The acoustic aspect of ossicular joints fixation in human temporal bones

Abstract

Problem: The middle ear mechanism involves the conduction of mechanical vibrations along the ossicular chain. Although the functional mechanism and ossicular motion of the middle ear have been revealed in many papers, the acoustic role of ossicular joints is still unclear in human temporal bones. Methods: Eleven fresh human temporal bones with intact middle and inner ear were measured for the peak-to-peak displacement and phase on umbo and stapes footplate using a laser Doppler vibrometer system. Sound was generated and controlled by a SYSid 6.5 audio analysis system and the stimulus was a sound input of 80 to 120 dB SPL at the tympanic membrane over the 0.1 to 10 kHz frequency range. After the baseline measurements, incus was carefully removed and replaced to ordinary position with cyanoacrylate glue. Measurements of umbo and stapes footplate motion were repeated. Results: The average time lag between umbo and footplate in the middle ear was 89 μs, which was constant at a measured frequency range from 0.1 to 10 kHz. After the fixation of ossicular joints, the delay was reduced to 35 μs for sound transmission between umbo and stapes footplate. With the incus removed and reglued, the footplate displacement was about 7 dB lower than for the intact middle ear at frequencies below 1 kHz. Conclusion: The time lag in the middle ear seems to be caused by the slippage of the ossicular joints, not only the I-S joint but also the M-I joint. The 89 μs delay means that when malleus moves inward, stapes footplate moves outward at 5.6 kHz. Significance: In middle ear surgery, the surgical exposure could be improved by resection of incus, because the acoustic damage of the incus reposition was less than 6 dB. Support: None reported.