Silicone-coated thin film array cochlear implantation in a feline model.

Cochlear implants (CI) are well established in the treatment of severe-to-profound hearing loss. Current US criteria for implantation include prelingual children 12 months or older with binaural profound sensorineural hearing loss and postlingual adults with binaural severe-to-profound sensorineural hearing loss with minimal speech perception using best aided conditions (1). Over the past several decades, subject performance postimplant has steadily increased (2). Current implants, however, are still limited in their functional ability to provide a high resolution of pitches (3). Therefore, subjects are unable to fully appreciate music or to understand speech in crowded environments (3). At the current time, 3 manufacturer's cochlear implant devices are available in the United States (3). These implants have electrodes that are 10 to 31 mm in length and use up to 22 intracochlear electrode contacts to deliver impulses tonotopically to the cochlear nerve (2,3). It has been proposed that increasing the number of electrode stimulating sites will improve music appreciation and speech recognition in crowded situations (3). Current electrode designs prevent increasing electrode density because of the small size of the human cochlea scala tympani (3). Expanding the number of electrodes would increase the overall diameter of current implants and subsequently inhibit successful, atraumatic insertion. Thin film arrays (TFAs) were proposed to overcome this problem. Early TFA electrode designs, although efficacious with respect to activating auditory nerve fibers, were unable to be advanced past the basal turn of the cochlea, limiting the portion of the cochlea available for stimulation (3). This limitation was thought to be due to the thin-film silicone substrate lacking the mechanical flexibility (4). To overcome this design constraint, researchers at the School of Electrical and Computer Engineering, Georgia Institute of Technology, in collaboration with Georgia Regents University created a prototype polymeric TFA coupled with an insertion test device in hopes of gaining successful insertion (3). This study is a continuation of a previous implantation trial of TFAs into cadaveric human temporal bones using an insertion test device (ITD) backed TFA electrode. The ITD was similar in size to a cochlear implant electrode but lacked the current conducting component. It had a maximal insertion length of 17 mm to a T-stopper, and the array did not extend beyond its length. Results showed successful insertion of the TFA past the basal turn of the cochlea with a mean insertion depth of 17 mm. The authors concluded that their TFA electrode could be successfully and reliably inserted into human cochlea with minimal trauma when using the insertion test device (3). The end goal for the TFA electrode is to create an improved implant for use in humans. Before implantable devices can be used as human prostheses, however, function, efficacy, and safety must be documented in an animal model. Several previous studies have noted successful cochlear implantation in cats, with reliably observed responses to sound and the ability to elicit evoked auditory brainstem responses (2,5,6). The aim of this study was to implant a modified silicone coated TFA electrode cochlear implant in live cats to test the in vivo characteristics.