Computed wall stress may predict the growth of abdominal aortic aneurysm

Growth rate of abdominal aortic aneurysm (AAA) is thought to be an important indicator of the potential risk of rupture. Wall stress is also thought to be a trigger for its rupture. However, stress change during the expansion of an AAA is unclear. Forty-four patients with AAAs were included in this longitudinal follow-up study. They were assessed by serial abdominal ultrasonography and computerized tomography (CT) scans if a critical size was reached or a rapid expansion occurred. Patient-specific 3-dimensional AAA geometries were reconstructed from the follow-up CT images. Structural analysis was performed to calculate the wall stresses of the AAA models at both baseline and final visit. A non-linear large-strain finite element method was used to compute the wall stress distribution. The average growth rate was 0.66cm/year (range 0-1.32 cm/year). A significantly positive correlation between shoulder tress at baseline and growth rate was found (r=0.342; p=0.02). A higher shoulder stress is associated with a rapidly expanding AAA. Therefore, it may be useful for estimating the growth expansion of AAAs and further risk stratification of patients with AAAs.

Stress analysis of carotid atheroma in transient ischemic attack patients: Evidence for extreme stress-induced plaque rupture

Plaque rupture has been considered to be the result of its structural failure. The aim of this study is to suggest a possible link between higher stresses and rupture sites observed from in vivo magnetic resonance imaging (MRI) of transient ischemic attack (TIA) patients, by using stress analysis methods. Three patients, who had recently suffered a TIA, underwent in vivo multi-spectral MR imaging. Based on plaque geometries reconstructed from the post-rupture status, six pre-rupture plaque models were generated for each patient dataset with different reconstructions of rupture sites to bridge the gap of fibrous cap from original MRI images. Stress analysis by fluid structure interaction simulation was performed on the models, followed by analysis of local stress concentration distribution and plaque rupture sites. Furthermore, the sensitivity of stress analysis to the pre-rupture plaque geometry reconstruction was examined. Local stress concentrations were found to be located at the plaque rupture sites for the three subjects studied. In the total of 18 models created, the locations of the stress concentration regions were similar in 17 models in which rupture sites were always associated with high stresses. The local stress concentration region moved from circumferential center to the shoulder region (slightly away from the rupture site) for a case with a thick fibrous cap. Plaque wall stress level in the rupture locations was found to be much higher than the value in non-rupture locations. The good correlation between local stress concentrations and plaque rupture sites, and generally higher plaque wall stress level in rupture locations in the subjects studied could provide indirect evidence for the extreme stress-induced plaque rupture hypothesis. Local stress concentration in the plaque region could be one of the factors contributing to plaque rupture.

Association between aneurysm shoulder stress and abdominal aortic aneurysm expansion: A longitudinal follow-up study

Background Aneurysm expansion rate is an important indicator of the potential risk of abdominal aortic aneurysm (AAA) rupture. Stress within the AAA wall is also thought to be a trigger for its rupture. However, the association between aneurysm wall stresses and expansion of AAA is unclear. Methods and Results Forty-four patients with AAAs were included in this longitudinal follow-up study. They were assessed by serial abdominal ultrasonography and computed tomography scans if a critical size was reached or a rapid expansion occurred. Patient-specific 3-dimensional AAA geometries were reconstructed from the follow-up computed tomography images. Structural analysis was performed to calculate the wall stresses of the AAA models at both baseline and final visit. A nonlinear large-strain finite element method was used to compute the wall-stress distribution. The relationship between wall stresses and expansion rate was investigated. Slowly and rapidly expanding aneurysms had comparable baseline maximum diameters (median, 4.35 cm [interquartile range, 4.12 to 5.0 cm] versus 4.6 cm [interquartile range, 4.2 to 5.0 cm]; P=0.32). Rapidly expanding AAAs had significantly higher shoulder stresses than slowly expanding AAAs (median, 300 kPa [interquartile range, 280 to 320 kPa] versus 225 kPa [interquartile range, 211 to 249 kPa]; P=0.0001). A good correlation between shoulder stress at baseline and expansion rate was found (r=0.71; P=0.0001). Conclusion A higher shoulder stress was found to have an association with a rapidly expanding AAA. Therefore, it may be useful for estimating the expansion of AAAs and improve risk stratification of patients with AAAs.

Stress analysis of carotid atheroma in a transient ischaemic attack patient using the MRI-based fluid-structure interaction method

Rupture of atherosclerotic plaque is a major cause of mortality. Plaque stress analysis, based on patient-specific multisequence in vivo MRI, can provide critical information for the understanding of plaque rupture and could eventually lead to plaque rupture prediction. However, the direct link between stress and plaque rupture is not fully understood. In the present study, the plaque from a patient who recently experienced a transient ischaemic attack (TIA) was studied using a fluid-structure interaction method to quantify stress distribution in the plaque region based on in vivo MR images. The results showed that wall shear stress is generally low in the artery with a slight increase at the plaque throat owing to minor luminal narrowing. The oscillatory shear index is much higher in the proximal part of the plaque. Both local wall stress concentrations and the relative stress variation distribution during a cardiac cycle indicate that the actual plaque rupture site is collocated with the highest rupture risk region in the studied patient.

Impact of calcification and intraluminal thrombus on the computed wall stresses of abdominal aortic aneurysm

Background: Increased biomechanical stresses within the abdominal aortic aneurysm (AAA) wall contribute to its rupture. Calcification and intraluminal thrombus can be commonly found in AAAs, but the relationship between calcification/intraluminal thrombus and AAA wall stress is not completely described. Methods: Patient-specific three-dimensional AAA geometries were reconstructed from computed tomographic images of 20 patients. Structural analysis was performed to calculate the wall stresses of the 20 AAA models and their altered models when calcification or intraluminal thrombus was not considered. A nonlinear large-strain finite element method was used to compute the wall stress distribution. The relationships between wall stresses and volumes of calcification and intraluminal thrombus were sought. Results: Maximum stress was not correlated with the percentage of calcification, and was negatively correlated with the percentage of intraluminal thrombus (r = -0.56; P = .011). Exclusion of calcification from analysis led to a significant decrease in maximum stress by a median of 14% (range, 2%-27%; P < .01). When intraluminal thrombus was eliminated, maximum stress increased significantly by a median of 24% (range, 5%-43%; P < .01). Conclusion: The presence of calcification increases AAA peak wall stress, suggesting that calcification decrease the biomechanical stability of AAA. In contrast, intraluminal thrombus reduces the maximum stress in AAA. Calcification and intraluminal thrombus should both be considered in the evaluation of wall stress for risk assessment of AAA rupture.

The stress in a slowly sheared granular column

We report measurements of the wall stress in a granular material sheared in a cylindrical Couette cell, as a function of the distance from the free surface. Our results shows that when the material is static, all components of the stress saturate to constant values within a short distance from the free surface, in conformity with earlier experiments and theoretical predictions. When the material is sheared by rotating the inner cylinder at a constant rate, the stresses are remarkably altered. The radial normal stress does not saturate, and increases even more rapidly with depth than the linear hydrostatic pressure profile. The axial shear stress changes sign on shearing, and its magnitude increases with depth. These results are discussed in the context of the predictions of the classical and Cosserat plasticity theories.

Mechanical behaviour and rupture of normal and pathological human ascending aortic wall

The mechanical properties of aortic wall, both healthy and pathological, are needed in order to develop and improve diagnostic and interventional criteria, and for the development of mechanical models to assess arterial integrity. This study focuses on the mechanical behaviour and rupture conditions of the human ascending aorta and its relationship with age and pathologies. Fresh ascending aortic specimens harvested from 23 healthy donors, 12 patients with bicuspid aortic valve (BAV) and 14 with aneurysm were tensile-tested in vitro under physiological conditions. Tensile strength, stretch at failure and elbow stress were measured. The obtained results showed that age causes a major reduction in the mechanical parameters of healthy ascending aortic tissue, and that no significant differences are found between the mechanical strength of aneurysmal or BAV aortic specimens and the corresponding age-matched control group. The physiological level of the stress in the circumferential direction was also computed to assess the physiological operation range of healthy and diseased ascending aortas. The mean physiological wall stress acting on pathologic aortas was found to be far from rupture, with factors of safety (defined as the ratio of tensile strength to the mean wall stress) larger than six. In contrast, the physiological operation of pathologic vessels lays in the stiff part of the response curve, losing part of its function of damping the pressure waves from the heart.

Xyloglucan endotransglucosylase/hydrolase (XTH) overexpression affects growth and cell wall mechanics in etiolated Arabidopsis hypocotyls.

Growth and biomechanics of etiolated hypocotyls from Arabidopsis thaliana lines overexpressing xyloglucan endotransglucosylase/hydrolase AtXTH18, AtXTH19, AtXTH20, and PttXET16-34 were studied. Overexpression of AtXTH18, AtXTH19, and AtXTH20 stimulated growth of hypocotyls, while PttXET16-34 overexpression did not show this effect. In vitro extension of frozen/thawed hypocotyls measured by a constant-load extensiometer started from a high-amplitude initial deformation followed by a slow time-dependent creep. Creep of growing XTH-overexpressing (OE) hypocotyls was more linear in time compared with the wild type at pH 5.0, reflecting their higher potential for long-term extension. XTH-OE plants deposited 65?84% more cell wall material per hypocotyl cross-sectional area than wild-type plants. As a result, their wall stress under each external load was lower than in the wild-type. Growing XTH-OE hypocotyls had higher values of initial deformation·stress?1 compared with the wild type. Plotting creep rates for each line under different loads against the respective wall stress values gave straight lines. Their slopes and intercepts with the abscissa correspond to ? (in vitro cell wall extensibility) and y (in vitro cell wall yield threshold) values characterizing cell wall material properties. The wall material in XTH-OE lines was more pliant than in the wild type due to lower y values. In contrast, the acid-induced wall extension in vitro resulted from increasing ? values. Thus, three factors contributed to the XTH-OE-stimulated growth in Arabidopsis hypocotyls: their more linear creep, higher values of initial deformation·stress?1, and lower y values.

Is quantitative interpretation likely to increase sensitivity of dobutamine stress echocardiography? A study of false-negative results

Background: False-negative interpretations of do-butamine stress echocardiography (DSE) may be associated with reduced wall stress. using measurements of contraction, we sought whether these segments were actually ischemic but unrecognized or showed normal contraction. Methods. We studied 48 patients (29 men; mean age 60 +/- 10 years) with normal regional function on the basis of standard qualitative interpretation of DSE. At coronary angiography within. 6 months of DSE, 32 were identified as having true-negative and 16 as having false-negative results of DSE. Three apical views were used to measure regional function with color Doppler tissue, integrated backscatter, and strain rate imaging. Cyclic variation of integrated backscatter was measured in 16 segments, and strain rate and peak systolic strain was calculated in 6 walls at rest and peak stress. Results. Segments with false-negative results of DSE were divided into 2 groups with and without low wall stress according to previously published cut-off values. Age, sex, left ventricular mass, left ventricular geometric pattern, and peak workload were not significantly different between patients with true and false-negative results of DSE. Importantly, no significant differences in cyclic variation and strain parameters at rest and peak stress were found among segments with true-and false-negative results of DSE with and without low wall stress. Stenosis severity had no influence on cyclic variation and strain parameters at peak stress. Conclusions: False-negative results of DSE reflect lack of ischemia rather than underinterpretation of regional left ventricular function. Quantitative markers are unlikely to increase the sensitivity of DSE.

Comparison between computational models of abdominal aortic aneurysm

Objective: To compare the differences in the hemodynamic parameters of abdominal aortic aneurysm (AAA) between fluid-structure interaction model (FSIM) and fluid-only model (FM), so as to discuss their application in the research of AAA. Methods: An idealized AAA model was created based on patient-specific AAA data. In FM, the flow, pressure and wall shear stress (WSS) were computed using finite volume method. In FSIM, an Arbitrary Lagrangian-Eulerian algorithm was used to solve the flow in a continuously deforming geometry. The hemodynamic parameters of both models were obtained for discussion. Results: Under the same inlet velocity, there were only two symmetrical vortexes in the AAA dilation area for FSIM. In contrast, four recirculation areas existed in FM; two were main vortexes and the other two were secondary flow, which were located between the main recirculation area and the arterial wall. Six local pressure concentrations occurred in the distal end of AAA and the recirculation area for FM. However, there were only two local pressure concentrations in FSIM. The vortex center of the recirculation area in FSIM was much more close to the distal end of AAA and the area was much larger because of AAA expansion. Four extreme values of WSS existed at the proximal of AAA, the point of boundary layer separation, the point of flow reattachment and the distal end of AAA, respectively, in both FM and FSIM. The maximum wall stress and the largest wall deformation were both located at the proximal and distal end of AAA. Conclusions: The number and center of the recirculation area for both models are different, while the change of vortex is closely associated with the AAA growth. The largest WSS of FSIM is 36% smaller than that of FM. Both the maximum wall stress and largest wall displacement shall increase with the outlet pressure increasing. FSIM needs to be considered for studying the relationship between AAA growth and shear stress.

Cumulus clouds and convective boundary layers: a tropical perspective on two turbulent shear flows

This paper is a review prepared for the second Marseille Colloquium on the mechanics of turbulence, held in 2011, 50 years after the first. The review covers recent developments in our understanding of the large-scale dynamics of cumulus cloud flows and of the atmospheric boundary layer in the low-wind convective regime that is often encountered in the tropics. It has recently been shown that a variety of cumulus cloud forms and life cycles can be experimentally realized in the laboratory, with the transient diabatic plume taken as the flow model for a cumulus cloud. The plume is subjected to diabatic heating scaled to be dynamically similar to heat release from phase changes in clouds. The experiments are complemented by exact numerical solutions of the Navier-Stokes-Boussinesq equations for plumes with scaled off-source heating. The results show that the Taylor entrainment coefficient first increases with heating, reaches a positive maximum and then drops rapidly to zero or even negative values. This reduction in entrainment is a consequence of structural changes in the flow, smoothing out the convoluted boundaries in the non-diabatic plume, including the tongues engulfing the ambient flow. This is accompanied by a greater degree of mixedness in the core flow because of lower dilution by the ambient fluid. The cloud forms generated depend strongly on the history of the diabatic heating profile in the vertical direction. The striking effects of heating on the flow are attributable to the operation of the baroclinic torque due to the temperature field. The mean baroclinic torque is shown to peak around a quasi-cylindrical sheet situated midway between the axis of the flow and the edges. This torque is shear-enhancing and folds down the engulfment tongues. The increase in mixedness can be traced to an explosive growth in the enstrophy, triggered by a strong fluctuating baroclinic torque that acts as a source, especially at the higher wave numbers, thus enhancing the mixedness. In convective boundary layers field measurements show that, under conditions prevailing in the tropics, the eddy fluxes of momentum and energy do not follow the Monin-Obukhov similarity. Instead, the eddy momentum flux is found to be linear in the wind speed at low winds; and the eddy heat flux is, to a first approximation, governed by free convection laws, with wind acting as a small perturbation on a regime of free convection. A new boundary layer code, based on heat flux scaling rather than wall-stress scaling, shows promising improvements in predictive skills of a general circulation model.

Dynamic compressive response of composite square honeycombs

Carbon fibre-epoxy composite square honeycombs, and the parent composite material, were tested in quasi-static compression at a strain rate of 10 -3 s -1 and in dynamic compression at strain rates of 10 3-10 4 s -1 using an instrumented Kolsky bar arrangement. Taken together, these tests provide an assessment of the potential of this composite topology for use as a lightweight sandwich core. The honeycombs had two relative densities, 0.12 and 0.24, and two material orientations, ±45° and 0/90° with respect to the prismatic, loading direction of the honeycomb. Honeycomb manufacture was by slotting, assembling and bonding together carbon fibre/epoxy woven plies of composite sheets of 2 × 2 twill weave construction. The peak value of wall stress in the honeycombs was about one third that of the parent material, for all strain rates. An elastic finite element analysis was used to trace the source of this knock-down in strength: a stress concentration exists at the root of the slots and leads to premature failure by microbuckling. Shock-wave effects were evident at impact velocities exceeding 50 ms -1 for the honeycomb of relative density 0.12. This was traced to stubbing of the buckled cell walls against the face of the Kolsky bar. © 2011 Elsevier Ltd. All rights reserved.