Kendall Teichert and Dr. Scott Thomson, Mechanical Engineering

Introduction

Ramig and Verdolini state: “it has been estimated that 3% to 9% of the total population in the United States has a voice disorder.” One way to better understand voice disorders is to learn about the development of the structures and properties, or morphology, they affect. Much study has been done on the morphology of the human vocal folds. As explained by Hirano et al the vocal fold is made up of two parts: a cover (mucosa) and body (muscle). Hirano further explains that the mucosa is composed of various layers which include the mucosa epithelium and the lamina propria. The deep parts of the mucosa, the intermediate and deep layers of the lamina propria, consist, respectively, of large amounts of elastic and collagenous fibers. These fibrous layers are known as the vocal ligament.

The vocal ligament develops with age. Hirano et al, has stated that the vocal ligament is unobservable in infants and only primitive ligaments can be seen at ages 1 to 4. It has been observed that this development continues into adolescence when eventually a complete ligament can be seen. The exact cause for this remodeling is unknown. Concerning development of tissue in general, Sato and Hirano state that mechanical stress is a key component to the formation of fiber materials, and in regards to the vocal folds, that phonation after birth may stimulate the fibroblasts in the macula flava (located at the anterior and posterior ends of the vocal fold). These fibroblasts are cells that are assumed to produce the collagenous and elastic fibers which are key components of the vocal ligament. Also of interest is that the macula flava of newborns have a higher density of fibroblast than those of adults, but a lower density of actual fibrous tissue.4 It is postulated that as the vocal folds are stressed by vibration, the fibroblast cells are activated in such a way as to produce the necessary components of the vocal ligament.

Project Description

This study was to explore the validity of this particular theory. A computer model and ADINA, a Finite Element Analysis program, were used to be used to determine the stress levels and concentrations in the vocal fold during vibration. An algorithm was to be developed and used that would change the stiffness of the vocal fold dependent on stress levels. It was anticipated that with multiple iterations of this approach a stiffening of the inner structure of the vocal fold would form, analogous to the development of the vocal ligament.

Procedure

Approach

Due to the complex nature of the problem and the software involved, I spent a large portion of time becoming acquainted with the project and software involved. With this basic understanding and ability, I began with a very simplified rectangular model to develop the computer code that would be used to change the stiffness of the material due to the stress levels. By doing this I was able to develop the code with fewer difficulties.

Once the initial coding was complete, a model of the infant vocal fold was developed. This was done by scaling an adult fold and changing the cover thickness to more closely represent an infant fold. This model was coupled with the algorithm mentioned previously. MATLab coding was also developed to take output from the simulation and plot this data on a color contour graph.

Significant Problems/Accomplishments

Many problems were encountered throughout this project of which only a few will be discussed here, in no particular order. First, it was desired to have the model vibrate naturally with only steady input pressure applied. This was seen in the adult model but was difficult to attain in the infant model due to the differences in geometry. No perfect solution has been found to this point, but alternatives, though less desirable, are available. Second, the stiffness of the cover (stress-strain relationship) of the vocal fold is given by a certain mathematical equation, however the best approximation of this equation is nonlinear. Much work was done to try to implement this nonlinear equation into the model. However it was finally determined that for now a linear approximation would be sufficient. Finally, it was recently discovered that strain rather than stress may be the driving stimuli for change, so the code has been adapted to so as to accommodate either proposed stimuli.

Further Research

To date few results have been gathered. However, Dr. Scott Thomson will be using the model that has been developed to run additional testing for gathering data for validation of the study and the theory under review.

Several improvements and advancements can be made to this model if further research in this area is done. Implementation of the nonlinear material would more accurately model the vocal fold, as well as possibly providing a stiffening effect that would increase the robustness of the model, conceivably allowing greater pressures to be applied. The MATLab coding could be developed to display not only the changing stiffness, but also the deflection of the vocal fold (this is now done only in ADINA). Also, as was originally anticipated but never fully realized, collaboration could be done with the National Center for Voice and Speech, Denver Colorado. This could include tissue engineering for validation of the computer model.

Presentation

This work along with the results produced by Dr. Thomson will be presented at the 5th Int. Conf. on Voice Physiology and Biomechanics, July 12–14, 2006, Tokyo, Japan. It will be presented under the title “Simulating Stress-Induced Human Vocal Ligament Morphogenesis,” Teichert, K. B., and Thomson, S. L. This will either be delivered as a presentation or poster.

References

• Ramig L, Verdolini K. Treatment efficacy: voice disorders. Journal of Speech & Hearing Research. 1998;41 Issue 1:101-116.
• Hirano M, Kakita Y. Cover-body theory of vocal fold vibration. 1985; 2-20.
• Hirano M, Kurita S, Hakashima T. Growth, development, and aging of human vocal folds. Vocal Fold Physiology: Contemporary Research and Clinical Issues. 1983; 23-43
• Sato K, Hirano M. Histologic investigation of the macula flava of the human newborn vocal fold. Ann Otol Rhinol Laryngol. 1995; 104:556-62.
• Hirano M, Sato K, Hakahima T. Fibroblasts in human vocal fold mucosa. Acta Otolaryngol (Stockh). 1999; 119:271-276.