Copyright © 2016 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. The Effect of Simulated Mastoid Obliteration on the Mechanical Output of Electromagnetic Transducers Martin Grosso ¨hmichen, Rolf Salcher, Thomas Lenarz, and Hannes Maier Cluster of Excellence Hearing4all, Department of Otolaryngology and Institute of Audioneurotechnology (VIANNA), Hannover Medical School, Hannover, Germany Background: The electromagnetic transducers of implanta- ble middle ear hearing devices or direct acoustic cochlear implants (DACIs) are intended for implantation in an air- filled middle ear cavity. When implanted in an obliterated radical mastoid cavity, they would be surrounded by fatty tissue of unknown elastic properties, potentially attenuating the mechanical output. Here, the elastic properties of this tissue were determined experimentally and the vibrational output of commonly used electromagnetic transducers in an obliterated radical mastoid cavity was investigated in vitro using a newly developed method. Methods: The Young’s moduli of human fatty tissue samples (3-mm diameter), taken fresh from the abdomen or from the radical mastoid cavity during revision surgeries, were determined by indentation tests. Two phantom materials having Young’s moduli similar to and higher than (worst case scenario) the tissue were identified. The dis- placement output of a DACI, a middle ear transducer (MET) and a floating mass transducer (FMT), was measured when embedded in the phantom materials in a model radical cavity and compared with the output of the nonembedded transducers. Results: The here-determined Young’s moduli of fresh human abdominal fatty tissue were comparable to the moduli of human breast fat tissue. When embedded in the phantom materials, the displacement output amplitude at 0.1 to 10 kHz of the DACI and MET was attenuated by maximally 5 dB. The attenuation of the output of the FMT was also minor at 0.5 to 10 kHz, but significantly reduced by up to 35 dB at lower frequencies. Conclusion: Using the method developed here, the Young’s moduli of small soft tissue samples could be estimated and the effect of obliteration on the mechanical output of electromagnetic transducers was investigated in vitro. Our results demonstrate that the decrease in vibrational output of the DACI and MET in obliterated mastoid cavities is expected to be minor, having no major impact on clinical indication. Although no major attenuation of vibrational output of the FMT was found for frequencies >0.5 kHz, for implantations in patients the attenuation at frequencies <0.5 kHz may have to be taken into account. Key Words: Active middle ear implant —AMEI—Codacs— DACI—Direct acoustic cochlear implant—FMT—IMEHD— Implantable middle ear hearing device —MET — Obliteration—Petrosectomy—Radical cavity—Subtotal. Otol Neurotol 37:xxx–xxx, 2016. One common treatment for chronic otitis media, chroni- cally discharging ears, or cholesteatoma is the subtotal petrosectomy (1) with the creation of a radical mastoid cavity and the obliteration with abdominal fat (2–6). The primary aim of this procedure is the elimination of the diseased tissue and the hearing rehabilitation in these patients may be challenging. One potential treatment is the implantation of an implantable middle ear hearing device (IMEHD), or a direct acoustic cochlear implant (DACI) in the obliterated mastoid cavity to substitute the middle ear functionally in combination with or without a sensorineural hearing loss. Such implantations have already been performed successfully in a limited number of patients (3–7). However, the transducers of these implants are intended to operate in the air-filled middle ear cavity. In an obliterated mastoid cavity they are surrounded by fatty tissue of unknown elastic properties, potentially attenuating the vibrational output. In this work, we characterized the elastic properties of this tissue and investigated its effect on the vibrational output of a DACI and two different IMEHDs experimentally. METHODS Potentially, the material characteristic of obliterated fatty tissue change postoperatively because of, e.g., fibrotic tissue growth. Therefore, two kinds of tissues were investigated: fresh Address correspondence and reprint requests to Martin Grosso ¨hmichen, M.Sc., Klinik fu ¨r Hals-Nasen-Ohrenheilkunde, Medizinische Hochschule Hannover, Stadtfelddamm 34, 30625 Hannover, Germany; E-mail: Grossoehmichen.Martin@mh-hannover.de M.G. and H.M. received travel support by Cochlear Ltd. to meetings. This work was supported by the DFG Cluster of Excellence EXC 1077/1 ‘‘Hearing4all.’’ This work is part of the doctoral thesis of M.G. and was supported by Cochlear Ltd. The authors disclose no other conflicts of interest. 1 Otology & Neurotology xx:xx–xx ß 2016, Otology & Neurotology, Inc.