Stresses in vocal folds: A numerical investigation under vibration, impact and surface adhesion conditions

Pinaki Bhattacharya, Purdue University

Abstract

The aim of this study is to determine and characterize mechanical stresses in the vocal fold (VF) tissues during the phonation process. In particular the phonation process is simulated under conditions of self-oscillation, high- amplitude vibration, collision and de-cohesion. A bi-directionally coupled computational model of the fluid-structure interaction between the VF structure and the glottal air flow is the main object of analysis in this investigation. Physically realistic biomechanical properties and constitutive models are used to define the VF tissue and glottal air flow behavior. Governing equations of each physical domain are solved using commercially-available software. Advanced capabilities in model design, execution and post-processing phase are taken leverage of. The segregated solution approach gives rise to self-oscillation of the VFs, wherein periodic oscillations are computed without external periodic forcing. Overall air flow and VF deformation characteristics are found to fall within the range of previously measured data. Stresses within the VF volume are considered to govern interstitial flow through the VF tissue from a biphasic theory based poroelastic model. A decomposition framework is introduced which considers the total stress field to comprise of a mean, a vibratory and a collision-induced component. The component fields that are obtained as a result of the decomposition are shown to scale with externally measurable dynamical quantities. This approach is separately validated with respect to experimental observations from a physical self-oscillating model of the VFs. Taken together, the decomposition framework and interstitial flow model enable isolating the effects of mean, vibratory and collision-induced deformation on systemic hydration of the VF. Progress is also made in explaining the effect of glottal surface adhesion due to the airway surface liquid (ASL) on VF deformation characteristics. An interface debonding model is used to define the constitutive behavior of the ASL. A range of parameters is considered to simulate different conditions of ASL biomechanical behavior. The previously developed collision model is expanded to include the development of tensile tractions on the VF surface. It is demonstrated that the ASL, even when considered to have relatively low energy content, is able to induce important changes in the local and global features of the interaction of the VF structure with the flow. Definitions of features of VF motion important in the context of surface adhesion are introduced. Changes in the features in dependence of ASL properties are characterized in detail in this study for the first time. Two exploratory investigation are conducted to understand the importance of functional gradation of the VF tissue modulus, and that of geometric detail of VF structure in the presence of heterogeneity and anisotropy. Results from the investigation of functional gradation show that variation of elastic modulus along the length of the VF can significantly alter the way the VF structure interacts with the air flow, even when collision effects are neglected. Detailed analysis of the behavior of the coupled system under varying degrees of functional gradation provide fundamental insight into understanding the cause of perceived differences in voice quality between smokers and non-smokers. The results from the investigation of geometric effects provide a rational basis for construction of a canonical model that can better represent the dynamical behavior of subject-specific VF structure.

Degree

Ph.D.

Advisors

Siegmund, Purdue University.

Subject Area

Mechanical engineering|Biomechanics

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