loss as a result of meningitis, otosclerosis or ossification Malformation or obstruction of the cochlea Air–bone gap > 15 dB HL External ear contra-indications to using amplification devices Hearing preservation surgery A special surgical technique is the key to preserving the residual hearing of the patient. In most routine cochlear implant surgeries, any residual hearing will likely be destroyed. The residual hearing preservation rate in cochlear implantation is influenced surgical factors. The residual median hearing preservation for children was identified to be better when perimodiolar electrodes were inserted with AOS through fenestral cochleostomy. Important factors for preserving residual hearing are: Special atraumatic soft electrodes Smallest possible acoustic trauma due to cochleostomy drilling Smallest possible mechanical trauma due to drilling and electrode insertion Avoidance of inflammatory and fibrotic reactions (contamination with blood, bone dust, middle ear bacteria, etc.) The use of drugs that will aid in the preservation of hearing Two methods are commonly used for inserting the electrode into the cochlea: Round-window insertion Cochleostomy insertion Most recently, round-window insertion has found a wider acceptance because it is considered to be less traumatic. The best results have been achieved with electrode insertion depths of 18 mm, which corresponds to the approximate place in the normal sized cochlea where 1000 Hz is processed. As a result, frequencies below 1000 Hz are stimulated acoustically, and above 1000 Hz are stimulated electrically. For those interested in gaining the latest expertise necessary for a successful treatment of EAS candidates, surgical and professional training courses are provided by companies that offer EAS products. EAS electrodes for cochlear implants The design of the electrode array (shape, length, and bundle flexibility) is one of the key factors for preserving residual hearing. The surgical technique used for electrode array insertion is also important: the smaller the force used to insert the electrode, the greater the chance of protecting the fragile structures within the cochlea. The round window approach has been shown to be more successful at preserving residual hearing. Today only lateral wall electrodes are used. Studies with pre-shaped (modiolus-hugging electrodes) have been proven to be not so effective. EAS audio processors EAS audio processors use one microphone for the input but have two separate digital sound processors for differentiated processing. The parallel processing of these signals, however, is performed separately and optimized for both acoustic hearing (focusing on low-frequency hearing) and cochlear implant stimulation (focusing on high-frequency hearing).