ABSTRACT
Objectives
To evaluate the effects of age and gender on electrode impedance during cochlear implant surgery and at 6 months after implantation, and to determine whether activation timing modifies these relationships across cochlear regions.
Methods
This retrospective longitudinal cohort study included cochlear implant recipients with normal cochlea implanted with the MED‐EL FLEX28. All electrodes were fully inserted via the round window approach. Participants underwent early activation (1 day postoperatively) or classical activation (within approximately 1 month). Electrode impedance was measured intraoperatively and at ≥ 6 months. Associations between age, gender, activation timing, and impedance were analyzed across apical, middle, basal, and overall regions.
Results
A total of 360 implanted ears were analyzed. Gender was not significantly associated with impedance. Implantation age demonstrated region‐specific associations with impedance behavior, with the strongest findings observed in activation‐stratified analyses of the apical electrodes. Mean apical impedance increased from 6.1 ± 2.1 kΩ intraoperatively to 6.9 ± 1.8 kΩ at 6 months. Younger recipients exhibited higher intraoperative apical impedance with a negative age–impedance association (β ≈ −0.01 to −0.02 per year, p < 0.001), which reversed at 6 months to a positive association (β ≈ +0.03 to +0.05 per year, p < 0.001). Age‐related associations were less pronounced in the middle and basal regions.
Conclusions
Implantation age demonstrated a dynamic and region‐specific association with impedance behavior, particularly in apical electrodes. Early activation modified postoperative impedance trajectories and attenuated midterm impedance elevation.
Level of Evidence
Level 3.
Keywords: age, cochlear implant, early activation, electrode impedance, gender, green cochlea
Implantation age is a key determinant of electrode impedance behavior, with a region‐specific effect most pronounced in the apical cochlea. Younger recipients exhibit higher intraoperative impedance followed by a time‐dependent reversal at 6 months. Early activation attenuates impedance elevation and promotes more stable postoperative impedance profiles.
1. Introduction
Cochlear implants (CIs) are among the most successful neural prostheses in contemporary medicine, providing effective auditory rehabilitation for individuals with severe‐to‐profound sensorineural hearing loss across a wide range of ages and clinical indications. Continuous advances in implant technology, electrode design, surgical techniques, and candidacy criteria have led to a steady expansion in CI utilization worldwide. Despite these developments, long‐term CI performance remains closely dependent on biological processes occurring within the cochlea following electrode insertion and chronic electrical stimulation [1].
Following implantation, the CI electrode array constitutes a foreign body within a biologically sensitive intracochlear environment. This presence initiates a cascade of electrochemical and tissue‐mediated responses, including protein adsorption at the electrode surface, alterations in the ionic composition of cochlear fluids, inflammatory cellular activity, fibrotic tissue proliferation, and, in some cases, new bone formation [2, 3]. These biological reactions collectively shape the electrode–tissue interface and play a central role in determining the efficiency and long‐term stability of electrical signal transmission within the cochlea [4].
Electrical impedance is a key electrophysiological parameter reflecting the functional status of this electrode–tissue interface. It represents the opposition to current flow from intracochlear electrode contacts into the surrounding cochlear environment [5]. Experimental models have demonstrated that protein adsorption and subsequent cellular growth at the electrode surface contribute differently to impedance behavior, with protein layers primarily increasing polarization impedance and cellular coverage predominantly increasing access resistance, highlighting impedance as a biologically meaningful marker of the electrode–tissue interface [6].
In routine clinical practice, impedance measurements are obtained intraoperatively to confirm electrode integrity and are serially monitored during follow‐up visits [7, 8]. These measurements guide programming decisions, facilitate early detection of abnormal electrode behavior, and help anticipate changes in stimulation requirements [9]. Persistently elevated impedance values are clinically relevant because they increase voltage demand, increase the risk of exceeding device compliance limits, and may adversely affect auditory performance and device energy consumption [10].
Importantly, persistently high impedance at 1 year post‐implantation was associated with poorer auditory and language development outcomes in a pediatric cohort [11].
Impedance typically follows a well‐described trajectory: it is lowest intraoperatively, rises at initial activation due to early inflammatory and fibrotic responses, and then gradually declines and stabilizes once continuous stimulation begins [11, 12, 13]. Several recipient‐related factors influence the magnitude and pattern of these changes, with age at implantation among the most consistently reported. Younger patients generally exhibit higher and more dynamic postoperative impedances, reflecting heightened inflammatory and reparative activity within the pediatric cochlea [14].
In contrast, the potential influence of gender on impedance has received little attention. Several anatomical studies have demonstrated that males have significantly longer cochlear duct lengths (CDL) than females across populations [15, 16, 17]. Whether these structural differences translate into measurable variation in impedance behavior remains unknown.
Another factor of growing interest is the activation mode. Emerging evidence suggests that early activation may attenuate the initial postoperative rise in impedance by limiting early protein deposition and soft‐tissue proliferation around the electrode, leading to a more stable electrode–tissue interface [13, 18, 19, 20, 21]. However, it remains unclear whether the activation strategy modifies the influence of demographic factors, such as age or gender, on impedance trajectories.
To address these gaps, the present study examines the effects of implantation age and gender on intraoperative and postoperative impedance across different cochlear regions, and whether early versus classical activation alters these associations.
2. Materials and Methods
2.1. Study Design and Setting
This retrospective longitudinal cohort study included cochlear implant recipients treated between 2015 and 2020 at a tertiary ear center. Institutional approval was obtained from the College of Medicine Institutional Review Board (Approval No. E‐25‐9549). Written informed consent for the use of clinical and research data was obtained at the time of implantation from all adult participants and from parents or legal guardians of pediatric participants. The study was conducted in accordance with institutional guidelines and the Declaration of Helsinki.
2.2. Participants
Eligible participants had normal cochlear anatomy as confirmed by preoperative CT and MRI. All underwent unilateral or bilateral cochlear implantation using standard non‐dexamethasone‐eluting FLEX28 lateral wall arrays (MED‐EL, Innsbruck, Austria). Electrode insertion was performed through the round window in all cases, with full insertion and normal intraoperative impedance profiles. Patients receiving steroids or exhibiting any inner ear malformation were excluded.
2.3. Variables and Outcome Measures
Demographic variables included age at implantation and gender. Surgical and device‐related characteristics—implantation date, surgical approach, implanted side, implant type, and array model—were recorded. Programming variables were collected from MAESTRO 11.0, including fitting date and follow‐up intervals. Based on the timing of processor activation, recipients were classified into early activation (EA; fitted 1 day postoperatively) or classical activation (CA; fitted within approximately 1 month). The primary outcome was electrode impedance (kΩ), measured intraoperatively and at ≥ 6‐month follow‐up. Impedance values were analyzed across all 12 channels and by region: apical (Channels 1–4), middle (Channels 5–8), and basal (Channels 9–12). The six‐month point was chosen because it was reported that electrode impedance stabilizes around this time [13].
2.4. Statistical Analysis
Descriptive statistics were reported as mean ± SD and range for continuous variables and as counts and percentages for categorical variables. Generalized estimating equations (GEEs) were used to evaluate the overall associations among age, gender, and impedance across time points. Because some participants contributed bilateral implanted ears, generalized estimating equations (GEEs) were used to account for within‐patient correlation. An exchangeable correlation structure was applied when appropriate; otherwise, an independence structure was used. Interaction terms were applied to assess effect modification by activation group. Simple linear regression analyses were performed separately for intraoperative and six‐month measurements.
All analyses were conducted for overall impedance and for each cochlear region (apical, middle, basal). The baseline comparisons between both groups of activations regarding patients' demographics were performed using a permutation t‐test for independent samples for continuous variables, while a chi‐squared test was applied for categorical ones. Statistical significance was defined as p ≤ 0.05. Analyses were performed using R statistical software (version 4.5.1).
3. Results
The study included 360 cochlear‐implanted (CI) ears; 56.1% were on the right side, and 43.9% on the left. Their average age at implantation was 7.2 ± 11.5 years. Males accounted for approximately 53.1% of participants, whereas females accounted for approximately 46.9%. All cases received the FLEX28 electrode array. Approximately 61.1% of the implants were activated using the classical method, and 38.9% were fitted early. Baseline demographic and surgical characteristics were generally comparable between activation groups, although bilateral implantation was more frequent in the early activation group (65.0% vs. 53.2%, p = 0.035). Impedance measurements were taken at two time points (Intraoperative and 6 months post‐surgery), with average apical impedances of 6.1 ± 2.1 kΩ and 6.9 ± 1.8 kΩ, respectively. The average middle impedance was 5.2 ± 2.4 kΩ and 5.3 ± 1.7 kΩ, while the basal impedance was 5.1 ± 2.5 kΩ and 5.2 ± 1.5 kΩ. The overall impedance was 5.6 ± 2.1 kΩ and 6.0 ± 2.0 kΩ at intra‐op and 6‐months post‐op, respectively, as shown in Table 1.
TABLE 1.
Descriptive analysis of patients' demographics and outcome measures.
| Patients' characteristics | Overall (N = 360) | Classical activation (n = 220) | Early activation (n = 140) | p |
|---|---|---|---|---|
| Age at implantation in years | ||||
| Mean (SD) | 7.2 (11.5) | 6.3 (11.0) | 8.5 (12.2) | 0.086 |
| Min—Max | 0.7–69.0 | 0.8–69.0 | 0.7–65.5 | |
| Gender | ||||
| Female | 169 (46.9%) | 106 (48.2%) | 63 (45.0%) | 0.630 |
| Male | 191 (53.1%) | 114 (51.8%) | 77 (55.0%) | |
| Side of the ear | ||||
| Left | 158 (43.9%) | 93 (42.3%) | 65 (46.4%) | 0.506 |
| Right | 202 (56.1%) | 127 (57.7%) | 75 (53.6%) | |
| Laterality | ||||
| Unilateral implanted | 152 (42.2%) | 103 (46.8%) | 49 (35.0%) | 0.035 |
| Bilateral implanted | 208 (57.8%) | 117 (53.2%) | 91 (65.0%) | |
| Baseline intraoperative overall impedance (kΩ) | ||||
| Mean (SD) | 5.6 (2.1) | 5.6 (2.0) | 5.8 (2.2) | 0.405 |
| Min—Max | 2.6–14.1 | 2.6–14.1 | 2.7–14.0 | |
| Impedance measures (kΩ) | Intra‐operative | 6‐months post‐operative |
|---|---|---|
| Apical impedance | ||
| Mean (SD) | 6.1 (2.1) | 6.9 (1.8) |
| Min—Max | 2.7–14.7 | 4.1–11.1 |
| Middle impedance | ||
| Mean (SD) | 5.2 (2.4) | 5.3 (1.7) |
| Min—Max | 2.2–13.5 | 3.8–10.6 |
| Basal impedance | ||
| Mean (SD) | 5.1 (2.5) | 5.2 (1.5) |
| Min—Max | 2.2–14.8 | 3.2–9.1 |
| Overall impedance | ||
| Mean (SD) | 5.6 (2.1) | 6.0 (2.0) |
| Min—Max | 2.6–14.1 | 3.8–13.9 |
Abbreviations: kΩ, kiloohms; SD, standard deviation.
3.1. Patients' Implantation Age and Gender With Impedance Measures
The age at implantation showed a non‐significant negative correlation with intraoperative apical impedance measures (Figure 1A), whereas a non‐significant positive correlation was observed at 6 months postoperatively. Gender was not significantly associated with apical impedance at any time point; however, males exhibited a 0.28 kΩ decrease intraoperatively and a 0.31 kΩ increase at 6 months postoperatively, compared to females (pvalues = 0.245 and 0.501, respectively), as shown in Figure 1B (Table S1).
FIGURE 1.
Association between impedance measures (intra‐op vs. 6‐months post‐op) and demographic characteristics. (A) with patients' age at implantation (years) at apical electrodes, (B) with patients' gender at apical electrodes, (C) with patients' age at implantation (years) at middle electrodes, (D) with patients' gender at middle electrodes, (E) with patients' age at implantation (years) at basal electrodes, (F) with patients' gender at basal electrodes, (G) with patients' age at implantation (years) across the 12 electrodes, and (H) with patients' gender across the 12 electrodes.
The middle impedance showed a non‐significant negative correlation with intraoperative implantation age (p = 0.149). However, no association was detected with age at implantation at 6 months postoperatively (p‐value = 0.914), as shown in Figure 1C. In contrast, males exhibited a significant intraoperative decrease of 0.73 kΩ (p = 0.008) and a non‐significant 6‐month postoperative reduction of 0.23 kΩ (p = 0.473), compared to females (Figure 1D).
For basal impedance, no correlations were observed intraoperatively or 6 months postoperatively with age at implantation, as shown in Figure 1E. In comparison, gender showed non‐significant associations with basal impedance, which decreased by 0.39 kΩ intraoperatively and 0.37 kΩ at 6 months postoperatively in males, compared with females (p values = 0.155 and 0.291, respectively), as shown in Figure 1F.
The overall impedance in Figure 1G showed a non‐significant negative correlation with the age at intraoperative implantation (p = 0.145), whereas a non‐significant correlation was observed 6 months postoperatively (p = 0.668). However, males showed non‐significant decreases of 0.41 kΩ and 0.24 kΩ intra‐op and 6 months post‐op, respectively (p values = 0.078 and 0.514, respectively), compared to females, as shown in Figure 1H.
The GEE interaction model for apical impedance indicated that, in both the classical and early‐activated groups, age at implantation was significantly associated with apical impedance, showing a negative relationship during surgery and a positive relationship at 6 months postoperatively (p < 0.001 for both groups), as depicted in Figure 2A. Additionally, classical‐activated males showed a non‐significant 0.11 kΩ increase during surgery and a non‐significant 0.20 kΩ decrease at 6 months postoperatively compared with classical‐activated females. Likewise, early activated males demonstrated a significant 0.84 kΩ decrease during surgery (p = 0.028) and a non‐significant 1.0 kΩ increase at 6 months post‐surgery compared with early‐activated females, as shown in Figure 2B.
FIGURE 2.
Relationship between impedance measures at apical electrodes (during surgery versus 6 months post‐surgery) and demographic characteristics based on the fitting group (classical vs. early activation), (A) with patients' age at implantation (years), and (B) with patients' gender.
For the middle impedance, Figure 3A showed that, in the classical activated group, age at implantation was significantly correlated with middle impedance, exhibiting a negative correlation intraoperatively and a positive one at 6 months postoperatively (p < 0.001). In the early‐activated group, significant negative correlations were observed either intraoperatively or at 6 months postoperatively (p < 0.001). Furthermore, classical activated males exhibited non‐significant decreases of 0.65 and 0.31 kΩ intraoperatively and at 6 months postoperatively, respectively, compared to classical activated females. Additionally, early activated males showed non‐significant decreases of 0.84 and 0.13 kΩ intraoperatively and at 6 months postoperatively, respectively, compared with early‐activated females, as shown in Figure 3B.
FIGURE 3.
Relationship between impedance measurements at middle electrodes (intra‐op versus 6 months post‐op) and demographic characteristics grouped by the fitting category (classical vs. early activation), (A) with patients' age at implantation (years), and (B) with patients' gender.
For basal impedance, Figure 4A showed that, in the classical activated group, age at implantation significantly correlated with basal impedance, displaying a negative correlation intraoperatively and a positive correlation at 6 months postoperatively (p < 0.001). In the early‐activated group, a significant negative correlation was observed at 6 months postoperatively (p < 0.001), whereas no association was observed intraoperatively. Meanwhile, classical activated males showed non‐significant decreases of 0.37 and 0.32 kΩ intraoperatively and at 6 months postoperatively, respectively, compared to classical activated females. Additionally, early activated males exhibited non‐significant decreases of 0.51 and 0.42 kΩ intraoperatively and at 6 months postoperatively, respectively, compared with early‐activated females, as shown in Figure 4B.
FIGURE 4.
Relationship between impedance measurements at basal electrodes (intra‐op vs. 6 months post‐op) and demographic characteristics categorized by the fitting group (classical vs. early activation), (A) with patients' age at implantation (years), and (B) with patients' gender.
Furthermore, the overall impedance in the classical activated group showed a significant negative correlation with age at the time of intraoperative implantation. In contrast, a significant positive correlation was observed at 6 months postoperatively (p < 0.001). In the early‐activated group, a significant negative correlation was found between age at implantation and intraoperative overall impedance (p < 0.001); however, no association was detected at 6 months postoperatively, as illustrated in Figure 5A. Additionally, the classical activated males exhibited non‐significant decreases of 0.09 and 0.28 kΩ intraoperatively and 6 months postoperatively, respectively, compared to classical activated females. Meanwhile, the early‐activated males showed a significant intraoperative decline of 0.91 kΩ (p = 0.020) and a non‐significant reduction of 0.19 kΩ at 6 months post‐op compared with early‐activated females, as shown in Figure 5B.
FIGURE 5.
Relationship between impedance measures across all 12 electrodes (intra‐op vs 6‐months post‐op) and demographic characteristics classified by the fitting group (classical vs. early activation), (A) with patients' age at implantation (years), and (B) with patients' gender.
4. Discussion
In this large cohort of cochlear implant recipients implanted with the FLEX28 electrode array, gender showed no consistent or clinically meaningful effect on electrode impedance, whereas implantation age demonstrated a clear, region‐specific association. Age‐related effects were most pronounced in the apical region, where younger recipients exhibited higher intraoperative impedance values (mean 6.1 ± 2.1 kΩ). In the interaction model, age at implantation was significantly associated with intraoperative apical impedance, with a negative association (β ≈ −0.01 to −0.02 per year; p < 0.001), suggesting that younger age is associated with higher early impedance levels at surgery. Notably, this relationship reversed at 6 months postoperatively, at which point apical impedance increased with advancing implantation age (mean 6.9 ± 1.8 kΩ; β ≈ +0.03 to +0.05 per year, p < 0.001), indicating a dynamic, time‐dependent effect of age on impedance behavior. These findings are consistent with previous work showing higher impedance values in children than in adults during the initial fitting period and the early postoperative weeks [22]. Similarly, higher impedance values have been reported in younger recipients compared with older patients, particularly in the middle and apical turns [14]. Furthermore, the apical segment's anatomical and microenvironmental characteristics—such as narrower intracochlear spaces, reduced perilymph volume, and a narrower scala tympani—may amplify the measurable impact of postoperative biological changes on impedance in this region [23, 24]. This interpretation is further supported by the expected deep insertion achieved with FLEX28 in normally formed cochleae—approximately 540° in an average‐sized cochlea [25].
When stratified by activation timing, both classical and early activation groups showed comparable directionality and strength of association between age at implantation and intraoperative impedance measures, as expected because intraoperative impedance precedes sustained electrical stimulation. At the six‐month postoperative time point, recipients activated on the first postoperative day exhibited an attenuated increase in impedance. This aligns with evidence from pediatric cohorts, in which early activation was associated with significantly lower impedance than controls during the first postoperative month [19]. In a more recent, larger series with greater age diversity, early activation was also associated with lower impedance across all electrode array sections at 1 month, 1 year, and 2 years post‐implantation [18]. The reduction in impedance following electrical stimulation is a well‐established phenomenon [12, 22] and similar patterns have been observed with DEX‐eluting arrays [4, 26], supporting the hypothesis that prompt stimulation may suppress soft‐tissue changes around the electrode array and thereby moderate impedance elevation. Although this study focused on FLEX28 lateral wall electrodes, impedance‐lowering effects of early activation have also been reported with other electrode designs, including perimodiolar arrays, with the greatest reductions described in apical and middle regions during the 3–6‐month postoperative period [27]. Because intraoperative impedance precedes electrical stimulation, the observed age‐related differences are unlikely to reflect an inherent, fixed property of age itself; rather, the stimulation‐induced suppression of fibrous tissue growth appears to interact with age, attenuating age‐related impedance differences in the early‐activated group while allowing them to persist in the classical‐activation group. This biological interaction explains why a significant age effect emerges only once the cohort is stratified by activation timing.
Collectively, these observations support the role of activation mode as a modifier of impedance behavior over time, particularly in apical cochlear regions. Beyond its biological impact, early activation may also reduce healthcare resource utilization, as early activation has been shown to decrease hospital visits and patient travel, thereby lowering overall system burden [28]. In the context of increasing emphasis on sustainable healthcare delivery, such streamlined care pathways align with the emerging Green Cochlea concept [29], highlighting the potential for clinically effective strategies to also support more resource‐efficient cochlear implant care.
Gender‐related analyses demonstrated largely non‐significant associations across cochlear regions and both time points. While males typically have slightly longer cochlear duct lengths than females across multiple populations [15, 16, 17], this anatomical variation did not translate into robust impedance differences in the present cohort, suggesting that postoperative tissue response and stimulation history are more influential determinants of impedance evolution than sex‐based structural differences. Nevertheless, small effect sizes observed in select comparisons indicate that subtle sex‐related influences cannot be fully excluded and may warrant further evaluation in larger datasets and with more frequent telemetry assessment.
Overall, these findings indicate that implantation age demonstrates a dynamic and region‐specific association with impedance behavior, particularly at apical electrodes, while activation timing appears to influence midterm impedance trajectories. Clinically, this may assist in interpreting impedance patterns across age groups and in informing expectations during postoperative follow‐up, particularly for younger recipients.
This study has some limitations. Its retrospective single‐center design may limit generalizability. In addition, activation allocation was not randomized, and impedance was evaluated at only two time points—intraoperatively and at 6 months postoperatively—limiting characterization of intermediate and long‐term impedance trajectories. Audiological outcome measures were also not included to correlate impedance behavior with clinical performance.
Future work should extend follow‐up beyond 6 months to determine whether the observed age‐dependent apical patterns persist or converge over time. Multicenter datasets with broader demographic representation, more frequent impedance monitoring, and comparisons with other impedance‐reducing strategies (e.g., steroid‐eluting electrodes) may further clarify mechanisms underlying impedance variability and guide optimization of postoperative activation protocols.
5. Conclusion
The findings of this study revealed that electrode impedance is mainly affected by implantation age rather than gender, with the apical region showing the greatest age‐related variability. Younger recipients showed higher intraoperative impedance values, which reversed at midterm follow‐up. Early activation helped reduce age‐related impedance changes and lower the midterm increase in impedance.
Funding
The authors have nothing to report.
Disclosure
Claude (Anthropic, Claude Sonnet 4.6) was used to assist with language editing and manuscript preparation. The authors reviewed, verified, and take full responsibility for all content in this manuscript.
Conflicts of Interest
The authors declare no conflicts of interest.
Supporting information
Table S1: Relationships between patients' implantation age and gender with electrode impedance measures during surgery versus 6 months postoperatively, presented overall and stratified by activation category.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Table S1: Relationships between patients' implantation age and gender with electrode impedance measures during surgery versus 6 months postoperatively, presented overall and stratified by activation category.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.