The Role of Cochlear Anatomy in Hearing Preservation After Cochlear Implantation in Adults: A Systematic Review

ABSTRACT

Objective

To evaluate the association between cochlear anatomical parameters and postoperative hearing preservation (HP) following cochlear implantation in adults with preoperative residual hearing (RH).

Methods

This systematic review followed PRISMA guidelines (PROSPERO: CRD420250630915). Embase, MEDLINE, PubMed, Scopus, and Web of Science were searched for studies published between 2010 and 2026. Eligible studies included adults (≥ 18 years) with preoperative RH undergoing cochlear implantation, assessed cochlear anatomy using computed tomography or magnetic resonance imaging, reported HP within 6 months postoperatively, and examined the relationship between cochlear anatomy parameter(s) and HP. Risk of bias was evaluated using the QUIPS tool. Data were synthesized narratively; where possible, a meta‐analysis using a random‐effects model was performed. Certainty of evidence was assessed using GRADE.

Results

Ten studies (487 implants) were included. Risk of bias was highest for study participation and confounding. Longer cochlear duct length and larger cochlear volume were suggested to be associated with better HP, though findings were inconsistent. Six studies examined angular insertion depth; four were pooled. Meta‐analysis indicated a small positive correlation (pooled r = 0.23; p = 0.004; 95% confidence interval: −0.14 to 0.54), suggesting poorer HP with deeper insertion, with moderate heterogeneity (I ² = 48.5%). Certainty of evidence for all outcomes was rated very low.

Conclusion

Cochlear anatomy may influence hearing preservation following cochlear implantation, with longer cochlear duct length and larger cochlear volume showing the most consistent associations with favorable outcomes. However, the certainty of evidence was very low, precluding firm conclusions regarding the predictive value of anatomical factors.

Level of Evidence

3.

Cochlear anatomy may influence hearing preservation following cochlear implantation in adults with residual hearing. Longer cochlear duct length and larger cochlear volume showed the most consistent associations with favorable hearing preservation, whereas deeper electrode insertion may be associated with poorer outcomes. However, the certainty of evidence was very low, limiting conclusions regarding the predictive value of anatomical factors.

1. Introduction

Cochlear implants (CI) are widely used to restore functional hearing in individuals with severe‐to‐profound sensorineural hearing loss by bypassing the dysfunctional Organ of Corti and directly stimulating the spiral ganglion neurons [1]. Continuous advancements in device technology and surgical techniques have led to improved postoperative outcomes [2, 3, 4]. In recent years, candidacy criteria have shifted from patients with profound hearing loss to those retaining greater degrees of preoperative residual hearing (RH), particularly in the low‐frequency range [5, 6, 7, 8]. These candidates are presumed to have a higher proportion of functional sensory cells compared to individuals without RH prior to implantation [9]. This trend has introduced hearing preservation (HP) as a secondary objective in cochlear implantation [10, 11]. HP refers to both structural and functional preservation of intracochlear integrity and is associated with enhanced speech perception, improved music appreciation, and superior sound quality due to additional low‐frequency auditory cues and increased spectral information [9, 10, 12].

Unfortunately, postoperative loss of RH is a well‐documented phenomenon. It may occur immediately after surgery, primarily due to acute insertion trauma and subsequent inflammatory responses, which can lead to sensory cell degeneration resulting in cell death and subsequent degeneration of the neural elements [13]. In addition, delayed loss of RH has been reported and is hypothesized to result from chronic inflammatory reactions, progressive fibrosis, or neo‐ossification [6, 8, 13, 14, 15, 16]. Both initial and long‐term HP rates remain highly variable [17, 18, 19], underscoring the multifactorial nature of postoperative HP outcomes.

The likelihood of postoperative RH loss is influenced by multiple factors, with individual cochlear morphology emerging as a key determinant alongside surgical approach and electrode array design [17]. Cochlear anatomy shows substantial interindividual variability, and the literature employs diverse anatomical parameters and measurement approaches to obtain detailed quantitative information [14, 19, 20, 21]. This variability is evident in both overall cochlear dimensions, such as basal diameters A and B, cochlear height H [22, 23, 24, 25], and intracochlear microanatomy, including cochlear duct length (CDL) and the dimensions of the organ of Corti [5, 18, 21, 25]. Such anatomical variability may partly account for the wide range of postoperative HP outcomes, yet the relative contribution of specific factors remains unclear.

A comprehensive understanding of which anatomical parameter(s) influence HP is essential for individualized surgical planning, optimized electrode insertion strategies, and the refinement of electrode array designs to accommodate anatomical variability and minimize insertion trauma [8, 21, 25, 26, 27]. In addition, such insights may also support the development of prognostic models for cochlear implantation that aim to reduce the risk of RH loss and improve both HP and speech perception outcomes. Despite increasing interest in this area, current evidence is fragmented, and no consensus exists regarding which anatomical determinants are most predictive. Therefore, a systematic synthesis of the available literature is warranted. This review aims to critically evaluate and integrate current evidence on the influence of cochlear anatomy on HP in adults undergoing cochlear implantation and, where possible, provide quantitative effect estimates. By identifying key anatomical predictors and clarifying their clinical relevance, this work seeks to inform evidence‐based surgical decision‐making and advance personalized cochlear implantation strategies.

2. Materials and Methods

2.1. Search Strategy and Study Selection

A systematic search was conducted in Embase (via Embase.com), MEDLINE (via Ovid), PubMed, Scopus (via Scopus.com), and Web of Science Core Collection (via Clarivate) from November 2024 until August 2026 to identify retrospective and prospective studies evaluating associations between cochlear anatomical parameters and HP in adult CI recipients with preoperative RH. Search terms included cochlear anatomy, cochlear implantation, and hearing preservation, supplemented with synonyms and related keywords (see Text document, Supplemental Digital Content 1 for full search string of all databases). Only English‐language articles published between 2010 and 2026 were considered.

Title and abstract screening were performed by one reviewer, with a second reviewer independently screening 50% of records. No discrepancies were identified. Full texts were screened independently by two reviewers. Reference lists of the included studies were examined using forward and backward snowballing.

The eligibility criteria were defined according to the PECOTS framework (Table 1). Studies were eligible if they included adults (≥ 18 years) undergoing cochlear implantation and examined at least one cochlear anatomical parameter in relation to HP. Accepted HP definitions included absolute threshold shift and the HEARRING Group formula [28]. Because this review focused on acute RH loss, only HP outcomes assessed within 6 months were extracted. Studies with longer follow‐up were included only if early postoperative data were available. Exclusion criteria were case reports, reviews, editorials, letters, conference abstracts, clinical protocols, animal studies, and cadaveric investigations.

TABLE 1.

Population, Exposure, Comparison, Outcomes, Study design (PECOTS) framework of the study.

PECOTS element	Specification
P—Population	Adults (≥ 18 years) with postlingual hearing loss and preoperative residual hearing undergoing cochlear implantation.
E—Exposure	Preoperative and/or perioperative radiological measures of cochlear anatomy potentially associated with hearing preservation (e.g., cochlear duct length).
C—Comparator	Participants with differing anatomical characteristics or radiological measurements, where applicable.
O—Outcome	Postoperative hearing preservation after cochlear implantation, reported as follows: −Absolute hearing threshold shift (dB HL). −Relative hearing preservation (%) according to the HEARRING Group formula.
T—Timing	Postoperative hearing assessment at short‐term follow‐up interval (up to 6 months after implantation).
S—Study design	Retro‐ and prospective cohort studies.

2.2. Risk of Bias Assessment

Risk of bias was independently assessed by two reviewers using the Quality In Prognosis Studies (QUIPS) tool, as recommended by the Cochrane Prognosis Methods Group. QUIPS evaluates six domains: study participation, study attrition, prognostic factor measurement, outcome measurement, study confounding, and statistical analysis/reporting. Each domain was rated as low, moderate, or high risk of bias. Confounders considered relevant for HP included hearing loss etiology, cochlear health, surgical approach, and electrode array design. Domain‐level ratings were summarized graphically using Microsoft Excel (version 2607).

2.3. Data Extraction

Data was extracted by one reviewer using a standardized Excel form. Extracted variables included:

1. Study characteristics: title, year, first author, design, study aim.

2. Population: number of implants, etiology of hearing loss, age at implantation, sex distribution, definition of preoperative RH, inclusion/exclusion criteria, post‐lingual deafness (yes/no).

3. Intervention: manufacturer, electrode array, surgical approach, imaging modality and timing, measurement/calculation methods, and administration of medication.

4. Outcomes: cochlear anatomical parameters (definition, values), and HP measures (definition, method, timing).

5. Statistics: analytic methods, effect estimates, and confounders collected/adjusted.

6. Author's conclusions.

Corresponding authors were contacted to obtain or confirm missing or unclear data.

2.4. Data Synthesis

Study characteristics and effect estimates were systematically summarized and presented in structured tables. To determine which studies were eligible for each synthesis, included studies were grouped based on the type of reported anatomical parameter, HP outcome, and statistical measure, with only those reporting correlations between AID and HP as absolute threshold shift contributing to the quantitative synthesis.

Due to substantial clinical and methodological heterogeneity among the included studies, narrative synthesis was the primary approach. For the subset of studies reporting Pearson correlation coefficients (r) between AID and HP as an absolute measure, a meta‐analysis was conducted using Meta‐Essentials (correlational module). A random‐effects model was selected to account for between‐study variability and expected heterogeneity. Individual study results were visually summarized using a forest plot.

Pooled effect sizes were reported with 95% confidence intervals and 95% prediction intervals. Statistical heterogeneity was quantified using the I ² statistic. No subgroup analyses or meta‐regressions were performed due to the limited number of comparable studies, and sensitivity analyses were not feasible for the same reason. Owing to the limited number of studies contributing to the quantitative synthesis, formal assessment of reporting bias was not appropriate.

2.5. Certainty of Evidence

The certainty of evidence for each outcome was assessed independently by two reviewers using the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) framework [29]. Certainty was graded as high, moderate, low, or very low based on five domains: risk of bias, inconsistency, indirectness, imprecision, and publication bias. Disagreements between reviewers were resolved by discussion, with involvement of a third reviewer when necessary.

3. Results

3.1. Study Selection

The database search yielded 469 records. After removal of 307 duplicates, 162 records remained for title and abstract screening. Twenty‐three full texts were assessed for eligibility, of which 10 met all inclusion criteria [30, 31, 32, 33, 34, 35, 36, 37, 38, 39]. Forward and backward citation screening based on these 10 studies identified nine additional records, all of which were excluded after full‐text evaluation. In total, 10 studies were included in this systematic review.

A PRISMA 2020 flow diagram summarizing the study selection process is presented in Figure 1.

FIGURE 1.

Flow diagram of the study selection according to 2020 PRISMA guidelines.

3.2. Study Characteristics

Characteristics of the 10 included studies are summarized in Table 2. Publication years ranged from 2016 to 2026. Eight studies used a retrospective design, and two appeared prospective based on data collection methods. Sample sizes ranged from 14 to 166 CIs.

TABLE 2.

Study characteristics of all 10 included studies.

Authors	Study design	Implants	Electrode arrays	Surgical approach	Imaging technique	Pharmacological intervention	Hearing preservation assessment
Canfarotta et al. [30]	Retrospective	45	FLEX24, FLEX28, FLEXSoft	RW	CB‐CT (pre‐ and postoperative)	Dexamethasone (intraoperative) and methylprednisolone (postoperative)	LFPTA shift (125, 250, and 500 Hz) at 1 and 6 months
Hollis et al. [31]	Retrospective	19	FLEXSoft	RW	X‐ray (intraoperative) or CT (postoperative)	Dexamethasone (intraoperative)	LFPTA shift and individual frequencies (125, 250, and 500 Hz) at 2 months and “most recent”
Nassiri et al. [32]	Retrospective	24	CI532	NR	CB‐CT (intraoperative)	NR	LFPTA shift (125, 250, and 500 Hz) at activation, 6 and 12 months
Nordfalk et al. [33]	NR	39	FLEX24, FLEX28, FLEXSoft	RW	CB‐CT (intraoperative)	Dexamethasone (preoperative) and methylprednisolone (intraoperative)	LFPTA shift and individual frequencies (125, 250, and 500 Hz) at 4–6 weeks
O'Connell et al. [34]	Retrospective	27	FLEX24, FLEX28, FLEXSoft	NR	CT (pre‐ and postoperative)	NR	LFPTA shift (125, 250, and 500 Hz) at activation
Räth et al. [35]	Retrospective	166	FLEX16, FLEX20, FLEX24, FLEX26, FLEX28, FLEXSoft	RW	CB‐CT (preoperative)	NR	LFPTA shift (125, 250, 500, 1000, and 1500 Hz) at 6 months
Skarzynski et al. [36]	Retrospective	54	CI422	RW	CB‐CT (pre‐ and intraoperative)	Dexamethasone (postoperative)	Relative hearing preservation (%) at activation, 12 and 24 months
Takahashi et al. [37]	Retrospective	46	FLEX28, FLEXSoft	RW	MRI and CT (preoperative)	Dexamethasone (intraoperative)	LFPTA shift (125, 250, and 500 Hz) at 3 months
Yoshimura et al. [38]	NR	44	FLEX24, FLEX28, FLEXSoft	RW	CT (preoperative)	“Steroids” (intra‐ and postoperative)	Relative hearing preservation (%) at 6 months
Dzan et al. [39]	Retrospective	14	SlimJ	RW	CB‐CT (pre‐and postoperative)	Dexamethasone (intraoperative) and methylprednisolone (postoperative)	LFPTA shift (125, 250, and 500 Hz) at 2–4 weeks and 6 months

Abbreviations: CB‐CT = cone beam computed tomography, LFPTA = low‐frequency pure‐tone average, MRI = magnetic resonance imaging, NR = not reported, RW = round window.

Across studies, nine electrode arrays were reported (FLEX24, FLEX28, FLEXSoft, FLEX26, FLEX20, FLEX16, CI422, CI532, and SlimJ) from three manufacturers. A round window approach was described in eight studies; the remaining two did not specify the surgical technique.

Preoperative radiologic imaging was performed in eight studies; four of these additionally used postoperative imaging, and four used intraoperative imaging. Imaging modalities included cone‐beam computed tomography (CB‐CT; n = 6, two using flat‐panel CT), conventional CT (n = 4), magnetic resonance imaging (MRI) for cochlear volume assessment (n = 1), and x‐ray (n = 2).

Seven studies reported perioperative pharmacological interventions. Dexamethasone was the most frequently used corticosteroid (n = 6), followed by methylprednisolone (n = 3). Timing of administration varied: preoperative (n = 1), intraoperative (n = 6), and postoperative (n = 4). HP outcomes were assessed at 1–3 months in eight studies and at 6 months in five studies. One evaluated HP “at activation” without specifying timing. Two studies extended follow‐up beyond 12 months, including one reporting outcomes at 24 months.

3.3. Risk of Bias

Risk of bias assessment using the QUIPS tool is summarized in Figure 2. Overall, risk of bias varied considerably across studies and domains. Study participation showed important concern, with all studies rated as moderate or high risk, reflecting limited representativeness of included populations. Study attrition demonstrated moderate concerns in several studies due to incomplete or insufficiently reported follow‐up. For prognostic factor measurement and outcome measurement, most studies were rated as low to moderate risk. Study confounding presented the most consistent methodological weakness: a large proportion of studies were rated high risk because key confounders were incompletely measured or not statistically adjusted. Statistical analysis and reporting were generally low to moderate risk. Overall, selection bias and inadequate control of confounding emerged as the most critical sources of potential bias across the included studies.

FIGURE 2.

Risk of bias assessment of included studies using the Quality‐in‐Prognostic‐Studies (QUIPS) tool. Risk of bias is presented as percentages across all included studies.

3.4. Hearing Preservation Measures

Eight studies assessed HP using the absolute threshold shift of the low‐frequency pure‐tone average (LFPTA). Seven calculated LFPTA based on thresholds at 125, 250, and 500 Hz, while one additionally included 1000 and 1500 Hz. Two studies also reported frequency‐specific threshold shifts. Two studies applied the HEARRING group formula to categorize HP as complete, partial, minimal, or loss. Measurement definitions, calculation approaches, and follow‐up intervals for all studies are summarized in Table 3.

TABLE 3.

Hearing preservation (HP) outcomes of the 10 included studies.

Authors	Preoperative residual hearing	Outcome measure	Outcome definition	Hearing preservation outcome
Canfarotta et al. [30]	Unaided HT ≤ 45 dB HL at 250 Hz	Absolute HP at 1 and 6 months	Functional hearing ≤ 80 dB HL for 250 Hz	34.1 dB HL ±17.9 33.5 dB HL ±17.8
Hollis et al. [31]	Unaided HT ≤ 65 dB HL at 125 Hz	Absolute HP at 2 months	Functional hearing ≤ 80 dB HL	27.6 dB HL ±12.8
Nassiri et al. [32]	Unaided LFPTA 125, 250, and 500 Hz < 80 dB	Absolute HP at 6 months	Functional hearing LFPTA ≤ 80 dB HL	21.8 dB HL ± NR
Nordfalk et al. [33]	Unaided HT ≤ 70 dB HL at 125 and 250 Hz and 90 dB HL at 500 Hz	Absolute HP at 4–6 weeks	NR	27 dB HL ±6 (FLEX24) 23 dB HL ±17 (FLEX28) 23 dB HL ±14 (FLEXSoft)
O'Connell et al. [34]	Unaided HT ≤ 80 dB at 250 Hz	Absolute HP at activation	Functional hearing ≤ 80 dB HL at 250 Hz	27 dB HL ±14
Räth et al. [35]	Measurable hearing between 125 and 1500 Hz (not further specified)	Absolute HP at 6 months	NR	21.9 dB HL ±12.8
Skarzynski et al. [36]	NR	Relative HP at 1 month	HEARRING Group classification system	75% complete HP
Takahashi et al. [37]	Unaided HT at 125, 250, and 500 Hz (criteria NR)	Absolute HP at 3 months	TS < 10 dB	9.5 dB HL ± NR
Yoshimura et al. [38]	Unaided LFPTA 125, 250, and 500 Hz < 80 dB	Relative HP at 6 months	HEARRING Group classification system	40% complete HP 38% partial HP 16% minimal HP 6% no measurable hearing
Dzan et al. [39]	Unaided HT ≤ 45 dB HL at 250 Hz	Absolute HP at 2 to 4 weeks and 6 months	Functional hearing ≤ 80 dB HL for 250 Hz	21 dB HL ± NR 22 dB HL ± NR

Note: ± = standard deviation. HEARRING Group classification system = complete HP (> 75%), partial HP (25%–75%), minimal HP (0%–25%), and no measurable hearing (< 0%).

Abbreviations: HP = hearing preservation, HT = hearing threshold, LFPTA = low‐frequency pure‐tone average, NR = not reported, TS = threshold shift.

3.5. Anatomical Predictors

An overview of all effect estimates for the 10 included studies per anatomical cochlear parameter is given in Table 4.

TABLE 4.

Effect estimates of included studies per anatomical parameter.

Anatomical parameter	Authors (n)	Statistical analysis	Effect estimate	p	95% Confidence interval	Adjusted for confounding	Author's conclusion
CDL (mm)	Canfarotta et al. [30] (45)	Pearson correlation	r = −0.32	0.031*	NR	No	Better HP with longer CDL.
		Pearson correlation	r = −0.46	0.009**	NR	No
		Linear mixed‐effects model	β = −1.78	0.367	NR	Yes	No significant main effect of CDL on LFPTA shift.
	Hollis et al. [31] (19)	Pearson correlation	r = −0.64	0.004**	NR	Yes	Better HP with longer CDL.
	Yoshimura et al. [38] (44)	Pearson correlation	r = −0.02	0.885	NR	No	No significant correlation.
V (mm3)	Canfarotta et al. [30] (45)	Pearson correlation	r = −0.33	0.028*	NR	No	Better HP with larger volumes.
		Pearson correlation	r = −0.46	0.007**	NR	No
		Linear mixed‐effects model	β = −1.56	0.044*	NR	Yes	Significant main effect of scala tympani volume on LFPTA shift, with better HP for CI recipients with larger volumes.
	Räth et al. [35] (166)	Pearson correlation	r = −0.280 to 0.041	0.133–0.663	NR	No	No significant correlation.
	Takahashi et al. [37] (46)	Multivariate logistic regression	OR = 12.2	0.009**	1.84–80.8	Yes	Cochlear volume ≥ 71 mm3 was a significant predictor of HP.
	Dzan et al. [39] (14)	Pearson correlation	R = 0.28	0.050	NR	No	Better HP with larger volumes.
AID (°)	Canfarotta et al. [30] (45)	Pearson correlation	r = −0.05	0.735	NR	No	No significant correlation.
		Pearson correlation	r = −0.23	0.208	NR	No
		Linear mixed‐effects model	β = 0.01	0.700	NR	Yes	No significant main effect of AID on LFPTA shift.
	Hollis et al. [31] (19)	Pearson correlation	r = 0.4	0.089	NR	No	No significant correlation.
	Nassiri et al. [32] (24)	Multivariate linear regression	R 2 = 0.18	0.24	NR	Yes	No significant correlation.
	Nordfalk et al. [33] (39)	Pearson correlation	r = 0.26	0.160	NR	No	No significant correlation.
	O'Connell et al. [34] (27)	Pearson correlation	r = 0.41	0.04*	NR	No	Worse HP with deeper AID.
	Skarzynski et al. [36] (54)	Pearson correlation	r = −0.23	0.100	NR	No	No significant correlation.

Abbreviations: AID = angular insertion depth, CDL = cochlear duct length, HP = hearing preservation, LFPTA = low‐frequency pure‐tone average, NR = not reported, V = volume.

^* p < 0.05.

^** p < 0.01.

3.5.1. Cochlear Duct Length (CDL)

Three studies evaluated CDL in relation to HP, with inconsistent findings. One reported no significant correlation between CDL and absolute threshold shifts [38]. In contrast, two others identified a negative correlation with LFPTA shift at activation (r = −0.32; p = 0.031) [30], at 2 months (r = −0.64; p = 0.004) [31], and at 6 months (r = −0.46; p = 0.009) [30, 31]. Overall, evidence suggests that longer CDL may be associated with smaller LFPTA shifts (i.e., better HP).

3.5.2. Cochlear Volume

Four studies examined cochlear volume, with mixed results. Three studies assessed total cochlear volume, and one investigated scala tympani (ST) volume. Two studies reported an association between larger cochlear volume and better HP outcomes. One study identified cochlear volume as a significant predictor of postoperative HP (OR = 12.2; 95% confidence interval: 1.84–80.8; p = 0.009) [37], while another reported significant correlations between cochlear volume and HP at activation (r = 0.28, p = 0.050) and at 6 months postoperatively (r = 0.29, p = 0.048) [39]. Similarly, the study examining ST volume found significant negative correlations between ST volume and LFPTA shifts both at activation (r = −0.33, p = 0.028) and at 6 months (r = −0.48, p = 0.007) [30]. In contrast, the remaining study reported no significant correlation between cochlear volume and HP outcomes [35]. Collectively, these findings suggest a potential association between larger cochlear or ST volume and better HP.

3.5.3. Angular Insertion Depth (AID)

Six studies evaluated the relationship between AID and HP, yielding mixed results. One study found no overall correlation but identified limited predictive value for threshold shift at 250 Hz (R ² = 0.20; p = 0.020) [33]. Another reported a significant positive correlation (r = 0.41; p = 0.040), indicating that deeper insertions were associated with larger LFPTA shifts (i.e., poorer HP) [34]. However, most studies reported no significant correlation [30, 31, 32, 36].

3.5.4. Other Anatomical Parameters

Two studies examined additional morphological predictors. One study evaluated electrode‐to‐modiolus distance and found no significant correlation with HP [32]. Another assessed multiple dimensional parameters, including basal diameter (A), cochlear width (B), height (H), basal duct length (BTLLW), total duct length along the lateral wall, and cross‐sectional area at the round window. Significant negative correlations were found between LFPTA shift and both B and BTLLW [35].

3.6. Meta‐Analysis of AID and HP

Of the six studies evaluating the association between AID and HP, four were included in the meta‐analysis [30, 31, 33, 34]. Two studies were excluded from pooling for methodological differences. One included patients who received a perimodiolar electrode instead of the straight arrays used in the other studies [32], while another assessed HP using the HEARRING Group formula rather than absolute threshold shift [36]. Individual and pooled effect estimates are shown in Figure 3.

FIGURE 3.

Forest plot of pooled effect estimates for AID on HP as an absolute measure (i.e., threshold shift). Each horizontal line represents the 95% confidence interval for an individual study, with the gray circles indicating the point estimate and its size proportional to the study weight in the meta‐analysis. The last circle represents the overall pooled estimate, its 95% confidence interval (bold line) and prediction interval (gray line), reflecting the expected range of effects in a new study. The vertical line indicates the line of no effect. Estimates were calculated using a random‐effects model. Heterogeneity was assessed using the I² statistic.

Using a random‐effects model, the pooled correlation coefficient was r = 0.23 (95% confidence interval: −0.14 to 0.54). Although the confidence interval included zero, the overall test for effect was statistically significant (z = 2.02; p = 0.044, two‐tailed). The 95% prediction interval (−0.39 to 0.71) indicated that the true effect in a future comparable study could vary widely in magnitude and direction. Moderate heterogeneity was observed (Q = 5.82; p = 0.121; I² = 48.47%), with between‐study variance estimated at τ ² = 0.03. Variability in RH definition and timing of HP assessment likely contributed to the observed heterogeneity.

Overall, the pooled estimate suggests a modest positive association between AID and LFPTA shift, indicating worse HP with deeper insertions. However, the wide confidence and prediction intervals, moderate heterogeneity, and limited number of studies substantially limit confidence in this finding. Consequently, the observed association should be interpreted cautiously.

3.7. Certainty of Evidence

The certainty of evidence for all outcomes was rated as very low according to the GRADE approach (see Table S1, Supplemental Digital Content 2 for summary of findings and certainty of evidence). The associations between CDL and HP, and between cochlear volume and HP were downgraded due to serious concerns regarding risk of bias, indirectness, and imprecision. The certainty of evidence for the association between AID and HP was also rated as very low, primarily due to risk of bias, imprecision, and inconsistency across studies. Overall, these ratings indicate substantial uncertainty regarding the true magnitude and direction of the reported associations.

4. Discussion

4.1. Summary of Evidence

This systematic review synthesized evidence from 10 studies assessing the influence of cochlear anatomy on HP in adults undergoing cochlear implantation. While several anatomical characteristics appeared to be associated with HP, findings were inconsistent across studies. AID was the only parameter suitable for meta‐analysis, yielding a small pooled positive correlation. However, moderate statistical heterogeneity and a wide prediction interval indicate substantial uncertainty regarding the true effect. Furthermore, the QUIPS assessment revealed important methodological limitations across studies, particularly relating to selection bias and inadequate adjustment for confounding factors. Consequently, the overall certainty of evidence was rated as very low across all outcomes according to GRADE.

4.2. Interpretation of Anatomical Predictors

4.2.1. CDL

The evidence regarding CDL was inconsistent but suggested a potential protective role. Studies reporting longer CDL associated with smaller LFPTA shifts align with the theoretical assumption that individuals with longer CDL may experience comparatively less effective insertion depth for a fixed‐length array, reducing the likelihood of intracochlear trauma apically [20]. Nonetheless, conflicting findings across studies and variability in CDL measurement protocols limit firm conclusions.

4.2.2. Cochlear Volume

Findings for ST and overall cochlear volume were similarly mixed. Studies reporting beneficial associations may reflect the biomechanical advantage of larger inner ear structures, which can accommodate electrodes with greater spatial tolerance and potentially reduce contact pressure or trauma [21]. However, the lack of consistent volumetric definitions complicates cross‐study comparisons. Divergent imaging and measurement techniques further amplify this variability. These factors underscore the need for standardized volumetric measurement protocols.

4.2.3. AID

Although deeper insertion is biomechanically plausible as a risk factor for trauma and subsequent loss of RH, empirical findings were inconsistent. Most studies reported no association between AID and HP, suggesting that insertion depth alone may be an insufficient predictor when considered independently. The small, pooled effect derived from the meta‐analysis should be interpreted cautiously given the wide confidence and prediction interval and statistical heterogeneity.

4.3. Methodological Considerations

Substantial methodological variation contributed to the observed heterogeneity and low certainty of evidence across studies. Definitions of preoperative RH varied widely, resulting in non‐comparable baseline populations and complicating interpretation of postoperative HP. Outcome measures as well as assessment timing also differed: some studies quantified absolute LFPTA shifts, others categorized HP using the HEARRING Group formula, and time interval ranged from activation to 6 months. Differences in imaging modality, anatomical definitions, electrode design, and perioperative steroid administration introduced additional variability. These inconsistencies hindered data pooling and likely attenuated detectable relationships with anatomical factors.

Statistical approaches also differed markedly. Several studies relied solely on bivariate correlations, which do not adjust for confounding variables such as electrode array and other surgery‐related factors. Multivariable models were used less frequently, but when implemented, results often diverged from bivariate findings, highlighting the importance of accounting for potential confounders.

Collectively, these sources of heterogeneity substantially limit the ability to draw firm conclusions regarding the independent contribution of cochlear anatomy to HP. Although some anatomical parameters demonstrated potentially favorable associations, it remains unclear whether these findings reflect true biological effects or residual confounding arising from differences in patient characteristics, electrode selection, and perioperative management.

4.4. Strengths and Limitations

Strengths of this review include a comprehensive search across multiple databases, independent screening, and use of established tools for risk of bias assessment and certainty appraisal. The review incorporated narrative synthesis, and meta‐analysis was conducted where feasible. To the best of our knowledge, this is the first systematic review investigating the relevance of cochlear anatomy for HP following cochlear implantation.

However, several limitations must be acknowledged. First, most included studies were retrospective observational studies, and several were judged to have moderate to high risk of bias. These limitations reduce confidence in the observed associations and preclude causal inference. Second, substantial heterogeneity across the included studies limited the ability to synthesize data quantitatively. Third, the small number of eligible studies for meta‐analysis constrained analytic power and precluded subgroup and sensitivity analyses. Furthermore, publication bias could not be formally assessed. Finally, the very low certainty of evidence across all outcomes restricts the strength of conclusions.

4.5. Clinical Implications and Future Research

From a clinical perspective, the currently available evidence is insufficient to support the routine use of cochlear anatomical parameters as standalone predictors for HP or to guide individualized electrode selection strategies. Surgeons should therefore continue employing atraumatic techniques while considering cochlear anatomy as one of multiple factors influencing HP.

Future studies should prioritize standardization of preoperative RH and HP outcome definitions. Larger, prospective studies and appropriately powered multivariable models are needed to clarify the independent contribution of anatomical parameters. Consistent methodologies will facilitate more reliable comparisons, enable robust meta‐analyses, and support development of predictive models for individualized cochlear implantation strategies.

5. Conclusion

This systematic review suggests that cochlear anatomy may influence HP following cochlear implantation. Longer CDL and larger cochlear volume showed the greatest potential as predictors of favorable hearing preservation outcomes, while evidence regarding AID remained inconclusive. However, due to very low certainty of evidence across the individual studies, no anatomical parameter can currently be considered a robust predictor of hearing preservation. The available evidence should therefore be regarded as exploratory and hypothesis‐generating rather than confirmatory. High‐quality prospective studies employing standardized anatomical measurements, harmonized hearing preservation outcomes, and appropriately adjusted multivariable analyses are required before firm conclusions can be drawn regarding the role of cochlear anatomy in hearing preservation.

Funding

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Data S1: Supplemental Digital Content 1. Full search strategy.

Data S2: Supplemental Digital Content 2. Certainty of evidence (GRADE).

Table S1: Summary of findings and certainty of evidence for the association between cochlear anatomy parameters and hearing preservation after cochlear implantation, assessed using the GRADE approach.

Data Availability Statement

Research data are not shared.