Material properties of components in human carotid atherosclerotic plaques: A uniaxial extension study

Computational modelling to calculate the mechanical loading within atherosclerotic plaques has been shown to be complementary to defining anatomical plaque features in determining plaque vulnerability. However, its application has been partially impeded by the lack of comprehensive knowledge about the mechanical properties of various tissues within the plaque. Twenty-one human carotid plaques were collected from endarterectomy. The plaque was cut into rings, and different type of atherosclerotic tissues, including media, fibrous cap (FC), lipid and intraplaque haemorrhage/thrombus (IPH/T) was dissected for uniaxial extension testing. In total, 65 media strips from 17 samples, 59 FC strips from 14 samples, 38 lipid strips from 11 samples, and 21 IPH/T strips from 11 samples were tested successfully. A modified Mooney–Rivlin strain energy density function was used to characterize the stretch–stress relationship. The stiffnesses of media and FC are comparable, as are lipid and IPH/T. However, both media and FC are stiffer than either lipid or IPH/T. The median values of incremental Young’s modulus of media, FC, lipid and IPH/T at λ = 1 are 290.1, 244.5, 104.4, 52.9, respectively; they increase to 1019.5, 817.4, 220.7 and 176.9 at λ = 1.1; and 4302.7, 3335.0, 533.4 and 268.8 at λ = 1.15 (unit, kPa; λ, stretch ratio). The material constants of each tissue type are suggested to be: media, c₁ = 0.138 kPa, D₁ = 3.833 kPa and D₂ = 18.803; FC, c₁ = 0.186 kPa, D₁ = 5.769 kPa and D₂ = 18.219; lipid, c₁ = 0.046 kPa, D₁ = 4.885 kPa and D₂ = 5.426; and IPH/T, c₁ = 0.212 kPa, D₁ = 4.260 kPa and D₂ = 5.312. It is concluded that all soft atherosclerotic tissues are non-linear, and both media and FC are stiffer than either lipid or IPH/T.     

Local mechanical and structural properties of healthy and diseased human ascending aorta tissue

ObjectiveThis study investigates the mechanics and histology of healthy and dilated human ascending aortas (AA). The regional variation in mechanical response and tissue structure were compared.MethodsRings of human AA from healthy (n=5), dilated tricuspid aortic valve (TAV, n=5), and dilated bicuspid aortic valve (BAV, n=6) patients were mechanically tested. Each aortic ring was sectioned into quadrants—anterior, posterior, medial (inner curvature) and lateral (outer curvature). Low- and high-stress elastic moduli were calculated from the equibiaxial stress strain curve to determine the local mechanical properties. Histological analysis was used to quantify the percent composition of elastin, collagen, and smooth muscle cells.ResultsBAV tissue was thinnest and contained the largest percent composition of collagen. Both TAV and BAV tissue had significantly less elastin than healthy tissue. At low strain in the circumferential direction, TAV tissue was on average the least stiff. The elastic modulus was dependent on quadrant and tissue type but not direction (isotropic). Generally, the lateral quadrant tissue was the stiffest and the medial quadrant the least stiff. There were no apparent local variations in the tissue histology.ConclusionsLocal variations in tissue thickness and mechanical properties were evident in all samples analyzed and may be linked to the type of aortic valve present.     

Altered structural and mechanical properties in decellularized rabbit carotid arteries

Recently, major achievements in creating decellularized whole tissue scaffolds have drawn considerable attention to decellularization as a promising approach for tissue engineering. Decellularized tissues are expected to have mechanical strength and structure similar to the native tissues from which they are derived. However, numerous studies have shown that mechanical properties change after decellularization. Often, tissue structure is observed by histology and electron microscopy, but the structural alterations that may have occurred are not always evident. Here, a variety of techniques were used to investigate changes in tissue structure and relate them to altered mechanical behavior in decellularized rabbit carotid arteries. Histology and scanning electron microscopy revealed that major extracellular matrix components were preserved and fibers appeared intact, although collagen appeared looser and less crimped after decellularization. Transmission electron microscopy confirmed the presence of proteoglycans (PG), but there was decreased PG density and increased spacing between collagen fibrils. Mechanical testing and opening angle measurements showed that decellularized arteries had significantly increased stiffness, decreased extensibility and decreased residual stress compared with native arteries. Small-angle light scattering revealed that fibers had increased mobility and that structural integrity was compromised in decellularized arteries. Taken together, these studies revealed structural alterations that could be related to changes in mechanical properties. Further studies are warranted to determine the specific effects of different decellularization methods on the structure and performance of decellularized arteries used as vascular grafts.     

Modulation of mechanical properties in multiple-component tissue adhesives

In vitro, ex vivo, and in vivo studies were performed to investigate the effect of mixing upon the mechanical properties of a two-component tissue adhesive. The hypothesis investigated was that a more complete mixing of the two components would yield an increase in the mechanical performance of the adhesive. This in turn would be demonstrated by improved outcomes in models of clinical sealant application. In vitro stereological analysis of tissue adhesive mixed and delivered by several different applicators demonstrated variation in the amount of mixing provided by each type of delivery system. Ex vivo tensile adhesive strength showed that there was a correlation between the amount of mixing and bonding strength; that is, more thorough mixing demonstrated higher adhesive strength. No significant difference was seen, however, between the different applicator types and impact on in vivo dermal incisional closure strength. There was a correlation, though, in amount of mixing and in vivo hemostasis. In a rabbit spleen incision model, a more thoroughly mixed sealant corresponded with a decrease in time to obtain complete hemostasis, as well as less sealant used. The effects of mixing on tissue-adhesive mechanical performance were influenced somewhat by the amount of mixing provided by the applicator. This effect, however, was dependent upon the sealant formulation and the type of in vivo application.     

Elastic properties of microstructural components of human bone tissue as measured by nanoindentation.

The elastic properties of several microstructural components of dry human vertebrae (T-12 and L-1) and tibiae have been investigated in the longitudinal and transverse directions using nanoindentation. The largest Young's modulus was that for the interstitial lamellae in the longitudinal direction (25.7 +/- 1.7 GPa). This was followed in decreasing order by osteons in the longitudinal direction (22.4 +/- 1.2 GPa), trabeculae in the longitudinal direction (19.4 +/- 2.3 GPa), an average over osteons and interstitial lamellae in the transverse direction [16.6 +/- 1.1 GPa (it was difficult to microstructurally distinguish osteons from interstitial lamellae in the transverse direction)], and trabeculae in the transverse direction (15.0 +/- 2.5 GPa). An ANOVA statistical analysis revealed that the values all are significantly different (p < 0.05). Since the elastic moduli in the longitudinal direction are all greater than in the transverse, measurable elastic anisotropies exist in the components. The hardnesses also varied among the microstructural components in the range 0.52-0.74 GPa.     

Computer simulation and geometric design of endarterectomized carotid artery bifurcations

The main goal of this computational study is to establish surgical guidelines for optimal geometries of carotid endarterectomy reconstructions that may measurably reduce postoperative complications, that is, thrombosis, stroke, and/or restenosis. The underlying hypotheses are that nonuniform hemodynamics, or "disturbed flows," are linked to arterial diseases and consequently that minimization of "disturbed flow" indicators leads to geometric bifurcation designs that lower postoperative complication rates. Considering transient 3-D laminar blood flow in partially occluded, in-plane, rigid-wall carotid artery bifurcations, the results presented include time-averaged indicators of "disturbed flow", such as the wall shear stress, spatial wall shear stress gradient, and wall shear stress angle deviation. In addition, trajectories and deposition patterns of critical blood particles (i.e., monocytes) are shown and evaluated. Within given physiological constraints, the vessel geometry was then changed in order to reduce the magnitudes of key indicators associated with thrombosis (i.e., blood clot formation) or restenosis (e.g., renewed atherosclerosis and/or hyperplasia). The quantitative results and knowledge base generated will be crucial for future clinical trials.     

Cyclically stretching developing tissue in vivo enhances mechanical strength and organization of vascular grafts

Tissue-engineered vascular grafts must have qualities that rival native vasculature, specifically the ability to remodel, the expression of functional endothelial components and a dynamic and functional extracellular matrix (ECM) that resists the forces of the arterial circulation. We have developed a device that when inserted into the peritoneal cavity, attracts cells around a tubular scaffold to generate autologous arterial grafts. The device is capable of cyclically stretching (by means of a pulsatile pump) developing tissue to increase the mechanical strength of the graft. Pulsed (n = 8) and unpulsed (n = 8) devices were implanted for 10 days in Lovenaar sheep (n = 8). Pulsation occurred for a period of 5–8 days before harvest. Thick unadhered autologous tissue with cells residing in a collagen ECM was produced in all devices. Collagen organization was greater in the circumferential direction of pulsed tissue. Immunohistochemical labelling revealed the hematopoietic origin of >90% cells and a significantly higher coexpression with vimentin in pulsed tissue. F-actin expression, mechanical failure strength and strain were also significantly increased by pulsation. Moreover, tissue could be grafted as carotid artery patches. This paper shows that unadhered tissue tubes with increased mechanical strength and differentiation in response to pulsation can be produced with every implant after a period of 10 days. However, these tissue tubes require a more fine-tuned exposure to pulsation to be suitable for use as vascular grafts.     

人骨拉伸和压缩力学的性能测试

背景：人体骨组织材料力学性能参数是进行骨骼内部应力计算分析、断裂损伤分析、应力骨改建研究等的基础数据,也是深入开展人体骨骼生物力学行为特性研究的必要已知条件之一,但目前国际上还没有统一的骨组织力学性能测试的试验标准。 目的：探讨骨组织拉压力学性能测试的试样制备、试样约束和夹持、试验数据处理和试验误差等问题的研究进展。 方法：在骨组织拉伸压缩试验无具体标准可循的环境下,对已验证的试验方法和途径进行介绍。总结并强调了骨组织拉压试验中的各项误差及其减少;以减少系统误差为目的,介绍了规范的骨组织宏观拉压测试方法。 结果与结论：通过拉伸压缩试验可获得骨组织的弹性模量、泊松比、拉伸强度和压缩强度等重要力学性能参数,测试关键环节涉及标准试样设计和加工、试件夹持、试验数据处理和力学参数的计算,各环节均需参照最新研究以减少试验误差。     

Connective tissue components of the normal and fibrotic human liver

Summary In this second part, clinical aspects of connective tissue metabolism in the liver will be described and two main aspects considered. The first is the possibility to monitor the activity of fibrosis by the use of metabolites and enzymes of connective tissue metabolism. In recent studies the qualification for this purpose of enzymes such as procollagen prolyl hydroxylase and lysosomal N-Acetyl--glucosaminidase and the N-terminal peptide of procollagen type III has been tested. The serum activities or concentrations of these substances in patients with chronic active liver diseases increase in due relation to the histologically estimated activity of liver fibrosis.The second aspect deals with therapeutic approaches to fibrosis in the liver by using connective tissue specific agents. So far none of the antifibrotic substances such as proline analogues, colchicine, lathyrogens and penicillamine has been used in longer-term antifibrotic treatment.Dedicated to Prof. Dr. J. Staubesand on the ocasion of his 60th birthday     

Flow in carotid bifurcations: effect of the superior thyroid artery.

The superior thyroid artery was ignored in previous fluid dynamics studies of carotid bifurcations. However, it is not clear to what extent the flow patterns within the carotid might be influenced by the presence of this outflow tract in reality. In this study, quantitative effects of the superior thyroid artery upon the flow patterns and wall shear stress in the carotid bifurcation were investigated in detail by a numerical simulation method. Comparisons of the maximum reversed flow velocities, flow patterns and wall shear stress were made between models with and without the superior thyroid artery. Results demonstrate that this small artery has only a marginal effect on the overall flow characteristics within the carotid sinus. However, it does have significant effects on flow patterns in the common-external side branch. An alternative approach is proposed to compensate for the absence of this small artery in numerical calculations.     

In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength

Background: Collagen‐rich tendons and ligaments are important for joint stability and force transmission, but the capacity to form new tendon is poorly understood. In the present study, we investigated mechanical strength, fibril size, and structure during development of tendon‐like tissue from adult human tenocytes (termed tendon constructs) in vitro over 5 weeks in 3D tissue culture. Results: The constructs displayed large elongated tendon cells aligned along the tendon axis together with collagen fibrils that increased in diameter by 50% from day 14 to 35, which approaches that observed in adult human tendon in vivo. The increase in diameter was accompanied by a 5‐fold increase in mechanical strength (0.9±0.1 MPa to 4.9±0.6 MPa) and Young's modulus (5.8±0.9 MPa to 32.3±4.2 MPa), while the maximal strain at failure (16%) remained constant throughout the 5‐week culture period. Conclusions: The present study demonstrates that 3D tendon constructs can be formed by isolated human tendon fibroblasts, and when these constructs are subjected to static self‐generated tension, the fibrils will grow in size and strength approaching that of adult human tendon in vivo. Developmental Dynamics 242:2–8, 2013. © 2012 Wiley Periodicals, Inc.     

Dissection and intussusception of the common carotid artery following endarterectomy

We describe a 67-year-old man who developed a dissection of the right common carotid arterial wall with intussusception two years following endarterectomy for atherosclerosis. We suggest that the previous surgical procedure resulted in a thinned and weakened media. With a disruption of the intima, a dissection into the inner third of the media formed, and blood in the false lumen buckled the intima into the lumen of the blood vessel. Hemodynamic forces then probably played a role in the propagation of the dissection and the formation of an intussusception. Complete occlusion by thrombus followed, resulting in a massive cerebral infarct, the likely terminal event. Carotid endarterectomy rarely may be followed by dissection of the vascular wall and intussusception.     

Chemoreceptor properties of glomus tissue found in the carotid region of the cat

1. ;Miniglomera' appearing as small masses of tissue with ample vascularization were found around the common carotid artery of the cat. Physiological, gross anatomical and electron microscopic studies were conducted on these tissues.2. The chemosensory function of each ;miniglomus' was evident from the behaviour of the afferent nerve fibres supplying the tissue: afferent responses became more active during asphyxia, when the blood flow through the tissue was reduced or blocked and when cyanide or ACh were applied. The afferent impulses became more infrequent during hyperventilation.3. Sensory frequency response curves constructed against percentage of inhaled O(2) showed that the impulses of single units increased in frequency with lowering of O(2) content of the inhaled gas.4. These miniglomera are innervated by afferent fibres emerging from the nodose ganglion; sometimes these fibres are contained in the aortic or common carotid baroreceptor nerves, but sometimes they emerge as independent nerves. None of the miniglomera are supplied by branches of the sinus nerve.5. The fine structure of the miniglomus is similar to that of the carotid body. The tissue contains two types of cells: glomus cells which contain dense cored granules, and sustentacular cells whose fine processes enclose the former. Membrane densifications occur where glomus cells lie adjacent to one another or where they are contacted by nerve terminals. Nerve fibres are common in the miniglomus but they contact glomus cells less frequently than in the carotid body.     

Mechanical and biochemical properties of human cervical tissue

The mechanical integrity of cervical tissue is crucial for maintaining a healthy gestation. Altered tissue biochemistry can cause drastic changes in the mechanical properties of the cervix and contribute to premature cervical dilation and delivery. We present an investigation of the mechanical and biochemical properties of cervical samples from human hysterectomy specimens. Three clinical cases were investigated: nonpregnant hysterectomy patients with previous vaginal deliveries; nonpregnant hysterectomy patients with no previous vaginal deliveries; and pregnant hysterectomy patients at time of cesarean section. Tissue samples were tested in confined compression, unconfined compression and tension. Cervical tissue samples for the three clinical cases were also subjected to biochemical analysis. Biochemical assays measured cervical tissue hydration, collagen content, collagen extractability and sulfated glycosaminoglycan (GAG) content. Results from the mechanical tests indicate that cervical stroma has a nonlinear, time-dependent stress response with varying degrees of conditioning and hysteresis depending on its obstetric background. It was found that the nonpregnant tissue was significantly stiffer than the pregnant tissue in both tension and compression. Further, collagen extractability, sulfated GAG content and hydration were substantially higher in the pregnant tissue. This study is the first important step towards the attainment of an improved understanding of the complex interplay between the molecular structure of cervical tissue and its macroscopic mechanical properties.     

Fractal dimension and mechanical properties of human cortical bone

Fractal dimension (FD) can be used to characterize microstructure of porous media, particularly bone tissue. The porous microstructure of cortical bone is observable in micro-CT (μCT) images. Estimations of fractal dimensions of μCT images of coupons of human cortical bone are obtained. The same samples were tested on a tensile test machine and Young's modulus (YM) and Failure stress were obtained. When both types of measures were compared, a clear correlation was found (R = ?81%, P < 0.01). Young's modulus of each sample and the FD of its μCT images are correlated. From the assumption that cortical bone is approximately a fractal set, a non-linear constitutive relation involving FD is obtained for YM. Experimental results show good agreement with this constitutive relation. Additional parameters in the non-linear relation between YM and FD have been estimated from experimental results and related to physical parameters.     

Biological properties of human leukocyte interferon components.

Human leukocyte interferon, purified approximately 1000-fold by affinity chromatography on immobilized anti-interferon globulins and SDS-Sephadex filtration, was resolved into one major and one minor component by adsorption chromatography on hydroxylapatite and electrophoresis in polyacrylamide gels. These components were indistinguishable in their capacity to protect bovine, porcine and murine cells, and the antiviral activities of both were equally susceptible to reduction by beta-mercaptoethanol. They were neutralized to the same degree of rabbit anti-leukocyte interferon but were not neutralized by rabbit antifibroblast interferon serum. Mice immunized with either component developed antibodies to both but failed to form antibodies against human fibroblast interferon. Our present evidence indicates that the two components posses at most only minor structural and antigenic dissimilarities.     

Mechanical,biological and structural characterization of in vitro ruptured human carotid plaque tissue

Recent experimental studies performed on human carotid plaques have focused on mechanical characterization for the purpose of developing material models for finite-element analysis without quantifying the tissue composition or relating mechanical behaviour to preoperative classification. This study characterizes the mechanical and biological properties of 25 human carotid plaques and also investigates the common features that lead to plaque rupture during mechanical testing by performing circumferential uniaxial tests, Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) on each specimen to relate plaque composition to mechanical behaviour. Mechanical results revealed large variations between plaque specimen behaviour with no correlation to preoperative ultrasound prediction. However, FTIR classification demonstrated a statistically significant relationship between stress and stretch values at rupture and the level of calcification (P = 0.002 and P = 0.009). Energy-dispersive X-ray spectroscopy was carried out to confirm that the calcium levels observed using FTIR analysis were accurate. This work demonstrates the potential of FTIR as an alternative method to ultrasound forpredicting plaque mechanical behaviour. SEM imaging at the rupture sites of each specimen highlighted voids created by the nodes of calcifications in the tissue structure which could lead to increased vulnerability of the plaque.