This is a prospective multicenter multinational randomized control trial. The duration of the study for the individual patient will be approximately 1 year (pre-operative assessments, 1 week, 3 months and 12 months follow-up assessments). Participants will be randomized to one of two surgical approaches: eRW or CO, with a 1:1 allocation in a parallel design.

Rationale: Preserving residual hearing in cochlear implant (CI) surgery has gained significant attention. It not only signifies minimally invasive implantation but also supports natural sound perception and enables electrical-acoustic stimulation, enhancing sound localization, music appreciation, and speech recognition in noisy environments. Using the Cochlear™ Nucleus® CI with Slim Modiolar electrode (Cochlear Ltd., Sydney, Australia), potential sites of residual hearing loss in surgery include approaches to the cochlear lumen, potential damage during entry, and damage during electrode insertion. Studies comparing different surgical approaches (cochleostomy (CO) and extended round window (eRW)) show varied results. Electrocochleography (ECochG) can be used to indicate intracochlear damage during electrode insertion, however, the variability observed in ECochG responses during cochlear implantation remains significant. Moreover, ECochG has not yet been used to monitor cochlear functions throughout every phase of surgery, including the insertion of the sheath or stabilization of the electrode lead.

Objective: The aim of this study is to investigate whether the type of surgical approach to the cochlea; CO or eRW using the CI632 affects the final residual hearing and secondarily, intracochlear trauma and electrode position, as determined by pure tone audiometry, EcochG and imaging.

Study population: Adult CI candidates with post-lingual onset of severe to profound hearing loss, who are scheduled to receive a Nucleus CI632 with a slim modiolar electrode array and have a preoperative audiometric low-frequency average air conduction threshold of <80 decibel (dB) hearing level at 500 Hz in the ear to be implanted.

Inclusion Criteria:

• Individuals aged 18 years and older who have clinically established post-linguistic sensorineural hearing loss.
• CI candidate based on local or national reimbursement criteria.
• Cochlear implantation with a CI632.
• Preoperative 500 Hz pure-tone air conduction threshold <80 dB hearing level (HL) in the ear to be implanted.
• Willing and able to provide written informed consent.

Exclusion Criteria:

• Previous or existing CI recipient.
• Ossification or other anatomical abnormalities of the cochlea or its windows possibly affecting normal electrode array insertion.
• Abnormal cochlear nerve anatomy on preoperative CT or MRI.
• Subjects who are unable to undergo CT or MRI.
• Deafness due to acoustic nerve or central auditory pathway lesions.
• Diagnosis of auditory neuropathy.
• Active middle ear infection.
• Additional handicaps that would prevent participation in study evaluations.
• Unrealistic expectations from the participant regarding the possible benefits, risks, and limitations inherent to the CI procedure and the investigation.
• Cochlear employees or employees of Contract Research Organisations or contractors engaged by Cochlear for the purposes of this investigation.