Mechanical Stresses in Vocal Fold Tissue During Voice Production

author:Heather Elspeth Gunter
adviser:Robert D. Howe
degree: Ph.D.
institution: Harvard University

Theoretical and experimental techniques are used to study the tissue mechanics governing vocal fold closure and collision during phonation in order to evaluate the development of stresses that may be risk factors for pathology development. An original three-dimensional finite element model of vocal fold tissue predicts these quantities with high spatial resolution. Models predict that compressive stress in three directions and vertical shear stress are increased during collision in the typical location of lesions (i.e. the center of the superior medial edge of the vocal fold in the middle of the vibrating and contact region). This supports the hypothesis that stress is a cause of vocal fold pathology, and suggests modes of tissue injury. Predictions of increased stress due to increased voicing intensity are consistent with the hypothesis and clinical observations that loud voicing is a risk factor for benign vocal fold pathology development. Additional finite element models include a representation of the superficial lamina propria (SLP), which is a soft tissue layer near the surface of the vocal folds that contributes to voice quality and vocal fold injury. Increases in SLP stiffness are associated with increases in compressive and shear stresses in both the epithelium and SLP during collision. Increases in SLP stiffness are also associated with decreases in longitudinal tensile stress in the epithelium prior to collision. These results support the proposed role of SLP stiffness in determining mechanical stress and injury risk, and guide design and selection of SLP replacements used in vocal fold augmentation surgery. In vivo vocal fold collision forces in humans are measured using a new low profile force sensor that minimizes measurement artifacts and maintains voice quality. Impact force correlates more strongly with voice intensity than pitch. The finite element models translate increased force magnitudes into increases in compressive stress, vertical shear stress and Von Mises stress magnitudes. Avoidance of the conditions of increased collision force may prevent development of lesions.

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