Abstract
The role of the ossicular joints in the mammalian middle ear is still debated. This work tests the hypothesis that the two synovial joints filter potentially damaging impulsive stimuli by transforming both the peak amplitude and width of these impulses before they reach the cochlea. The three-dimensional (3D) velocity along the ossicular chain in unaltered cadaveric human temporal bones (N = 9), stimulated with acoustic impulses, is measured in the time domain using a Polytec (Waldbronn, Germany) CLV-3D laser Doppler vibrometer. The measurements are repeated after fusing one or both of the ossicular joints with dental cement. Sound transmission is characterized by measuring the amplitude, width, and delay of the impulsive velocity profile as it travels from the eardrum to the cochlea. On average, fusing both ossicular joints causes the stapes velocity amplitude and width to change by a factor of 1.77 (p = 0.0057) and 0.78 (p = 0.011), respectively. Fusing just the incudomalleolar joint has a larger effect on amplitude (a factor of 2.37), while fusing just the incudostapedial joint decreases the stapes velocity on average. The 3D motion of the ossicles is altered by fusing the joints. Finally, the ability of current computational models to predict this behavior is also evaluated.
Highlights
The middle ear of terrestrial vertebrates has been understood to serve a simple purpose: to act as an impedance transformer to maximize the transfer of acoustic energy from the environment into the specialized mechanotransduction organs in the fluid-filled cochlea (e.g., Helmholtz, 1912; Wever et al, 1948; Wever and Lawrence, 1950)
The velocity at the malleus head shows a small delay relative to the umbo, but is reflected about the x axis and has a smaller peak amplitude, indicating a seesaw-like motion of the malleus
A noticeable delay is evident between the malleus head and incus body, corresponding to delay in the incudomalleolar joint (IMJ)
Summary
The middle ear of terrestrial vertebrates has been understood to serve a simple purpose: to act as an impedance transformer to maximize the transfer of acoustic energy from the environment into the specialized mechanotransduction organs in the fluid-filled cochlea (e.g., Helmholtz, 1912; Wever et al, 1948; Wever and Lawrence, 1950). This impedance transformation is largely accomplished by the area ratio between a large tympanic membrane (TM) and a smaller cochlear opening. Of particular a)Portions of this work were presented in “Ossicular joint flexibility protects the cochlea by reducing damaging impulsive peak amplitudes,” at ARO 2017, in Baltimore, MD
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