Mechanical stress during phonation in a self-oscillating finite-element vocal fold model

The stress information during phonation in the vocal folds is helpful in understanding the etiologies of vocal trauma and its related vocal diseases, such as nodules. In this paper, a self-oscillating finite-element model, which combines aerodynamic properties, tissue mechanics, airflow-tissue interactions, and vocal fold collisions, was used to simulate the vocal fold vibration during phonation. The spatial and temporal characteristics of mechanical stress in the vocal folds were predicted by this model. Temporally, it was found that mechanical stress periodically undulates with vibration of the vocal folds and that vocal fold impact causes a jump in the normal stress value. Spatially, the normal stress is significantly higher on the vocal fold surface than inside of the vocal folds. At the midpoint of the medial surface, the peak-to-peak amplitude of the normal stress reaches its maximum value. Using different lung pressures (0-1.5kPa) to drive the self-oscillating model, we found that lower lung pressure can effectively decrease the mechanical stress in the vocal folds. This study supports the fatigue damage hypothesis of vocal trauma. With this hypothesis and the numerical simulation in this study, the clinical observations of vocal fold trauma risk can be explained. This implies the mechanical stress predicted by this self-oscillating model could be valuable for predicting, preventing, and treating vocal fold injury.     

Investigating the Three-dimensional Flow Separation Induced by a Model Vocal Fold Polyp

The fluid-structure energy exchange process for normal speech has been studied extensively, but it is not well understood for pathological conditions. Polyps and nodules, which are geometric abnormalities that form on the medial surface of the vocal folds, can disrupt vocal fold dynamics and thus can have devastating consequences on a patient''s ability to communicate. Our laboratory has reported particle image velocimetry (PIV) measurements, within an investigation of a model polyp located on the medial surface of an in vitro driven vocal fold model, which show that such a geometric abnormality considerably disrupts the glottal jet behavior. This flow field adjustment is a likely reason for the severe degradation of the vocal quality in patients with polyps. A more complete understanding of the formation and propagation of vortical structures from a geometric protuberance, such as a vocal fold polyp, and the resulting influence on the aerodynamic loadings that drive the vocal fold dynamics, is necessary for advancing the treatment of this pathological condition. The present investigation concerns the three-dimensional flow separation induced by a wall-mounted prolate hemispheroid with a 2:1 aspect ratio in cross flow, i.e. a model vocal fold polyp, using an oil-film visualization technique. Unsteady, three-dimensional flow separation and its impact of the wall pressure loading are examined using skin friction line visualization and wall pressure measurements.     

Non-invasive in vivo measurement of the shear modulus of human vocal fold tissue

Voice is the essential part of singing and speech communication. Voice disorders significantly affect the quality of life. The viscoelastic mechanical properties of the vocal fold mucosa determine the characteristics of the vocal folds oscillations, and thereby voice quality. In the present study, a non-invasive method was developed to determine the shear modulus of human vocal fold tissue in vivo via measurements of the mucosal wave propagation speed during phonation. Images of four human subjects' vocal folds were captured using high speed digital imaging (HSDI) and magnetic resonance imaging (MRI) for different phonation pitches, specifically fundamental frequencies between 110 and 440 Hz. The MRI images were used to obtain the morphometric dimensions of each subject's vocal folds in order to determine the pixel size in the high-speed images. The mucosal wave propagation speed was determined for each subject and at each pitch value using an automated image processing algorithm. The transverse shear modulus of the vocal fold mucosa was then calculated from a surface (Rayleigh) wave propagation dispersion equation using the measured wave speeds. It was found that the mucosal wave propagation speed and therefore the shear modulus of the vocal fold tissue were generally greater at higher pitches. The results were in good agreement with those from other studies obtained via in vitro measurements, thereby supporting the validity of the proposed measurement method. This method offers the potential for in vivo clinical assessments of vocal folds viscoelasticity from HSDI.     

Geometry of human vocal folds and glottal channel for mathematical and biomechanical modeling of voice production

Current models of the vocal folds derive their shape from approximate information rather than from exactly measured data. The objective of this study was to obtain detailed measurements on the geometry of human vocal folds and the glottal channel in phonatory position. A non-destructive casting methodology was developed to capture the vocal fold shape from excised human larynges on both medial and superior surfaces. Two female larynges, each in two different phonatory configurations corresponding to low and high fundamental frequency of the vocal fold vibrations, were measured. A coordinate measuring machine was used to digitize the casts yielding 3D computer models of the vocal fold shape. The coronal sections were located in the models, extracted and fitted by piecewise-defined cubic functions allowing a mathematical expression of the 2D shape of the glottal channel. Left-right differences between the cross-sectional shapes of the vocal folds were found in both the larynges.     

Adapted to roar: functional morphology of tiger and lion vocal folds

Vocal production requires active control of the respiratory system, larynx and vocal tract. Vocal sounds in mammals are produced by flow-induced vocal fold oscillation, which requires vocal fold tissue that can sustain the mechanical stress during phonation. Our understanding of the relationship between morphology and vocal function of vocal folds is very limited. Here we tested the hypothesis that vocal fold morphology and viscoelastic properties allow a prediction of fundamental frequency range of sounds that can be produced, and minimal lung pressure necessary to initiate phonation. We tested the hypothesis in lions and tigers who are well-known for producing low frequency and very loud roaring sounds that expose vocal folds to large stresses. In histological sections, we found that the Panthera vocal fold lamina propria consists of a lateral region with adipocytes embedded in a network of collagen and elastin fibers and hyaluronan. There is also a medial region that contains only fibrous proteins and hyaluronan but no fat cells. Young's moduli range between 10 and 2000 kPa for strains up to 60%. Shear moduli ranged between 0.1 and 2 kPa and differed between layers. Biomechanical and morphological data were used to make predictions of fundamental frequency and subglottal pressure ranges. Such predictions agreed well with measurements from natural phonation and phonation of excised larynges, respectively. We assume that fat shapes Panthera vocal folds into an advantageous geometry for phonation and it protects vocal folds. Its primary function is probably not to increase vocal fold mass as suggested previously. The large square-shaped Panthera vocal fold eases phonation onset and thereby extends the dynamic range of the voice.     

Elasticity and stress relaxation of a very small vocal fold

Across mammals many vocal sounds are produced by airflow induced vocal fold oscillation. We tested the hypothesis that stress-strain and stress-relaxation behavior of rat vocal folds can be used to predict the fundamental frequency range of the species' vocal repertoire. In a first approximation vocal fold oscillation has been modeled by the string model but it is not known whether this concept equally applies to large and small species. The shorter the vocal fold, the more the ideal string law may underestimate normal mode frequencies. To accommodate the very small size of the tissue specimen, a custom-built miniaturized tensile test apparatus was developed. Tissue properties of 6 male rat vocal folds were measured. Rat vocal folds demonstrated the typical linear stress-strain behavior in the low strain region and an exponential stress response at strains larger than about 40%. Approximating the rat's vocal fold oscillation with the string model suggests that fundamental frequencies up to about 6 kHz can be produced, which agrees with frequencies reported for audible rat vocalization. Individual differences and time-dependent changes in the tissue properties parallel findings in other species, and are interpreted as universal features of the laryngeal sound source.     

Cervids with different vocal behavior demonstrate different viscoelastic properties of their vocal folds

The authors test the hypothesis that vocal fold morphology and biomechanical properties covary with species‐specific vocal function. They investigate mule deer (Odocoileus hemionus) vocal folds, building on, and extending data on a related cervid, the Rocky Mountain elk (Cervus elaphus nelsoni). The mule deer, in contrast to the elk, is a species with relatively little vocal activity in adult animals. Mule deer and elk vocal folds show the typical three components of the mammalian vocal fold (epithelium, lamina propria and thyroarytenoid muscle). The vocal fold epithelium and the lamina propria were investigated in two sets of tensile tests. First, creep rupture tests demonstrated that ultimate stress in mule deer lamina propria is of the same magnitude as in elk. Second, cyclic loading tests revealed similar elastic moduli for the vocal fold epithelium in mule deer and elk. The elastic modulus of the lamina propria is also similar between the two species in the low‐strain region, but differs at strains larger than 0.3. Sex differences in the stress–strain response, which have been reported for elk and human vocal folds, were not found for mule deer vocal folds. The laminae propriae in mule deer and elk vocal folds are comparatively large. In general, a thick and uniformly stiff lamina propria does not self‐oscillate well, even when high subglottic pressure is applied. If the less stiff vocal fold seen in elk is associated with a differentiated lamina propria it would allow the vocal fold to vibrate at high tension and high subglottic pressure. The results of this study support the hypothesis that viscoelastic properties of vocal folds varies with function and vocal behavior. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

A coupled sharp-interface immersed boundary-finite-element method for flow-structure interaction with application to human phonation

A new flow-structure interaction method is presented, which couples a sharp-interface immersed boundary method flow solver with a finite-element method based solid dynamics solver. The coupled method provides robust and high-fidelity solution for complex flow-structure interaction (FSI) problems such as those involving three-dimensional flow and viscoelastic solids. The FSI solver is used to simulate flow-induced vibrations of the vocal folds during phonation. Both two- and three-dimensional models have been examined and qualitative, as well as quantitative comparisons, have been made with established results in order to validate the solver. The solver is used to study the onset of phonation in a two-dimensional laryngeal model and the dynamics of the glottal jet in a three-dimensional model and results from these studies are also presented.     

Influence of asymmetric stiffness on the structural and aerodynamic response of synthetic vocal fold models

The influence of asymmetric vocal fold stiffness on voice production was evaluated using life-sized, self-oscillating vocal fold models with an idealized geometry based on the human vocal folds. The models were fabricated using flexible, materially-linear silicone compounds with Young's modulus values comparable to that of vocal fold tissue. The models included a two-layer design to simulate the vocal fold layered structure. The respective Young's moduli of elasticity of the “left” and “right” vocal fold models were varied to create asymmetric conditions. High-speed videokymography was used to measure maximum vocal fold excursion, vibration frequency, and left–right phase shift, all of which were significantly influenced by asymmetry. Onset pressure, a measure of vocal effort, increased with asymmetry. Particle image velocimetry (PIV) analysis showed significantly greater skewing of the glottal jet in the direction of the stiffer vocal fold model. Potential applications to various clinical conditions are mentioned, and suggestions for future related studies are presented.     

Effect of the ventricular folds in a synthetic larynx model

Within the human larynx, the ventricular folds serve primarily as a protecting valve during swallowing. They are located directly above the sound-generating vocal folds. During normal phonation, the ventricular folds are passive structures that are not excited to periodical oscillations. However, the impact of the ventricular folds on the phonation process has not yet been finally clarified.An experimental synthetic human larynx model was used to investigate the effect of the ventricular folds on the phonation process. The model includes self-oscillating vocal fold models and allows the comparison of the pressure distribution at multiple locations in the larynx for configurations with and without ventricular folds.The results indicate that the ventricular folds increase the efficiency of the phonation process by reducing the phonation threshold level of the pressure below the vocal folds. Two effects caused by the ventricular folds could be identified as reasons: (1) a decrease in the mean pressure level in the region between vocal and ventricular folds (ventricles) and (2) an increase in the glottal flow resistance.The reason for the first effect is a reduction of the pressure level in the ventricles due to the jet entrainment and the low static pressure in the glottal jet. The second effect results from an increase in the glottal flow resistance that enhances the aerodynamic energy transfer into the vocal folds. This effect reduces the onset threshold of the pressure difference across the glottis.     

Differential cell adhesion to vocal fold extracellular matrix constituents.

The human vocal folds are a complex layering of cells and extracellular matrix. Vocal fold extracellular matrix uniquely contributes to the biomechanical viscoelasticity required for human phonation. We investigated the adhesion of vocal fold stellate cells, a novel cell type first cultured by our laboratory, and fibroblasts to eight vocal fold extracellular matrix components: elastin, decorin, fibronectin, hyaluronic acid, laminin and collagen types I, III and IV. Our data demonstrate that these cells adhere differentially to said substrates at 5 to 120 min. Cells were treated with hyaluronidase and Y-27632, a p160ROCK-specific inhibitor, to test the role of pericellular hyaluronan and Rho-ROCK activation in early and mature adhesion. Reduced adhesion resulted; greater inhibition of fibroblast adhesion was observed. We modulated the fibronectin affinity exhibited by both cell types using Nimesulide, an inhibitor of fibronectin integrin receptors alpha5beta1 and alphavbeta3. Our results are important in understanding vocal fold pathologies, wound healing, scarring, and in developing an accurate organotypic model of the vocal folds.     

声带组织切片方法的比较

目的:探讨犬声带冠状位切片与水平位切片各自的特点,为声带实验提供合适的切片方法。方法:家犬4只,2只取材后行冠状位石蜡切片,2只取材后行水平位石蜡切片。通过HE染色观察声带固有层的一般组织结构,Masson三色染色观察固有层中胶原的排列情况。结果:HE染色示冠状位、水平位切片均可见声带表面被覆复层鳞状上皮,固有层内有大量排列紧密的纤维组织,纤维组织中夹杂少量腺体,固有层下方为肌层。冠状位切片可观察声带某一点冠状面固有层的情况,若观察整个声带的情况需声带连续切片;水平位切片可在一张切片中观察到前联合、声带膜部及声带突部位的固有层情况,解剖标志明显,利于定位。Masson三色染色示冠状位、水平位切片均可见固有层浅层有较细的胶原纤维束,中层有较粗的纤维束与较细的纤维束交织排列,深层纤维束排列更紧密。结论:冠状位切片可观察声带某一点冠状面固有层的整体情况,水平位切片可在一张切片中观察到前联合、膜部及声带突部位的固有层情况。     

Regeneration of Vocal Fold Mucosa Using Tissue-Engineered Structures with Oral Mucosal Cells

Objectives

Scarred vocal folds result in irregular vibrations during phonation due to stiffness of the vocal fold mucosa. To date, a completely satisfactory corrective procedure has yet to be achieved. We hypothesize that a potential treatment option for this disease is to replace scarred vocal folds with organotypic mucosa. The purpose of this study is to regenerate vocal fold mucosa using a tissue-engineered structure with autologous oral mucosal cells.

Study Design

Animal experiment using eight beagles (including three controls).

Methods

A 3 mm by 3 mm specimen of canine oral mucosa was surgically excised and divided into epithelial and subepithelial tissues. Epithelial cells and fibroblasts were isolated and cultured separately. The proliferated epithelial cells were co-cultured on oriented collagen gels containing the proliferated fibroblasts for an additional two weeks. The organotypic cultured tissues were transplanted to the mucosa-deficient vocal folds. Two months after transplantation, vocal fold vibrations and morphological characteristics were observed.

Results

A tissue-engineered vocal fold mucosa, consisting of stratified epithelium and lamina propria, was successfully fabricated to closely resemble the normal layered vocal fold mucosa. Laryngeal stroboscopy revealed regular but slightly small mucosal waves at the transplanted site. Immunohistochemically, stratified epithelium expressed cytokeratin, and the distributed cells in the lamina propria expressed vimentin. Elastic Van Gieson staining revealed a decreased number of elastic fibers in the lamina propria of the transplanted site.

Conclusion

The fabricated mucosa with autologous oral mucosal cells successfully restored the vocal fold mucosa. This reconstruction technique could offer substantial clinical advantages for treating intractable diseases such as scarring of the vocal folds.     

Medial epicanthoplasty combined with plication of the medial canthal tendon in Asian eyelids

The authors present a new technique for the correction of the medial epicanthal fold using the Y-W-plasty or inverted Y-V-plasty combined with plication of the medial canthal tendon. From January of 1996 to April of 2001, 10 patients with epicanthal folds received a medial epicanthoplasty combined with plication of the medial canthal tendon. The patients ranged in age from 20 to 49 years (average, 27.3 years). Eight patients with epicanthal folds received the Y-W-plasty with plication of the medial canthal tendon. Two patients were operated by inverted Y-V-plasty. The follow-up period ranged from 5 months to 2 years. Neither injury of the lacrimal apparatus nor asymmetry of the eyes was noted. Fibrosis and redness of the operated scar was noted in the first 2 to 3 months. However, the scar maturated by 3 months in all patients, and the hypertrophic scar was unnoticeable in all patients. Two key modifications of this technique are plication of the outer leaflet of the medial canthal ligament and lateral advancement of the central triangular flap. These modifications remarkably reduce the tension along the skin suture line. This method is very effective for the correction of the moderate-to-severe epicanthal fold. In addition, these modifications can be applied in most other medial epicanthoplasty techniques.     

A numerical analysis of phonation using a two-dimensional flexible channel model of the vocal folds.

A two-dimensional flexible channel model of the vocal folds coupled with an unsteady one-dimensional flow model is presented for an analysis of the mechanism of phonation. The vocal fold is approximated by springs and dampers distributed in the main flow direction that are enveloped with an elastic cover. In order to approximate three-dimensional collision of the vocal folds using the two-dimensional model, threshold values for the glottal width are introduced. The numerical results show that the collision plays an important role in speech sound, especially for higher resonant frequency components, because it causes the source sound to include high-frequency components.     

Morphological aspects of the euphonic voice

The high quality of a euphonic voice is the result of complex interactions between many organs and systems. Vibrating vocal folds play a crucial role in this process. Their physiological motion is conditioned by the presence of the layered structure of laryngeal mucosa. In this study, we assessed the degree of dysphonia according to the Union of European Phoniatrics (UEP) scale. Videoendoscopy (VLS) and videostroboscopic (VLSS) examination of the larynx was used to visualize the vibration of the vocal folds. Morphological assessment of the inter-membranous part of the vocal fold mucosa was carried out using material collected after surgical treatment (60%) or obtained from autopsy (40%). The samples were examined by light microscopy and transmission electron microscopy. In euphonic voices, 1° of dysphonia (UEP) and the physiological endoscopic (VLS) and stroboscopic (VLSS) findings of vocal folds were registered. No morphological or ultramorphological changes were observed in the cells of the multilayered flat epithelium, basal membrane or in the stroma. Unchanged epithelial cells were situated on the basal membrane with folds. Moreover, numerous pericytes, vessels with multiplication of basal membranes, scanty collagenous fibers, plasmatic cells and lymphocytes were seen. Morphological changes with signs of atrophy and polypoid degeneration of the vocal fold mucosa were found in only 3 (15%) patients.     

Paired versus two-group experimental design for rheological studies of vocal fold tissues

Vibratory function of the vocal folds is largely determined by the rheological properties or viscoelastic shear properties of the vocal fold lamina propria. To date, investigation of the sample size estimation and statistical experimental design for vocal fold rheological studies is nonexistent. The current work provides the closed-form sample size formulas for two major study designs (i.e. paired and two-group designs) in vocal fold research. Our results demonstrated that the paired design could greatly increase the statistical power compared to the two-group design. By comparing the variance of estimated treatment effect, this study also confirms that ignoring within-subject and within-vocal fold correlations during rheological data analysis will likely increase type I errors. Finally, viscoelastic shear properties of intact and scarred rabbit vocal fold lamina propria were measured and used to illustrate theoretical findings in a realistic scenario and project sample size requirement for future studies.