Basic studies of the restoration of sound conducting system by tympanoplasty

Abstract

Part 1. On the Conduction Capacity of the ColumellaAfter extracting the temporal bone of cats the author gave sound stimuli to this temporal bone through the external auditory canal and measured the vibration amplitude of the labyrinthine side by means of a capacitative probe and determined the conduction capacity of the sound conducting system at 250, 500, 1, 000 and 2, 000cps when the lever function of the ossicular chain is obliterated. The results are as follows:1. When the malleus and incus are removed from the normal sound conducting system, the sound conduction capactiy decreases about 28db in average.2. After the removal of the malleus and incus from the normal sound conducting system:a) When the portion between the capitulum of the stapes and the part of the tympanic membrane corresponding to the short process, is communicated with an artificial columella, the sound conduction capacity increases about 21db as compared with that in the item 1, and shows a decrease in the capacity by 7db in average as compared with the normal.b) When the central part of the tympanic membrane and the capitulum of the stapes are communicated with an artificial columella, the sound conduction capacity increases 16db in average as compared with that of the iteml.c) when the part of the tympanic membrane corresponding to the umbo and the capitulum of the stapes are communicated with an artificial columella, there can be observed an increase in the sound conduction capacity by about 14db as compared with that of the item 1, showing a decrease of about 14db as compared with the normal.d) When the skin graft is placed on the capitulum of the stapes as to touch each other in the case where the tympanic membrane was deliberately removed, the sound conduction capacity decreases about 10db in average as compared with the normal.3. The weight ratio of the artifcial columellas used in the model experiments is 1:3:5, and it seems that the lighter is the weight of the columella the better the restoration of the sound conduction.4. It has been found that in the application of artificial columella the decrease in the sound conduction capacity is more marked in the high tone range than in the low tone range.Part 2. Sound Conduction Capacity in Various Sound Conducting Systems by ManipulationWith the temporal bone removed from human sudjects, sound stimuli were given through the external auditory canal, and the vibration amplitude of the labyrinthine side was measured with a capacitative probe in order to estimate the efficacy of various sound pathways reconstructed by tympa noplasty. The following are the results:1. When the antrum is blocked, the sound conduction capacity is raised by about 5 db in the range below 1, 000cps, and is decreased by about 5 db in the range of 2, 000 cps.2. In the attic, when there is an adhesion between the skin graft on onehand, the head of malleus and the incus on the other, the sound conduction capacity is diminished and it is especially marked in the range above 1, 000cps, 3. In order to maintain a better sound conduction capacity after tympanoplasty, it is desirable to leave a bony bridge and/or a vertical bony bridge to prevent the skin graft from adhesion.4. When the long process of the incus and the stapes are connected with a polyethylene tube after separating the incudostapedial joint, the decrease in sound conduction capacity proves to be only about 2db in average.