Why the middle ear piston prosthesis is not effective enough and how to change it?

Abstract

AbstractPiston prostheses of the middle ear do not ensure sufficient audibility of high and low sounds. To find out the reason, the amplitudes of the vibrations for the stapes footplate and the piston end were compared. It was seen that for a given force that oscillates with a low frequency, the amplitude of the piston end was higher than the amplitude of the stapes footplate. This means that the stiffness of the tissue sealing up the piston in the footplate hole is lower than the stiffness of the stapes plate suspension. It was shown that as a result, for the case of the higher frequencies, the amplitude of the piston vibrations drops several times. Next, it was compared a sound propagation in the healthy ear with that in the ear after the stapedotomy. To do it, a previously prepared model of sound propagation in the ear was used. The model is simplified, but it gives all parameters of the sound wave in the cochlear fluid. According to it, a motion of the stapes footplate forms a plane wave, while the piston motion initially gives a wave similar to the spherical one. A part of the spherical wave with the front directed towards the apex forms the primary wave running in the cochlea. However, the rest of this wave has the front directed towards the stapes footplate. This wave part, after a reflection from the stapes footplate, creates a secondary wave that follows the primary wave. A splitting of the wavefront around the edge of the piston end is a source of disruptions in the sound perception. The shift of the secondary wave reduces the power of the primary wave; it disturbs the waving of the basilar membrane and may cause extra noise. To justify it, a graph of the level of the cochlear amplification for the ear with the piston prosthesis was shown. The result compared with a simulation for the healthy ear gave the values 5 dB lower. To remove these drawbacks, it was proposed to place the piston end, not inside the cochlea, but in a guide in the form of a tube ended with a funnel fixed in the hole made in the stapes footplate. The piston was suspended in a guide tube on an O-ring formed of silicone gel. It was shown that when the piston is in the guide, the level of cochlear amplification was the same as that in the healthy ear. Some design details of the new piston guide are given. It enables us to make the new piston prosthesis easily and put it into practice.