**1. Introduction**

Cochlear implants (CI) are the most successful neural prosthetic devices to date that provide hearing to profoundly hearing-impaired people around the world. CIs work by bypassing hair cell functionality and applying electrical stimulation to the auditory nerve fibers directly via a multichannel electrode array ideally implanted in the scala tympani (ST). Among other factors, CIs were shown to provide better hearing outcomes, e.g., word recognition scores for patients with greater neural survival [1,2]. In recent years, patients with low-frequency residual hearing also became eligible for CIs [3,4], and the CIs have shown a superior performance compared to those in profoundly deaf users [5–9]. Preservation of neural structures and residual hearing is therefore of high importance as it can provide additional auditory cues and improve speech understanding. There are several factors that can affect the preservation of residual hearing during cochlear implant surgery. These include the surgical approach, the type of cochlear implant being used and the skill of the surgeon. Soft surgery, with its smaller incisions and less invasive approach, may be more likely to preserve residual hearing compared to traditional surgery [10]. However, each patient's situation is unique, and the best approach for preserving residual hearing will

**Citation:** Hussain, R.; Frater, A.; Calixto, R.; Karoui, C.; Margeta, J.; Wang, Z.; Hoen, M.; Delingette, H.; Patou, F.; Raffaelli, C.; et al. Anatomical Variations of the Human Cochlea Using an Image Analysis Tool. *J. Clin. Med.* **2023**, *12*, 509. https://doi.org/10.3390/

Academic Editor: Christof Röösli

Received: 14 December 2022 Revised: 30 December 2022 Accepted: 4 January 2023 Published: 8 January 2023

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**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

depend on the individual's specific needs and circumstances. The delicate process of CI electrode insertion is nevertheless prone to introducing damage to cochlear structures [11–13]. Cochlear damage was shown to relate to long-term neural degeneration [14,15] and was also associated with the loss of residual hearing [16–19].

Cochlear damage due to electrode insertion may be mitigated by less traumatic surgical procedures [20,21] and by the improvement of CI electrode array designs [22–24]. Manufacturers may offer electrode arrays that best match the needs of individuals by providing electrodes with different dimensions that are the most suitable for the candidates. However, cochlear size and morphology are known to have large inter-individual variability [25–27]. To guide electrode development, detailed information is required about the variability of parameters that describe the cochlear size and shape [27]. These parameters can be obtained from computed tomography (CT) images, which are routinely available from CI candidates [28].

Recent studies relating cochlear morphology to CI electrode insertion focused on the establishment of normative datasets and reliable cochlear size measures [28], quantification of internal cochlear dimensions with high precision [27], evaluation of electrode mechanical properties in relation to induced cochlear trauma [29] or the establishment of a mathematical model that describes the shape of the cochlea [30]. In this study, variability and correlation of cochlear parameters, extracted via 3D reconstruction by the Oticon Medical Nautilus software [31], are investigated in a large set of 1099 cochleae. Additionally, intra-patient, inter-patient and inter-sex similarities are also analyzed.

#### **2. Materials and Methods**
