EXPERIMENTAL STUDIES OF THE TYMPANOPLASTY BY ELECTRO-COCHLEAR RESPONSES

Abstract

(1) Separation of the Ossicular Chain (2) Reconstruction of the Broken Ossicular Chain (3) Granulation in the Tympanic Cavity adjacent to the Ossicular Chain In order to evaluate the effects of each of these three factors on the sound conduction mecha- nism and to contribute fundamental knowledge for the performance of tympanoplasty, a series of experiments, has been conducted by manipulating the incus and its surrounding region in various manners, and the following results were obtained: 1. When a small piece of muscle is inserted into the region adjacent to the ossicular chain around the incus a loss of cochlear potential in horizontal form of about 20db has been observed in the entire frequency range examined at 250-8, 000cps. 2. When only the incus is removed without injuring other ossicles, the maximal loss in the potential (59.0db at 1, 000cps) has been observed. This loss decreases in the region either near the low tone or near the high tone. However, in comparison with the loss shown in the case of com- plete separation of the incudostapedial joint, the loss in the potential at the time of removal of the incus is slightly smaller. 3. When a small piece of muscle is inserted into the tympanic cavity which incus has been removed, the loss in the potential shows the maximum (45.5db) in the low tone range; and when it is closer to the high tone range, the loss tends to decrease accordingly. Comparing this value with the potential loss in the removal of the incus, in the later the cochlear potential is greatest at 1, 000cps and the potential either in the low tone or in the high tone range is less than 1, 000cps. 4. When the neck of the malleus and the head of the stapes are connected by bridging with acryl cement bar after removing the incus, there is a loss of about 15db in the cochlear potential but no marked dip in frequency. Comparing the potential with that in which incus has been remoned, the increase in the potential is greatest in the middle tone, followed by the low tone and then high tone range. 5. For the cases where the incus is missing, it would be Letter to consider some suitable material for bridging the malleus and stapes aside from the conventional operation using the stapes as columella, in which there is often an attendant danger of skin flap adhesion. Therefore, it is hoped that there will appear some suitable material for the connecting of ossicles. 6. As for the influences of the granulation tissue in the tympanic cavity to the sound condu- ction, the granulation tissue itself is assisting the sound conduction in some degree when the ossicular chain is broken; and when the chain is intact it interferes the conduction in various degrees. From these aspects, in the cases where the conductive system is connected by granulation tissue, the removal of granulation tissue diminishes the hearing capacity. Therefore, it is concluded that a consideration needs to be made before removing such granulation tissue.