Sound transmission by chamber prosthesis of the middle ear

Abstract

Tests done on specimens cut from the temporal bones show, that the stapedotomy can be more effective, if instead of the piston prosthesis, the ear chamber prosthesis is used. In that case, the vibrations of the eardrum are transferred to a plate with attachment sticked to the incus. The plate is suspended on a membrane stretched on the base of conical chamber which is filled with a fluid and placed in the middle ear cave. The sound wave caused by a vibrating plate, is focused at the chamber outlet placed in a small hole drilled in the stapes footplate. As in the case of the piston prosthesis behavior of the round window membrane differs from that observed in the normal ear. The flow through a narrow outlet of the conical chamber makes a more deflection of the central part of the round window membrane. The properties of the prosthesis elements are close to those of the removed parts of the middle ear. In spite of this, one can observe a different sound transmission inside the ear. When the sound is higher than 1000 Hz, the vibration amplitude of the plate is 5-10 dB higher than that for the stapes footplate in the healthy ear. However, when the sound is lower than 1000 Hz, this amplitude is lower than that for the stapes footplate. To explain it, a simplified model of the sound propagation in the ear given in the prior work is used. To get a better agreement with the test results, the model takes into account a damping of the sound wave by the round window membrane. Next, the model is adapted to the ear with chamber prosthesis. The factors that may have an effect on the behavior of the sound wave are examined. The first is shortening of the incus. It increases the leverage of the ossicles and the force acting on the prosthesis plate compared to that in the normal ear. Next factor is a reduction of the mass of the vibrating plate what makes a growth of its resonance frequency. This slightly reduces the amplitude of the plate for the low sounds and increases it for the medium and the higher sounds. At end, the lack of the influence of the flow through the conical chamber on the sound wave energy is shown. The assumed model gives the rules for amplitudes of the chamber plate as functions of the sound frequency. Their values for the sound frequency from 400 Hz to 8000 Hz and its graphs are shown and compared with those for the stapes footplate in the normal ear. One can see that if the sound frequency is higher than 1000 Hz, then the chamber prosthesis makes higher amplitudes of the sound wave than the normal ear. To explain their drop for frequencies lower than 1000 Hz, needs more tests in this range.