Experimantal study in vibratony hearing

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1. Mechanics of the middle ear for the acoustic sound has been investigated and vibrational relationship between stapes and round window membrane also have been well studied by many authors. In this study we investigated a new stimulation site with hearing threshold by stimulating round window membrane, long process of incus, promontory, or stapes were investigated by using piezoelectric bimorph element vibrator which were made of lead titanate zirconete (Pb[ZrTi]OィイD23ィエD2) and the output sensitivity of the vibrator was 107dBSPL/1Vrms at 2 kHz. The tip of the vibrator was touched on the midst of long processus of incus, promontory, round window membrane and stapes. In long processus of the incus, the threshold was same as that of in the stapes. In case of stimulating the round window, the threshold was median of the that of promontory and the stapes. The value was 0.68 Vp-p in 12.5 kHz. This result indicates that stimulation of the round window membrane is effective for the stimulation site of Implantable hearing aid.2. In next experiment we studied a new surgical technique for PIHA implantation by using animals with experimental stapes fixation. Eight rabbits with normal hearing were used in this study. Tympanic membrane and ossicles except stapes were removed to set the vibrator of the PIHA. Cochleostomy was performed on the basal turn of the cochlea by 0.5 mm width with preserving membranous labyrinth. Stapes fixation was made by cementing the stapes with glass cement. The stapes and the opening of cochlea were stimulated by the vibrator and hearing thresholds were measured respectively. This study showed no hearing loss by cochleostomy and good sensitivity of the vibrator in stimulating the opening of cochlea even in stapes fixation. We concluded that cochleostomy was a possible procedure for implantation of the PIHA in patients with stapes fixation.