Finite element analysis of the coupling between ossicular chain and mass loading for evaluation of implantable hearing device.

Finite element (FE) model is used to analyze the coupling effects between ossicular chain and transducer of implantable middle-ear hearing devices. The mass loading of the transducer is attached to the long process of the incus in the form of floating mass transducer (FMT) or applied near the incus-stapes joint by a magnet of contactless electromagnetic transducer (CLT). By changing placement of the transducer, crimping connection and damping parameter of the crimping mechanism, theoretical performances of the transducers were investigated on mechanical characteristics in two aspects: (1) displacement change at the stapes footplate, which describes the change in hearing due to placement of the transducer; (2) the equivalent pressure output of the transducer, which relates the footplate displacement driven by transducer to the sound pressure applied to a normal ear to produce that displacement. For the FMT with a less tight crimping connection or low supporting rigidity, a large drop of the sound-induced stapes displacement occurs at a specific frequency, with a peak reduction about 25.8 dB. A tight connection or high supporting rigidity shifts the drop of the stapes displacement to higher frequency. For the CLT, an electromagnetic transducer of 25 mg placed near the incus-stapes joint produces a maximum decrease of the stapes displacement around 16.5 dB. The equivalent sound pressure output and electromagnetic force requirement are proposed to produce the stapes displacement equivalent to that ear canal sound stimulus. The drop of the footplate displacement caused by mass loading effect can be recovered by the transducer stimulation over frequency range from 1500 Hz to 4000 Hz. The FE analysis reveals that enhancing the coupling stiffness between the clip and the ossicular chain is much helpful for maximizing the efficiency of the transducer stimulation.