Biomechanical influences on the vibration of human vocal fold tissue

Jordan E Kelleher, Purdue University

Abstract

This investigation attempts to shed light on phonation from a solid mechanics (i.e. biomechanics) perspective. The global hypothesis is that the biomechanical properties of the vocal fold lamina propria are critical to understanding phonatory mechanisms. The contributions of this research can be summarized into two main categories: (1) using optical measurements of vocal fold tensile deformation to determine spatial heterogeneities in the biomechanical properties and the implications of said heterogeneities to phonation, and (2) measuring the vocal fold biomechanical anisotropy and subsequently exploring the effects of the anisotropy on vocal fold vibration through models (computational and analytical). The biomechanical properties were examined using in vitro experiments of human vocal fold cover and vocal ligament specimens isolated from excised larynges. In one study, an improved uniaxial testing protocol was used involving optical measurements of the actual tissue deformation (not simply the applied displacement). The initial longitudinal elastic modulus was found to be considerably higher if determined based on optical displacement measurements than typical values reported in the literature. Also, digital image correlation (DIC) was used to obtain the entire spatial deformation field for a ligament specimen. DIC results revealed that the elastic modulus was very heterogeneous, being approximately 10 times higher at the mid-point of the vocal ligament than in the anterior and posterior macula flavae regions. However, it was discovered that the modulus heterogeneity was much greater in specimens from non-smokers as opposed to those from smokers. Additionally, microstructural images revealed that the collagen fibers were less aligned and less dense at the mid-membranous location in smokers than in non-smokers, which may support the biomechanical heterogeneity findings. The strong modulus gradient is proposed to have a functional role of enabling more complete glottal close which was verified through a finite element eigenmode analysis. A new protocol was developed to enable empirical measurements of the biomechanical anisotropy (i.e. longitudinal elastic modulus to longitudinal shear modulus ratio) within a single specimen by conducting uniaxial tensile and transverse indentation experiments. In the unstretched state, the anisotropy was discovered to be substantially impact the vibration characteristics through transversely isotropic finite element models and analytical Timoshenko beam models. Finally, the anisotropy was represented in a microstructural, anisotropic, nonlinear, hyperelastic constitutive model (Gasser-Ogden-Holzapfel model). Multiphoton microscopy images of a vocal ligament specimen's micro-architecture were analyzed and the degree of collagen fiber dispersion was quantified. The natural frequencies of vibration were predicted using Euler-Bernoulli and Timoshenko beam models in dependence of the neuromuscular elongation of the ligament. The primary factor influencing the natural frequencies as the anisotropy increased was transverse shear deformation.

Degree

Ph.D.

Advisors

Siegmund, Purdue University.

Subject Area

Biomedical engineering|Mechanical engineering|Biomechanics

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