Safety and Efficacy of a Drug Eluting Slim Modiolar Electrode Array

Inclusion Criteria: * 18 years or older, at time of consent. * Postlinguistic moderately severe to profound sensorineural hearing loss, defined by a four frequency (500, 1000, 2000, 4000 Hz) pure-tone average unaided threshold (PTA4) ≥ 60 decibels hearing level (dB HL), in the ear to be implanted (For this study, moderately severe is defined by a four-frequency, pure-tone average threshold (PTA4) over the range 500 through 4000 Hz of more than 55 dB HL through 70 dB HL, inclusive. Profound is defined by a PTA4 of more than 90 dB HL). * Pure-tone average unaided threshold (500 through 4000 Hz) ≥ 30 dB HL, in the contralateral ear. * Preoperative aided word score ≤ 40% correct in the ear to be implanted. * Evidence of pneumococcal vaccination (e.g., Pneumovax) according to local guidelines. * Candidate is proficient in the language used to assess speech perception performance. * Willing and able to provide written informed consent. Exclusion Criteria: * Intra-axial (within the brain) lesions or deafness due to lesions of the acoustic nerve affecting the ear to be implanted. * Middle ear infection (including acute otitis media, chronic otitis media, suppurative otitis media, or serous drainage) in the ear to be implanted either at the time of surgery or within 3 months prior to enrolment. * Active autoimmune disease or active immunosuppressive therapy. * Presence of a tympanic membrane perforation or a history of otologic surgery within 3 months prior to enrolment, in the ear to be implanted. * Previously reported diagnosis of auditory neuropathy, in the ear to be implanted. * Previously reported diagnosis, in the ear to be implanted, of Large Vestibular Aqueduct Syndrome (LVAS), Meniere's disease, or cochlear hydrops. * Ossification, otosclerosis, malformation or any other cochlear anomaly, such as common cavity, that might prevent complete insertion of the electrode array, in the ear to be implanted. * Current cerebrospinal fluid (CSF) shunts or drains, existing perilymph fistula, skull fracture or CSF leaks. * Previously reported diagnosis of bacterial meningitis. * Known allergic reaction or contraindication to dexamethasone or corticosteroids. * Severe, or poorer, bilateral sensorineural hearing loss prior to 5 years of age, as reported by the subject. (For this study, severe is defined by a PTA4 of more than 70 dB HL through 90 dB HL, inclusive). * Severe, or poorer, sensorineural hearing loss for more than 20 years, as reported by the subject, in the ear to be implanted. * Prior cochlear implantation in either ear. * Medical plan for cochlear implantation during the clinical investigation, contralateral to the ear to be implanted. * Medical or psychological conditions that contraindicate general anaesthesia, surgery, or participation in the clinical investigation as determined by the Investigator. * Unrealistic expectations on the part of the subject regarding the possible benefits, risks, and limitations inherent to the surgical procedure(s) and prosthetic devices, as determined by the Investigator. * Women who are pregnant. * Additional disabilities that may affect the subject's participation or safety during the clinical investigation. * Unable or unwilling to comply with all the requirements of the clinical investigation as determined by the investigator. * Investigator site personnel directly affiliated with this study and/or their immediate families. Immediate family is defined as spouse, parent, child, or sibling. * Cochlear employees or employees of Contract Research Organisations or contractors engaged by Cochlear for the purposes of this investigation. * Current participation, or participation in another interventional clinical study/trial in the past 30 days, involving an investigational drug or device unless Cochlear sponsored and determined by Investigator or Sponsor to not impact this investigation.