Ossicular vibration in human temporal bones

Abstract

Vibration mode of the ossicles was investigated in twelve fresh human temporal bones using a video measuring system (VMS, Technical Instrument). This system allows one to observe the ossicular vibration and to measure its vibration amplitude (up to 0.2 micron) and phase angle. In this study the inner and middle ear was kept intact except for two small holes in the tympanic tegmen. These holes were for the observation of ossicular movement and were covered with a thin cover glass during the experiment. The vibration amplitude and phase angle of the umbo, malleus head, lenticular process and stapes head were measured at 19 frequencies between 0.1 kHz and 4.5 kHz. The umbo moved piston-like at 0.1-0.8 kHz and 2.6-4.5 kHz but in an ellipse at 1.0-2.4 kHz. The malleus head showed elliptical movement with its long axis anteriorly tilted around 45 degrees from the direction of the umbo vibration at 0.1 kHz. Both the lenticular process and stapes head showed similar movement; piston-like in lower frequencies and elliptical in higher frequencies. The umbo, lenticular process and stapes head vibrated parallel at lower frequencies. The position, displacement and phase angle of the rotation axis of the ossicles was calculated based on the displacement and phase angle of the umbo, malleus head and lenticular process. The axis was around the level of the neck of the malleus in frequencies lower than the resonant frequency, beneath the level of the short process near the resonant frequency and at the top of the malleus head in higher frequencies. The average axis displacement was 0.9 microns at 1 kHz but much smaller at frequencies lower than 0.8 kHz or higher than 2 kHz. This suggests that such axis movement may reduce the efficiency of the middle ear sound transmission.