Biomechanical Flow Amplification Arising From the Variable Deformation of the Subglottic Mucosa.

OBJECTIVE: This study mapped the variation in tissue elasticity of the subglottic mucosa, applied these data to provide initial models of the likely deformation of the mucosa during the myoelastic cycle, and hypothesized as to the impact on the process of phonation. STUDY DESIGN: Six donor human larynges were dissected along the sagittal plane to expose the vocal folds and subglottic mucosa. A linear skin rheometer was used to apply a controlled shear force, and the resultant displacement was measured. These data provided a measure of the stress/strain characteristics of the tissue at each anatomic point. A series of measurements were taken at 2-mm interval inferior of the vocal folds, and the change in elasticity was determined. RESULTS: It was found that the elasticity of the mucosa in the subglottic region increased linearly with distance from the vocal folds in all 12 samples. A simple deformation model indicated that under low pressure conditions the subglottic mucosa will deform to form a cone, which could result in a higher velocity, thus amplifying the low pressure effect resulting from the Venturi principle, and could assist in maintaining laminar flow. CONCLUSIONS: This study indicated that the deformation of the subglottic mucosa could play a significant role in the delivery of a low pressure airflow over the vocal folds. A large scale study will now be undertaken to secure more data to evaluate this hypothesis, and using computational fluid dynamics based on actual three-dimensional structure obtained from computed tomography scans the aerodynamics of this region will be investigated.

Quantification of change in vocal fold tissue stiffness relative to depth of artificial damage.

OBJECTIVES: To quantify changes in the biomechanical properties of human excised vocal folds with defined artificial damage. METHODS: The linear skin rheometer (LSR) was used to obtain a series of rheological measurements of shear modulus from the surface of 30 human cadaver vocal folds. The tissue samples were initially measured in a native condition and then following varying intensities of thermal damage. Histological examination of each vocal fold was used to determine the depth of artificial alteration. The measured changes in stiffness were correlated with the depth of cell damage. RESULTS: For vocal folds in a pre-damage state the shear modulus values ranged from 537 Pa to 1,651 Pa (female) and from 583 Pa to 1,193 Pa (male). With increasing depth of damage from the intermediate layer of the lamina propria (LP), tissue stiffness increased consistently (compared with native values) following application of thermal damage to the vocal folds. The measurement showed an increase of tissue stiffness when the depth of tissue damage was extending from the intermediate LP layer downwards. CONCLUSIONS: Changes in the elastic characteristics of human vocal fold tissue following damage at defined depths were demonstrated in an in vitro experiment. In future, reproducible in vivo measurements of elastic vocal fold tissue alterations may enable phonosurgeons to infer the extent of subepithelial damage from changes in surface elasticity.

The anisotropic nature of the human vocal fold: an ex vivo study.

The purpose of this study was to measure the relationship between the shear elastic properties of vocal fold with respect to the direction of applied stress. There is extensive published material that quantifies the shear viscoelastic properties of the vocal fold, but as much of these data were obtained using rotating parallel plate rheometers, which are unable to resolve out difference of the shear elastic behaviour with respect to direction, there is very little data that indicates anisotropic behaviour. To overcome this gap in knowledge, the team devised an apparatus that is capable of applying a shear stress in a known direction. A series of measurements were taken at the mid-membranous position, in the transverse and longitudinal directions. Point-specific measurements were performed using fourteen human cadaver excised larynges, which were hemi-sectioned to expose the vocal fold. An extremely low sinusoidal shear force of 1 g was applied tangentially to the membrane surface in both the longitudinal and transverse direction, and the resultant shear strain was measured. With the probe applied to the intact vocal fold, the average ratio of the elasticity in the transverse with respect to the longitudinal direction was 0.55. Further investigation using histological staining of collagens in the lamina propria indicates that there is a visible difference in the general alignment of collagen fibres when comparing the coronal and the sagittal sections. Our conclusion is that there is a quantifiable difference between the shear elastic response of the lamina propria in the longitudinal and transverse directions, and that this could be explained by the difference in alignment of collagen fibres within the lamina propria.

Gradation of stiffness of the mucosa inferior to the vocal fold.

During phonation, energy is transferred from the subglottal airflow through the air/mucosa interface that results in the propagation of the mucosal wave in the vocal fold. The vocal fold is soft, and the subglottal mucosa is stiff. We hypothesize that it is highly improbable that there is a rigid boundary between the tissue structures, with a sudden drop in stiffness; and that a gradual change would be more likely to support the efficient transfer of energy from the airflow to the mucosal wave. Our objective was to test this hypothesis by quantifying the change in mucosa stiffness with respect to anatomical position. In this initial study, using five pig larynges, a series of point-specific measurements of mucosa stiffness were taken in a line from the midpoint of the vocal fold toward the trachea. A modified linear skin rheometer adapted for laryngeal elasticity measurement applied shear stress to a series of seven positions at 2-mm intervals starting from the midmembranous vocal fold medial surface. A sinusoidal shear force of 1g was applied at each point, and resultant displacement curve logged. Using a regression algorithm, the stiffness of the tissue was derived in units of grams force per millimeter displacement. Five readings were taken at each position. The results indicate that there is a linear increase in stiffness with respect to position, increasing as the measurements are taken further from the vocal fold. There is a gradual change in stiffness of the subglottal mucosa of a pig larynx.

Control of vocal fold cover stiffness by laryngeal muscles: a preliminary study.

OBJECTIVES: To perform preliminary measurements of the shear modulus of the vocal fold cover layer during intrinsic laryngeal muscle contraction. STUDY DESIGN: Shear modulus was measured in an in vivo canine larynx and an ex vivo human larynx. METHODS: Shear stress was applied to the transverse axis of the vocal fold using a modified linear skin rheometer (LSR) via an attached suction probe. The probe displacement in response to the applied force was measured at various levels of laryngeal muscle contraction. The force-displacement data were used to derive the shear modulus using a simple shear model. In the ex vivo human larynx, lateral cricoarytenoid (LCA) muscle and cricothyroid (CT) muscle activity was simulated with arytenoid adduction and cricothyroid approximation sutures, respectively. In the in vivo canine, adductor muscle and CT muscle contraction was induced with graded stimulation of the recurrent laryngeal nerve (RLN) and the superior laryngeal nerves (SLN), respectively. RESULTS: : Baseline shear modulus was between 1,076 and 1,307 Pascals. In the ex vivo human larynx, the shear modulus increased gradually to a maximum of 1.6 times baseline value with graded arytenoid adduction and 3.7 times baseline value with cricothyroid approximation. In the in vivo larynx, the shear modulus increased to a maximum of 1.6 times baseline value with RLN stimulation and 2.5 times baseline value with SLN stimulation. CONCLUSIONS: Findings are in agreement with the cover-body model in that cricothyroid muscle activity generates a greater change in cover stiffness than laryngeal adductors. The role of the individual laryngeal adductors (thyroarytenoid [TA] vs. LCA) in control of vocal fold cover stiffness remains to be further elucidated.

Viscoelastic measurements of vocal folds using the linear skin rheometer.

As the number of interventions for vocal fold scar grows and with the advancement of mathematical modeling, greater accuracy and precision in the measurement of vocal fold pliability will become essential. Although indirect pliability measures have been used successfully, direct measurement of tissue pliability is essential. Indirect measurement with parallel plate technology has limitations; it requires the tissue to be removed from the surrounding framework, allows no site specificity, and offers no future for in vivo use in animals or humans. We tested the linear skin rheometer (LSR) in the evaluation of vocal fold pliability. We measured site-specific rheology of vocal folds thereby creating "pliability maps" in human, dog, and rat cadaveric larynges under conditions of altered stiffness; the canine vocal folds possessed sulci, the rat vocal fold was stiff secondary to controlled biopsy, and the human vocal fold was injected with trichloroacetic acid. Histology was performed to confirm the site and type of canine sulci. We found that the LSR reliably detected stiffness in the vocal folds of all species and created "pliability maps" consistent with previous data and clinical observations. The LSR should prove useful in the evaluation of vocal fold pliability for ex vivo and ultimately for in vivo applications.

The shear modulus of the human vocal fold in a transverse direction.

The aim of this study was to measure the shear modulus of the vocal fold in a human hemilarynx, such that the data can be related to direction of applied stress and anatomical context. Dynamic spring rate data were collected using a modified linear skin rheometer using human hemilarynges, and converted to estimated shear modulus via application of a simple shear model. The measurement probe was attached to the epithelial layer of the vocal fold cover using suction. A sinusoidal force of 3g was applied to the epithelium, and the resultant displacement logged at a rate of 1kHz. Force measurement accuracy was 20microg and position measurement accuracy was 4microm. The force was applied in a transverse direction at the midmembranous point between the vocal process and the anterior commissure. The shear modulus of the three female vocal folds ranged from 814 to 1232Pa. The shear modulus of the three male vocal folds ranged from 1021 to 1796Pa. These data demonstrate that it is possible to obtain estimates for the shear modulus of the vocal fold while preserving anatomical context. The modulus values reported here are higher than those reported using parallel plate rheometry. This is to be expected as the tissue is attached to surrounding structures, and is under natural tension.

In vivo measurement of the shear modulus of the human vocal fold: interim results from eight patients.

The shear modulus of the vocal fold is an essential parameter required to enhance our understanding of how the vocal fold operates, to develop mathematical models of phonatation, and to provide benchmarks to quantify the effectiveness of surgical procedures. The authors announced the successful deployment of an instrument to measure vocal fold elasticity in vivo last year, and now present the data taken from eight patients in vivo. The shear modulus was measured at the mid-membranous point, in a transverse direction with respect to the axis drawn between the anterior commissure and vocal process. The range of mean shear modulus results is 701-2,225 Pa, with a mean value of 1,371 Pa.

The shear modulus of the human vocal fold, preliminary results from 20 larynxes.

Quantification of the elastic properties of the human vocal fold provides invaluable data for researchers deriving mathematical models of phonation, developing tissue engineering therapies, and as normative data for comparison between healthy and scarred tissue. This study measured the shear modulus of excised cadaver vocal folds from 20 subjects. Twenty freshly excised human larynxes were evaluated less than four days post-mortem. They were split along the saggital plane and mounted without tension. Shear modulus was obtained by two different methods. For method 1 cyclical shear stress was applied transversely to the mid-membranous portion of the vocal fold, and shear modulus derived by applying a simple shear model. For method 2 the apparatus was configured as an indentometer, and shear modulus obtained from the stress/strain data by applying an established analytical technique. Method 1 shear model for male larynxes yielded a range from 246 to 3,356 Pa, with a mean value of 1,008 and SD of 380. The range for female larynxes was 286-3,332 Pa, with a mean value of 1,237 and SD of 768. Method 2 indentometer model for male larynxes yielded a range from 552 to 2,741 Pa, with a mean value of 1,000 and SD of 460. The range for female larynxes was 509-1,989 Pa, with a mean value of 1,332 and SD of 428. We have successfully demonstrated two methodologies that are capable of directly measuring the shear modulus of the human vocal fold, without dissecting out the vocal fold cover tissue. The sample size of nine female and 11 male larynxes is too small to validate a general conclusion. The high degree of variability in this small cohort of subjects indicates that factors such as age, health status, and post-mortem delay may be significant; and that there is range of 'normality' for vocal fold tissue.

Measurements of vocal fold elasticity using the linear skin rheometer.

OBJECTIVE: The linear skin rheometer (LSR), which measures skin visco-elasticity, was adapted for measurements of vocal fold properties. A series of studies was performed on animal and human excised larynges to determine if the LSR technique can be applied to the vocal fold. METHODS: In excised larynges, small patches of mucosa were driven sinusoidally at 0.3 Hz over distances of 1-2 mm using a small probe. Forces in the order of 1 g equivalent gave optimal measurements. Stiffness and viscosity values were derived from stress/strain data. RESULTS: The instrument was able to measure the visco-elasticity of the tissue in a repeatable manner and it could detect areas where the tissue was artificially stiffened. Two-dimensional maps of the mechanical properties of the laryngeal mucosa were obtained showing local variations in elasticity both parallel and perpendicular to the vocal fold edge. Initial studies were undertaken using animal tissue; more recently, the LSR has been successfully used to obtain similar data from human tissue. CONCLUSION: The LSR was been demonstrated to be capable of measuring the elastic properties of the vocal fold in a repeatable and reliable manner. Further studies will now be undertaken to obtain data from a larger sample of human tissue.

In vivo measurement of the elastic properties of the human vocal fold.

The ability to measure the biomechanical properties of the vocal fold in vivo is both an aid to diagnosis and enhances our knowledge of how the vocal folds operate. This paper details a new instrument that is capable of taking readings of the spring rate of the vocal fold in a repeatable manner. We also present three sets of readings taken from two volunteer patients. Patient 1 was suffering from polyp growth, and the data presented are taken from both the damaged vocal fold and the healthy vocal fold. The third set of readings was obtained from a similar volunteer and taken from a healthy vocal fold. It can be seen that the data obtained from the healthy vocal folds are similar and that the data obtained from the diseased vocal fold is at variance.