Layer-Specific Properties of the Human Infra-Renal Aorta During Aging Considering Pre/Post-Failure Damage.

There is little information on the layer-specific failure properties of the adult human abdominal aorta, and there has been no quantification of postfailure damage. Infra-renal aortas were thus taken from forty-seven autopsy subjects and cut into 870 intact-wall and layer strips that underwent uni-axial-tensile testing. Intact-wall failure stress did not differ significantly (p > 0.05) from the medial value longitudinally, nor from the intimal and medial values circumferentially, which were the lowest recorded values. Intact-wall failure stretch did not differ (p > 0.05) from the medial value in either direction. Intact-wall prefailure stretch (defined as failure stretch-stretch at the initiation of the concave phase of the stress-stretch response) did not differ (p > 0.05) from the intimal and medial values, and intact-wall postfailure stretch (viz., full-rupture stretch-failure stretch) did not differ (p > 0.05) from the adventitial value since the adventitia was the last layer to rupture, being most extensible albeit under residual tension. Intact-wall failure stress and stretch declined from 20 to 60 years, explained by steady declines throughout the lifetime of their medial counterparts, implicating beyond 60 years the less age-varying failure properties of the intima under minimal residual compression. The positive correlation of postfailure stretch with age counteracted the declining failure stretch, serving as a compensatory mechanism against rupture. Hypertension, diabetes, and coronary artery disease adversely affected the intact-wall and layer-specific failure stretches while increasing stiffness.

Residual strains in ascending thoracic aortic aneurysms: The effect of valve type, layer, and circumferential quadrant.

The stress distribution in ascending thoracic aortic aneurysms is determined by the mechanical properties, geometry, loading conditions, and zero-stress state of the aneurysmal aorta. Our objective was to fully characterize the zero-stress state of the aneurysmal aorta in twelve tricuspid aortic valve patients and eight (age/aortic diameter-matched) bicuspid aortic valve patients, for which little data are available. Opening angles and residual stretches were measured for the intact wall and individual layers according to quadrant and were similar in the two patient groups. The intact-wall and medial opening angles were comparable; their circumferential but not their axial ones peaked in the left lateral quadrant, with non-significant regional differences in the other layers. The intima's circumferential opening angles were the highest of all layers (∼300 deg) and the adventitia's the lowest (∼165 deg), with lesser layer differences in the axial opening angles. Upon radially cutting aortic rings, the released circumferential residual stretches were tensile (of large magnitude) externally and compressive (of small magnitude) internally, unlike the axial residual stretches released when cutting intact-wall strips, whose magnitude was small externally and large internally. Nevertheless, large circumferential compressive residual stretches were released in the adventitia upon layer dissection, counteracting the large circumferential tensile stretches of the intact wall externally. Moreover, the large axial tensile residual stretches of the intima counteracted the large axial compressive stretches of the intact wall internally. These layer-specific residual stretches may moderate the in-vivo stress gradients across wall thickness, serving as a protective mechanism against aortic dissection or rupture.

Layer-Specific Tensile Strength of the Human Aorta: Segmental Variations.

Knowledge of the failure properties of the aorta is essential to understand the mechanisms of dissection and rupture. Limited information is, however, available in humans or experimental animals about the layer-specific properties and their segmental variations have not been determined. In this paper, the failure properties of the intima, media, and adventitia were studied in nine consecutive aortic segments and two principal directions. Detailed biomechanical tests were performed with a tensile-testing device on 756 layer strips, harvested from fourteen cadaveric subjects aged 21-82 years. Intimal and medial strength in either direction remained invariant along the aorta, and their extensibility longitudinally decreased, whereas adventitial strength and extensibility longitudinally increased, explaining why the preferential sites for the development of aortic dissection or traumatic rupture are in the proximal aorta. The media was stronger circumferentially than longitudinally in all segments, accounting for the typically transverse tearing in dissection/rupture. The adventitial properties were significantly higher than the intimal and medial in most segments. Still, the intima had similar strength but lower extensibility compared to the media in both directions, and higher maximum stiffness longitudinally in several segments. The rupture surface of all layers was not perpendicular to the loading axis, more so in the circumferential strips compared to longitudinal ones. Aging impaired the extensibility and strength of all layers, particularly the media, but did not affect the maximum stiffness and rupture-surface direction. Females were rarely associated with different failure properties compared to age-matched males.

Regional delamination strength in the human aorta underlies the anatomical localization of the dissection channel.

Aortic dissection is a life-threatening event, during which a primary tear propagates along the aorta causing catastrophic delamination of the inner (intima with most of the media) from the outer layers (leftover media with adventitia). Our understanding of mode-I fracture resistance at different aortic regions is incomplete, although the anatomical localization of the dissection channel may be assigned to this factor. To determine whether the susceptibility to dissection propagation varied with aortic region, the average and standard deviation of peel tension (indices of adhesive strength between layers when pulled apart and its fluctuation) were measured in 24 cadaveric subjects. Measurements were made in the inner and outer quadrants of 9 consecutive regions. Strong regional heterogeneity was established that was age-related based on the following evidence: (1) the average and standard deviation of peel tension peaked in the ascending aorta, decreasing to almost constant values in the descending thoracic aorta, but increasing across the abdominal aorta; (2) axial differences were more pronounced in the inner quadrant, with differences among quadrants reaching significance proximally; (3) the average peel tension was greatly impaired from <40 to 40-60 but much less to >60-year-old subjects at most regions/quadrants, leading to non-uniform axial variations in all age groups; (4) gender affected little the data. This comprehensive series of delamination tests explains the clinical observation of most dissections initiating in the ascending aorta to extend distally and of few dissections initiating in the descending thoracic aorta to extend proximally, while supporting the increased vulnerability in aged subjects.

Regional and directional variations in the layer-specific resistance to tear propagation in ascending thoracic aortic aneurysms.

Aortic dissection often initiates a few centimeters distal to the coronary ostia in the right lateral wall, with an intimal-medial tear that tends to be transversely directed and occupy half of the aortic circumference, sometimes less, but seldom the entire circumference. To elucidate these clinical observations, tear tests were presently used to determine the layer-specific resistance to tear propagation in ascending thoracic aortic aneurysms, assessing variations over the four circumferential quadrants and two directions. Aneurysmal tissue strips of standardized dimensions from sixteen patients were anatomically separated into layers (seven hundred and twelve) and an incision made along one-third of their length. They underwent tear testing via uniaxial loading and then unloading before crack propagation had proceeded along their complete length. The average tear tension and tear energy per reference area generated were many-fold greater in outer- (adventitial) compared to inner- (intimal with small medial portion) and middle-layer (medial) strips, explaining why the tear is restricted to the inner wall. They were greater in inner- compared to middle-layer strips of the anterior and left lateral quadrants, suggesting that the tear will propagate to the less-resistant media even if initiated in the intima. In most longitudinally-cut middle- and inner-layer strips, the cracks deviated toward the circumferential direction and tore out through the side, justifying the circumferential course of the tear. Both fracture parameters were significantly higher in the right than the left lateral quadrant in outer-layer strips and the anterior quadrant in middle-layer strips, potentially affecting the circumferential extent of the tear.

Improved Repopulation Efficacy of Decellularized Small Diameter Vascular Grafts Utilizing the Cord Blood Platelet Lysate.

BACKGROUND: The development of functional bioengineered small-diameter vascular grafts (SDVGs), represents a major challenge of tissue engineering. This study aimed to evaluate the repopulation efficacy of biological vessels, utilizing the cord blood platelet lysate (CBPL). METHODS: Human umbilical arteries (hUAs, n = 10) were submitted to decellularization. Then, an evaluation of decellularized hUAs, involving histological, biochemical and biomechanical analysis, was performed. Wharton's Jelly (WJ) Mesenchymal Stromal Cells (MSCs) were isolated and characterized for their properties. Then, WJ-MSCs (1.5 × 106 cells) were seeded on decellularized hUAs (n = 5) and cultivated with (Group A) or without the presence of the CBPL, (Group B) for 30 days. Histological analysis involving immunohistochemistry (against Ki67, for determination of cell proliferation) and indirect immunofluorescence (against activated MAP kinase, additional marker for cell growth and proliferation) was performed. RESULTS: The decellularized hUAs retained their initial vessel's properties, in terms of key-specific proteins, the biochemical and biomechanical characteristics were preserved. The evaluation of the repopulation process indicated a more uniform distribution of WJ-MSCs in group A compared to group B. The repopulated vascular grafts of group B were characterized by greater Ki67 and MAP kinase expression compared to group A. CONCLUSION: The results of this study indicated that the CBPL may improve the repopulation efficacy, thus bringing the biological SDVGs one step closer to clinical application.

Failure properties of ascending thoracic aortic aneurysms with dysfunctional tricuspid aortic valves.

OBJECTIVES: Ascending thoracic aortic aneurysms (ATAAs) often coexist with dysfunctional tricuspid aortic valves (TAVs). How valvular pathology relates to the aortic wall mechanical properties requires detailed examination. METHODS: Intact-wall and layer-specific mechanical properties from 40 and 21 patients with TAV-ATAAs, respectively, were studied using uniaxial tensile testing, longitudinally and circumferentially. Failure stress (tensile strength), failure stretch (extensibility) and peak elastic modulus (stiffness) measurements, along with histological assays of thickness and elastin/collagen contents, were compared among patients with no valvular pathology (NVP), aortic stenosis (AS) or aortic insufficiency (AI). RESULTS: Intact-wall stiffness longitudinally and medial strength and stiffness, in either direction, were significantly lower in AI patients than in AS and NVP patients. Intact-wall/medial thickness and extensibility in either direction were significantly lower in AS patients than in AI and NVP patients. In contrast, intact-wall/medial stiffness circumferentially was significantly higher in AS patients than in NVP patients, consistent with the significantly increased medial collagen in AS patients. Failure properties and medial thickness and elastin/collagen contents were significantly lower (more impaired) in females. The left lateral was the thickest quadrant in NVP patients, but the 4 quadrants were equally thick in AS and AI patients. There were significant differences in strength and stiffness among quadrants, which varied however in the 3 patient groups. CONCLUSIONS: The aortic wall load-bearing capacity was impaired in patients with ATAA in the presence of TAV stenosis or insufficiency. These findings lend biomechanical support to the current guidelines suggesting lower thresholds for elective ascending aorta replacement in cases of aortic valve surgery.

Layer-Specific Residual Deformations and Their Variation Along the Human Aorta.

This study described the regional distribution of layer-specific residual deformations in fifteen human aortas collected during autopsy. Circumferentially and axially cut strips of standardized dimensions from the anterior quadrant of nine consecutive aortic levels were photographed to obtain the zero-stress state for the intact wall. The strips were then dissected into layers that were also photographed to obtain their zero-stress state. Changes in layer-specific opening angle, residual stretches, and thickness at each aortic level and direction were determined via image analysis. The circumferential and axial opening angles of the intima were ∼240 deg and ∼30 deg, respectively, throughout the aorta; those of the adventitia were ∼150 deg and -20 deg to 70 deg. The opening angles of the intact wall and media were similar in either direction. The circumferential residual stretches of the intima and the axial residual stretches of the media showed high values in the aortic arch, decreasing in the descending thoracic aorta and increasing toward the iliac artery bifurcation, while the axial residual stretches of the adventitia increased distally. The remaining residual stretches did not vary significantly with aortic level, suggesting an intimal role in determining circumferential, as well as medial and adventitial roles in determining axial residual stretches. We conclude that the tensile residual stretches released in the intima and media upon separation, and the compressive residual stretches released in the adventitia may moderate the inverse transmural stress gradients under physiologic loads, resulting from the >180 deg circumferential opening angle of the intact wall.

Variation of Passive Biomechanical Properties of the Small Intestine along Its Length: Microstructure-Based Characterization.

Multiaxial testing of the small intestinal wall is critical for understanding its biomechanical properties and defining material models, but limited data and material models are available. The aim of the present study was to develop a microstructure-based material model for the small intestine and test whether there was a significant variation in the passive biomechanical properties along the length of the organ. Rat tissue was cut into eight segments that underwent inflation/extension testing, and their nonlinearly hyper-elastic and anisotropic response was characterized by a fiber-reinforced model. Extensive parametric analysis showed a non-significant contribution to the model of the isotropic matrix and circumferential-fiber family, leading also to severe over-parameterization. Such issues were not apparent with the reduced neo-Hookean and (axial and diagonal)-fiber family model, that provided equally accurate fitting results. Absence from the model of either the axial or diagonal-fiber families led to ill representations of the force- and pressure-diameter data, respectively. The primary direction of anisotropy, designated by the estimated orientation angle of diagonal-fiber families, was about 35° to the axial direction, corroborating prior microscopic observations of submucosal collagen-fiber orientation. The estimated model parameters varied across and within the duodenum, jejunum, and ileum, corroborating histologically assessed segmental differences in layer thicknesses.

Ascending aorta mechanics in bicuspid aortopathy: controversy or fact?

Bicuspid aortic valve is the most common congenital cardiovascular defect, often associated with proximal aortic dilatation, and the ideal management strategy is debated. The inconsistency in previous and present guideline recommendations emphasizes the insufficiency of the maximal diameter as the sole criterion for prophylactic repair. Our ability to guide clinical decisions may improve through an understanding of the mechanical properties of ascending thoracic aortic aneurysms in bicuspid compared to tricuspid aortic valve patients and non-aneurysmal aortas, because dissection and rupture are aortic wall mechanical failures. Such an understanding of the mechanical properties has been attempted by several authors, and this article addresses whether there is a controversy in the accumulated knowledge. The available mechanical studies are briefly reviewed, discussing factors such as age, sex, and the region of mechanical examination that may be responsible for the lack of unanimity in the reported findings. The rationale for acquiring layer-specific properties is presented along with the main results from our recent study. No mechanical vulnerability of ascending thoracic aortic aneurysms was evidenced in bicuspid aortic valve patients, corroborating present conservative guidelines concerning the management of bicuspid aortopathy. Weakening and additional vulnerability was evidenced in aged patients and those with coexisting valve pathology, aortic root dilatation, hypertension, and hyperlipidemia. Discussion of these results from age- and sex-matched subjects, accounting for the region- and layer-specific aortic heterogeneity, in relation to intact wall results and histologic confirmation, helps to reconcile previous findings and affords a universal interpretation of ascending aorta mechanics in bicuspid aortopathy.

Time-course of axial residual strain remodeling and layer-specific thickening during aging along the human aorta.

Detailed estimation of axial residual strains in the human aorta is necessary when performing biomechanical analyses of physiologic functions and pathologic conditions. We recently published such data for autopsied aortas and the present aim was to measure axial residual stretches at different wall depths, along with layer thicknesses on images borrowed from that work. Residual stretches at the external surface and medial-adventitial interface increased along the aorta's ascending course, decreased along its descending course, and increased from the diaphragm toward the iliac arteries. Residual stretches at the intimal-medial interface and internal surface decreased down the distal one-third of the aorta. A continuous decrease in medial thickness was witnessed along the vessel, whereas intimal and adventitial thickness remained fairly stable. At some axial locations, smaller were the axial residual stretches of the outer than those of the other quadrants, with minor differences in layer-specific thicknesses among quadrants. Adventitial thickness did not vary with age, while the intima and media thickened considerably with different time-courses. The observed intimal thickening solely between young (≤40 yr) and middle-aged subjects (40-60 yr) is consistent with the increased circumferential residual stretches previously established by our group between those subject groups and the minimal further increase in old subjects (≥60 yr). The observed medial thickening between middle-aged and old subjects was accompanied by decreased axial residual stretches that were not seen between young and middle-aged subjects. These observations suggest distinct roles for the intima and media in determining circumferential and axial residual stretches that merit further attention.

Insights into Biomechanical and Proteomic Characteristics of Small Diameter Vascular Grafts Utilizing the Human Umbilical Artery.

The gold standard vascular substitutes, used in cardiovascular surgery, are the Dacron or expanded polytetrafluoroethylene (ePTFE)-derived grafts. However, major adverse reactions accompany their use. For this purpose, decellularized human umbilical arteries (hUAs) may be proven as a significant source for the development of small diameter conduits. The aim of this study was the evaluation of a decellularization protocol in hUAs. To study the effect of the decellularization to the hUAs, histological analysis was performed. Then, native and decellularized hUAs were biochemically and biomechanically evaluated. Finally, broad proteomic analysis was applied. Histological analysis revealed the successful decellularization of the hUAs. Furthermore, a great amount of DNA was removed from the decellularized hUAs. Biomechanical analysis revealed statistically significant differences in longitudinal direction only in maximum stress (p < 0.013) and strain (p < 0.001). On the contrary, all parameters tested for circumferential direction exhibited significant differences (p < 0.05). Proteomic analysis showed the preservation of the extracellular matrix and cytoskeletal proteins in both groups. Proteomic data are available via ProteomeXchange with identifier PXD020187. The above results indicated that hUAs were efficiently decellularized. The tissue function properties of these conduits were well retained, making them ideal candidates for the development of small diameter vascular grafts.

Alterations with age in the biomechanical behavior of human ureteral wall: Microstructure-based modeling.

The human ureters have not been thoroughly explored from the biomechanics perspective, despite the wealth of such data for other soft-tissue types. This study was motivated by the need to use relevant biomechanical data from human ureters and microstructure-based material formulations for simulations of ureteral peristalsis and stenting. Our starting choice was the four-fiber family model that has proven its validity as a descriptor of the multiaxial response of cardiovascular tissues. The degree of model complexity, required for rigorous fits to passive quasi-static pressure-diameter-force data at several axial stretches, was systematically investigated. Ureteral segments from sixteen human autopsy subjects were evaluated. A diagonal and axial family model allowed equally-good fits as the full model for all age groups and ureteral regions; considerably better than those allowed by the phenomenological Fung-type model whose root-mean-square error of fitting was three-fold greater. This reduced model mimicked the structure seen in histologic sections, namely plentiful diagonal collagen fibers in the lamina propria and axial fibers in the muscle and adventitia. The paucity of elastin fibers and mixed muscle orientation justified the use of isotropic muscle-dominated matrix with small neo-Hookean parameter values. The significantly thicker lamina propria in the lower than the upper ureter of young subjects (312 ± 27 vs. 232 ± 26 µm; mean ± standard error) corroborated the significant regional differences in diagonal-fiber family parameter values. The significant muscle thickening with age (upper ureter: 373 ± 48 vs. 527 ± 67 µm; middle: 388 ± 29 vs. 575 ± 69 µm; lower: 440 ± 21 vs. 602 ± 71 µm) corroborated the significant age-related increase in axial-fiber family parameter values.

Variation of Axial Residual Strains Along the Course and Circumference of Human Aorta Considering Age and Gender.

Our understanding of aortic biomechanics is customarily limited by lack of information on the axial residual stretches of the vessel in both humans and experimental animals that would facilitate the identification of its actual zero-stress state. The aim of this study was thus to acquire hitherto unreported quantitative knowledge of axial opening angle and residual stretches in different segments and quadrants of the human aorta according to age and gender. Twenty-three aortas were harvested during autopsy from the aortic root to the iliac bifurcation and were divided into ≥12 segments and 4 quadrants. Morphometric measurements were taken in the excised/curled configuration of rectangular strips considered to be under zero-stress using image-analysis software to study the axial/circumferential variation of axial opening angle, internal/external residual stretch, and thickness of the aortic wall. The measured data demonstrated: (1) an axial opening angle peak at the arch branches, decreasing toward the ascending and to a near-constant value in the descending thoracic aorta, and increasing in the abdominal aorta; (2) the variation of residual stretches resembled that of opening angle, but axial differences in external residual stretch were more prominent; (3) wall thickness showed a progressive diminution along the vessel; (4) the highest opening angle/residual stretches were found in the inner quadrant and the lowest in the outer quadrant; (5) the anterior was the thinnest quadrant throughout the aorta; (6) age caused thickening but greatly reduced axial opening angle/residual stretches, without differences between males and females.

Regional distribution of layer-specific circumferential residual deformations and opening angles in the porcine aorta.

Information on the layer-specific residual deformations of aortic tissue and how these vary throughout the vessel is important for understanding the regionally-varying aortic functions and pathophysiology, but not so much can be found in the literature. Toward this end, porcine aortas were sectioned into eighteen rings, with one ring from each anatomical position radially cut to obtain the zero-stress state for the intact wall and the other ring dissected into intimal-medial and adventitial layers; these rings were then radially cut to reach the zero-stress state for the intima-media and adventitia. Peripheral variations in internal/external circumferences, thickness, and opening angle of the intact wall and its layers were measured through image analysis at the no-load and zero-stress states. Intact wall and layer circumferences at both states significantly declined along the aorta, as did intact wall and intimal-medial but not adventitial thickness. Adventitia exhibited the greatest opening angles, approaching 180 deg all over the aorta. The opening angles of the intima-media and intact wall were quite similar, with the highest values in the ascending aorta, the lowest at the diaphragm, and increasing subsequently. Bending-related residual stretches were released by radial cutting that were compressive internally and tensile externally, displaying distinct axial variation for the intima-media and intact wall, and non-significant variation for the adventitia. Evidence is provided for the release upon layer separation of compressive stretches in the intima-media and of tensile stretches in the adventitia, whose values were smallest in the descending thoracic aorta and highest near the iliac artery bifurcation.

Regional distribution of delamination strength in ascending thoracic aortic aneurysms.

Considering that the biomechanical factors underlying aortic dissection remain poorly understood as does the rationale for the anatomic localization of the dissection channel, we have attempted to determine the regional distribution of delamination/tensile strengths of ascending thoracic aortic aneurysm wall tissue. Whole aneurysms were taken from seventeen patients undergoing elective surgery and cut into a couple of specimens per quadrant and direction. The specimens were subjected to delamination- and tensile-testing, from which average peel tension (=delamination strength) and failure stress (=tensile strength) were assessed. Histology revealed no differences with region and direction in the roughness of the delaminated surfaces of the inner (intima with most of media) and outer layers (leftover media with adventitia). Compared to the right, the left lateral region exhibited significantly higher thickness and peel tension in both directions, but lower failure stress of the inner and outer layers longitudinally. Failure stress of the inner layers mostly but also of the outer layers was significantly higher circumferentially than longitudinally, with significantly higher values for the outer compared to the inner layers. Differing heterogeneity was evidenced in the delamination and tensile strength of aneurysmal tissue, with important implications for dissection propagation. Particularly, the increased resistance to propagation in the left lateral region helps address the question why part of the circumference, characteristically the right lateral wall, is involved by the dissection and the other part of the aortic wall remains intact. The deterioration of delamination strength with aging suggests the greater potential of aged individuals for dissection propagation.

In vitro study of age-related changes in human ureteral failure properties according to region, direction, and layer.

Knowledge of the capacity of the ureteral wall to withstand urodynamic or external stresses is essential to understand ureteral injury and rupture that mostly occur following trauma, but may also be secondary to obstructive uropathy. It has clinical significance as well in the prevention of iatrogenic injury, for example, during ureteroscopy, but no information is available with regard to the age-related failure properties and regional differences have not been systematically described. Uniaxial tensile testing was performed on 166 ureteral rings and strips from 21 humans free of overt urologic disease; histological evaluation was performed. The degree of layer participation to the intact wall failure stress (=tissue strength), peak elastic modulus (=stiffness), and failure stretch (=extensibility) was assessed by examining layer-specific ruptures in the stress-stretch data. Failure stress at and peak elastic modulus before the first (muscle/adventitial) rupture correlated inversely less with age ( p < 0.05 in few regions/directions) than failure stress at the second (mucosal) rupture ( p < 0.05 in the middle and lower ureter), consistent with the decreased mucosal thickness in ≥50-year-old subjects. Failure stretch at both ruptures did not correlate with age ( p > 0.05 in most regions/directions), paralleling elastin content. Correlations with age were more significant in females than males. Failure stress at the second rupture point was higher ( p < 0.05) distally in <50-year-old but not in ≥50-year-old subjects, justified by the increased collagen distally in the former. Directional differences in failure stretches ( p < 0.05 at all ages/regions/genders) were justified by preferentially axial collagen reinforcement. The presented results may establish the foundation for computational models of iatrogenic/accidental ureteral trauma.

Identification of regional/layer differences in failure properties and thickness as important biomechanical factors responsible for the initiation of aortic dissections.

Thoracic aortic dissections involving the ascending aorta represent one of the most dramatic and lethal emergencies in cardiovascular surgery. It is therefore critical to identify the mechanisms driving them and biomechanical analyses hold great clinical promise, since rupture/dissection occur when aortic wall strength is unable to withstand hemodynamic stresses. Although several studies have been done on the biomechanical properties of thoracic aortic aneurysms, few data are available about thoracic aortic dissections. Detailed mechanical tests with measurement of tissue thickness and failure properties were performed with a tensile-testing device on 445 standardized specimens, corresponding to 19 measurement sites per inner (intima with most of media)/outer layer (leftover media with adventitia); harvested from twelve patients undergoing emergent surgical repair for type A dissection. Our data suggested inherent differences in tissue properties between the origin of dissection and distal locations, i.e. thinner and stiffer inner layers that might render them more vulnerable to tearing despite their increased strength. The strength of tissue circumferentially was greater than that longitudinally, likely determining the direction of tear. The relative strengths of the inner: ∼{65,40}N/cm2 and outer layer: ∼{350,270}N/cm2 in the two principal directions of dissected tissue were differentiated from the intima: ∼{100,75}N/cm2, media: ∼{150,55}N/cm2, and adventitia: ∼{270,190}N/cm2 of non-dissected ascending aortic aneurysms (Sokolis et al., 2012), in favor of weaker inner and stronger outer layers, allowing an explanation as to why the presently-studied tissue suffered dissection, i.e. tear of the inner layers, and not rupture, i.e. full tearing across the entire wall thickness.

Effect of Aneurysm and Bicuspid Aortic Valve on Layer-Specific Ascending Aorta Mechanics.

BACKGROUND: Previous studies have not examined the participation of intimal, medial, and adventitial layers in providing mechanical strength to the ascending thoracic aortic aneurysm (ATAA) wall compared with the nonaneurysmal aorta. In this study we compared the mechanical properties of intact wall and its layers among ATAAs and nonaneurysmal aortas, with explicit consideration of the effects of valve morphology; that is, bicuspid aortic valve (BAV) versus tricuspid aortic valve (TAV), and aortic quadrant. METHODS: Whole ATAAs were taken from patients undergoing elective repair and nonaneurysmal aortas from age-matched autopsy subjects. These were cut into 2 circumferential and longitudinal tissue strips for the intact wall and its layers per quadrant, permitting examination of the aortic wall as a multilayered structure. Tissue underwent tensile testing for determination of failure properties. RESULTS: Intact wall and layer-specific failure stretches (ie, extensibilities) were significantly greater in nonaneurysmal and BAV-ATAA than in TAV-ATAA, unaccounted for by elastin/collagen content changes. Intact wall failure stress (ie, strength) was significantly greater in BAV-ATAA than in TAV-ATAA, in analogy with medial failure stress. Failure stress and stretch associated negatively with age in most subject groups, layers, and intact wall, but failure stretch correlated positively with residual stretch (ie, structural bonds between layers). CONCLUSIONS: No mechanical vulnerability of BAV-ATAA was found, corroborating current conservative guidelines regarding the management of bicuspid aortopathy. Weakening and added vulnerability was found in patients with valvular deficiency, aortic root aneurysm, hypertension, and hyperlipidemia. Aging led to increased susceptibility to dissection initiation or full rupture, or both, in both patient classes.

Regional and age-dependent residual strains, curvature, and dimensions of the human ureter.

The ureters are retroperitoneal structures controlling urine transport from the kidneys to the bladder. Because of the relative scarcity of data on the biomechanical properties of human ureter and the established importance of knowing these properties for understanding its physiology, we initiated biomechanical studies in cadaveric tissue. Herein, we report definite zero-stress/no-load geometrical characterization at 15 regions along the ureter of human cadavers aged 23-82 years, estimating the opening angle, circumferential residual strains, axial curvature, and dimensional parameters. Opening angle decreased along the proximal 25% of ureter, increased and reached a maximum near the mid-ureter, and then gradually decreased toward the vesicoureteral junction (young: p < 0.05; middle-aged: p < 0.05; old: p > 0.05; males: p < 0.05; females: p < 0.05). Similar were the regional distributions of residual strain at the interface between epithelium-lamina propria and muscle and of internal but not external residual strain. Wall thickness increased steadily with aging ( p < 0.05 at few regions), while ureteral circumference did not ( p > 0.05 at most regions) and opening angle decreased ( p < 0.05 at several regions). Consistent with Fung's stress-growth law, the muscle layer thickened with age unlike the epithelium-lamina propria that thinned ( p < 0.05 at most regions for both thicknesses). Moderate-to-strong direct correlations of residual strain difference (= external - internal) with opening angle, wall thickness, and curvature were found in most ureters. The presented data will provide insight into the biomechanical response of ureter under zero/low-stress conditions and the relationship between ureteral remodeling and aging. Importantly, they may also be used to inform finite element models and computational studies simulating the ureter.