Acoustics and aerodynamic effects following glottal and infraglottal medialization in an excised larynx model.

OBJECTIVE: This study aimed to investigate the impact of the implant's vertical location during Type 1 Thyroplasty (T1T) on acoustics and glottal aerodynamics using excised canine larynx model, providing insights into the optimal technique for treating unilateral vocal fold paralysis (UVFP). METHODS: Measurements were conducted in six excised canine larynges using Silastic implants. Two implant locations, glottal and infraglottal, were tested for each larynx at low and high subglottal pressure levels. Acoustic and intraglottal flow velocity field measurements were taken to assess vocal efficiency (VE), cepstral peak prominence (CPP), and the development of intraglottal vortices. RESULTS: The results indicated that the implant's vertical location significantly influenced vocal efficiency (p = 0.045), with the infraglottal implant generally yielding higher VE values. The effect on CPP was not statistically significant (p = 0.234). Intraglottal velocity field measurements demonstrated larger glottal divergence angles and stronger vortices with the infraglottal implant. CONCLUSION: The findings suggest that medializing the paralyzed fold at the infraglottal level rather than the glottal level can lead to improved vocal efficiency. The observed larger divergence angles and stronger intraglottal vortices with infraglottal medialization may enhance voice outcomes in UVFP patients. These findings have important implications for optimizing T1T procedures and improving voice quality in individuals with UVFP. Further research is warranted to validate these results in clinical settings.

The Effects of Negative Pressure Induced by Flow Separation Vortices on Vocal Fold Dynamics during Voice Production.

This study used a two-dimensional flow-structure-interaction computer model to investigate the effects of flow-separation-vortex-induced negative pressure on vocal fold vibration and flow dynamics during vocal fold vibration. The study found that negative pressure induced by flow separation vortices enhances vocal fold vibration by increasing aeroelastic energy transfer during vibration. The result showed that the intraglottal pressure was predominantly negative after flow separation before gradually recovering to zero at the glottis exit. When the negative pressure was removed, the vibration amplitude and flow rate were reduced by up to 20%, and the closing speed, flow skewness quotient, and maximum flow declination rate were reduced by up to 40%. The study provides insights into the complex interactions between flow dynamics, vocal fold vibration, and energy transfer during voice production.

An Ex-vivo Model Examining Acoustics and Aerodynamic Effects Following Medialization With and Without Arytenoid Adduction.

OBJECTIVES/HYPOTHESIS: Quantify differences in acoustics and intraglottal flow fields between Thyroplasty Type 1 (TT1) with and without arytenoid adduction (AA) using excised canine larynx model. STUDY DESIGN: Basic science experiments using excised larynges. METHODS: Surgical procedures were implemented in eight excised canine larynges. Acoustics and intraglottal flow measurements were taken at low and high subglottal pressures in each experimental setup. RESULTS: In all larynges, vocal efficiency (VE) and cepstrum peak prominence (CPP) were higher, and the mean phonatory flow rate was lower in TT1 with AA than without AA. The glottal asymmetry is reduced with AA and promotes the formation of stronger vortices in the glottal flow during the closing phase of the vibrating folds. CONCLUSIONS: Findings suggest a clear acoustic and aerodynamic benefit to the addition of AA when performing TT1. It shows significant improvement in CPP, translating to decreased breathiness and dysphonia and increased VE, leading to easier and more sustainable phonation. Stronger intraglottal vortices are known to be correlated with the loudness of voice produced by phonation. LEVEL OF EVIDENCE: N/A Laryngoscope, 133:621-627, 2023.

Fluid-Structure Interaction Analysis of Aerodynamic and Elasticity Forces During Vocal Fold Vibration.

The effect of the intraglottal vortices on the glottal flow waveform was explored using flow-structure-interaction (FSI) modeling. These vortices form near the superior aspect of the vocal folds during the closing phase of the folds' vibration. The geometry of the vocal fold was based on the well-known M5 model. The model did not include a vocal tract to remove its inertance effect on the glottal flow. Material properties for the cover and body layers of the folds were set using curve fit to experimental data of tissue elasticity. A commercially available FSI solver was used to perform simulations at low and high values of subglottal input pressure. Validation of the FSI results showed a good agreement for the glottal flow and the vocal fold displacement data with measurements taken in the excised canine larynx model. The simulations result further support the hypothesis that intraglottal vortices can affect the glottal flow waveform, specifically its maximum flow declination rate (MFDR). It showed that MFDR occurs at the same phase when the highest intraglottal vortical strength and the negative pressure occur. It also showed that when MFDR occurs, the magnitude of the aerodynamic force acting on the glottal wall is greater than the elastic recoil force predicted in the tissue. These findings are significant because nearly all theoretical and computational models that study the vocal fold vibrations mechanism do not consider the intraglottal negative pressure caused by the vortices as an additional closing force acting on the folds.

Computational Modeling of Voice Production Using Excised Canine Larynx.

A combined experimental-numerical work was conducted to comprehensively validate a subject-specific continuum model of voice production in larynx using excised canine laryngeal experiments. The computational model is a coupling of the Navier-Stokes equations for glottal flow dynamics and a finite element model of vocal fold dynamics. The numerical simulations employed a cover-body vocal fold structure with the geometry reconstructed from magnetic resonance imaging scans and the material properties determined through an optimization-based inverse process of experimental indentation measurement. The results showed that the simulations predicted key features of the dynamics observed in the experiments, including the skewing of the glottal flow waveform, mucosal wave propagation, continuous increase of the divergent angle and intraglottal swirl strength during glottal closing, and flow recirculation between glottal jet and vocal fold. The simulations also predicted the increase of the divergent angle, glottal jet speed, and intraglottal flow swirl strength with the subglottal pressure, same as in the experiments. Quantitatively, the simulations over-predicted the frequency and jet speed and under-predicted the flow rate and divergent angle for the larynx under study. The limitations of the model and their implications were discussed.

Impact of Vertical Stiffness Gradient on the Maximum Divergence Angle.

INTRODUCTION: During vocal fold vibration, the medial surface of both folds forms a convergent shape during opening and a divergent shape during closing. A greater maximum divergence angle is associated with greater closing forces which will increase the closing speed of the glottis. An increased closing speed results in a greater acoustic intensity and greater vocal efficiency. Indentation testing showed that as the strain increases, the inferior aspect of the folds becomes stiffer than the superior aspect, resulting in the vertical stiffness gradient (VSG). We hypothesize that a reduction of the vertical stiffness gradient will reduce the maximum divergence angle. METHODS: Four excised canine larynges were tested. Stress-strain curves of the superior and inferior aspects of the fold in the mid membranous plane of the baseline larynges were taken using the indentation method. Calcium hydroxylapatite (CaHA) crystals were then injected into the superior aspect of the fold. The stress-strain tests were repeated. Particle imaging velocimetry (PIV) of the intraglottal velocity fields was performed in three larynges at different subglottal pressures in the mid coronal plane for the baseline and CaHA-injected larynges. RESULTS: CaHA injection reduced the inferior-superior stiffness gradient in all larynges. The maximal divergence angle was markedly reduced. In some cases, there was not a divergent angle. DISCUSSION: Marked reduction of the vertical stiffness gradient significantly reduces the maximum divergence angle. Clinical implications will be discussed. LEVEL OF EVIDENCE: NA Laryngoscope, 131:E1934-E1940, 2021.

Effects of False Vocal Folds on Intraglottal Velocity Fields.

Previous models have theorized that, during phonation, skewing of the glottal waveform (which is correlated with acoustic intensity) occurred because of inertance of the vocal tract. Later, we reported that skewing of the flow rate waveform can occur without the presence of a vocal tract in an excised canine larynx. We hypothesized that in the absence of a vocal tract, the skewing formed when dynamic pressures acted on the glottal wall during the closing phase; such pressures were greatly affected by formation of intraglottal vortices. In this study, we aim to identify how changes in false vocal folds constriction can affect the acoustics and intraglottal flow dynamics. The intraglottal flow measurements were made using particle image velocimetry in an excised canine larynx where a vocal tract model was placed above the larynx and the constriction between the false vocal folds was varied. Our results show that for similar values of subglottal pressures, the skewing of the glottal waveform, strength of the intraglottal vortices, and acoustic energy increased as the constriction between the false vocal folds was increased. These preliminary findings suggest that acoustic intensity during phonation can be increased by the addition of a vocal tract with false fold constriction.

Quantification of the Intraglottal Pressure Induced by Flow Separation Vortices Using Large Eddy Simulation.

The greatest rate of change in the glottal flow rate during phonation is a rapid decrease that occurs during the latter part of the glottal closing. Previous works showed that intraglottal flow separation vortices form in a divergent glottis, produce negative gauge pressures (below atmospheric) during closing. It is hypothesized here that flow separation vortices contribute to the rapid closing mechanism of the true vocal folds during phonation. Four idealized static models (M5) of the human larynx were investigated using large eddy simulation: 2 models featured parallel folds that did not enable flow separation in the glottis and 2 models involved a divergent glottis. The influence of the ventricular gap (narrow/wide) is evaluated. An unsteady pressure inlet representing a voicing cycle was applied to the sub-glottal region to mimic the time-varying glottal flow. Intraglottal vortex structures formed downstream of the separation point in a divergent glottis. Their existence caused a higher closing force that was applied onto the vocal folds. A narrow ventricular gap strengthens this effect. Strength of the intraglottal vortices increased with the maximum flow declination rate. Therefore, a more divergent shape of the glottis during glottal closing will be one of the main contributors to the voice quality.

Volume velocity in a canine larynx model using time­resolved tomographic particle image velocimetry.

In the classic source-filter theory, the source of sound is flow modulation. "Flow" is the flow rate (Q) and flow modulation is dQ/dt. Other investigators have argued, using theoretical, computational, and mechanical models of the larynx, that there are additional sources of sound. To determine the acoustic role of dQ/dt in a tissue model, Q needs to be accurately measured within a few millimeters of the glottal exit; however, no direct measures of Q currently exist. The goal of this study is to obtain this waveform in an excised canine larynx model using time-resolved tomographic particle image velocimetry. The flow rate data are captured simultaneously with acoustic measurements to determine relations with vocal characteristics. The results show that glottal waveform characteristics such as maximum flow declination rate are proportional to the subglottal pressure, fundamental frequency, and acoustic intensity. These findings are important as they use direct measurements of the volume flow at the glottal exit to validate some of the assumptions used in the source-filter theory. In addition, future work will address the accuracy of indirect clinical measurement techniques, such as the Rothenberg mask.

Computational study of false vocal folds effects on unsteady airflows through static models of the human larynx.

Compressible large eddy simulation is employed to numerically investigate the laryngeal flow. Symmetric static models of the human larynx with a divergent glottis are considered, with the presence of false vocal folds (FVFs). The compressible study agrees well with that of the incompressible study. Due to the high enough Reynolds number, the flow is unsteady and develops asymmetric states downstream of the glottis. The glottal jet curvature decreases with the presence of FVFs or the ventricular folds. The gap between the FVFs stretches the flow structure and reduces the jet curvature. The presence of FVFs has a significant effect on the laryngeal flow resistance. The intra-glottal vortex structures are formed on the divergent wall of the glottis, immediately downstream of the separation point. The vortices are then convected downstream and characterized by a significant negative static pressure. The FVFs are a main factor in the generation of stronger vortices, and thus on the closure of the TVFs. The direct link between the FVFs geometry and the motion of the TVFs, and by extension to the voice production, is of interest for medical applications as well as future research works. The presence of the FVFs also changes the dominant frequencies in the velocity and pressure spectra.