The biochemical properties of the human vocal fold

Phonation is a complex process requiring the controlled exhalation of air through the larynx. Within the larynx there is a specialist tissue structure known as the vocal folds, which under muscular control captures energy within the airflow and transfers it to a dynamic phenomena, analogous to a static fluid wave, known as the mucosal wave. This mucosal wave causes the vocal folds to open and close rhythmically, thus modulating the airflow, which can then be manipulated in the vocal tract to create the sounds that we know as speech. The purpose of the research detailed in this thesis is the quantification of the biomechanical properties of the vocal folds. There is a major gap in knowledge relating to the elastic properties of the vocal fold as the only reliable apparatus available to determine these properties rely on dissecting the tissue out of anatomical context. The author's research is dedicated to developing methods to measure these properties from intact larynges, and from patients in vivo. This is to enable a better understanding of how this complex tissue structure works; to assist with the derivation of mathematical models of phonation; and to provide methods to assess objectively the effectiveness of tissue engineering therapies used to repair scarred vocal folds. The author devised a new and novel apparatus to obtain data from excised tissue and in vivo. A key principle of these devices is that they directly measure the mechanical properties of intact larynges, which contrasts to methods reported by the majority of other researchers. The author also managed a number of research grant funded projects, in his capacity as PI, which deployed the devices. The author developed most of the software and the mathematical techniques used to analyse the data. Details of the apparatus devised to obtain data from both excised larynges and in vivo are given, which required the derivation of devices capable of measuring micrograms of force and displacement resolutions at micron level. Also given are the mathematical models used to transform the raw data into the fundamental material property known as shear modulus. The results include measurements f the shear modulus of a group of 20 excised vocal folds, of varying ages and both sexes. Also given are the results of similar data obtained from eight volunteer patients in vivo. The anisotropic nature of vocal fold tissue is quantified and iso-contour maps presented showing the variation of elasticity with respect to anatomical position. Early results are given that quantify the change in vocal fold tension with respect to electrical stimulation of the recurrent and superior laryngeal nerves in a canine model. Also given are the results of a study that demonstrated that hyaluronic acid tissue augmentation could restore vocal fold pliability in a rabbit model.EPSRC, The Royal Society and The Royal Academy of Engineering