7-8. Quantification of tympanic membrane elasticity parameters from in situ measurements*
Jef Aernouts, Joris A.M. Soons, Joris J.J. Dirckx
Affiliation: Laboratory of Biomedical Physics, University of Antwerp
ABSTRACT:
Correct quantitative parameters to describe tympanic membrane elasticity are an important input for realistic modeling of middle ear mechanics. In the past, several attempts have been made to determine tympanic membrane elasticity from tensile experiments on cut-out strips. The strains and stresses in such experiments may be far out of the physiologically relevant range and the elasticity parameters are only partially determined.
We developed a setup to determine tympanic membrane elasticity in situ, using a combination of point micro-indentation and Moire profilometry. The measuring method was tested on latex phantom models of the tympanic membrane, and our results show that the correct parameters can be determined. These parameters were calculated by finite element simulation of the indentation experiment and parameter optimization routines. As latex is a strongly nonlinear material, a hyperelastic constitutive model was used. When the apparatus was used for rabbit tympanic membranes, Moire profilometry showed that there is no measurable motion of the manubrium during the small indentations. This result greatly simplifies boundary conditions, as we may regard both the annulus and the manubrium as fixed without having to rely on fixation interventions. The technique allows us to determine linear elastic material parameters of a tympanic membrane in situ. In this way our method takes into account the complex geometry of the membrane, and parameters are obtained in a physiologically relevant range of strain.
Total Time: 12 minutes
Comments
7-8
Dear Dr. Aernouts,
Your latex model does not integrate the concept of the 2D collagene fibres as the main structure of the tympanic membrane? The superb performance of the drum is due to the mechanical characteristics of this construction with the high tensile strength of the fibres, which results in a completely different behaviour as compared to a simple latex membrane.
7-8: Layered composite eardrum model is needed
Dr Aernouts,
I agree with Dr Huttenbrink that the layered model including collagen fibers is critical for testing and verifying any modern mathematical modeling approach for the eardrum. Does your model include the four layer construction of the eardrum? We have done this in our previous work (Fay et al 2005 JBM; Fay et al 2006 PNAS). Without the layered model you are testing and verifying the wrong mathematical approach in my opinion.
Sunil Puria, PhD
http://www.stanford.edu/people/puria
Author reply
Dear Dr Huttenbrink and Dr Puria
I agree, and I know that the tympanic membrane is not at all a linear homogeneous isotropic elastic material. In future studies, I will certainly consider elastic models that take into account anisotropic effects. I will also consider nonlinear elasticity, strain rate dependency, inhomogeneities (e.g. the border and the pars flaccida), viscoelasticity ...
However, the presented paper has two main parts. In part one, the measurement technique (doing point indentation measurements and inverse modeling) is validated on a simple homogeneous latex rubber. The output of the indentation approach was in good agreement with that of a standard elastic characterization routine (uniaxial testing). Thus, we showed that our approach gives good results. It is a validation, it should not be interpreted as a comparison between rubber and the tympanic membrane.
In part two, a preliminary experiment is shown on a rabbit tympanic membrane. In this first approach, a simple linear elastic model is used. The output of my approach is in good agreement with that of numerous previous studies. Studies in which the tympanic membrane was also considered as simple linear elastic.
As I wrote in the beginning of this comment, what I'm going to do now is studying more specific characteristics. And the point is that my technique will allow me to measure these specific properties. Properties that are difficult to study with standard characterization procedures.
Kind regards
Jef