Abstract
An anatomically based three-dimensional finite-element human middle-ear (ME) model is used to test the sensitivity of ME sound transmission to tympanic-membrane (TM) material properties. The baseline properties produce responses comparable to published measurements of ear-canal input impedance and power reflectance, stapes velocity normalized by ear-canal pressure (PEC), and middle-ear pressure gain (MEG), i.e., cochlear-vestibule pressure (PV) normalized by PEC. The mass, Young's modulus (ETM), and shear modulus (GTM) of the TM are varied, independently and in combination, over a wide range of values, with soft and bony TM-annulus boundary conditions. MEG is recomputed and plotted for each case, along with summaries of the magnitude and group-delay deviations from the baseline over low (below 0.75 kHz), mid (0.75-5 kHz), and high (above 5 kHz) frequencies. The MEG magnitude varies inversely with increasing TM mass at high frequencies. Increasing ETM boosts high frequencies and attenuates low and mid frequencies, especially with a bony TM annulus and when GTM varies in proportion to ETM, as for an isotropic material. Increasing GTM on its own attenuates low and mid frequencies and boosts high frequencies. The sensitivity of MEG to TM material properties has implications for model development and the interpretation of experimental observations.
Highlights
Acting as the interface between incoming sound and the ossicular chain, the tympanic membrane (TM) plays an important role in sound transmission to the cochlea
The shear modulus (GTM), mass (MTM), boundary a)Portions of this work were presented in “Varying the material properties of the human tympanic membrane and ossicular chain in a 3D finite-element model to examine their effects on middle-ear sound transmission,” IUTAM Symposium on Advances in Biomechanics, May 2016, Stuttgart, Germany
The ETM values that produce the closest similarity to the baseline are 20 MPa for the truly isotropic cases and between 75 and 140 MPa for the fixed-GTM cases. The latter result indicates that, provided that GTM remains fixed at a relatively soft value, having the same 140 MPa ETM stiffness value in the radial (R), transverse (h), and circumferential (U) directions produces very similar results to the baseline case, in which only the radial (R) direction has that value and the h and U directions have respective values of 20 and 75 MPa. These results suggest that treating the human TM as orthotropic in the baseline model may make very little difference for sound transmission compared with using a single value in all three directions for ETM between 75 and 140 MPa when everything else is left the same
Summary
Acting as the interface between incoming sound and the ossicular chain, the tympanic membrane (TM) plays an important role in sound transmission to the cochlea. Testing the functional implications of controlled changes to the material properties of the TM and other middle-ear (ME) structures can be difficult or impossible, so many researchers have turned to computational models of the ME to test the effects of precisely defined changes in well-controlled virtual experiments (e.g., Funnell and Laszlo, 1978; Wada et al, 1992; Beer et al, 1999; Gan et al, 2002). Assuming they have realistic anatomy and material properties, reproduce responses similar to available experimental measurements, and employ reasonable simplifying assumptions when necessary, computational models can serve as a powerful scientific tool for investigating structure–function relationships in the ME (Decraemer and Funnell, 2008; Funnell et al, 2013; De Greef et al, 2017)
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