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Individual Hearing Preservation Cochlear Implantation Using the Concept of Partial Insertion

Lenarz, Thomas; Timm, Max Eike; Salcher, Rolf; Büchner, Andreas

Author Information
doi: 10.1097/MAO.0000000000002127


Cochlear implantation (CI) is a successful therapy of total or severe-profound hearing loss. Even patients with a total or profound hearing loss in the high frequencies and residual hearing in the low frequencies can benefit from this therapy. In the last decades the indication criteria were expanded to this patient group, and systems for combined electricacoustic stimulation (EAS) in one device were developed. Thin straight, flexible and shorter electrodes of 16, 20, and 24 mm length were developed to preserve residual hearing and to cover high-frequency range of the cochlea up to 1 kHz for electrical stimulation (ES).

Using the preserved acoustic functional residual hearing in the low frequencies in addition to ES, EAS patients can benefit in challenging listening situations like speech in noise recognition (1–9), music listening (10) and report better sound quality results (11,12).

For EAS treatment hearing preservation is important. The influence of the electrode length on hearing preservation was investigated in different studies (13–16) showing that the risk of significant postoperative hearing loss is related with electrode length. Although hearing preservation is possible with longer electrodes (17–20) the percentage of patients with hearing preservation is higher for shorter electrodes (13,15) and appears to be stable in most cases (13,15,21).

However, if residual hearing is progressive over time, the patients have to rely on ES-only. For patients using ES-only longer electrodes provide significant better speech understanding in noise (1). Furthermore, other studies showed that a larger insertion angle (IA) results in a better speech understanding for ES-only (22,23). These findings show a disadvantage for patients treated with a shorter electrode if residual hearing is lost or cannot be used for EAS.

Therefore, the choice of the right electrode length for patients with residual hearing and the aim to use EAS is a trade-off between best preservation of residual hearing and best performance with ES. This led to the motivation and concept of a partial insertion of longer, atraumatic electrodes to preserve residual hearing and to use the advantage of EAS at its best. Additionally, the possibility of afterloading the electrode if hearing loss progresses could be used to achieve deeper insertions and accordingly higher levels of speech perception for ES-only.


This retrospective study included adult patients with usable residual low-frequency hearing. All patients fulfilled the standard criteria for cochlear implantation at our clinic. The ethics committee of the Hannover Medical School, Germany, approved this retrospective study. Due to the retrospective design, no written information was given to the patients of the study group. All patient data were anonymized and de-identified prior the retrospective analysis.

Preparatory Test

By the partial insertion of a longer electrode the insertion depth can be reduced, but on the same time less electrodes are inside cochlea and can be activated for electrical stimulation.

Therefore, a preparatory test was performed in n = 6 EAS users with a MED-EL (MED-EL, Innsbruck, Austria) Concerto or Synchrony Implant and a fully inserted shorter custom made device (CMD) electrode of 16 mm length. It was investigated whether less than 12 activate contacts along an insertion depth of 16 mm will lead to equal or reduced speech understanding in EAS.

All patients had a preoperative hearing loss better than 65 dB up to 1 kHz. They used their clinical EAS fitting with SONNET EAS processor since first activation with stable speech performance. The maps had no overlapping between the acoustic and electric stimulation and individual crossover frequencies (CF) based on the latest audiogram.

In addition to the clinical map a second EAS condition with only 6 active electrodes in the electrical map was acutely configured and tested 3 months post first activation. Every second contact was deactivated to simulate a reduced number electrodes and a bigger contact distance like it is the case for a longer electrode being partially implanted. The other parameters were set as automatically recalculated by the fitting software. For acoustic fitting the clinical acoustic fitting was kept.

The subjects were unilaterally tested in free field in both conditions using the Freiburg monosyllabic word test (FBM) in quiet and the German language Hochmair–Desoyer, Schulz, Moser sentence test (HSM sentence test) (24) in noise with a fixed signal to noise ratio (SNR) of 10 dB at 0° azimuth (S0N0). They had no training for the new condition with 6 channels active. The contralateral ear was masked appropriately.

Partial Insertion—Surgical Concept

To achieve a insertion depth of 16 mm or 20 mm a MED-EL FLEX24 or FLEX28 was partially inserted into the cochlea. To visualize the insertion depth during surgery the closest contact to the aimed insertion depth (e.g., 16 mm and 20 mm) should be used as a marker at the round window (see Fig. 1A): for the 24 mm long electrode the 9th contact located 16.4 mm away from the electrode-tip and for the 28 mm long the 10th located at 20.1 mm.

FIG. 1
FIG. 1:
Surgical approach for partial insertion (exemplary for a FLEX28 electrode). A, Fascia ring around electrode contact at desired insertion depth. B, Insertion until fascia ring lies on round window membrane. Electrode contacts 11 and 12 are located extracochlearly. Electrode fixation groove in facial recess, drilled with a 0.5 mm diamond burr can be seen. C, Electrode fixation in drilled slit. D, electrode position in post op CBCT. Electrode contacts 11 and 12 can be identified outside the cochlear. CBCT indicates cone beam CT.

Patients Treated with Partial Insertion

Around 6 patients (4 females and 2 males) with a mean age of 54.8 ± 17.4 years were implanted with a partially inserted lateral wall electrode at the Hannover Medical School (MHH). All patients had preoperative residual functional hearing better than 65 dB up to 500 kHz. 4 patients used hearing aids preoperatively in the ear to be implanted, in the other 2 cases a hearing aid was used without sufficient benefit. The decision for the insertion depth was based on the amount and slope of the preoperative residual hearing of the patients in the lower frequencies. The three subjects for whom a 16 mm insertion was chosen had preoperative residual hearing ≤ 65 dB HL up to 750 Hz and a more gentle slope toward the higher frequencies. They fulfilled the EAS indication range set by the manufacture up to 2 kHz (see Fig. 2A). The three patients who received a 20 mm insertion had air-conduction thresholds <65 dB HL up to 500 Hz and more steep slope beyond this frequency. So, they did not fulfill the EAS indication range at 750 Hz. To achieve a higher electrical coverage for EAS a longer insertion was chosen. The patient specific cochlear length was not taken into account for the decision on the insertion depth in this study group.


All patients had surgery under general anesthesia using the standard surgical procedure for hearing preservation CI with transmastoid approach and posterior tympanatomy to expose the round window niche. The round window membrane was exposed by removing the bony overhang. A bone bed was drilled to secure the implant. A bone slit was drilled in the inferior part of the facial recess to fix the electrode after insertion and prevent it from migrating. The contact limiting the desired insertion depth was marked with a fascia ring (see Fig. 1A). After incision of the round window membrane the electrode was inserted in a slow and smooth manner to preserve residual hearing until the preset insertion depth was reached (Fig. 1B). The electrode lead was then placed in the prepared slit (Fig. 1C). The previously attached fascia or freshly taken venous blood was used to close the round window to prevent perilymph fistula and infection.

Cochlea Geometry

A cone beam CT (CBCT) of the temporal is routinely done pre- and postoperatively in all CI patients at the Hannover Medical School. For the six subjects using the DICOM viewer Osirix MD (version 2.5.1 64bit, Pixmeo SARL, Switzerland) different dimensions of the cochlear and the electrode array were measured (see Fig. 3): in the preoperative scans the length of the cochlea (CL) measured as described in (25–27); in the postoperative scans the inserted electrode length (IEL) and IA were measured from the entrance through the round window up to the most apical contact displayed in the CBCT (see Fig. 3). Following the lateral wall up to IA then yielded the covered cochlea length (CCL) (28). The inserted electrode depth (IED) was calculated by adding the electrode specific tip length which is not visible in CBCT to IEL (see as depicted in Fig. 3).

FIG. 3
FIG. 3:
A, Partially inserted FLEX24 (8 contacts inside the cochlea) also showing the CL, measured at the lateral wall of the cochlea (from RW to apex), and the IED (from RW to electrode tip). B, Partially inserted FLEX28 (nine contacts inside the cochlea) showing the corresponding IEL, CCL and IA (all of which reach from RW to the most apical electrode contact. CCL indicates covered cochlea length; IA, insertion angle; IED, inserted electrode depth.


The hearing status of the patients was evaluated during their routine appointments at the clinic. This included the preoperative (1–2 days prior to surgery) and the postoperative appointments: first activation (4–6 weeks after implantation), 3 and 6 months post activation. During the preoperative appointment unaided hearing thresholds and speech recognition aided with hearing aids were tested. During every postoperative appointment unaided pure-tone audiometry, fitting and speech recognition tests with EAS were performed.

Pure-tone Audiometry

Air-conduction thresholds of all six subjects were measured between 125 Hz and 8 kHz with Audiometer CAS AD17 using headphones. If no hearing could be measured up to the measurement limit of the audiometer, the thresholds were set to audiometer limit + 5 dB. This is a best-case assumption and corresponds to the audiometer limit (see Fig. 2) + the minimum audiometer step size. The frequency specific median pre- and postoperative air-conduction thresholds are shown in Fig. 2A. To document hearing preservation outcomes, differences between the pre- and postoperative unaided air-conducted pure tone thresholds in low frequencies (125 Hz, 250 Hz, 500 Hz, 1 kHz, 1.5 kHz) (PTALF) were determined for each visit. The individual data is shown as scatterplots (Fig. 2B). The median pre- to postoperative hearing loss was classified into: PTALF shifts ≤15 dB (good hearing preservation), >15 to ≤ 30 dB (moderate hearing preservation), and PTALF shift >30 dB (hearing loss) as described previously by Suhling (13,15). Additionally, the hearing preservation rate was documented by calculating the S-Factor as described by Skarzynsky et al. (29). Functional residual hearing was reported as defined by Wanna et al. (30).

FIG. 2
FIG. 2:
A, Air-conducted hearing thresholds at (1) first activation, (2) 3 months, and (3) 6 months for 16 mm insertion (n = 3) and 20 mm insertion (n = 3). B, Pre- to postoperative change in low frequencies (125 Hz – 1.5 kHz) at (1) first activation, (2) 3 months (3M), and (3) 6 months (6M) for 16 mm insertion (n = 3) and 20 mm insertion (n = 3).


The first activation of the audio processor and the determination of the acoustic and electric fitting parameters were performed with the fitting software Maestro 6. All patients used the SONNET EAS processor including an amplification of their acoustic hearing and an individually CF based on the actually measured audiogram. Based on the fitting guideline provided by the manufacturer CF was set to first intersection of air conduction thresholds with 65 dB HL from right to left in audiogram.

Speech Audiometry

For speech perception testing the FBM in quiet and the HSM sentence test (24) in noise at 10 dB SNR were used. All tests were performed in free field S0N0 with monosyllables and sentences presented at 65 dB SPL. The results were determined as words correct in percentage. The patients were tested unilaterally in the aided situation using their every-day condition of the implanted ear. To prevent the influence of the residual hearing of the contralateral ear, it was masked appropriately.


To compare speech data results in-between one patient group at different time points or for different conditions statistical tests for dependent samples were used. If the data were normally distributed, tested using the Kolomorov–Smirnoff test, a t-test was used. If normal distribution was not the case, the Wilcoxon signed-rank test for dependent samples was used. All data were analyzed statistically using IBM SPSS Statistics 22.


Preparatory Tests

The average FBM scores were 76.7% ± 16.0% with the EAS clinical map and 69.2% ± 13.6% with the EAS map with six contacts active. The speech scores in HSM sentence test at 10 dB SNR were in average 81.0% ± 15.6% with the EAS clinical map and 75.1% ± 20.5 with the EAS map with six contacts active. Between both test conditions no significant differences were found: FBM (T-test: p = 0.06); HSM 10 dB SNR (T-test: p = 0.423) (see supplement Fig. 1,



The geometry data are shown in Table 1. The mean CL in the study group is 39.2 mm ± 3.2 mm.

Inserted (based on imaging) and activated electrode contacts of each patient.

For patients with the 16 mm insertion depth (n = 3) 8 electrodes were fully inserted in the cochlea. For this group a mean CL of 38.6 mm ± 4.8 mm, a mean IED of 14.8 mm ± 0.9 mm was measured. A mean insertion angle of 239.1° ± 47.3 and a mean CC of 42.3% ± 11.5% were reached.

For the patients with 20 mm insertion depth in two cases 9 electrodes and in one case 10 contacts were inserted in the cochlea. The mean CL of this group is 39.8 mm ± 1.1 mm and the mean IED of 20.4 mm ± 2.2 mm. A mean IA of 332.7° ± 27° and a mean CC of 55.7% ± 4.2% is reached.

Partial InsertionHearing Preservation

Median preoperative and postoperative air-conduction thresholds are shown in Fig. 2. All patients had preserved functional residual hearing defined as HL≤80 dB HL at 250 Hz at first activation and 6 months post first activation.

Median PTALF on the implanted ear was prior to implantation in median 54 dB HL (n = 6), 65 dB HL (n = 6) at first activation and 61 dB HL (n = 6) at 3 months (see Fig. 2).

The hearing preservation rate depicted by the S-factor was in median 72% at first activation, 75% at 3 months and 77% at 6 months. In no case a complete hearing loss (>30 dB) occurred. In the 16 mm insertion group the median hearing loss between 125 Hz and 1500 Hz was 6–7 dB (n = 3) at the different appointments (see Table 2). In all three cases, complete preservation of hearing (≤ 15 dB) at all appointments up to 6 months visit (see Table 2 and Fig. 2) was achieved. In the 20 mm insertion group the median hearing loss between 125 Hz and 1500 Hz was 17 dB at first activation and the 3 months visit and improved to 11 dB at 6 months. In one case total hearing preservation (≤ 15 dB) was reached. In two cases the residual hearing could be preserved with a hearing loss between 15 and 30 dB. At 6 months, one subject recovered and switched to the group of total hearing preservation (≤ 15 dB).

Results of hearing preservation with partial insertion.


In Table 1, the inserted contacts based on imaging and the activated contacts at 6 months after implantation are compared, the extracochlear contacts were globally deactivated. In one patient (P2), an electrode was activated where imaging indicates a position at the round window. In another patient (P3) an intracochlear contact was deactivated due to a missing hearing impression. Corresponding to the described fitting procedure the SONNET EAS was adjusted at first activation and the follow-up appointments for each patient. In all patients the coding strategy FS4 was used at all time points. The CF varied between 325 and 1375 Hz (see Table 1). Only in one patient the CF had to be adapted at every appointment due to unstable residual hearing.

Speech Comprehension

The mean preoperative speech understanding in FBM at 65 dB in quiet (Fig. 4A) was 24.2% ± 13.9% (n = 6) in the best-aided condition. The postoperative speech understanding with EAS improved significantly with a mean score of 58.3% ± 27.5% (n = 6, p = 008) at first activation, of 61.7% ± 30.8% (n = 6, p = 0.020) at 3 months and of 65.8% ± 29.7% (n = 6, p = 0.022) at 6 months in FBM compared to the preoperative situation with hearing aids.

FIG. 4
FIG. 4:
Median, interquartile, minimum and maximum scores of A, Freiburg Monosyllables, B, Hochmair–Desoyer, Schulz, Moser (HSM) sentence test in noise 10 dB SNR preoperative measured with hearing aids and at first activation, 3 months and 6 months in electric-acoustic stimulation (EAS), statistical significance is marked with for p < 0.05 and ∗∗ for p < 0.01.

The speech understanding results in HSM (10 dB SNR) are shown Fig. 4B for four patients, where a preoperative measurement with hearing aids was available: preoperatively a mean score of 20.1% ± 18.6% (n = 4) was achieved, which significantly improved to 58.3% ± 31.4% (n = 4, p = 0.04) at first activation and to 65.0% ± 38.4% (n = 4, p = 0.033) at 3 months with EAS treatment. At 6 months the mean score was 76.0% ± 24.1% (n = 4, p = 0.068) for speech understanding. The results of all six patients showed a mean speech understanding of 60.2% ± 24.5% at first activation which improved up to 73.9% ± 32.9% at 3 months visit and to 71.7% ± 25.9% at 6 months post first activation.


Hearing preservation CI with shorter electrodes of 16 to 24 mm length and EAS is a proven therapy for patients with high-frequency hearing loss. If the preserved residual hearing can be acoustically amplified and used for EAS, it shows superior results compared to ES-only in several studies (1–9).

However, hearing loss might be progressive and the patients have then to rely on a shallower insertion depth for ES. As shown by Büchner et al. (1), patients treated with shorter electrodes using ES-only achieve significant worse results than patients with longer electrodes in ES-only stimulation. In order to overcome this trade-off, the concept of a partial insertion of longer electrodes with the option for deeper afterload insertion if the hearing loss progresses was developed and clinically evaluated.

If a longer electrode is partially inserted, fewer intracochlear contacts are available for electrical stimulation in the cochlea which might result in poorer performance compared to full number of intracochlear contacts using a shorter array. Therefore, it was investigated whether the availability of less electrode contacts for electrical stimulation has a negative effect on auditory performance. In a test group of patients implanted with a 16 mm electrode array every second contact was switched-off. No significant differences in monosyllables and HSM sentences in noise were found between the full map and the map with only six activated contacts (see supplement Fig. 1, at acute testing of 3 months post first activation. Concluding, as even without acclimatization time the patients achieved comparable results, a treatment with partially inserted electrode array would not lead to a disadvantage for hearing in EAS. As the contact spacing was also changed by this procedure, it might also be suggested that a greater distance between the contacts leads to less channel interaction and could even be beneficial (31,32).

Respecting the radiologic imaging there is variation in the IED (see Table 1). The differences can be explained by the surgical procedure (see Fig. 1). Depending on the view, the visibility and the surgeon's technique this might to lead to a different IED. Moreover, the electrode array should not be inserted deeper when noticing resistance. In one case the electrode array is inserted deeper, which can be explained by the preoperative hearing threshold and the comparatively large cochlea (see Table 1). As hearing could be preserved in this patient, it did not cause a negative impact for the results of this patient.

One basic requirement for EAS is the preservation of their residual hearing after CI. In all 6 subjects the functional residual hearing was preserved (see Fig. 2). Complete hearing preservation was achieved in 66.7% of cases at first activation and 83.3% at 6 months. Median hearing loss was 7.0 dB at first activation and 6 months for the group with 16 mm insertion and improved from 17.0 to 11.0 dB for the group with 20 mm insertion.

These results are in the same range as the results for a of fully inserted short electrode arrays of 16 mm length as reported by Jurawitz et al. (15) (median hearing loss 10 dB n = 97 at initial fitting and 8.8 dB at 6 months). Looking on the results of the 20 mm insertion group separately, slightly better results compared to a full implanted FLEX20 electrode (median hearing loss 17.5 n = 46 at initial fitting) (13) were achieved. As only six patients are included in this evaluation, the comparison to the results of studies with a higher patient group is limited. However, it is remarkable that no total hearing loss of >30 dB was seen. Published data show a percentage of patients with total hearing loss of 9.8% for Hybrid-L, 28.3% for FLEX20, 17.7% for FLEX24 and 32.1% for CI422 at 6 months (13,15). The surgical approach and concept of partial insertion does not lead to higher hearing loss rates compared to fully implanted electrodes of comparable intracochlear length. A higher patient number is needed to statistically strengthen the results and our conclusion. Furthermore, it might even be the case that the tip of flexible electrode is less traumatic due to less contacts and wires on the same insertion length. More in-depth analysis is needed to answer this question. Moreover, all patients show stable hearing thresholds (see Table 2 and Fig. 2) and there was no need of a deeper insertion of the electrode. If the hearing loss would progress over time the array could be deeper inserted later on. This surgical procedure seems to be a possible solution since reposition with preservation of residual hearing was successfully performed in other patient groups. It might be limited by the amount of fibrous tissue built around the electrode after surgery. While there have not been any cases of a deeper insertion for patients with partial inserted so far, an afterload was performed in our clinic in patients where an electrode migration occurred after cochlear implantation (n = 8). In n = 6 patients of that patient group, the same electrode could be inserted again into the cochlea. In none of these cases an infection could be observed. Moreover, the speech performance reached the initial level again. The revision surgery was done soon after the migration was diagnosed with probably less fibrous tissue. The most challenging aspect in this kind of surgery will be not to damage the electrode, as it happened in the two other cases. A possible future method could be a position control system implanted with the cochlea implant (33).

All patients use their preserved functional residual hearing for EAS and benefit from the CI. They improved by 42% in monosyllables and by 56% HSM in noise scores by the intervention (Fig. 4). They had slightly superior results (FBM = 65.8%, HSM 10 dB = 71.7% (n = 6)) compared to the results of EAS patients at 6 months (mean scores: FBMs = 56.9%, HSM 10 dB = 59.8%) (1). This might due to the better pre- and postoperative hearing thresholds in this patient group.

The decision of chosen insertion depth was mainly based on the preoperative residual hearing. However, the known variation in CL (25,27,34–36) leads to different IA and CC for comparable IED (see Table 1). This could also be found in other studies (27,36–38). Therefore, the cochlea size together with the preoperative residual hearing and intraoperative cochlear monitoring using CM recording during insertion should be taken into account to adapt the IED individually. Evidence-based decision making could give the possibility to predict the expected postoperative development of residual hearing and hearing performance. Based on this the best solution for the individual patient could be chosen preoperatively.


Partial insertion could solve the trade-off between better hearing preservation results with shorter electrodes and better performance with longer electrodes using ES-only. This treatment offers the advantage of an initial EAS treatment and if hearing loss is progressive the electrode can be further advanced, and the patients can benefit from longer electrode insertion depth and higher cochlear coverage for ES.

Future investigation is needed to find the individual timepoint and approach for a probable further insertion.


We thank Cornelia Batsoulis, PhD, Sarah Vormelcher, Max Fröhlich and Daniel Schurzig, PhD, all MED-EL Medical Electronics Hannover, for their scientific support in this study.


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Acoustic stimulation; Cochlear implantation; Electric; Hearing preservation; Lateral wall electrodes; Partial insertion; Residual hearing

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