Biomechanical effects of hydration in vocal fold tissues

OBJECTIVE: It has often been hypothesized, with little empirical support, that vocal fold hydration affects voice production by mediating changes in vocal fold tissue rheology. To test this hypothesis, we attempted in this study to quantify the effects of hydration on the viscoelastic shear properties of vocal fold tissues in vitro. STUDY DESIGN: Osmotic changes in hydration (dehydration and rehydration) of 6 excised canine larynges were induced by sequential incubation of the tissues in isotonic, hypertonic, and hypotonic solutions. Elastic shear modulus (G'), dynamic viscosity eta' and the damping ratio of the vocal fold mucosa (lamina propria) were measured as a function of frequency (0.01 to 15 Hz) with a torsional rheometer. RESULTS: Vocal fold tissue stiffness (G') and viscosity (eta) increased significantly (by 4 to 7 times) with the osmotically induced dehydration, whereas they decreased by 22% to 38% on the induced rehydration. Damping ratio (zeta) also increased with dehydration and decreased with rehydration, but the detected differences were not statistically significant at all frequencies. CONCLUSION: These findings support the long-standing hypothesis that hydration affects vocal fold vibration by altering tissue rheologic (or viscoelastic) properties. SIGNIFICANCE. Our results demonstrated the biomechanical importance of hydration in vocal fold tissues and suggested that hydration approaches may potentially improve the biomechanics of phonation in vocal fold lesions involving disordered fluid balance.