In multivariable analysis, surgical approach (round window vs. cochleostomy) had no significant influence on these measurements.

## Objective implant analysis

The mean impedance at 1 month was 9.1 ± 1.8 kΩ in the PM group and 10.7 ± 2.4 kΩ in the ST electrode group. The difference between the groups was statistically significant (*P* < 0.001, Table 2, Fig. 2A), and was more pronounced for apical electrodes than for basal ones. At 12 months, a significant decrease in

impedances was observed only in the PM group. The average impedance dropped to 7.7 ± 1.4 kΩ (*P* < 0.001), with the reduction again being more marked in apical electrodes. In contrast, the average impedance in the ST electrode group remained stable (11.3 ± 3.1 kΩ). Surgical approach (RW vs cochleostomy insertion) had no significant effect on mean and basal impedances.

NRT threshold levels are summarized in Table 2. At 6 months post implantation, these values were significantly lower in the PM group (171 ± 16 CU) than in the ST array group (184 ± 15 CU; *P* < 0.001; Fig. 2B). As for the impedance, the differences were also more pronounced for apical electrodes than for basal ones.

T/C levels at activation, 6, and 12 months are shown in Table 3. No significant differences were observed between groups across time points. However, mean pulse width was significantly shorter in the PM group (33.5 ± 10 vs 43.4 ± 11 μs; *P* < 0.001), resulting in significantly lower charge-per-phase (nC) for stimulation (Fig. 2C).

## Hearing outcomes

PTA improved significantly in both groups from activation to 6 and 12 months. In the PM group, PTA threshold (dB HL) was 93.2 ± 17.3 at activation, 40.5 ± 12.9 at 6 months, and 31.9 ± 6.3 at 12 months post implantation (*P* < 0.001). In the ST electrode group, PTA threshold was 93.3 ± 18.8 at activation, 41.6 ± 19 at 6 months, and 34.9 ± 10.7 at 12 months post implantation (*P* < 0.001). There was no significant difference in
<table><caption>**Table 1**
Demographic characteristics of the studied cohort</caption><thead><tr><th></th><th>Slim Modiolar; 632/<br>532 (n = 65), n (%)</th><th>Slim Straight; 622/<br>522 (n = 57), n (%)</th></tr></thead><tbody><tr><td>Sex</td><td></td><td></td></tr><tr><td>Female</td><td>34 (53)</td><td>21 (37)</td></tr><tr><td>Male</td><td>31 (47)</td><td>36 (63)</td></tr><tr><td>Mean age at implantation (y)</td><td>3.1 ± 2.9</td><td>5.6 ± 4.9<sup>b</sup></td></tr><tr><td>Implantation strategy</td><td></td><td></td></tr><tr><td>Unilateral implantation</td><td>18 (28)</td><td>21 (37)</td></tr><tr><td>First implant</td><td>17 (26)</td><td>12 (21)</td></tr><tr><td>Second implant (sequential)</td><td>24 (37)</td><td>20 (35)</td></tr><tr><td>Simultaneous bilateral</td><td>6 (9)</td><td>4 (7)</td></tr><tr><td>Surgical technique</td><td></td><td></td></tr><tr><td>Cochleostomy</td><td>38 (58)</td><td>48 (84)<sup>a</sup></td></tr><tr><td>Round window</td><td>27 (42)</td><td>9 (16)</td></tr><tr><td>Electrode array</td><td></td><td></td></tr><tr><td>Slim 4×2 series</td><td>—</td><td>15 (38)</td></tr><tr><td>Slim 5×2 series</td><td>12 (19)</td><td>17 (44)</td></tr><tr><td>Slim 6×2 series</td><td>53 (81)</td><td>25 (18)</td></tr><tr><td>Cochlear morphology<sup>a</sup></td><td></td><td></td></tr><tr><td>Normal cochlea</td><td>58 (89)</td><td>37 (65)</td></tr><tr><td>Isolated vestibular<br>malformation</td><td>5 (8)</td><td>6 (10)</td></tr><tr><td>Enlarged vestibular aqueduct</td><td>0</td><td>10 (18)</td></tr><tr><td>Incomplete partition type 2<br>(Mondini)</td><td>2 (3)</td><td>3 (5)</td></tr><tr><td>Incomplete partition type 3</td><td>0</td><td>1 (2)</td></tr><tr><td>Cause of hearing loss</td><td></td><td></td></tr><tr><td>Genetic</td><td>32 (49)</td><td>21 (37)</td></tr><tr><td>Cytomegalovirus</td><td>12 (18)</td><td>6 (10)</td></tr><tr><td>Traumatic (fracture)</td><td>1 (2)</td><td>0</td></tr><tr><td>Neonatal hypoxia</td><td>2 (3)</td><td>6 (10)</td></tr><tr><td>Unknown</td><td>18 (28)</td><td>24 (42)</td></tr></tbody></table>

the mean PTA progression between groups. However, analysis of variability showed significant differences over time. At activation, variability within groups was similar. By 6 months, variability at mid-to-high frequencies (1000 to 4000 Hz) was significantly lower in the PM group (P < 0.05), and this persisted at 12 months (P < 0.02; Table 4; Fig. 2D), suggesting more consistent outcomes with PM electrodes.

## Speech and language outcomes

Speech and language performance was analyzed in 72 children receiving their first implant (68 unilateral or sequential bilateral cases and 4 simultaneous bilateral cases, 38 PM arrays and 34 ST arrays). At 2 years post implantation, children implanted at 3 years of age or younger with PM arrays achieved significantly better outcomes than those with ST arrays. Mean APCEI scores were 3.54 ± 1.03 for the PM group (n = 26) versus 2.60 ± 1.24 for the ST group (n = 15), with a multivariable effect of β = -1.33 (95% CI: -2.27 to -0.38, P = 0.006). Mean CAP scores were 4.69 ± 1.01 for PM (n = 26) versus 3.47 ± 1.46 for ST (n = 15), with a multivariable effect of β = -1.25 (95% CI: -2.36 to -0.14, P = 0.028). In contrast, among children implanted at >3 years, no significant differences were observed between electrode types. Mean APCEI scores were 3.42 ± 1.38 for PM (n = 12) and 4.26 ± 1.05 for ST (n = 19), with β = +0.33 (95% CI: -0.58 to +1.25, P = 0.48). Mean CAP scores were 4.17 ± 1.40

for PM (n = 12) and 4.70 ± 1.16 for ST (n = 19), with β = +0.29 (95% CI: -0.96 to +1.55, P = 0.65). In this subgroup, age at implantation showed only a nonsignificant trend toward higher APCEI score with increasing age (β = +0.09/y, P = 0.059), while electrode type had no effect. Other covariates, such as round window versus cochleostomy approach or the presence of inner ear malformations, were not significantly associated with speech/language outcomes.

## Discussion

### Study significance

Hearing loss in children is a major global public health concern. Continuous advances in CI aim to improve auditory outcomes while minimizing intracochlear trauma. This study compared two recently introduced electrode arrays from Cochlear Ltd (Sydney, Australia): the slim perimodiolar (CI532/CI632) and the slim straight (CI422/CI522/CI622) electrode arrays. Previous studies comparing PM and ST electrodes in postlingually deafened adults have reported contradictory results. Some found no significant differences between the two types[25,26], whereas others reported faster acquisition of auditory results[17] or better speech perception with PM arrays.[27-29] Pediatric data remain limited, largely focused on electrophysiological parameters.[30] To our knowledge, this is the first large pediatric series comparing the two electrode designs using combined radiographic, electrophysiological, and functional outcomes.

### Study design and cohort characteristics

This retrospective cohort included 88 children (122 ears) with homogeneous distributions in terms of sex and surgical approach, but heterogeneous etiologies of deafness. Children with inner ear malformations (eg, incomplete partition of the cochlea, enlarged vestibular aqueduct) were deliberately included to better reflect real-world pediatric practice. Importantly, electrode selection was not randomized but clinically guided. This practice-based decision, although clinically justified, represents an inherent limitation that may introduce selection bias. Children implanted with PM electrodes were significantly younger at the time of surgery (Table 1), reflecting evolving implantation criteria, the introduction of slim PM designs, and the implementation of universal newborn hearing screening in France. Conversely, children with progressive hearing loss related to cochlear malformations were often implanted later, preferentially receiving slim ST electrodes.

### Electrode insertion techniques and cochlear trauma

Insertion-related cochlear trauma remains a critical issue in pediatric CI, especially in the context of hearing and cochlear structure preservation, and potential future therapies such as gene therapy. In this cohort, cochleostomy was the predominant approach, largely due to limited anatomic exposure in younger children, despite evidence suggesting superior hearing preservation with round window approaches in adults.[31-33] Although hearing preservation was not directly assessed, indirect markers of cochlear trauma were evaluated using electrophysiological measures and imaging analysis. Multivariate analyses showed no significant influence of surgical approach on electrophysiological parameters or audiological outcomes.

<footnote><sup>a</sup>P < 0.01.</footnote>

<footnote><sup>b</sup>P < 0.001.</footnote>
**Table 2**

<table><caption>Impedance values at 1 month and 1 year, and NRT thresholds at activation and 6 months in the perimodiolar (PM) and straight electrode (ST) groups</caption><thead><tr><th rowspan="2">Electrodes</th><th colspan="2">PM Impedances (kOhm)</th><th colspan="2">ST Impedances (kOhm)</th><th colspan="2">PM NRT Thresholds (CU)</th><th colspan="2">ST NRT Thresholds (CU)</th></tr><tr><th>1 mo</th><th>1 y</th><th>1 mo</th><th>1 y</th><th>Activation</th><th>6 mo</th><th>Activation</th><th>6 mo</th></tr></thead><tbody><tr><td>2</td><td>9.6 ± 2.3</td><td>8.9 ± 1.7</td><td>10.3 ± 2.7</td><td>10.9 ± 4.1<sup>b</sup></td><td>194 ± 23</td><td>187 ± 15</td><td>202 ± 24</td><td>195 ± 23</td></tr><tr><td>7</td><td>8.9 ± 2.3</td><td>8.3 ± 2.2</td><td>9.9 ± 2.0</td><td>10.5 ± 3.6<sup>b</sup></td><td>184 ± 20</td><td>182 ± 12</td><td>194 ± 18</td><td>188 ± 15</td></tr><tr><td>12</td><td>8.4 ± 2.2</td><td>7.0 ± 2.3<sup>a</sup></td><td>10.4 ± 2.9<sup>b</sup></td><td>10.8 ± 3.7<sup>b</sup></td><td>178 ± 24</td><td>177 ± 25</td><td>192 ± 19</td><td>193 ± 19</td></tr><tr><td>17</td><td>9.7 ± 2.3</td><td>7.3 ± 1.7<sup>a</sup></td><td>11.5 ± 3.1<sup>b</sup></td><td>11.7 ± 3.8<sup>b</sup></td><td>162 ± 25</td><td>158 ± 28</td><td>178 ± 21</td><td>177 ± 19</td></tr><tr><td>22</td><td>8.8 ± 2.4</td><td>6.9 ± 1.8<sup>a</sup></td><td>12.4 ± 3.5<sup>b</sup></td><td>12.6 ± 3.8<sup>b</sup></td><td>162 ± 21</td><td>159 ± 19</td><td>171 ± 22</td><td>171 ± 18</td></tr><tr><td>Mean</td><td>9.1 ± 1.9</td><td>7.7 ± 1.4<sup>a</sup></td><td>10.9 ± 2.4<sup>b</sup></td><td>11.3 ± 3.1<sup>b</sup></td><td>176 ± 16</td><td>171 ± 16</td><td>187 ± 16<sup>b</sup></td><td>185 ± 15<sup>b</sup></td></tr></tbody></table>

**Objective implant measures**

Comparison of impedance, NRT, T/C levels measurements revealed significant intergroup differences. Impedance, defined as the electrical resistance between the electrodes and the

surrounding cochlear environment, is influenced by electrode design, surface properties, inflammatory responses, fibrotic tissue formation, and encapsulation.[34] It typically increases during the initial 2 weeks following implantation and stabilizes thereafter,

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Comparison between PM (x32) and ST (x22) electrodes. A, Impedances in kΩ at 1 month and 1 year. B, NRT thresholds in clinical units (CU) at activation and 6 months. C, Delivered charge per phase (nC/phase) at activation, 6 months, and 12 months. D, Averages Pure Tone Audiometry (500-4000 Hz) at activation, 6 months, and 12 months. The grey boxplots represent the distribution of PM electrode values. The white boxplots represent the distribution of ST electrode values. The height of each box corresponds to the interquartile range (IQR), the horizontal line represents the median, the vertical line indicates the SD, and the open circles denote outlier values beyond the whiskers.</figcaption></figure>

<footnote><sup>a</sup>A significant difference within the same category of electrode, *P* < 0.001.</footnote>

<footnote><sup>b</sup>A significant difference between PM and ST.</footnote>
<table><caption>**Table 3**

T(threshold), C(comfort) levels at activation, 6 and 12 months post CI in the perimodiolar (PM) and straight electrode (ST) groups</caption><thead><tr><th rowspan="2">Implant Group</th><th colspan="2">PM Electrodes</th><th colspan="2">ST Electrodes</th></tr><tr><th>in CU</th><th>nC/Phase</th><th>in CU</th><th>nC/Phase</th></tr></thead><tbody><tr><td>T levels at activation</td><td>99 ± 15</td><td>3.1 ± 1</td><td>99 ± 23</td><td>4.3 ± 1.9<sup>a</sup></td></tr><tr><td>T levels at 6 mo</td><td>128 ± 13</td><td>5.4 ± 1.3</td><td>120 ± 19</td><td>6.4 ± 2.3<sup>b</sup></td></tr><tr><td>T levels at 12 mo</td><td>125 ± 16</td><td>5.7 ± 1.6</td><td>117 ± 16</td><td>6.6 ± 2.4<sup>b</sup></td></tr><tr><td>C levels at activation</td><td>141 ± 15</td><td>6.6 ± 2.1</td><td>140 ± 23</td><td>9.1 ± 3.9<sup>a</sup></td></tr><tr><td>C levels at 6 mo</td><td>169 ± 13</td><td>11.5 ± 2.8</td><td>161 ± 20</td><td>13.5 ± 5<sup>b</sup></td></tr><tr><td>C levels at 12 mo</td><td>167 ± 16</td><td>12.1 ± 3.5</td><td>160 ± 16</td><td>14.3 ± 5.2<sup>b</sup></td></tr><tr><td>Mean pulse duration</td><td>33.5 ± 10 μs</td><td></td><td>43.4 ± 11<sup>b</sup> μs</td><td></td></tr></tbody></table>

particularly after the first programming sessions.[13,17] At 1-month post implantation, the PM group exhibited significantly lower impedance values, especially at the apical contacts, in accordance with previous reports.[29] This divergence in impedance profiles between PM and ST arrays became even more marked at 12 months, with a continued decline observed in the PM group, in contrast to the stability noted in the ST electrode group. These differences are unlikely to be explained by disparities in electrode surface areas, as the slim PM (x32) and slim ST (x22) arrays have comparable contact dimensions (0.16 to 0.20 mm² from apex to base in both models), unlike earlier contour advanced PM arrays (x12, 0.21 to 0.23 mm²). They may instead reflect the biomechanical and biological advantages conferred by the modiolar-hugging trajectory of PM arrays within the scala tympani. Closer proximity to the modiolus likely reduces current path length and spread and could potentially limit fibrotic tissue formation and encapsulation around the electrode contacts. Importantly, surgical approach (RW vs cochleostomy) did not affect impedance profiles, even for basal electrodes, which are more exposed to fibrotic reactions. The BID was found similar for both electrode designs.

NRT, which quantifies electrically evoked auditory nerve responses, is a key tool in pediatric CI programming.[35] At 6 months, NRT thresholds were significantly lower in the PM group, suggesting reduced cochlear trauma and improved neural activation, as suggested in earlier studies.[12,35] These lower thresholds correlated with reduced stimulation levels observed in the same group.

While T/C levels did not differ significantly across time points at activation, 6, or 12 months, energy consumption (delivered charge-per-phase) was significantly lower in the PM group due to shorter pulse widths. This offered meaningful gains in battery life, a notable benefit for children during school hours.

### Electrode positioning and radiographic analysis

Intracochlear electrode positioning is an important determinant of auditory outcomes. Ideal positioning allows focused neural stimulation and minimizes current spread, reducing unintended activation of adjacent neurons, which ultimately improves auditory performance.[5] Our radiographic assessment confirmed that slim PM arrays consistently hugged the modiolus more closely than ST arrays, enhancing the electro-neural interface, reducing pulse-width requirements, and improving battery efficiency. However, this closer proximity may increase the risk of complications, such as tip fold-over, particularly in malformed cochleae. In our series, one patient with an incomplete partition type II (Mondini) experienced a tip fold-over with a CI632 implant, necessitating reimplantation with a slim straight CI622 array. This 1.5% complication rate aligns with previously reported tip fold-over rate of 4% to 7% for slim PM arrays.[35-37]

Insertion depth (aDOI) did not differ significantly between groups, indicating no increased risk of overinsertion with PM arrays or associated trauma compared with ST electrodes. Although the role of basal insertion parameter remains debated,[23] our BID analysis, and thus the position of the most basal electrodes, was comparable between designs, suggesting no impact on high-frequency coding. In contrast, mICPI clearly demonstrated closer modiolus-hugging trajectory of PM electrodes compared with ST designs.[24,38] This proximity has been correlated with improved speech perception outcomes in adult studies.[28]

Although cone beam CT (CBCT) offers greater precision in assessing electrode position and depth of insertion, its use is often constrained in pediatric hospitals by logistical and radiologic limitations. The mICPI method, relying solely on plain radiographs, offers a practical, low-radiation alternative suitable for routine clinical application. By integrating Stenvers reconstructions obtained from the preoperative CT and postoperative radiographic assessments, we obtained robust positioning data while minimizing radiation exposure in this vulnerable population.

<table><caption>**Table 4**

PTA results for each frequency, mean PTA thresholds (500-4000 Hz) and mean PTA thresholds (1000-2000-4000 Hz)</caption><thead><tr><th rowspan="2">Implant Group (Frequencies)</th><th colspan="3">PM Electrodes PTA Thresholds (dB)</th><th colspan="3">ST Electrodes Thresholds (dB)</th></tr><tr><th>Initial</th><th>At 6 mo</th><th>At 12 mo</th><th>Initial</th><th>At 6 mo</th><th>At 12 mo</th></tr></thead><tbody><tr><td>250 Hz</td><td>76.7 ± 14.8</td><td>40.5 ± 12</td><td>33.3 ± 6.5</td><td>77.8 ± 16</td><td>43.7 ± 15.1</td><td>37.2 ± 9.2<sup>a</sup></td></tr><tr><td>500 Hz</td><td>84.2 ± 17.4</td><td>39.8 ± 13.3</td><td>31.7 ± 6.5</td><td>82.7 ± 20.6</td><td>40.3 ± 15.7</td><td>34.3 ± 9.6<sup>b</sup></td></tr><tr><td>1000 Hz</td><td>91.3 ± 19</td><td>37.5 ± 10.3</td><td>31.8 ± 6.7</td><td>90.7 ± 24.7</td><td>39 ± 17.2<sup>a</sup></td><td>33.7 ± 8</td></tr><tr><td>2000 Hz</td><td>99.1 ± 18</td><td>37.9 ± 11.2</td><td>31.1 ± 7.7</td><td>98.1 ± 21.3</td><td>41.9 ± 22.8<sup>b</sup></td><td>34.2 ± 13<sup>a</sup></td></tr><tr><td>4000 Hz</td><td>106.7 ± 17.7</td><td>41.9 ± 14.5</td><td>32.7 ± 7.1</td><td>103.8 ± 19.4</td><td>45.2 ± 25.4<sup>a</sup></td><td>38.2 ± 16.7<sup>a</sup></td></tr><tr><td>Mean PTA</td><td><strong>93.2 ± 17.3</strong></td><td><strong>40.5 ± 12.9</strong></td><td><strong>31.9 ± 6.3</strong></td><td><strong>93.3 ± 18.8</strong></td><td><strong>41.6 ± 19</strong></td><td><strong>34.9 ± 10.7<sup>a</sup></strong></td></tr><tr><td>Mean PTA (1000-2000-4000 Hz)</td><td><strong>96.8 ± 18</strong></td><td><strong>39.3 ± 11.3</strong></td><td><strong>32 ± 6.7</strong></td><td><strong>97.6 ± 19.5</strong></td><td><strong>42.5 ± 21.1<sup>a</sup></strong></td><td><strong>35.3 ± 11.6<sup>b</sup></strong></td></tr></tbody></table>

Comparison of variability between perimodiolar and straight groups using Brown-Forsythe and Fligner-Killeen tests, P-values.

<footnote><sup>a</sup>P < 0.001.</footnote>

<footnote><sup>b</sup>P < 0.01.</footnote>

<footnote><sup>a</sup>P < 0.05.</footnote>

<footnote><sup>b</sup>P < 0.01.</footnote>
## Auditory and speech-language outcomes

Beyond objective and radiographic parameters, PTA thresholds, CAP, and APCEI scales quantified functional performance. PTA thresholds improved significantly in both groups at 6 and 12 months, with no significant mean differences between groups. However, variability analysis revealed greater consistency in the PM group, particularly across speech-relevant frequencies (1000 to 4000 Hz) at 6 and 12 months. This suggests more predictable outcomes, where reliability is essential in pediatric populations. These results support prior observations in adults[17] and highlight the potential of PM designs to deliver stable auditory input in early postimplantation phases.

At 2 years, stratified multivariable analyses showed that children implanted at ≤3 years with slim PM arrays achieved significantly higher CAP and APCEI scores (*P* < 0.03 and 0.006, respectively) compared with slim ST arrays, even after adjustment of covariates such as surgical technique or cochlear malformations. In children implanted >3 years, electrode type had no significant impact, age at implantation remaining the strongest determinant of outcomes. While the CAP scale reflects functional auditory performance, the APCEI provides a broader assessment encompassing auditory perception as well as early language skills such as comprehension, expression, and speech intelligibility.[20,21] Our results highlight the dual influence of electrode design and early age at implantation, with PM electrodes offering particular advantages in younger children. While, to our knowledge, no previous pediatric studies have confirmed this, adult studies have reported superior speech recognition with precurved arrays.[28,29]

## Implications and conclusion

This comparative study of cochlear nucleus slim modiolar (CIx32) and slim straight (CIx22) arrays in a pediatric population demonstrates that electrode design influences objective measures, radiographic positioning, and functional outcomes. PM electrodes consistently achieved closer modiolar proximity, lower impedances and neural thresholds, reduced energy consumption, and more consistent early auditory thresholds, translating into significantly better speech and language outcomes in children implanted at 3 years or younger of age.

Nevertheless, several limitations must be acknowledged. The retrospective, single-center design carries potential biases, including the nonrandomized selection of electrode type, heterogeneity of etiologies, and evolution of deafness and different surgical approaches (including anatomic variability, cochlear malformations). Audiological data were limited to PTA, and standardized speech perception testing could not be systematically performed in very young children, leading to the use of more global performance scales. Furthermore, electrode design is intrinsically linked to radiographic and electrophysiological parameters such as mICPI and impedances, raising concerns of collinearity and suggesting that part of the electrode effect may be mediated by these factors. Finally, the 2-year follow-up limits conclusions about long-term language, educational outcomes, and hearing preservation.

Despite these limitations, our findings support the preferential use of slim PM arrays in young children and highlight the importance of early implantation. Gains in battery efficiency represent additional advantages for school-aged children. While PM arrays offer clear short-term benefits, long-term outcomes remain strongly influenced by patient-specific factors, such as age at implantation, neuroplasticity, and cognitive development.

From a technical standpoint, PM arrays, thanks to their peri-modiolar positioning and lower stimulation levels, may help preserve the cochlear and neural integrity, which could be advantageous for future intracochlear gene or cell-based therapies. Long-term longitudinal studies, including electro-physiological assessments and standardized evaluations of speech perception and production, will be needed to confirm whether these benefits persist, both in terms of cochlear microstructure preservation, relevant for emerging therapies, and in auditory performance, language acquisition, and quality of life.

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