Evaluation of the Stress-Growth Hypothesis in Saphenous Vein Perfusion Culture

The great saphenous vein (GSV) has served as a coronary artery bypass graft (CABG) conduit for over 50 years. Despite prevalent use, first-year failure rates remain high compared to arterial autograft options. Amongst other factors, vein graft failure can be attributed to material and mechanical mismatching that lead to apoptosis, inflammation, and intimal-medial hyperplasia. Through the implementation of the continuum mechanical-based theory of "stress-mediated growth and remodeling," we hypothesize that the mechanical properties of porcine GSV grafts can be favorably tuned for CABG applications prior to implantation using a prolonged but gradual transition from venous to arterial loading conditions in an inflammatory and thrombogenic deficient environment. To test this hypothesis, we used a hemodynamic-mimetic perfusion bioreactor to guide remodeling through stepwise incremental changes in pressure and flow over the course of 21-day cultures. Biaxial mechanical testing of vessels pre- and post-remodeling was performed, with results fit to structurally-motivated constitutive models using non-parametric bootstrapping. The theory of "small-on-large" was used to describe appropriate stiffness moduli, while histology and viability assays confirmed microstructural adaptations and vessel viability. Results suggest that stepwise transition from venous-to-arterial conditions results in a partial restoration of circumferential stretch and circumferential, but not axial, stress through vessel dilation and wall thickening in a primarily outward remodeling process. These remodeled tissues also exhibited decreased mechanical isotropy and circumferential, but not axial, stiffening. In contrast, only increases in axial stiffness were observed using culture under venous perfusion conditions and those tissues experienced moderate intimal resorption.

Transvaginal Ultrasound Vibro-elastography for Measuring Uterine Viscoelasticity: A Phantom Study

The purpose of this research was to determine the feasibility of a transvaginal ultrasound vibro-elastography (TUVE) technique for generating and measuring shear wave propagation in the uterus. In TUVE, a 0.1-s harmonic vibration at a low frequency is generated on the abdomen of a subject via a handheld vibrator. A transvaginal ultrasound probe is used to measure the resulting shear wave propagation in the uterus. TUVE was evaluated on a female ultrasound phantom. The shear wave speeds in the region of interest of the uterus of the female ultrasound phantom were measured in the frequency range of 100-300 Hz. The viscoelasticity was analyzed based on the wave speed dispersion with frequency. The measurement of shear wave speed suggests that the uterus of this female ultrasound phantom is much stiffer than the human uterus. This research illustrates the feasibility of TUVE for generating and measuring shear wave propagation in the uterus of a female ultrasound phantom. We will further evaluate TUVE in patients, both normal controls and those with uterine diseases such as adenomyosis.

Comparative mechanics of diverse mammalian carotid arteries

The prevalence of diverse animal models as surrogates for human vascular pathologies necessitate a comprehensive understanding of the differences that exist between species. Comparative passive mechanics are presented here for the common carotid arteries taken from bovine, porcine, ovine, leporine, murine-rat, and murine-mouse specimens. Data is generated using a scalable biaxial mechanical testing device following consistent circumferential (pressure-diameter) and axial (force-length) testing protocols. The structural mechanical response of carotids under equivalent loading, quantified by the deformed inner radius, deformed wall thickness, lumen area compliance and axial force, varies significantly among species but generally follows allometric scaling. Conversely, descriptors of the local mechanical response within the deformed arterial wall, including mean circumferential stress, mid-wall circumferential stretch, and mean axial stress, are relatively consistent across species. Unlike the larger animals studied, the diameter distensibility curves of murine specimens are non-monotonic and have a significantly higher value at 100 mmHg. Taken together, our results provide baseline structural and mechanical information for carotid arteries across a broad range of common animal models.

Contractile Smooth Muscle and Active Stress Generation in Porcine Common Carotids

The mechanical response of intact blood vessels to applied loads can be delineated into passive and active components using an isometric decomposition approach. Whereas the passive response is due predominantly to the extracellular matrix (ECM) proteins and amorphous ground substance, the active response depends on the presence of smooth muscle cells (SMCs) and the contractile machinery activated within those cells. To better understand determinants of active stress generation within the vascular wall, we subjected porcine common carotid arteries (CCAs) to biaxial inflation-extension testing under maximally contracted or passive SMC conditions and semiquantitatively measured two known markers of the contractile SMC phenotype: smoothelin and smooth muscle-myosin heavy chain (SM-MHC). Using isometric decomposition and established constitutive models, an intuitive but novel correlation between the magnitude of active stress generation and the relative abundance of smoothelin and SM-MHC emerged. Our results reiterate the importance of stretch-dependent active stress generation to the total mechanical response. Overall these findings can be used to decouple the mechanical contribution of SMCs from the ECM and is therefore a powerful tool in the analysis of disease states and potential therapies where both constituent are altered.

A Novel Noninvasive Ultrasound Vibro-elastography Technique for Assessing Patients With Erectile Dysfunction and Peyronie Disease

OBJECTIVE

To translate a novel ultrasound vibro-elastography (UVE) technique for noninvasively measuring viscoelasticity of the penis.

METHODS

A pilot study of UVE was performed in men with erectile dysfunction or Peyronie disease. Assessments were performed in triplicate on the lateral aspect of the penis (bilaterally) at 100, 150, and 200 Hz before and after erectogenic injection administration. Viscoelasticity of the corpora was also calculated and compared before and after injection and against measures of erectile function, including the International Index of Erectile Function-Erectile Function Domain, and the total erectogenic medication volume required for achieving a firm erection.

RESULTS

Significant increases in viscoelasticity were found after erectogenic injection, validating the ability of UVE to measure dynamic changes with erections. Baseline measures also significantly correlated with the volume of erectogenic medication required to achieve an erection (100 Hz, parameter estimate [PE] 2.21, P <.001; 150 Hz, PE 0.53, P = .03; 200 Hz, PE 0.34, P = .07) but not with age and International Index of Erectile Function-Erectile Function Domain. As erectogenic medications likely represent the most accurate measure of erectile function, these findings suggest a potential role for UVE as a viable diagnostic modality for erectile dysfunction.

CONCLUSION

This first report of the use of elastography with erectile function in humans demonstrates significant associations with responsiveness to erectogenic injection medications. These data have significant potential implications for broader clinical practice and merit further study and validation.

Determination of Viscoelastic Properties of human Carotid Atherosclerotic Plaque by Inverse Boundary Value Analysis

In this study, we assessed the mechanical response of samples from human atherosclerotic diseased media and fibrous cap via uniaxial tensile testing. Results show a pronounced hysteresis phenomenon caused by viscoelasticity during the loading-unloading process. An inverse analysis method with finite element modeling was employed to identify the material parameter values for a viscoelastic anisotropic (VA) constitutive model through matching simulation predictions of load-displacement curves with experimental measurements. The identified material parameter values can be used in simulation studies of diseased human carotid arteries, including investigations of inflation processes associated with stenting or angioplasty.

Numerical modeling of experimental human fibrous cap delamination

Fibrous cap delamination is a critical process during the rupture of atherosclerotic plaque, which often leads to severe life-threatening clinical consequences such as myocardial infarction or stroke. In this study a finite element modeling and simulation approach is presented that enables the study of fibrous cap delamination experiments for the purpose of understanding the fibrous cap delamination process. A cohesive zone model (CZM) approach is applied to simulate delamination of the fibrous cap from the underlying plaque tissue. A viscoelastic anisotropic (VA) model for the bulk arterial material behavior is extended from existing studies so that the hysteresis phenomenon observed in the fibrous cap delamination experiments can be captured. A finite element model is developed for the fibrous cap delamination experiments, in which arterial layers (including the fibrous cap and the underlying plaque tissue) are represented by solid elements based on the VA model and the fibrous cap-underlying plaque tissue interface is characterized by interfacial CZM elements. In the CZM, the delamination process is governed by an exponential traction-separation law which utilizes critical energy release rates obtained directly from the fibrous cap delamination experiments. A set of VA model parameter values and CZM parameter values is determined based on values suggested in the literature and through matching simulation predictions of the load vs. load-point displacement curve with one set of experimental measurements. Using this set of parameter values, simulation predictions for other sets of experimental measurements are obtained and good agreement between simulation predictions and experimental measurements is observed. Results of this study demonstrate the applicability of the viscoelastic anisotropic model and the CZM approach for the simulation of diseased arterial tissue failure processes.

A mechanical argument for the differential performance of coronary artery grafts

Coronary artery bypass grafting (CABG) acutely disturbs the homeostatic state of the transplanted vessel making retention of graft patency dependent on chronic remodeling processes. The time course and extent to which remodeling restores vessel homeostasis will depend, in part, on the nature and magnitude of the mechanical disturbances induced upon transplantation. In this investigation, biaxial mechanical testing and histology were performed on the porcine left anterior descending artery (LAD) and analogs of common autografts, including the internal thoracic artery (ITA), radial artery (RA), great saphenous vein (GSV) and lateral saphenous vein (LSV). Experimental data were used to quantify the parameters of a structure-based constitutive model enabling prediction of the acute vessel mechanical response pre-transplantation and under coronary loading conditions. A novel metric Ξ was developed to quantify mechanical differences between each graft vessel in situ and the LAD in situ, while a second metric Ω compares the graft vessels in situ to their state under coronary loading. The relative values of these metrics among candidate autograft sources are consistent with vessel-specific variations in CABG clinical success rates with the ITA as the superior and GSV the inferior graft choices based on mechanical performance. This approach can be used to evaluate other candidate tissues for grafting or to aid in the development of synthetic and tissue engineered alternatives.

The biaxial active mechanical properties of the porcine primary renal artery

The mechanical response of arteries under physiological loads can be delineated into passive and active components. The passive response is governed by the load-bearing constituents within the arterial wall, elastin, collagen, and water, while the active response is a result of vascular smooth muscle cell (SMC) contraction. In muscular blood vessels, such as the primary renal artery, high SMC wall content suggests an elevated importance of the active response in determining overall vessel behavior. This study is a continuation of our previous investigation, in which a four-fiber constitutive model of the passive response of the primary porcine renal artery was identified. Here we focus on the active response of this vessel, specifically in the case of maximal SMC contraction, and develop a constitutive model of the active stress-stretch relations. The results of this study demonstrate the existence of biaxial active stress in the vessel wall, and suggest the active mechanical response is a critical component of renal arterial performance.