Variation of the cochlear anatomy and cochlea duct length: analysis with a new tablet-based software

Purpose In cochlear implantation, thorough preoperative planning together with measurement of the cochlear duct length (CDL) assists in choosing the correct electrode length. For measuring the CDL, different techniques have been introduced in the past century along with the then available technology. A tablet-based software offers an easy and intuitive way to visualize and analyze the anatomy of the temporal bone, its proportions and measure the CDL. Therefore, we investigated the calculation technique of the CDL via a tablet-based software on our own cohort retrospectively. Methods One hundred and eight preoperative computed tomography scans of the temporal bone (slice thickness < 0.7 mm) of already implanted FLEX28™ and FLEXSOFT™ patients were found eligible for analysis with the OTOPLAN software. Measurements were performed by two trained investigators independently. CDL, angular insertion depth (AID), and cochlear coverage were calculated and compared between groups of electrode types, sex, sides, and age. Results Mean CDL was 36.2 ± 1.8 mm with significant differences between sex (female: 35.8 ± 0.3 mm; male: 36.5 ± 0.2 mm; p = 0.037), but none concerning side or age. Differences in mean AID (FLEX28: 525.4 ± 46.4°; FLEXSOFT: 615.4 ± 47.6°), and cochlear coverage (FLEX28: 63.9 ± 5.6%; FLEXSOFT: 75.8 ± 4.3%) were significant (p < 0.001). Conclusion A broad range of CDL was observed with significant larger values in male, but no significant differences concerning side or age. Almost every cochlea was measured longer than 31.0 mm. Preoperative assessment aids in prevention of complications (incomplete insertion, kinking, tipfoldover), attempt of atraumatic insertion, and addressing individual necessities (hearing preservation, cochlear malformation). The preferred AID of 720° (two turns of the cochlea) was never reached, opening the discussion for the requirement of longer CI-electrodes versus a debatable audiological benefit for the patient in his/her everyday life. Supplementary Information The online version contains supplementary material available at 10.1007/s00405-021-06889-0.


Klinik und Poliklinik für Hals-Nasen-Ohren-Heilkunde Campus Großhadern Campus Innenstadt
Page 3/8 [3] Lazard DS, Vincent C, Venail F, Van   Accordingly, we have changed the following paragraph in the discussion part, conclusion, and the conclusion part of the abstract (References, please see manuscript): Abstract/Conclusion (page 1): "A broad range of CDL was observed with significant larger values in male, but no significant differences concerning side or age. Almost every cochlea was measured longer than 31.0 mm.
Preoperative assessment aids in prevention of complications (incomplete insertion, kinking, tipfoldover), attempt of atraumatic insertion, and addressing individual necessities (hearing preservation, cochlear malformation). The  (2) At the end of the introduction please write the objectives of the study.

Ad (2):
As recommended, we added the objectives of the study in the introduction part as follows (page 4): "Objectives of the study were to evaluate the range of CDL, find differences in different patient groups (sex, age, type of electrode), and to assess the angular insertion depth (AID) for the cochlear coverage."

(3) Methods "Patients who received a FLEX 28 electrode were implanted with the aim of hearing preservation"
This sentence is very strange and needs explanation. Implants that preserve hearing are traditionally up to 24 mm.

Ad (3):
We agree that traditionally a 20 to 24 mm electrode would be an option to achieve hearing preservation with the aim of electric acoustic stimulation. Nevertheless, the choice for the length of the electrode is influenced by individual anatomy, residual hearing, and the philosophy of the surgeon. The analyzed cohort of Flex 28 patients have all been implanted by the same surgeon with either the aim of prevention of vertigo or hearing preservation in patients with minimal residual hearing. In all of those patients the residual hearing was within the lower frequencies, however beyond the indication range for electrical acoustic stimulation. If the reviewer feels this sentence is irrelevant for the conclusion of the study, since the study merely focusses on the evaluated anatomic data and not on the hearing outcome, we will omit this sentence.

Klinik und Poliklinik für Hals-Nasen-Ohren-Heilkunde Campus Großhadern Campus Innenstadt
Page 7/8 (4) In Data analysis: "a slice thickness ≥ 0.7 mm were excluded from the study" this is redundant as it was stated earlier in Methods Ad (4): We omitted the following sentence (page 6): "CT scans of patients suffering from inner ear anomalies and/or with a slice thickness ≥ 0.7 mm were excluded from the study." Thank you for pointing out the unclear phrasing of the sentence. In order to clarify, we have rephrased it (page 8): "Due to a high rate of scans with slice thickness of 0.7 mm or more, only a total of 72 FLEX28 implanted ears and 36 FLEXSOFT implanted ears were included into the investigation (Figure 1)." (6) Fig. 1  We have changed it to "≤ 0.6mm" in Figure 1 (7) I would think of another implication of the study. We perform medical tests in order to make better decisions. When looking into the literature using different or even the same measuring technique, a broad range of CDL is reported (see Table 2). In addition, a number of studies report of individuals with shorter CDL than 31.0 mm. Thus, preoperative measuring of the CDL contributes to the surgical management, in particular, when the cochlea seems smaller/shorter/flatter in the CT scan at first glance. Without getting familiar with the surgical anatomy preoperatively, intraoperative complications like trying to insert a 31.0 mm long electrode into a smaller cochlea which could damage the lamina spiralis or result in incomplete insertion with kinking or tipfoldover of the electrode.
We changed the following paragraph to the paper (discussion and conclusion): Discussion (page 10): "Even if the morphology seems normal at first glance in the computed tomography, pitfalls might occur intraoperatively, like incomplete insertion in patients with shorter cochlea with kinking or tipfoldover of the electrode, or damage of the lamina spiralis. Interestingly, two cochleae were measured shorter than 32.0 mm, meaning that in the remaining 106 patients the insertion of an electrode of 31.0 mm length, would have been feasible. Moreover, with regard to further implications and improvements of CI, the morphology, CDL and AID play an essential role." Conclusion (page 12): "Analysis with the tablet-based software OTOPLAN showed a broad range of CDL with a variation over 30% and significant differences in sex, but none in age or side. This broad range in CDL should be considered preoperatively for issues like avoidance of complications (incomplete insertion, kinking or tipfoldover of the electrode), attempt of atraumatic insertion, individual necessities (hearing preservation, cochlear malformation), and tonotopic matching of electrical stimulation site."