35 research outputs found
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
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 (Al).
RESULTS: Intact-wall stiffness longitudinally and medial strength and
stiffness, in either direction, were significantly lower in Al patients
than in AS and NVP patients. Intact-wall/medial thickness and
extensibility in either direction were significantly lower in AS
patients than in Al 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 Al 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
Regional distribution of layer-specific circumferential residual deformations and opening angles in the porcine aorta
Alterations with age in the biomechanical behavior of human ureteral wall: Microstructure-based modeling
Layer-dependent wall properties of abdominal aortic aneurysms: Experimental study and material characterization
Mechanical testing and in-depth characterization of the abdominal aortic
aneurysm wall from fifteen patients undergoing open surgery was
performed to establish the layer-dependent tissue properties that are
non-available in the literature. Quantitative microscopic evaluation was
performed to identify the spatial organization of collagen-fiber
network. Among a number of candidate models, the four-fiber family
(microstructure-motivated) model, especially that including dispersions
of fiber angles about the main directions, was superior to the Fung- and
Gasser-type models in the fitting quality allowed, though it presented a
practical difficulty in parameter estimation, so that an analysis was
conducted aiding the identification of a more specific diagonal- and
circumferential-fiber family model for all three layers. The adventitia
was stiffer and stronger than the other layers, owing to its increased
collagen content, and its contribution to the response of the intact
wall was augmented being under greater residual tension than the media,
whereas the intima was under residual compression. All layers were
stiffer circumferentially than longitudinally, due to preferential
collagen arrangement along that axis. The histologically-guided material
characterization of layered wall presented herein is expected to assist
clinical decision, by developing reliable criteria to predict the
rupture risk of abdominal aortic aneurysms, and optimize endovascular
interventions. (C) 2015 Elsevier Ltd. All rights reserved
Variation of Axial Residual Strains Along the Course and Circumference of Human Aorta Considering Age and Gender
Impaired mechanics and matrix metalloproteinases/inhibitors expression in female ascending thoracic aortic aneurysms
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 similar to 240 deg and similar to 30
deg, respectively, throughout the aorta; those of the adventitia were
similar to 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