MECHANICAL STRESSES ASSOCIATED WITH FLATTENING OF HUMAN FEMOROPOPLITEAL ARTERY SPECIMENS DURING PLANAR BIAXIAL TESTING AND THEIR EFFECTS ON THE CALCULATED PHYSIOLOGIC STRESS-STRETCH STATE

Planar biaxial testing is commonly used to characterize the mechanical properties of arteries, but stresses associated with specimen flattening during this test are unknown. We quantified flattening effects in human femoropopliteal arteries (FPAs) of different ages, and determined how they affect the calculated arterial physiologic stress-stretch state. Human FPAs from 472 tissue donors (age 12-82 years, mean 53±16 years) were tested using planar biaxial extension, and morphometric and mechanical characteristics were used to assess the flattening effects. Constitutive parameters for the invariant-based model were adjusted to account for specimen flattening, and used to calculate the physiologic stresses, stretches, axial force, circumferential stiffness, and stored energy for the FPAs in 7 age groups. Flattened specimens were overall 12±4% stiffer longitudinally and 19±11% stiffer circumferentially when biaxially tested. Differences between the stress-stretch curves adjusted and non-adjusted for the effects of flattening were relatively constant across all age groups longitudinally, but increased with age circumferentially. In all age groups these differences were smaller than the intersubject variability. Physiologic stresses, stretches, axial force, circumferential stiffness, and stored energy were all qualitatively and quantitatively similar when calculated with and without the flattening effects. Stresses, stretches, axial force, and stored energy reduced with age, but circumferential stiffness remained relatively constant between 25 and 65 years of age suggesting a homeostatic target of 0.75±0.02 MPa. Flattening effects associated with planar biaxial testing are smaller than the intersubject variability, and have little influence on the calculated physiologic stress-stretch state of human FPAs

Feasibility of Fluoroscopy-Free Endovascular Navigation in Subjects of Different Ages

https://digitalcommons.unmc.edu/emet_posters/1013/thumbnail.jp

A method of assessing peripheral stent abrasiveness under cyclic deformations experienced during limb movement

Poor outcomes of peripheral arterial disease stenting are often attributed to the inability of stents to accommodate the complex biomechanics of the flexed lower limb. Abrasion damage caused by rubbing of the stent against the artery wall during limb movement plays a significant role in reconstruction failure but has not been characterized. Our goals were to develop a method of assessing the abrasiveness of peripheral nitinol stents and apply it to several commercial devices. Misago, AbsolutePro, Innova, Zilver, SmartControl, SmartFlex, and Supera stents were deployed inside electrospun nanofibrillar tubes with femoropopliteal artery-mimicking mechanical properties and subjected to cyclic axial compression (25%), bending (90°), and torsion (26°/cm) equivalent to five life-years of severe limb flexions. Abrasion was assessed using an abrasion damage score (ADS, range 1–7) for each deformation mode. Misago produced the least abrasion and no stent fractures (ADS 3). Innova caused small abrasion under compression and torsion but large damage under bending (ADS 7). Supera performed well under bending and compression but caused damage under torsion (ADS 8). AbsolutePro produced significant abrasion under bending and compression but less damage under torsion (ADS 12). Zilver fractured under all three deformations and severely abraded the tube under bending and compression (ADS 15). SmartControl and SmartFlex fractured under all three deformations and produced significant abrasion due to strut penetration (ADS 20 and 21). ADS strongly correlated with clinical 12- month primary patency and target lesion revascularization rates, and the described method of assessing peripheral stent abrasiveness can guide device selection and development

In vivo three-dimensional blood velocity profile shapes in the human common, internal, and external carotid arteries

Objective: True understanding of carotid bifurcation pathophysiology requires a detailed knowledge of the hemodynamic conditions within the arteries. Data on carotid artery hemodynamics are usually based on simplified, computer-based, or in vitro experimental models, most of which assume that the velocity profiles are axially symmetric away from the carotid bulb. Modeling accuracy and, more importantly, our understanding of the pathophysiology of carotid bifurcation disease could be considerably improved by more precise knowledge of the in vivo flow properties within the human carotid artery. The purpose of this work was to determine the three-dimensional pulsatile velocity profiles of human carotid arteries. Methods: Flow velocities were measured over the cardiac cycle using duplex ultrasonography, before and after endarterectomy, in the surgically exposed common (CCA), internal (ICA), and external (ECA) carotid arteries (n = 16) proximal and distal to the stenosis/endarterectomy zone. These measurements were linked to a standardized grid across the flow lumina of the CCA, ICA, and ECA. The individual velocities were then used to build mean three-dimensional pulsatile velocity profiles for each of the carotid artery branches. Results: Pulsatile velocity profiles in all arteries were asymmetric about the arterial centerline. Posterior velocities were higher than anterior velocities in all arteries. In the CCA and ECA, velocities were higher laterally, while in the ICA, velocities were higher medially. Pre- and postendarterectomy velocity profiles were significantly different. After endarterectomy, velocity values increased in the common and internal and decreased in the external carotid artery. Conclusions: The in vivo hemodynamics of the human carotid artery are different from those used in most current computer-based and in vitro models. The new information on three-dimensional blood velocity profiles can be used to design models that more closely replicate the actual hemodynamic conditions within the carotid bifurcation. Such models can be used to further improve our understanding of the pathophysiologic processes leading to stroke and for the rational design of medical and interventional therapies

The choice of a constitutive formulation for modeling limb flexion-induced deformations and stresses in the human femoropopliteal arteries of different ages

Open and endovascular treatments for peripheral arterial disease are notorious for high failure rates. Severe mechanical deformations experienced by the femoropopliteal artery (FPA) during limb flexion and interactions between the artery and repair materials play important roles and may contribute to poor clinical outcomes. Computational modeling can help optimize FPA repair, but these simulations heavily depend on the choice of constitutive model describing the arterial behavior. In this study finite element model of the FPA in the standing (straight) and gardening (acutely bent) postures was built using computed tomography data, longitudinal pre-stretch and biaxially determined mechanical properties. Springs and dashpots were used to represent surrounding tissue forces associated with limb flexion-induced deformations. These forces were then used with age-specific longitudinal pre-stretch and mechanical properties to obtain deformed FPA configurations for seven age groups. Four commonly used invariant-based constitutive models were compared to determine the accuracy of capturing deformations and stresses in each age group. The four-fiber FPA model most accurately portrayed arterial behavior in all ages, but in subjects younger than 40 years, the performance of all constitutive formulations was similar. In older subjects, Demiray (Delfino) and classic two-fiber Holzapfel–Gasser–Ogden formulations were better than the Neo-Hookean model for predicting deformations due to limb flexion, but both significantly overestimated principal stresses compared to the FPA or Neo-Hookean models

In situ longitudinal pre-stretch in the human femoropopliteal artery

In situ longitudinal (axial) pre-stretch (LPS) plays a fundamental role in the mechanics of the femoropopliteal artery (FPA). It conserves energy during pulsation and prevents buckling of the artery during limb movement. We investigated how LPS is affected by demographics and risk factors, and how these patient characteristics associate with the structural and physiologic features of the FPA. LPS was measured in n = 148 fresh human FPAs (14–80 years old). Mechanical properties were characterized with biaxial extension and histopathological characteristics were quantified with Verhoeff–Van Gieson Staining. Constitutive modeling was used to calculate physiological stresses and stretches which were then analyzed in the context of demographics, risk factors and structural characteristics. Age had the strongest negative effect (r = −0.812, p \u3c 0.01) on LPS and could alone explain 66% of LPS variability. Male gender, higher body mass index, hypertension, diabetes, coronary artery disease, dyslipidemia and tobacco use had negative effects on LPS, but only the effect of tobacco was not associated with aging. FPAs with less pre-stretch had thicker medial layers, but thinner intramural elastic fibers with less dense and more fragmented external elastic laminae. Elastin degradation was associated with decreased physiological tethering force and longitudinal stress, while circumferential stress remained constant. FPA wall pathology was negatively associated with LPS (r = −0.553, p \u3c 0.01), but the effect was due primarily to aging. LPS in the FPA may serve as an energy reserve for adaptive remodeling. Reduction of LPS due to degradation and fragmentation of intramural longitudinal elastin during aging can be accelerated in tobacco users

In situ longitudinal pre-stretch in the human femoropopliteal artery

In situ longitudinal (axial) pre-stretch (LPS) plays a fundamental role in the mechanics of the femoropopliteal artery (FPA). It conserves energy during pulsation and prevents buckling of the artery during limb movement. We investigated how LPS is affected by demographics and risk factors, and how these patient characteristics associate with the structural and physiologic features of the FPA. LPS was measured in n = 148 fresh human FPAs (14–80 years old). Mechanical properties were characterized with biaxial extension and histopathological characteristics were quantified with Verhoeff–Van Gieson Staining. Constitutive modeling was used to calculate physiological stresses and stretches which were then analyzed in the context of demographics, risk factors and structural characteristics. Age had the strongest negative effect (r = −0.812, p \u3c 0.01) on LPS and could alone explain 66% of LPS variability. Male gender, higher body mass index, hypertension, diabetes, coronary artery disease, dyslipidemia and tobacco use had negative effects on LPS, but only the effect of tobacco was not associated with aging. FPAs with less pre-stretch had thicker medial layers, but thinner intramural elastic fibers with less dense and more fragmented external elastic laminae. Elastin degradation was associated with decreased physiological tethering force and longitudinal stress, while circumferential stress remained constant. FPA wall pathology was negatively associated with LPS (r = −0.553, p \u3c 0.01), but the effect was due primarily to aging. LPS in the FPA may serve as an energy reserve for adaptive remodeling. Reduction of LPS due to degradation and fragmentation of intramural longitudinal elastin during aging can be accelerated in tobacco users

Non-Destructive Characterization of Peripheral Arteries using Intravascular Ultrasound

Peripheral Artery Disease (PAD) is the chronic obstruction of blood flow to the extremities caused by plaque buildup. Poor circulation results in exertional pain, numbness, and weakness, and in severe cases, can manifest critical conditions, including gangrene and limb loss. PAD affects approximately 8.5 million Americans and costs the United States $21 billion annually in direct medical expenses. High expenditures are attributed to operation and intervention failures resulting in frequent need for revascularization. Treatment of PAD typically involves lifestyle/diet adjustments, bypass surgery, or angioplasty/stenting. Unfortunately, repeated limb deformation during locomotion often results in adverse repair device-artery interactions, which hinder the long-term efficacy of endovascular therapies. Patient and lesion-specific device selection guided by computational modeling can help improve clinical outcomes, but these models rely heavily on accurately recorded three-dimensional arterial geometry and plaque composition. Intravascular ultrasound (IVUS) is a minimally invasive method of endovascular imaging that allows evaluation of the geometry and composition of the arterial wall, but its two-dimensional nature is often insufficient to capture complex three-dimensional plaques. We have developed a method of obtaining three-dimensional arterial geometry from two-dimensional IVUS images to build Computer-Aided Design models of calcified human femoropopliteal arteries. Our imaging method will allow for the characterization of calcium, necrotic core, fibrofatty, and fibrous tissue using IVUS. Correlation of IVUS images with conventional histology, micro-CT imaging, and clinical CTA data will help inform computational models.https://digitalcommons.unmc.edu/surp2021/1025/thumbnail.jp

Coupled hemodynamics and mechanics of the repaired human carotid artery

Restenosis is a major complication of the existing surgical techniques used to treat atherosclerosis of the carotid artery. Recently, atherosclerosis has been linked to certain mechanical factors. The objective of this work was to develop a comprehensive model of a reconstructed human carotid and to use it to study relevant mechanical characteristics as functions of repair technique and material. Three-dimensional geometry of the blood vessel was constructed using computerized tomography data. The observed geometry was more tortuous and with less bifurcation angle than it had been commonly assumed before. Mechanical properties of the carotid artery wall and common repair materials were evaluated by biaxial mechanical testing. It was found that the behavior of tissues is highly non-linear and anisotropic. All tested patching materials were substantially stiffer than the carotid walls. Hemodynamics of the blood flow at the boundaries of the carotid artery was studied using duplex ultrasound method. It was found that velocity profiles in the carotid artery are not symmetric. A comprehensive finite element model of a patched carotid was developed that takes into account the interaction between the blood flow and the arterial wall, non-linear anisotropic mechanical properties of the wall, complex three-dimensional vessel geometry, real pulsatile blood velocity profiles, non-Newtonian blood rheology, and closure with suture. Analysis of the results of modeling showed that certain techniques and materials have advantages and some have shortcomings. It was found that increase in the patch width had negative effect on the behavior of the artery. Lateral placement of the patch decreased stresses in the arterial wall but at the same time broadened the low wall shear stress region and boosted the cyclic strain. Patch material had significant effect on behavior of the repaired artery. Overall, analysis of the existing repair techniques showed that the ideal patch and/or repair method are yet to be found. The developed model can be used to study and optimize the results of surgical interventions and can help develop improved repair techniques and materials