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

Recurring exposure to low humidity induces transcriptional and protein level changes in the vocal folds of rabbits

Voice disorders are an important human health condition. Hydration is a commonly recommended preventive measure for voice disorders though it is unclear how vocal fold dehydration is harmful at the cellular level. Airway surface dehydration can result from exposure to low humidity air. Here we have induced airway surface dehydration in New Zealand White rabbits exposed to a recurring 8-h low humidity environment over 15 days. This model mimics an occupational exposure to a low humidity environment. Exposure to moderate humidity was the control condition. Full thickness soft-tissue samples, including the vocal folds and surrounding laryngeal tissue, were collected for molecular analysis. RT-qPCR demonstrated a significant upregulation of MUC4 (mucin 4) and SCL26A9 (chloride channel) and a large fold-change though statistically non-significant upregulation of SCNNA1 (epithelial sodium channel). Proteomic analysis demonstrated differential regulation of proteins clustering into prospective functional groups of muscle structure and function, oxidative stress response, and protein chaperonin stress response. Together, the data demonstrate that recurring exposure to low humidity is sufficient to induce both transcriptional and translational level changes in laryngeal tissue and suggest that low humidity exposure induces cellular stress at the level of the vocal folds.

Shrinkage of specimens after CO2 laser cordectomy: an objective intraoperative evaluation

PURPOSE

The margin status after CO₂ laser cordectomy for glottic carcinoma may influence prognosis. There are no studies assessing the possible bias due to anatomic changes of the specimens for shrinkage. The authors evaluated the intraoperative shrinkage of specimens immediately after transoral CO₂ laser microsurgery (CO₂ TOLMS) to improve the understanding and the interpretation of surgical margins.

METHODS

This is a prospective study involving a consecutive cohort of 23 patients (19 males, 4 females, mean age 69.9 years, range 42-83 years) with early glottic carcinoma treated by CO₂ TOLMS from February 2017 to April 2019. The anteroposterior shrinkage of the specimen, of the tumor, and of the anterior and posterior margins was measured intraoperatively with a cross table reticle glass micrometer ruler, inserted into the eyepiece of the surgical microscope.

RESULTS

The mean shrinkage of the mucosal specimen from intralaryngeal measurement to post-resection measurement was 3.8 ± 0.3 mm, resulting in an average loss of 29% of the anteroposterior length (p < 0.01). The anteroposterior length of both the tumor and the margins after resection significantly decreased, but the shrinkage of the anterior and posterior margins was significantly greater than the shrinkage of the tumor (49% versus 20% and 45% versus 20%, p < 0.01).

CONCLUSION

The present study demonstrates significant shrinkage of specimens after CO₂ TOLMS, especially in the anteroposterior length of the vocal cords, and justifies the good oncological results for specimens with close and single positive superficial margins. Follow-up versus a second surgical look policy could be safely suggested in cases of close superficial and single positive superficial margins.

RNA sequencing identifies transcriptional changes in the rabbit larynx in response to low humidity challenge

BACKGROUND

Voice disorders are a worldwide problem impacting human health, particularly for occupational voice users. Avoidance of surface dehydration is commonly prescribed as a protective factor against the development of dysphonia. The available literature inconclusively supports this practice and a biological mechanism for how surface dehydration of the laryngeal tissue affects voice has not been described. In this study, we used an in vivo male New Zealand white rabbit model to elucidate biological changes based on gene expression within the vocal folds from surface dehydration. Surface dehydration was induced by exposure to low humidity air (18.6% + 4.3%) for 8 h. Exposure to moderate humidity (43.0% + 4.3%) served as the control condition. Ilumina-based RNA sequencing was performed and used for transcriptome analysis with validation by RT-qPCR.

RESULTS

There were 103 statistically significant differentially expressed genes identified through Cuffdiff with 61 genes meeting significance by both false discovery rate and fold change. Functional annotation enrichment and predicted protein interaction mapping showed enrichment of various loci, including cellular stress and inflammatory response, ciliary function, and keratinocyte development. Eight genes were selected for RT-qPCR validation. Matrix metalloproteinase 12 (MMP12) and macrophage cationic peptide 1 (MCP1) were significantly upregulated and an epithelial chloride channel protein (ECCP) was significantly downregulated after surface dehydration by RNA-Seq and RT-qPCR. Suprabasin (SPBN) and zinc activated cationic channel (ZACN) were marginally, but non-significantly down- and upregulated as evidenced by RT-qPCR, respectively.

CONCLUSIONS

The data together support the notion that surface dehydration induces physiological changes in the vocal folds and justifies targeted analysis to further explore the underlying biology of compensatory fluid/ion flux and inflammatory mediators in response to airway surface dehydration.

Mechanics of human vocal folds layers during finite strains in tension, compression and shear

During phonation, human vocal fold tissues are subjected to combined tension, compression and shear loading modes from small to large finite strains. Their mechanical behaviour is however still not well understood. Herein, we complete the existing mechanical database of these soft tissues, by characterising, for the first time, the cyclic and finite strains behaviour of the lamina propria and vocalis layers under these loading modes. To minimise the inter or intra-individual variability, particular attention was paid to subject each tissue sample successively to the three loadings. A non-linear mechanical behaviour is observed for all loading modes: a J-shape strain stiffening in longitudinal tension and transverse compression, albeit far less pronounced in shear, stress accommodation and stress hysteresis whatever the loading mode. In addition, recorded stress levels during longitudinal tension are much higher for the lamina propria than for the vocalis. Conversely, the responses of the lamina propria and the vocalis in transverse compression as well as transverse and longitudinal shears are of the same orders of magnitude. We also highlight the strain rate sensitivity of the tissues, as well as their anisotropic properties.

Quantitative second harmonic generation imaging of leporine, canine, and porcine vocal fold collagen

OBJECTIVES/HYPOTHESIS

Vocal fold collagen composition is an important determinant of material properties and mucosal wave propagation. Collagen alignment and straightness are quantitatively characterized by second harmonic generation (SHG) imaging. We examined leporine, canined and porcine vocal folds showing collagen composition variation that is species, location, and strain specific.

STUDY DESIGN

Animal model.

METHODS

Leporine (n = 5), canine (n = 5), and porcine (n = 5) larynges were harvested and fixed in situ. Samples were transversely sectioned, and SHG images were collected for two inferior-superior sections along five anterior-posterior locations. Additional porcine samples were fixed and imaged under tensile strain (0%, 5%, 10%, 15%, 20%, n = 5 per group). Two-way repeated measures (RM) analysis of variance (ANOVA) tested for section and location differences in each species. Multiway RM-ANOVA tested for section, location, and strain differences in porcine samples.

RESULTS

Alignment and straightness were higher inferiorly in the porcine (P = .0047, P = .002) and canine (P = .0011, P < .001) vocal folds, but not in leporine samples (P = .67652, P = .4831). There were significant interactions between elongation and superior-inferior section for both alignment (P = .0047) and straightness (P = .0371).

CONCLUSIONS

Our results correspond well to findings in the literature that the inferior vocal fold lip is stiffer in porcine and canine larynges. The absence of a collagen gradient in the leporine vocal fold is notable because rabbits are less vocal animals, indicating the collagen gradient may be a result of voice use and an important consideration in model selection when extracellular matrix is of interest. Strain results were also consistent with the role of collagen in strain stiffening behavior of vocal fold tissue.

LEVEL OF EVIDENCE

NA Laryngoscope, 129:2549-2556, 2019.

Viscoelastic properties of human aryepiglottic fold and ventricular fold tissues at phonatory frequencies

OBJECTIVE

The purpose of this study was to quantify the viscoelastic shear properties of the human ventricular fold (or false vocal fold) mucosa and aryepiglottic fold mucosa at frequencies of phonation.

METHODS

Linear viscoelastic shear properties of the mucosa of false vocal fold and aryepiglottic fold specimens from seven cadaveric subjects were determined as functions of frequency (5-250 Hz) and compared to those of the true vocal fold cover. Measurements of elastic shear modulus (G') and dynamic viscosity (η') were made with a controlled-strain simple-shear rheometer. Linear least-squares regression was conducted to curve-fit log G' and log η' versus log frequency, and statistical analysis was performed with one-way analysis of variance.

RESULTS

All specimens showed similar frequency dependence of the viscoelastic functions G' and η', with G' gradually increasing with frequency and η' monotonically decreasing with frequency. The magnitudes of G' and η' of the false fold mucosa were generally higher than those of the aryepiglottic fold mucosa and true vocal fold cover, but there were no significant differences in G' and η' among the false fold, aryepiglottic fold, and true vocal fold.

CONCLUSION

The false vocal fold and aryepiglottic fold mucosa showed similar frequency dependence and a similar range of tissue viscoelastic behavior as the true vocal fold. These preliminary findings suggested that such tissues could become candidates for the replacement of the true vocal fold lamina propria in patients with significant tissue loss and deficiencies, for those requiring laryngeal reconstruction following partial laryngectomy or airway reconstruction.

LEVEL OF EVIDENCE

NA. Laryngoscope, E296-E301, 2018.

Comparison of a fiber-gel finite element model of vocal fold vibration to a transversely isotropic stiffness model

A fiber-gel vocal fold model is compared to a transversely isotropic stiffness model in terms of normal mode vibration. The fiber-gel finite element model (FG-FEM) consists of a series of gel slices, each with a two-dimensional finite element mesh, in a plane transverse to the tissue fibers. The gel slices are coupled with fibers under tension in the anterior-posterior dimension. No vibrational displacement in the fiber-length direction is allowed, resulting in a plane strain state. This is consistent with the assumption of transverse displacement of a simple string, offering a wide range of natural frequencies (well into the kHz region) with variable tension. For low frequencies, the results compare favorably with the natural frequencies of a transversely isotropic elastic stiffness model (TISM) in which the shear modulus in the longitudinal plane is used to approximate the effect of fiber tension. For high frequencies, however, the natural frequencies do not approach the string mode frequencies unless plane strain is imposed on the TISM model. The simplifying assumption of plane strain, as well as the use of analytical closed-form shape functions, allow for substantial savings in computational time, which is important in clinical and exploratory applications of the FG-FEM model.

Functional assessment of the ex vivo vocal folds through biomechanical testing: A review

The human vocal folds are complex structures made up of distinct layers that vary in cellular and extracellular composition. The mechanical properties of vocal fold tissue are fundamental to the study of both the acoustics and biomechanics of voice production. To date, quantitative methods have been applied to characterize the vocal fold tissue in both normal and pathologic conditions. This review describes, summarizes, and discusses the most commonly employed methods for vocal fold biomechanical testing. Force-elongation, torsional parallel plate rheometry, simple-shear parallel plate rheometry, linear skin rheometry, and indentation are the most frequently employed biomechanical tests for vocal fold tissues and each provide material properties data that can be used to compare native tissue to diseased or treated tissue. Force-elongation testing is clinically useful, as it allows for functional unit testing, while rheometry provides physiologically relevant shear data, and nanoindentation permits micrometer scale testing across different areas of the vocal fold as well as whole organ testing. Thoughtful selection of the testing technique during experimental design to evaluate a hypothesis is critical to optimize biomechanical testing of vocal fold tissues.