**1. Introduction**

Hearing loss and deafness are associated with severe consequences for the affected patients, such as insufficient speech development, anxiety, depression, as well as lower educational and career opportunities due to social isolation [1,2] and an increased risk for the development of dementia [3]. Thus, the early diagnosis and treatment of hearing loss has a high socio-economical value.

Patients with severe sensorineural hearing loss are treated with a cochlear implant (CI) [4]. The surgical technique and the insertion technique are largely standardised, and regular CI electrodes can be inserted in the majority of cases. Special cases, however, such as the implantation of patients with anatomical malformations, obliterations of the cochlea, or re-implantations pose a challenge in cochlear implantation and may require special devices. Obliteration of the cochlea, for example, occurs after meningitis, trauma, or infection which result in hearing loss and subsequent intracochlear tissue growth, such as connective tissue or bone formation. When the cochlea is obliterated or ossified to a particularly significant degree, the conventional insertion of the mechanically flexible electrode array may be impossible [5], and alternative surgical techniques, such as incomplete insertion, the

**Citation:** Hoffmann, J.A.C.; Warnecke, A.; Timm, M.E.; Kludt, E.; Prenzler, N.K.; Gärtner, L.; Lenarz, T.; Salcher, R.B. Cochlear Implantation in Obliterated Cochlea: A Retrospective Analysis and Comparison between the IES Stiff Custom-Made Device and the Split-Array and Regular Electrodes. *J. Clin. Med.* **2022**, *11*, 6090. https://doi.org/10.3390/jcm11206090

Academic Editor: Nicolas Guevara

Received: 19 September 2022 Accepted: 13 October 2022 Published: 16 October 2022

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implantation of double arrays [6], implantation into the scala vestibuli [7,8], or a radical cochleostomy, must be considered. All these methods are associated with some disadvantages, such as the poor performance of the implant. However, despite a significantly higher risk for injuring the facial nerve, the internal carotid artery, or the modiolus [9], such alternative procedures are recommended in cases of partial and complete ossification.

To enable insertion even in cases with abnormal cochlear anatomies, special electrode arrays have been developed. MED-EL (Innsbruck, Austria), for example, created a compressed array as well as a Split-Array CMD for special requirements. The compressed array features 12 pairs of electrodes with an active stimulation range (ASR) of 12.1 mm (standard array ASR 31 mm), allowing the array to be placed in close proximity to the neurons, especially in malformed or partially ossified cochleae [10]. The split electrode array (MED-EL) with a double-branch electrode array was designed for the severe ossification of the cochlea. It contains five and seven electrode pairs on separated arrays on an ASR of 4.4 and 6.6 mm, respectively, which can be inserted through two cochleostomies (one in the basal and the second in the medial part of the cochlea) to increase the number of completely inserted electrode contacts in the ossified cochlea. Nevertheless, it can be shown that the speech performance of patients treated with this type of electrode is in the lower range of the spectrum that can be achieved by patients with a regular cochlear anatomy implanted with MED-EL's STANDARD and FLEX electrodes [10,11].

Since August 2015, some of those challenging cases were also treated with a custommade device (CMD, MED-EL) in our clinic. The CMD comprises a stiff insertion electrode (insertion electrode stiff, IES) and was prescribed in individual cases when the patients received a flexible lateral wall electrode array and there is an obstruction of the scala tympani (e.g., due to fibrosis or ossification). In such cases, the device is inserted prior to electrode insertion to dilate the cochlear lumen. This retrospective analysis evaluates speech performance data and impedance values as well as postoperative symptoms of patients treated with the IES CMD and compares them to the current available treatment options, a Split-Array CMD or a normal insertion.

## **2. Materials and Methods**

#### *2.1. Patients*

A retrospective analysis of all patients treated between August 2015 and March 2019 with a MED-EL device revealed 33 patient ears which were treated with lateral wall CI electrodes using the IES CMD prior to electrode insertion (the IES group). Demographic and clinical data, impedance values, hearing results, and speech performance results of the patients were collected retrospectively. We also retrospectively identified patients treated with the STANDARD or FLEX electrode arrays to be included in our analysis as part of a comparative control group (the REGULAR group). The REGULAR group consisted of patients selected to match the patient's age (±5 years), the patient's gender, the electrode carrier, and the type of implantation (first implantation or re-implantation) of the IES group. As an additional control, we identified and included patients treated with a Split-Array CMD (the SPLIT group). In these patients, implantation with regular electrode arrays was impossible. Due to the special electrode carrier, there were no suitable match patients for the SPLIT group to form a comparative collective.
