Biomedical subjects

Roger W Chan

Publications and source records attributed to Roger W Chan.

16 recordsLinked to original sources

PubMed2006

Viscoelastic shear properties of the fresh porcine lens.

AIM: To determine the viscoelastic properties of the porcine lens METHODS: Linear viscoelastic shear properties of the stroma of four porcine lenses were measured within 5 hours post-mortem, using sinusoidal oscillatory shear deformation. The elastic shear modulus, viscous shear modulus, dynamic viscosity, damping ratio, and phase shift of the lenses were quantified by a controlled-strain, linear simple-shear rheometer at frequencies of 10-50 Hz. RESULTS: The mean viscoelastic properties and their standard deviations across the frequencies examined were: the elastic shear modulus, G' = 6.2+/-4.0 Pa, the viscous shear modulus, G'' = 19.2+/-2.5 Pa, the dynamic viscosity, eta' = 0.16+/-0.1 Pa x sec, the damping ratio zeta = 4.06+/-1.25, and the phase shift, delta = 76 degrees +/- 5.6 degrees. CONCLUSIONS: The measured viscoelastic shear properties of the porcine lens reflect a low dynamic viscosity with a high damping ratio. The porcine lens is viscoelastic and is more viscous than elastic. The magnitude of the complex shear modulus of the porcine lens, |G*|, is similar to the shear modulus of the young human lens. Understanding these viscoelastic properties of the natural lens may provide guidance in developing a lens substitute capable of accommodation in the post cataract patient.

Animals↗

PubMed2006

A constitutive model of the human vocal fold cover for fundamental frequency regulation.

The elastic as well as time-dependent mechanical response of the vocal fold cover (epithelium and superficial layer of the lamina propria) under tension is one key variable in regulating the fundamental frequency. This study examines the hyperelastic and time-dependent tensile deformation behavior of a group of human vocal fold cover specimens (six male and five female). The primary goal is to formulate a constitutive model that could describe empirical trends in speaking fundamental frequency with reasonable confidence. The constitutive model for the tissue mechanical behavior consists of a hyperelastic equilibrium network in parallel with an inelastic, time-dependent network and is combined with the ideal string model for phonation. Results showed that hyperelastic and time-dependent parameters of the constitutive model can be related to observed age-related and gender-related differences in speaking fundamental frequency. The implications of these findings on fundamental frequency regulation are described. Limitations of the current constitutive model are discussed.

Adolescent↗

PubMed2006

Dependence of phonation threshold pressure on vocal tract acoustics and vocal fold tissue mechanics.

Analytical and computer simulation studies have shown that the acoustic impedance of the vocal tract as well as the viscoelastic properties of vocal fold tissues are critical for determining the dynamics and the energy transfer mechanism of vocal fold oscillation. In the present study, a linear, small-amplitude oscillation theory was revised by taking into account the propagation of a mucosal wave and the inertive reactance (inertance) of the supraglottal vocal tract as the major energy transfer mechanisms for flow-induced self-oscillation of the vocal fold. Specifically, analytical results predicted that phonation threshold pressure (Pth) increases with the viscous shear properties of the vocal fold, but decreases with vocal tract inertance. This theory was empirically tested using a physical model of the larynx, where biological materials (fat, hyaluronic acid, and fibronectin) were implanted into the vocal fold cover to investigate the effect of vocal fold tissue viscoelasticity on Pth. A uniform-tube supraglottal vocal tract was also introduced to examine the effect of vocal tract inertance on Pth. Results showed that Pth decreased with the inertive impedance of the vocal tract and increased with the viscous shear modulus (G") or dynamic viscosity (eta') of the vocal fold cover, consistent with theoretical predictions. These findings supported the potential biomechanical benefits of hyaluronic acid as a surgical bioimplant for repairing voice disorders involving the superficial layer of the lamina propria, such as scarring, sulcus vocalis, atrophy, and Reinke's edema.

Biomechanical Phenomena↗

PubMed2006

Elasticity of the human false vocal fold.

OBJECTIVES: Very little is known about the elasticity of the human ventricular fold (false vocal fold). To better understand the potential role of the false fold in the fluid dynamics and aeroacoustics of phonation, we made some measurements on the elastic properties of human ventricular fold tissues in vitro. METHODS: Uniaxial tensile stress-strain characteristics of 6 male and 6 female false fold specimens were quantified with sinusoidal stretch-release deformation. Midcoronal sections of 3 specimens were examined to quantify the relative densities of collagen, elastin, seromucous glandular tissue, and adipose tissue by digital image analysis. RESULTS: Nonlinear stress-strain curves with hysteresis (viscous energy loss) were observed, with large interindividual differences. A hybrid linear-exponential model was used to determine the elastic modulus (tangent Young's modulus) of the false fold. On average, the male false fold was twice as stiff as the female at a tensile strain of 20% to 30%. CONCLUSIONS: This preliminary gender-related difference in elasticity could be attributed to a higher proportion of glandular tissue in the female false fold, due to the lower elastic modulus of glands. The present data allow one to develop a more comprehensive biomechanical model of phonation, for optimizing postoperative voice production following laryngeal reconstruction procedures.

Aged↗

PubMed2004

Characterization of chronic vocal fold scarring in a rabbit model.

The purpose of the current study was to assess the histologic and rheologic properties of the scarred vocal fold lamina propria during a chronic phase of wound repair in a rabbit model. Eighteen rabbit larynges were scarred using a procedure that involved stripping the vocal fold lamina propria down to the thyroarytenoid muscle, using 3-mm microforceps. The approximate dimension of injury to the vocal fold was 3 x 1.5 x 0.5 mm [length x width x depth]. At 6 months postoperatively, histologic analysis of the scarred and control lamina propria in eight of these rabbits was completed for collagen, procollagen, elastin, and hyaluronic acid. Compared with control samples, scarred tissue samples revealed fragmented and disorganized elastin fibers. Additionally, collagen was significantly increased, organized, and formed thick bundles in the scarred vocal fold lamina propria. Measurements of the viscoelastic shear properties of the scarred and control lamina propria in the remaining 10 rabbits revealed increased elastic shear modulus (G') in 8 of 10 scarred samples and increased dynamic viscosity (eta') in 9 of 10 scarred samples. Although rheologic differences were not statistically significant, they revealed that on average, scarred samples were stiffer and more viscous than the normal controls. Histologic data are interpreted as indicating that by 6 months postinjury, the scarred rabbit vocal fold has reached a mature phase of wound repair, characterized by an increased, organized, and thick bundle collagen matrix. Rheologic data are interpreted as providing support for the potential role of increased, thick bundle collagen, and a disorganized elastin network on shear stiffness and dynamic viscosity in the chronic vocal fold scar. Based on these results, a 6-month postoperative time frame is proposed for future studies of chronic vocal fold scarring using the rabbit animal model.

Animals↗

PubMed2004

Prevention of vocal fold scarring by topical injection of hepatocyte growth factor in a rabbit model.

OBJECTIVES/HYPOTHESIS: Vocal fold scarring disrupts the layer structure of the vocal fold lamina propria that is essential for optimal mucosal vibration. Prevention of vocal fold scarring remains challenging. Hepatocyte growth factor (HGF) has strong antifibrotic activity. The authors' previous studies have found that HGF stimulates hyaluronic acid production and suppresses collagen production from vocal fold fibroblasts, suggesting that HGF has therapeutic potential in prevention of vocal fold scarring. The present study aimed to demonstrate the effects of HGF on vocal fold scarring in an in vivo rabbit model. STUDY DESIGN: Animal experiment. METHODS: The vocal fold mucosa was stripped unilaterally in 20 rabbits, then HGF or saline (sham-treated group) was immediately injected into the injured site. At 6 months after the procedure, histological, rheological, and physiological examinations of vibratory behavior were completed. RESULTS: Histological examination revealed excessive collagen deposition and disorganized elastin in the sham-treated group, whereas the HGF-treated group presented with better wound healing exhibiting less collagen deposition. Contraction of the injured vocal folds observed in the sham-treated group did not occur in the HGF-treated group. Rheological data indicated that the HGF-treated vocal folds were less stiff and viscous compared with the sham-treated group. Mucosal vibration of HGF-treated vocal folds appeared much better than the sham-treated group in terms of phonation threshold pressure, vocal efficiency, mucosal wave amplitude, and glottal closure. CONCLUSION: Hepatocyte growth factor proved to be useful in preventing vocal fold scarring and maintaining viscoelastic shear properties of the vocal fold.

Animals↗

PubMed2004

Vocal fold tissue failure: preliminary data and constitutive modeling.

In human voice production (phonation), linear small-amplitude vocal fold oscillation occurs only under restricted conditions. Physiologically, phonation more often involves large-amplitude oscillation associated with tissue stresses and strains beyond their linear viscoelastic limits, particularly in the lamina propria extracellular matrix (ECM). This study reports some preliminary measurements of tissue deformation and failure response of the vocal fold ECM under large-strain shear The primary goal was to formulate and test a novel constitutive model for vocal fold tissue failure, based on a standard-linear cohesive-zone (SL-CZ) approach. Tissue specimens of the sheep vocal fold mucosa were subjected to torsional deformation in vitro, at constant strain rates corresponding to twist rates of 0.01, 0.1, and 1.0 rad/s. The vocal fold ECM demonstrated nonlinear stress-strain and rate-dependent failure response with a failure strain as low as 0.40 rad. A finite-element implementation of the SL-CZ model was capable of capturing the rate dependence in these preliminary data, demonstrating the model's potential for describing tissue failure. Further studies with additional tissue specimens and model improvements are needed to better understand vocal fold tissue failure.

Animals↗

PubMed2004

Measurements of vocal fold tissue viscoelasticity: approaching the male phonatory frequency range.

Viscoelastic shear properties of human vocal fold tissues have been reported previously. However, data have only been obtained at very low frequencies (< or = 15 Hz). This necessitates data extrapolation to the frequency range of phonation based on constitutive modeling and time-temperature superposition. This study attempted to obtain empirical measurements at higher frequencies with the use of a controlled strain torsional rheometer, with a design of directly controlling input strain that introduced significantly smaller system inertial errors compared to controlled stress rheometry. Linear viscoelastic shear properties of the vocal fold mucosa (cover) from 17 canine larynges were quantified at frequencies of up to 50 Hz. Consistent with previous data, results showed that the elastic shear modulus (G'), viscous shear modulus (G"), and damping ratio (zeta) of the vocal fold mucosa were relatively constant across 0.016-50 Hz, whereas the dynamic viscosity (eta') decreased monotonically with frequency. Constitutive characterization of the empirical data by a quasilinear viscoelastic model and a statistical network model demonstrated trends of viscoelastic behavior at higher frequencies generally following those observed at lower frequencies. These findings supported the use of controlled strain rheometry for future investigations of the viscoelasticity of vocal fold tissues and phonosurgical biomaterials at phonatory frequencies.

Animals↗

PubMed2003

An investigation of cricoarytenoid joint mechanics using simulated muscle forces.

Rotational and translational stiffnesses were calculated for arytenoid motion about the cricoarytenoid joint. These calculations were obtained from measurements on five excised human larynxes. For each larynx, known forces were applied to the arytenoid cartilage, and three markers were tracked as a function of applied forces. Assuming rigid body motion, arytenoid translations and rotations were computed for each applied force. Translational stiffnesses were obtained by plotting force versus displacement, and rotational stiffnesses were calculated by plotting torque versus angular rotation. A major finding was that the translational stiffness along the anterior-posterior direction was three times as great as the translational stiffnesses in the other two directions. This nonisotropic nature of the stiffnesses may be an important consideration for phonosurgeons who wish to avoid subluxation of the cricoarytenoid joint in patients. The computed rotational and translational stiffnesses currently are being implemented in 2D and 3D models. These stiffness parameters play a vital role in prephonatory glottal shaping, which in turn exerts a majorinfluence on all aspects of vocal fold vibration, including fundamental frequency, voice quality, voice register, and phonation threshold pressure.

Adult↗

PubMed2003

Geometrical deformation of vocal fold tissues induced by formalin fixation.

OBJECTIVES/HYPOTHESIS: Many existing studies of vocal fold geometry are based on anatomical measurements made on histologically fixed laryngeal tissues using formalin. However, the validity of these geometric data is questionable because of the potentially significant tissue deformation associated with formalin fixation, particularly tissue shrinkage. Previous experiments have shown that valid geometric data may be obtained on vocal fold tissue samples quickly frozen with liquid nitrogen. Based on this finding, the present study attempted to quantify the geometric deformation of formalin-fixed vocal fold tissues with respect to quick-frozen tissues. METHODS: Six freshly harvested canine larynges were quickly frozen with liquid nitrogen and sectioned at the mid-membranous coronal plane. Each larynx was thawed and divided into halves along the midsagittal plane, one of which was not fixed; the other was fixed with formalin, from which histological tissue sections were also prepared. Measurements of vocal fold geometry were made on digital images of mid-membranous coronal sections of the tissue samples, based on linear dimensions of vocal fold depth and thickness defined functionally for biomechanical applications. RESULTS: Significant shrinkage of the vocal fold (particularly the vocal fold body) and considerable distortion of the vocal fold contour (particularly at the free edge) were observed for the formalin-fixed samples and the histological sections in comparison with the unfixed samples. CONCLUSIONS: Results of the present study suggested that significant geometric artifacts are induced by conventional histological fixation of laryngeal tissues using formalin. These artifacts should be carefully considered for interpreting any vocal fold geometric data obtained through formalin fixation.

Animals↗

PubMed2003

Characterization of vocal fold scarring in a canine model.

OBJECTIVE: The objective was to assess the histological and viscoelastic shear tissue properties of the scarred vocal fold lamina propria at 2 and 6 months postoperatively in a canine model. STUDY DESIGN: Experimental, nonrandomized prospective study. METHODS: Six canine larynges were injured using a vocal fold stripping procedure. At 2 and 6 months postoperatively, histological analyses of the scarred and control lamina propria samples were completed for collagen, procollagen, elastin, and hyaluronic acid. RESULTS: In canines killed at 2 months, scarred tissue samples contained increased procollagen and decreased elastin. Elastin fibers in the scarred lamina propria were characteristically tangled and disorganized. In canines killed at 6 months, scarred tissue samples showed decreased elastin and increased collagen. Collagen fibers formed thick, disorganized bundles, and elastin fibers were disorganized throughout the entire scarred vocal fold lamina propria. Viscoelastic shear tissue measurements revealed increased stiffness and viscosity in one of three cases at 2 months and in all three cases at 6 months, indicating increased stiffness and resistance to shear flow during oscillatory shear deformation for scarred tissue samples. No differences were observed between the two postoperative times. CONCLUSIONS: Results indicated that viscoelastic tissue changes may take place before scar maturation in the scarred vocal fold lamina propria and that, although abundant collagen deposition may influence viscoelastic shear tissue properties, disorganization of collagen and elastin fibers, thick bundle collagen formation, or the interplay of several of these factors might also play a contributing role.

Animals↗

PubMed2003

Effect of postmortem changes and freezing on the viscoelastic properties of vocal fold tissues.

It is common practice in laryngeal research laboratories to store excised larynges and vocal fold tissue specimens frozen, presumably to preserve viability and structural integrity of the specimens for physiologic and biomechanical experiments. However, little is known about the biomechanical effects of postmortem changes and frozen storage in vocal fold tissues. This study attempted to quantify the effects of postmortem changes and freezing on the viscoelastic shear properties of the canine vocal fold mucosa. Sixteen larynges were excised from adult dogs immediately postmortem in a viable state. Using a torsional rheometer, the complex shear modulus (G*) of the mucosal tissue from one vocal fold of each larynx was measured as a function of frequency (0.01-15 Hz) by linear small-amplitude oscillation experiments. Measurement was repeated for ten mucosal specimens after 24 h of postmortem storage in saline solution at room temperature. Eleven of the 16 larynges were frozen and stored at -20 degrees C, six of them at a slow rate of cooling (storage in a regular freezer) and five others by quick freezing (chilling by liquid nitrogen prior to frozen storage). The larynges were thawed slowly overnight after one month and the viscoelastic shear properties of the contralateral vocal fold mucosa were measured. Results showed that the elastic shear modulus (G') and dynamic viscosity (eta') of the vocal fold mucosa did not seem to change significantly after 24 h of storage in saline at room temperature, nor after one month of frozen storage following quick freezing, whereas both G' and eta' decreased significantly for tissues that were slowly frozen. These findings supported the feasibility of using quick freezing to preserve laryngeal tissues for in vitro biomechanical testing, for excised larynx experiments, and for tissue engineering applications.

Animals↗

PubMed2002

Histologic and rheologic characterization of vocal fold scarring.

Scarring of the vocal fold causes considerable dysphonia and presents significant treatment challenges. A rabbit model was developed to investigate the histologic ultrastructure and rheologic properties of the scarred vocal fold lamina propria. Eleven rabbit larynges were scarred by means of forcep biopsy. Sixty days postoperatively, the rabbits were sacrificed and their vocal folds were harvested. Histological analysis of the scarred and normal lamina propria was completed for collagen, procollagen, elastin, and hyaluronic acid. Linear viscoelastic shear properties of the tissues were also measured, including elastic shear modulus and dynamic viscosity. Compared to normal vocal fold lamina propria, scarred tissues demonstrated significantly less collagen, an increase in procollagen, and a decrease in elastin. Rheologically, both elastic shear modulus and dynamic viscosity were significantly higher for the scarred tissues. Increased stiffness and viscosity do not appear to result from an increase in collagen, but rather appear to be related to the presence of new, disorganized collagen scaffolding. Results are interpreted in terms of the possible role of interstitial proteins in the etiology of increased stiffness and viscosity, which requires further investigation. This animal model should allow for systematic future investigations of vocal fold scarring and its treatment.

Animals↗

PubMed2002

Biomechanical effects of hydration in vocal fold tissues.

OBJECTIVE: It has often been hypothesized, with little empirical support, that vocal fold hydration affects voice production by mediating changes in vocal fold tissue rheology. To test this hypothesis, we attempted in this study to quantify the effects of hydration on the viscoelastic shear properties of vocal fold tissues in vitro. STUDY DESIGN: Osmotic changes in hydration (dehydration and rehydration) of 5 excised canine larynges were induced by sequential incubation of the tissues in isotonic, hypertonic, and hypotonic solutions. Elastic shear modulus (G'), dynamic viscosity eta' and the damping ratio zeta of the vocal fold mucosa (lamina propria) were measured as a function of frequency (0.01 to 15 Hz) with a torsional rheometer. RESULTS: Vocal fold tissue stiffness (G') and viscosity (eta) increased significantly (by 4 to 7 times) with the osmotically induced dehydration, whereas they decreased by 22% to 38% on the induced rehydration. Damping ratio (zeta) also increased with dehydration and decreased with rehydration, but the detected differences were not statistically significant at all frequencies. CONCLUSION: These findings support the long-standing hypothesis that hydration affects vocal fold vibration by altering tissue rheologic (or viscoelastic) properties. SIGNIFICANCE: Our results demonstrated the biomechanical importance of hydration in vocal fold tissues and suggested that hydration approaches may potentially improve the biomechanics of phonation in vocal fold lesions involving disordered fluid balance.

Animals↗

PubMed2002

Functional definitions of vocal fold geometry for laryngeal biomechanical modeling.

Precise geometric data on vocal fold dimensions are necessary for defining the vocal fold boundaries with respect to the laryngeal framework in physiological and biomechanical models of the larynx (eg, finite-element models). In the mid-membranous coronal section, vocal fold depth can be defined as the horizontal distance from the vocal fold medial surface to the thyroid cartilage, whereas vocal fold thickness can be defined as the vertical distance from the inferior border of the thyroarytenoid muscle to the vocal fold superior surface. Traditionally, such geometric data have been obtained from measurements made on histologic tissue sections. Unfortunately, it is very difficult to obtain reliable data by this method, unless the effects of sample preparation on vocal fold geometry are quantified. Significant tissue deformations are often induced by histologic processes such as fixation and dehydration, sometimes producing shrinkages as large as 30%. In this study, reliable geometric data of the canine vocal fold were obtained by the alternative method of quick-freezing for sample preparation, using liquid nitrogen. Coronal sections of quick-frozen larynges were thawed gradually in saline solution. Images of the mid-membranous coronal sections at various thawing stages were captured by a digital camera. Measurements of operationally defined vocal fold dimensions (depth and thickness) useful for biomechanical modeling were made with a graphics software package. The results showed that geometric changes of the vocal fold induced by freezing are likely reversed by thawing, such that the measurements made on thawed larynges are reliable approximations of the actual vocal fold dimensions.

Animals↗

PubMed2002

Treatment of vocal fold scarring: rheological and histological measures of homologous collagen matrix.

The current treatment options for dysphonia secondary to vocal fold scarring are limited. Few studies address changes in the lamina propria, which is critical to vocal fold biomechanical properties and voice production. Using rheological and histological measures of homologous collagen matrix (HCM)-injected vocal folds, we assessed HCM's potential for providing bulk and restoring biomechanical performance. Twenty rabbits underwent bilateral vocal fold scarring. After 10 weeks of scar maturation, the rabbits had unilateral injections of HCM or saline solution. Ten weeks after the injections, histological studies revealed well-defined collagen globules distributed throughout the lamina propria and underlying muscular tissue. Significantly more procollagen was observed in the HCM-treated group. No significant differences in elastic shear modulus or dynamic viscosity were found between the treatment groups. This study demonstrates that HCM is an inert, relatively stable injectate that may serve well for medialization but does not appear to improve the dynamic properties of the lamina propria.

Animals↗