Viscoelastic measurements after vocal fold scarring in rabbits--short-term results after hyaluronan injection

Ultrafast Laser Microlaryngeal Surgery for In Vivo Subepithelial Void Creation in Canine Vocal Folds

BACKGROUND/OBJECTIVES

Tightly-focused ultrafast laser pulses (pulse widths of 100 fs-10 ps) provide high peak intensities to produce a spatially confined tissue ablation effect. The creation of sub-epithelial voids within scarred vocal folds (VFs) via ultrafast laser ablation may help to localize injectable biomaterials to treat VF scarring. Here, we demonstrate the feasibility of this technique in an animal model using a custom-designed endolaryngeal laser surgery probe.

METHODS

Unilateral VF mucosal injuries were created in two canines. Four months later, ultrashort laser pulses (5 ps pulses at 500 kHz) were delivered via the custom laser probe to create sub-epithelial voids of ~3 × 3-mm2 in both healthy and scarred VFs. PEG-rhodamine was injected into these voids. Ex vivo optical imaging and histology were used to assess void morphology and biomaterial localization.

RESULTS

Large sub-epithelial voids were observed in both healthy and scarred VFs immediately following in vivo laser treatment. Two-photon imaging and histology confirmed ~3-mm wide subsurface voids in healthy and scarred VFs of canine #2. Biomaterial localization within a void created in the scarred VF of canine #2 was confirmed with fluorescence imaging but was not visualized during follow-up two-photon imaging. As an alternative, the biomaterial was injected into the excised VF and could be observed to localize within the void.

CONCLUSIONS

We demonstrated sub-epithelial void formation and the ability to inject biomaterials into voids in a chronic VF scarring model. This proof-of-concept study provides preliminary evidence towards the clinical feasibility of such an approach to treating VF scarring using injectable biomaterials.

LEVEL OF EVIDENCES

N/A Laryngoscope, 133:3042-3048, 2023.

Ultrafast laser surgery probe for sub-surface ablation to enable biomaterial injection in vocal folds

Creation of sub-epithelial voids within scarred vocal folds via ultrafast laser ablation may help in localization of injectable therapeutic biomaterials towards an improved treatment for vocal fold scarring. Several ultrafast laser surgery probes have been developed for precise ablation of surface tissues; however, these probes lack the tight beam focusing required for sub-surface ablation in highly scattering tissues such as vocal folds. Here, we present a miniaturized ultrafast laser surgery probe designed to perform sub-epithelial ablation in vocal folds. The requirement of high numerical aperture for sub-surface ablation, in addition to the small form factor and side-firing architecture required for clinical use, made for a challenging optical design. An Inhibited Coupling guiding Kagome hollow core photonic crystal fiber delivered micro-Joule level ultrashort pulses from a high repetition rate fiber laser towards a custom-built miniaturized objective, producing a 1/e2 focal beam radius of 1.12 ± 0.10 μm and covering a 46 × 46 μm2 scan area. The probe could deliver up to 3.8 μJ pulses to the tissue surface at 40% transmission efficiency through the entire system, providing significantly higher fluences at the focal plane than were required for sub-epithelial ablation. To assess surgical performance, we performed ablation studies on freshly excised porcine hemi-larynges and found that large area sub-epithelial voids could be created within vocal folds by mechanically translating the probe tip across the tissue surface using external stages. Finally, injection of a model biomaterial into a 1 × 2 mm2 void created 114 ± 30 μm beneath the vocal fold epithelium surface indicated improved localization when compared to direct injection into the tissue without a void, suggesting that our probe may be useful for pre-clinical evaluation of injectable therapeutic biomaterials for vocal fold scarring therapy. With future developments, the surgical system presented here may enable treatment of vocal fold scarring in a clinical setting.

In vivo hamster cheek pouch subepithelial ablation, biomaterial injection, and localization: pilot study

SIGNIFICANCE

The creation of subepithelial voids within scarred vocal folds via ultrafast laser ablation may help in localization of injectable biomaterials toward a clinically viable therapy for vocal fold scarring.

AIM

We aim to prove that subepithelial voids can be created in a live animal model and that the ablation process does not engender additional scar formation. We demonstrate localization and long-term retention of an injectable biomaterial within subepithelial voids.

APPROACH

A benchtop nonlinear microscope was used to create subepithelial voids within healthy and scarred cheek pouches of four Syrian hamsters. A model biomaterial, polyethylene glycol tagged with rhodamine dye, was then injected into these voids using a custom injection setup. Follow-up imaging studies at 1- and 2-week time points were performed using the same benchtop nonlinear microscope. Subsequent histology assessed void morphology and biomaterial retention.

RESULTS

Focused ultrashort pulses can be used to create large subepithelial voids in vivo. Our analysis suggests that the ablation process does not introduce any scar formation. Moreover, these studies indicate localization, and, more importantly, long-term retention of the model biomaterial injected into these voids. Both nonlinear microscopy and histological examination indicate the presence of biomaterial-filled voids in healthy and scarred cheek pouches 2 weeks postoperation.

CONCLUSIONS

We successfully demonstrated subepithelial void formation, biomaterial injection, and biomaterial retention in a live animal model. This pilot study is an important step toward clinical acceptance of a new type of therapy for vocal fold scarring. Future long-term studies on large animals will utilize a miniaturized surgical probe to further assess the clinical viability of such a therapy.

Hyaluronan Hydrogels for the Local Delivery of Mesenchymal Stromal Cells to the Injured Vocal Fold

Mesenchymal stromal cells (MSCs) promote wound healing by expediting the inflammatory phase. Local injection of MSCs into injured vocal folds (VFs) is effective in animal models, suggesting suitability for clinical translation. Despite their therapeutic potential, MSCs do not persist within the VF. This study evaluates whether hyaluronan (HA) hydrogels offer a safe delivery vehicle for local injection of MSCs into VFs, and increase longevity of the cells within the injured tissue. MSCs ± HA hydrogel were exposed to interleukin (IL)1β, IL8, and chemokine (C-C motif) ligand 4, and evaluated for mRNA expression of matrix remodeling genes and secretion of immunomodulatory/prohealing factors. Chemotaxis/invasion in response to inflammation was evaluated. A lapin model of VF injury evaluated in vivo effects of MSCs ± HA hydrogel on enhancing VF healing. Histological evaluation of inflammation, type I collagen expression, HA hydrogel resorption, and MSC persistence was evaluated at 3 and 25 days after injury. MSCs within HA hydrogel were responsive to their extracellular environment, upregulating immunomodulatory factors when exposed to inflammation. Despite delayed migration out of the gel in vitro, the MSCs did not persist longer within the injured tissue in vivo. MSCs ± HA hydrogel exerted equivalent dampening of inflammation in vivo. The gel was resorbed within 25 days and no edema was evident. HA hydrogels can be safely used in the delivery of MSCs to injured VFs, minimizing leakage of administered cells. MSCs within the HA hydrogel did not persist longer than those in suspension, but did exert comparable therapeutic effects.

Pathophysiology of Fibrosis in the Vocal Fold: Current Research, Future Treatment Strategies, and Obstacles to Restoring Vocal Fold Pliability

Communication by voice depends on symmetrical vibrations within the vocal folds (VFs) and is indispensable for various occupations. VF scarring is one of the main reasons for permanent dysphonia and results from injury to the unique layered structure of the VFs. The increased collagen and decreased hyaluronic acid within VF scars lead to a loss of pliability of the VFs and significantly decreases their capacity to vibrate. As there is currently no definitive treatment for VF scarring, regenerative medicine and tissue engineering have become increasingly important research areas within otolaryngology. Several recent reviews have described the problem of VF scarring and various possible solutions, including tissue engineered cells and tissues, biomaterial implants, stem cells, growth factors, anti-inflammatory cytokines antifibrotic agents. Despite considerable research progress, these technical advances have not been established as routine clinical procedures. This review focuses on emerging techniques for restoring VF pliability using various approaches. We discuss our studies on interactions among adipose-derived stem/stromal cells, antifibrotic agents, and VF fibroblasts using an in vitro model. We also identify some obstacles to advances in research.

Mesenchymal stromal cells modulate tissue repair responses within the injured vocal fold

OBJECTIVES/HYPOTHESIS

This study aimed to determine whether local injection of human mesenchymal stromal cells (MSC) could modulate the early inflammatory response within injured vocal folds (VFs) to promote wound-healing processes.

STUDY DESIGN

Experimental xenograft model.

METHODS

VF injury was surgically induced by bilateral resection of the lamina propria of rabbits, and MSC were immediately injected into the injured area of both VFs. Animals were sacrificed on days 2, 4, and 24. Histological analyses were performed by hematoxylin and eosin, Masson's Trichrome, and elastin staining. Cell death was visualized by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and the M2 macrophage marker, CD163, detected by immunohistochemistry. Persistence of injected MSC was evaluated by fluorescent in situ hybridization (FISH). Quantitative polymerase chain reaction was performed on the contralateral VF.

RESULTS

Histological examination at days 2 and 4 indicated that MSC were able to reduce tissue inflammation, with gene expression analysis confirming a significant reduction of proinflammatory markers, interleukin (IL)-1β, and IL-8. FISH demonstrated low-level persistence of injected MSC at both time points, and TUNEL confirmed localized cell death at the injury site. Increased levels of CD163+ anti-inflammatory macrophages indicated a change in the immune milieu, supporting wound resolution. Evidence of a more organized collagen matrix suggests that MSC may enhance the production of a functional repair tissue after injury, despite their low-level persistence within the tissue.

CONCLUSIONS

This study demonstrates that MSC are able to positively modulate the early wound-healing response through resolution of the inflammatory phase and promotion of tissue repair.

LEVEL OF EVIDENCE

NA Laryngoscope, 130:E21-E29, 2020.

Cell therapy and vocal fold scarring

Vocal fold microstructure is complex and can be affected by laryngeal microsurgery, inducing scarring that prevents mechanical uncoupling of epithelium and muscle, leading to vibration disorder and disabling dysphonia. Treatment options presently are few, and often without efficacy for vibration, having only an impact on volume to reduce glottal closure defect. The present review of the literature had two aims: (i) to report the current state of the literature on cell therapy in vocal fold scarring; and (ii) to analyze the therapeutic interest of the adipose-derived stromal vascular fraction in the existing therapeutic armamentarium. A PubMed^® search conducted in September 2016 retrieved English or French-language original articles on the use of stem cells to treat vocal fold scarring. Twenty-seven articles published between 2003 and 2016 met the study selection criteria. Mesenchymal stem cells were most widely used, mainly derived from bone marrow or adipose tissue. Four studies were performed in vitro on fibroblasts, and 18 in vivo on animals. End-points comprised: (i) scar analysis (macro- and micro-scopic morphology, viscoelastic properties, extracellular matrix, fibroblasts); and (ii) assessment of stem cell survival and differentiation. The studies testified to the benefit of mesenchymal stem cells, and especially those of adipose derivation. The stromal vascular fraction exhibits properties that might improve results by facilitating production logistics.

Parameters From the Complete Phonatory Range of an Excised Rabbit Larynx

OBJECTIVE

This study aims to collect data throughout the complete phonatory range using rabbit larynges.

STUDY DESIGN

This is a methodological excised rabbit larynx study.

METHODS

Seven rabbit larynges were dissected and mounted on a modified excised laryngeal apparatus. Phonation was initiated at phonation threshold pressure (PTP) and airflow was increased by consistent increments until phonation instability pressure (PIP) was reached. At each airflow level, aerodynamic measurements, acoustic recordings, and high-speed videos were recorded. This procedure was repeated at multiple elongation conditions to further explore the parameters. Data were then compared across subjects and elongation conditions.

RESULTS

At PTP, subglottal pressure, fundamental frequency, and sound pressure level were found to increase significantly as elongation was increased. As elongation was increased at PIP, airflow was found to significantly decrease, whereas fundamental frequency was found to significantly increase. Vibratory amplitude decreased at both PTP and PIP as elongation increased. Also, as elongation increased, the range of all parameters was found to decrease significantly.

CONCLUSIONS

The results obtained, combined with the similarities of the histologic structure of the vocal fold lamina propria between rabbits and humans, validate the rabbit larynx as an effective and reliable model for tissue inflammation studies.

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.

Stem Cell Therapy in Injured Vocal Folds: A Three-Month Xenograft Analysis of Human Embryonic Stem Cells

We have previously shown that human embryonic stem cell (hESC) therapy to injured rabbit vocal folds (VFs) induces human tissue generation with regained VF vibratory capacity. The aims of this study were to test the sustainability of such effect and to what extent derivatives of the transplanted hESCs are propagated in the VFs. The VFs of 14 New Zealand rabbits were injured by a localized resection. HESCs were transplanted to 22 VFs which were analyzed for persistence of hESCs after six weeks and after three months. At three months, the VFs were also analyzed for viscoelasticity, measured as dynamic viscosity and elastic modulus, for the lamina propria (Lp) thickness and relative content of collagen type I. Three months after hESC cell therapy, the dynamic viscosity and elastic modulus of the hESC treated VFs were similar to normal controls and lower than untreated VFs (p ≤ 0.011). A normalized VF architecture, reduction in collagen type I, and Lp thickness were found compared with untreated VFs (p ≤ 0.031). At three months, no derivatives of hESCs were detected. HESCs transplanted to injured rabbit VFs restored the vibratory characteristics of the VFs, with maintained restored function for three months without remaining hESCs or derivatives.

Safety and length of benefit of restylane for office-based injection medialization-a retrospective review of one institution's experience

OBJECTS

Restylane is a relatively new material being used for temporary vocal fold medialization. Few studies have evaluated the safety, complication rate, and length of benefit derived from Restylane injection medialization. We report on 64 patients who underwent office-based injection medialization with Restylane to establish the safety of and length of benefit from this material.

STUDY DESIGN

Retrospective review.

METHODS

Retrospective review of patients who underwent office-based injection medialization with Restylane during a 2-year period was performed.

RESULTS

Eighty-two injections in 64 patients were reviewed. Five total adverse events occurred (3.88%). The average length of benefit from injection medialization with Restylane was 12.2 weeks. No inflammatory or hypersensitivity reactions to Restylane occurred. A trend toward avoidance of permanent medialization was observed in patients injected <6 months from time of injury compared with patients injected >6 months after (P = 0.0511).

CONCLUSION

Restylane is a safe and effective temporary material for office-based injection medialization with an average length of benefit of 12.2 weeks. This represents the largest series to date using Restylane and no hypersensitivity or granulomatous reactions were observed. Our data showed a trend toward avoidance of permanent medialization in patients who were injected <6 months after nerve injury, which supports the findings of others.

Vocal fold scars: current concepts and future directions. Consensus report of the Phonosurgery Committee of the European Laryngological Society

Scarring of the vocal folds leads to a deterioration of the highly complex micro-structure with consecutively impaired vibratory pattern and glottic insufficiency. The resulting dysphonia is predominantly characterized by a reduced vocal capacity. Despite the considerable progress in understanding of the underlying pathophysiology, the treatment of scarred vocal folds is still an unresolved chapter in laryngology and phonosurgery. Essential for a successful treatment is an individual, multi-dimensional concept that comprises the whole armamentarium of surgical and non-surgical (i.p. voice therapy) modalities. An ideal approach would be to soften the scar, because the reduced pliability and consequently the increased vibratory rigidity impede the easiness of vibration. The chosen phonosurgical method is determined by the main clinical feature: Medialization techniques for the treatment of glottic gap, or epithelium freeing techniques for improvement of vibration characteristics often combined with injection augmentation or implantation. In severe cases, buccal mucosa grafting can be an option. New developments, include treatment with anxiolytic lasers, laser technology with ultrafine excision/ablation properties avoiding coagulation (Picosecond infrared laser, PIRL), or techniques of tissue engineering. However, despite the promising results by in vitro experiments, animal studies and first clinical trials, the step into clinical routine application has yet to be taken.

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.

Injection of human mesenchymal stem cells improves healing of vocal folds after scar excision--a xenograft analysis

OBJECTIVES

Using a xenograft model the aim was to analyze if injection of human mesenchymal stem cells (hMSC) into the rabbit vocal fold (VF), after excision of an established scar, can improve the functional healing of the VF.

STUDY DESIGN

Prospective design with an experimental xenograft model.

METHODS

The VFs of 12 New Zealand rabbits were injured by a bilateral localized resection. After 9 weeks the scar after the resection was excised and hMSC were injected into the VFs. After another 10 weeks 10 VFs were dissected and stained for histology. Lamina propria thickness and relative content of collagen type I were measured. Viscoelasticity of 14 VFs at phonatory frequencies was quantified by a simple-shear rheometer. The hMSC survival was determined using a human DNA specific reference probe, that is, FISH analysis.

RESULTS

The viscoelastic measurements, that is, dynamic viscosity and elastic shear modulus for the hMSC-treated VFs, were found to be similar to those of normal controls and were significantly lower than those of untreated controls (P < .05). A significant reduction in lamina propria thickness was also shown for the hMSC treated VFs compared with the untreated VFs (P < .05). This histologic finding corresponded with the viscoelastic results. No hMSC survived 10 weeks after the injection.

CONCLUSIONS

Human mesenchymal stem cells injected into the rabbit VF following the excision of a chronic scar, were found to enhance the functional healing of the VF with reduced lamina propria thickness and restored viscoelastic shear properties.

Assessment of local vocal fold deformation characteristics in an in vitro static tensile test

Voice quality is strongly dependent on vocal fold dynamics, which in turn are dependent on lung pressure and vocal fold biomechanics. Numerical and physical models are often used to investigate the interactions of these different subsystems. However, the utility of numerical and physical models is limited unless appropriately validated with data from physiological models. Hence a method that enables analysis of local vocal fold deformations along the entire surface is presented. In static tensile tests, forces are applied to distinctive working points being located in cover and muscle, respectively, so that specific layer properties can be investigated. The forces are directed vertically upward and are applied along or above the vocal fold edge. The resulting deformations are analyzed using multiple perspectives and three-dimensional reconstruction. Deformation characteristics of four human vocal folds were investigated. Preliminary results showed two phases of deformation: a range with a small slope for small deformations fading into a significant nonlinear deformation trend with a high slope. An increase of tissue stiffness from posterior to anterior was detected. This trend is more significant for muscle and in the mid-anterior half of the vocal fold.

Hyaluronic acid for the treatment of vocal fold scars

PURPOSE OF REVIEW

In vocal fold scars the lamina propria layer is lost or deficient. Lamina propria replacement therapy remains a clinical challenge because this layer has a highly specialized three-dimensional organization of extracellular matrix molecules and unique viscoelastic properties. Use of a polymer such as hyaluronic acid appears most promising for replacement therapy because it has the optimal viscoelasticity and also plays a role in the maturation and maintenance of vocal fold lamina propria.

RECENT FINDINGS

A variety of cross-linked hyaluronic acid formulations and growth factor therapies targeted to increase hyaluronic acid production have been used in the treatment of both acute and established vocal fold scars. Therapeutic strategies have focused on prevention of scar at the time of initial injury, and rejuvenation of lamina propria layer in established scars. Both strategies show improved histologic, viscoelastic, acoustic, and aerodynamic measures.

SUMMARY

Cross-linked hyaluronic acid formulations appear useful in the treatment of vocal fold scarring. Their use at the time of acute injury especially appears to lessen the degree of long-term scar formation and appears promising. While animal studies have demonstrated the safety profile of many hyaluronic acid formulations, further improvement in these materials and well designed and controlled human trials are needed to further establish the safety and efficacy of these materials and therapeutic approaches.

Histologic study of perifascial areolar tissue implanted in rabbit vocal folds: an experimental study

OBJECTIVES

Perifascial areolar tissue (PAT) consists of loose areolar tissue with viscoelastic properties that are similar to those found in tissues in the superficial layer of the vocal fold. The aim of this study was to quantify the inflammatory process and the collagen content of the graft, as well as that of the host tissue, after placement of a strip of PAT into the rabbit vocal fold.

METHODS

Surgeries were performed on 30 rabbits. The grafts were implanted in pockets that were surgically created in the right vocal fold. The left vocal fold (control group) was subjected only to surgical manipulation. The animals were divided into 3 groups for evaluations at 15 days, 3 months, and 6 months, and their larynx tissues were subsequently reviewed by histology.

RESULTS

The grafts were characterized by disorganized and thick collagen bundles and were identified in all study groups. The collagen density stayed constant over time. There was an acute inflammatory response induced by the graft at 15 days that did not exist in the specimens taken at 3 and 6 months. Deposition of collagen fibers in the lamina propria was observed starting at 15 days after the operation and was more intense in the experimental vocal fold than in the control vocal fold.

CONCLUSIONS

Our findings indicated that PAT has a low tendency for promoting an inflammatory response. However, there was a loss of the original architecture of the graft tissue and a greater deposition of collagen in the implanted vocal folds than in the control group.

Injection of human mesenchymal stem cells improves healing of scarred vocal folds: analysis using a xenograft model

OBJECTIVES/HYPOTHESIS

The aims were to analyze if improved histological and viscoelastic properties seen after injection of human mesenchymal stem cells (hMSCs) in scarred vocal folds (VFs) of rabbits are sustainable and if the injected hMSCs survive 3 months in the VFs.

STUDY DESIGN

Experimental xenograft model.

METHODS

Eighteen VFs of 11 New Zealand white rabbits were scarred by a bilateral localized resection. After 3 months the animals were sacrificed. Twelve VFs were dissected and stained for histology, lamina propria thickness, and relative collagen type I analyses. The hMSCs survival was analyzed using a human DNA-specific reference probe, that is, fluorescence in situ hybridization staining. Viscoelasticity, measured as the dynamic viscosity and elastic modulus, was analyzed in a parallel-plate rheometer for 10 VFs.

RESULTS

The dynamic viscosity and elastic modulus of hMSC-treated VFs were similar to that of normal controls and significantly improved compared to untreated controls (P < .05). A reduction in lamina propria thickness and relative collagen type 1 content were also shown for the hMSC-treated VFs compared to the untreated VFs (P < .05). The histological pictures corresponded well to the viscoelastic results. No hMSCs survived.

CONCLUSIONS

Human mesenchymal stem cells injected into a scarred vocal fold of rabbit enhance healing of the vocal fold with reduced lamina propria thickness and collagen type I content and restore the viscoelastic function.

Effects of substrate stiffness and cell density on primary hippocampal cultures

Previous studies have shown that dendrites are influenced by substrate stiffness when neurons are plated in either pure or mixed cultures. However, because substrate rigidity can also affect other aspects of culture development known to impact dendrite branching, such as overall cell number, it is unclear whether substrate stiffness exerts a direct or indirect effect on dendrite morphology. In this study, we determine whether substrate stiffness plays a critical role in regulating dendrite branching independent of cell number. We plated primary mixed hippocampal cultures on soft and stiff gels, with Young's moduli of 1 kPa and 7 kPa, respectively. We found that neurons plated on stiffer substrates showed increased branching relative to neurons grown on softer substrates at the same cell number. On the stiff gels, we also observed a cell number-dependent effect, in which increasing initial plating density decreased dendrite branching. This change correlates with an increase in extracellular glutamate. We concluded that both cell number and substrate stiffness play roles in determining dendrite branching, and that the two effects are independent of one another.

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.

Injection of embryonic stem cells into scarred rabbit vocal folds enhances healing and improves viscoelasticity: short-term results

OBJECTIVES

Scarring caused by trauma, postcancer treatment, or inflammation in the vocal folds is associated with stiffness of the lamina propria and results in severe voice problems. Currently there is no effective treatment. Human embryonic stem cells (hESC) have been recognized as providing a potential resource for cell transplantations, but in the undifferentiated state, they are generally not considered for therapeutic use due to risk of inadvertent development. This study assesses the functional potential of hESC to prevent or diminish scarring and improve viscoelasticity following grafting into scarred rabbit vocal folds.

STUDY DESIGN

hESC were injected into 22 scarred vocal folds of New Zealand rabbits. After 1 month, the vocal folds were dissected and analyzed for persistence of hESC by fluorescence in situ hybridization using a human specific probe, and for differentiation by evaluation in hematoxylin-eosin-stained tissues. Parallel-plate rheometry was used to evaluate the functional effects, i.e., viscoelastic properties, after treatment with hESC.

RESULTS

The results revealed significantly improved viscoelasticity in the hESC-treated vs. non-treated vocal folds. An average of 5.1% engraftment of human cells was found 1 month after hESC injection. In the hESC-injected folds, development compatible with cartilage, muscle and epithelia in close proximity or inter-mixed with the appropriate native rabbit tissue was detected in combination with less scarring and improved viscoelasticity.

CONCLUSIONS

The histology and location of the surviving hESC-derived cells strongly indicate that the functional improvement was caused by the injected cells, which were regenerating scarred tissue. The findings point toward a strong impact from the host microenvironment, resulting in a regional specific in vivo hESC differentiation and regeneration of three types of tissue in scarred vocal folds of adult rabbits.

Modern injection augmentation for glottic insufficiency

PURPOSE OF REVIEW

A number of technological advancements in recent years have spurred renewed interest in vocal fold injection augmentation. The present review discusses the characteristics of currently available short-term and long-term injection materials, and the advantages and disadvantages of each.

RECENT FINDINGS

Many of the newer laryngeal injectable substances were originally used as dermal fillers born out of the plastic surgery and dermatologic literature. Clinical outcomes have improved as a result of exciting advancements in vocal fold injection material availability and design. New substances now closely mimic the native viscoelastic properties of the vocal folds, but the search for the ideal material is ongoing.

SUMMARY

The wide array of injectable materials available is both daunting and exciting. Many variables must be considered in choosing the best temporary or long-term injectable material to meet each patient's needs. Surgeons will be better able to optimize patient outcomes by understanding the advantages and disadvantages each has to offer.

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.