Sensitivity of vocal fold vibratory modes to their three-layer structure: implications for computational modeling of phonation

Examining the influence of epithelium layer modeling approaches on vocal fold kinematics and kinetics

Grouping the thin epithelium and thicker superficial lamina propria layers into a single cover layer has been widely adopted in finite element vocal fold models. Recent silicone vocal fold studies have suggested, however, that inclusion of a distinct epithelial layer leads to more physiologically representative motion. This study systematically explores the ramifications of incorporating an epithelial layer into a cover grouping for finite element vocal fold modeling. A membrane model for the epithelium is introduced to facilitate parametric investigation by reducing the mesh density requirement of the epithelium into a single infinitesimally thin layer. Excluding the epithelium entirely leads to increased energy in higher order modes and larger inferior-superior excursion of the folds. Integrating the epithelium into a cover layer with volume-weighted average stiffness results in similar kinematics to that of a model treating the epithelium as a distinct layer. However, the internal stress/strain and contact pressure during collision are higher, and viscous dissipation is lower, when the epithelium is integrated into the cover. Thus, careful treatment of the epithelium is recommended for finite element studies, particularly when employing the models for estimating measures dependent upon internal stress/strain and/or collision pressure, such as vocal dose.

A three-dimensional vocal fold posturing model based on muscle mechanics and magnetic resonance imaging of a canine larynx

In this work, a high-fidelity three-dimensional continuum model of the canine laryngeal framework was developed for simulating laryngeal posturing. By building each muscle and cartilage from magnetic resonance imaging (MRI), the model is highly realistic in anatomy. The muscle mechanics is modeled using the finite-element method. The model was tested by simulating vocal fold postures under systematic activations of individual as well as groups of laryngeal muscles, and it accurately predicted vocal fold posturing parameters reported from in vivo canine larynges. As a demonstration of its application, the model was then used to investigate muscle controls of arytenoid movements, medial surface morphology, and vocal fold abduction. The results show that the traditionally categorized adductor and abductor muscles can have opposite effects on vocal fold posturing, making highly complex laryngeal adjustments in speech and singing possible. These results demonstrate that a realistic comprehensive larynx model is feasible, which is a critical step toward a causal physics-based model of voice production.

Effect of Longitudinal Variation of Vocal Fold Inner Layer Thickness on Fluid-Structure Interaction During Voice Production

A three-dimensional fluid-structure interaction computational model was used to investigate the effect of the longitudinal variation of vocal fold inner layer thickness on voice production. The computational model coupled a finite element method based continuum vocal fold model and a Navier-Stokes equation based incompressible flow model. Four vocal fold models, one with constant layer thickness and the others with different degrees of layer thickness variation in the longitudinal direction, were studied. It was found that the varied thickness resulted in up to 24% stiffness reduction at the middle and up to 47% stiffness increase near the anterior and posterior ends of the vocal fold; however, the average stiffness was not affected. The fluid-structure interaction simulations on the four models showed that the thickness variation did not affect vibration amplitude, glottal flow rate, and the waveform related parameters. However, it increased glottal angles at the middle of the vocal fold, suggesting that vocal fold vibration amplitude was determined by the average stiffness of the vocal fold, while the glottal angle was determined by the local stiffness. The models with longitudinal variation of layer thickness consumed less energy during the vibrations compared with the constant layer thickness one.

Influence of vocal fold cover layer thickness on its vibratory dynamics during voice production

The influence of vocal fold cover layer thickness on the flow-induced vibration and voice production was studied by using a continuum-mechanics based computational model. The cover-body thickness ratio of a three-layer vocal fold was systematically varied. The effect on the vocal fold stiffness, eigenfrequencies and eigenmodes, fundamental frequencies, glottal flow rate, vocal fold vibratory dynamics, and synchronization of the eigenmodes were analyzed by using the structure eigen analysis and flow-structure interaction simulations. It was found that the cover-body layer thickness ratio significantly affected the strength and synchronization of the eigenmodes during flow-structure interactions, and ultimately affected the fundamental frequency and vibration pattern. With the increasing cover-body thickness ratio, the strength of the wave-type higher-eigenfrequency modes increased, and that resulted in a nonlinear bifurcation of the system in which the system evolved from a regular periodic vibration to a periodic doubling vibration and then back to a regular periodic vibration with increased fundamental frequencies. During the transition, the system vibrated chaotically. Because of the increased strength of the wave-type modes, the maximum divergent angle of the glottis was also increased with the increasing cover-body thickness ratio.

Computational Modeling of Fluid-Structure-Acoustics Interaction during Voice Production

The paper presented a three-dimensional, first-principle based fluid-structure-acoustics interaction computer model of voice production, which employed a more realistic human laryngeal and vocal tract geometries. Self-sustained vibrations, important convergent-divergent vibration pattern of the vocal folds, and entrainment of the two dominant vibratory modes were captured. Voice quality-associated parameters including the frequency, open quotient, skewness quotient, and flow rate of the glottal flow waveform were found to be well within the normal physiological ranges. The analogy between the vocal tract and a quarter-wave resonator was demonstrated. The acoustic perturbed flux and pressure inside the glottis were found to be at the same order with their incompressible counterparts, suggesting strong source-filter interactions during voice production. Such high fidelity computational model will be useful for investigating a variety of pathological conditions that involve complex vibrations, such as vocal fold paralysis, vocal nodules, and vocal polyps. The model is also an important step toward a patient-specific surgical planning tool that can serve as a no-risk trial and error platform for different procedures, such as injection of biomaterials and thyroplastic medialization.

Subject-specific computational modeling of human phonation

A direct numerical simulation of flow-structure interaction is carried out in a subject-specific larynx model to study human phonation under physiological conditions. The simulation results compare well to the established human data. The resulting glottal flow and waveform are found to be within the normal physiological ranges. The effects of realistic geometry on the vocal fold dynamics and the glottal flow are extensively examined. It is found that the asymmetric anterior-posterior laryngeal configuration produces strong anterior-posterior asymmetries in both vocal fold vibration and glottal flow which has not been captured in the simplified models. It needs to be pointed out that the observations from the current numerical simulation are only valid for the flow conditions investigated. The limitations of the study are also discussed.

The influence of thyroarytenoid and cricothyroid muscle activation on vocal fold stiffness and eigenfrequencies

The influence of the thyroarytenoid (TA) and cricothyroid (CT) muscle activation on vocal fold stiffness and eigenfrequencies was investigated in a muscularly controlled continuum model of the vocal folds. Unlike the general understanding that vocal fold fundamental frequency was determined by vocal fold tension, this study showed that vocal fold eigenfrequencies were primarily determined by vocal fold stiffness. This study further showed that, with reference to the resting state of zero strain, vocal fold stiffness in both body and cover layers increased with either vocal fold elongation or shortening. As a result, whether vocal fold eigenfrequencies increased or decreased with CT/TA activation depended on how the CT/TA interaction influenced vocal fold deformation. For conditions of strong CT activation and thus an elongated vocal fold, increasing TA contraction reduced the degree of vocal fold elongation and thus reduced vocal fold eigenfrequencies. For conditions of no CT activation and thus a resting or slightly shortened vocal fold, increasing TA contraction increased the degree of vocal fold shortening and thus increased vocal fold eigenfrequencies. In the transition region of a slightly elongated vocal fold, increasing TA contraction first decreased and then increased vocal fold eigenfrequencies.

Computational modeling of phonatory dynamics in a tubular three-dimensional model of the human larynx

Simulation of the phonatory flow-structure interaction has been conducted in a three-dimensional, tubular shaped laryngeal model that has been designed with a high level of realism with respect to the human laryngeal anatomy. A non-linear spring-based contact force model is also implemented for the purpose of representing contact in more general conditions, especially those associated with three-dimensional modeling of phonation in the presence of vocal fold pathologies. The model is used to study the effects of a moderate (20%) vocal-fold tension imbalance on the phonatory dynamics. The characteristic features of phonation for normal as well as tension-imbalanced vocal folds, such as glottal waveform, glottal jet evolution, mucosal wave-type vocal-fold motion, modal entrainment, and asymmetric glottal jet deflection have been discussed in detail and compared to established data. It is found that while a moderate level of tension asymmetry does not change the vibratory dynamics significantly, it can potentially lead to measurable deterioration in voice quality.