Spatial Distribution of Abdominal Aortic Aneurysm Surface Expansion and Correlation With Maximum Diameter and Volume Growth.

BACKGROUND: Abdominal aortic aneurysm (AAA) growth rate, measured as maximum diameter (Dmax) change over time, is used as a surrogate marker of rupture risk. However, AAA expansion presents significant spatial variability. We aim to record the spatial distribution of regional wall surface expansion. METHODS: Thirty AAAs were retrospectively studied. Each AAA had one baseline and at least one follow-up computed tomography scan. Three-dimensional AAA models were reconstructed, and change in Dmax and total aneurysm volume was recorded to calculate annual growth rates. Regional surface growth was quantified using the VascForm algorithm, which is based on nonrigid point cloud registration and iterative closest point analysis. Maximum and average surface growths were calculated and correlated with the diameter/volume growth rates. Furthermore, to identify potential correlation between maximum thrombus (intraluminal thrombus) thickness and maximum surface growth, as well as between peak wall stress (PWS) and surface growth, their colocalization was examined. RESULTS: The median average annual surface growth was 6% (0%-28%), and the maximum surface growth 24% (11%-238%). There was strong evidence of a moderate correlation between Dmax and average as well as maximum surface growth. Regarding volumes, there was strong evidence of a very strong association with average surface growth rate and a moderate association with maximum surface growth rate (rho: 0.91, P < 0.001; rho: 0.7, P < 0.001, respectively). In 51.6% of the follow-ups, maximum surface growth occurred away from Dmax site. Sixteen cases presented maximum surface growth away and fifteen at the region of maximum initial intraluminal thrombus thickness. AAAs in the former group had significantly thinner initial intraluminal thrombus thickness (11.3 vs 19.5 mm, P < 0.001) than those in the latter. Apart from a single case, maximum surface growth did not occur at the PWS region. CONCLUSIONS: More than half of the lesions display maximum growth away from Dmax, suggesting that a more accurate method of analyzing AAA growth needs to be established in clinical practice that will take into account local surface growth.

Correlation of Intraluminal Thrombus Deposition, Biomechanics, and Hemodynamics with Surface Growth and Rupture in Abdominal Aortic Aneurysm-Application in a Clinical Paradigm.

BACKGROUND: The natural history of abdominal aortic aneurysm (AAA) can be investigated through longitudinal evaluation of localized aneurysm characteristics exploiting clinical images. The major challenge is to identify corresponding regions between follow-ups. We have recently developed an algorithm (VascForm) based on nonrigid registration that can obtain surface correspondence and quantify surface growth distribution. METHODS: A ruptured AAA with an initial computed tomography scan 2 years ago was studied. Following 3-dimensional reconstruction of outer wall and luminal surfaces, the wall/thrombus thickness was obtained. Wall stress distribution was computed with finite element analysis, and computational fluid dynamics simulation was performed. VascForm was applied and allowed for the ruptured wall site to be traced back to the initial wall surface and be correlated with local initial intraluminal thrombus thickness, wall stress, and hemodynamic parameters. It also allowed for the quantification of wall surface growth based on surface element growth. RESULTS: Rupture occurred at the posterolateral side. Initial wall surface growth was in most regions 40%. However, a large section of the posterior wall presented 110% growth. Initial thrombus deposition was more prevalent anteriorly, and a posterior thrombus-free isle was present. Peak wall stress (initial and follow-up) occurred at AAA neck. Nonrigid registration revealed that rupture originated from the vicinity of the initial thrombus-free isle. Furthermore, rupture occurred at the wall region with the largest growth (110%). No clear correlation between hemodynamics and rupture site could be identified. CONCLUSIONS: High local surface growth correlates with rupture site and could therefore potentially become a marker of rupture risk. The ongoing application of this methodology to a large cohort of AAA patients will focus on identifying characteristic features of AAA regions that present high surface growth in follow-up evaluations, to assist in improved rupture risk estimation.

Routine use of an aortic balloon to resolve possible inflow stenosis induced by the inflatable ring fixation mechanism of the Ovation endograft.

PURPOSE: To investigate if the routine use of an aortic balloon within 15-30 min after Ovation stent graft ring inflation would resolve any inflow stenosis, which may reach 60 %, at the level of the sealing rings. Moreover, we estimated the potential hemodynamic compromise in these patients during rest and exercise. METHODS: Following 3-dimensional reconstruction of AAA models, cross-sectional area of the infrarenal aorta just proximal the sealing mechanism (A aort, R aort, respectively) and internal area at the site of stenosis (A int, R int, respectively) were measured for 83. Forty-nine patients were managed without and 34 with an aortic balloon use. Pressure drop during rest and exercise was estimated. RESULTS: Technical success was 98 % and there were no perioperative deaths, one type-I endoleak, and 12 (14.5 %) type-II endoleaks. Median A int and R int were significantly reduced compared to A aort [55 % reduction, 143 (range 28-380) mm2 vs 314 (range 177-531) mm2, P value <0.001] and R aort [42 % reduction, 6.75 (range 3-11) mm vs 10 (range 7.5-13) mm, P value <0.001]. The observed stenosis was significantly less for patients in whom an aortic balloon was used intraoperatively (area reduction 36 vs 59 %, P value = 0.009). This stenosis caused a statistically significant, but clinically insignificant ΔP in both groups during rest (0.13 vs 0.06 mmHg, P value = 0.02) and exercise (1 vs 0.5 mmHg, P value = 0.02). CONCLUSION: The advantages of the unique sealing mechanism of the Ovation device seem to be accompanied by an inflow stenosis which is significantly reduced when neck molding with an aortic balloon is used. Overall, the hemodynamic impact of this abnormality seems to be clinically insignificant at 1-month follow-up.

Effect of intraluminal thrombus asymmetrical deposition on abdominal aortic aneurysm growth rate.

PURPOSE: To determine the relationship between asymmetrical intraluminal thrombus (ILT) deposition in abdominal aortic aneurysm (AAA) and growth rate and to explore its biomechanical perspective. METHODS: Thirty-four patients with AAA underwent at least 2 computed tomography scans during surveillance. The volumes of the AAA (VAAA) and thrombus (VILT) and the maximum thrombus thickness (ILTthick) were computed. Thrombus distribution was evaluated by introducing the asymmetrical thrombus deposition index (ATDI), with positive and negative values (-1

Graft inflow stenosis induced by the inflatable ring fixation mechanism of the Ovation stent-graft system: hemodynamic and clinical implications.

PURPOSE: To investigate the observed inflow stenosis at the O-rings of the Ovation stent-graft and evaluate its hemodynamic and clinical impact. METHODS: The study involved 49 consecutive patients (48 men; mean age 71.2 ± 7.7 years) treated successfully with the Ovation abdominal aortic stent-graft between June 2011 and January 2014 at a single center. Cross-sectional area and radius measurements of the infrarenal aorta just proximal to the sealing mechanism, as well at the site of stenosis, were measured from 3D reconstructions of the 1-month postoperative computed tomographic angiograms. Based on Poiseuille's law, the predicted pressure drop was calculated for each patient based on the length of the stenosis. Invasive blood pressure measurements at 3 levels (proximal to the inflatable rings, halfway inside the stenosis, and distal to the stenosis) were obtained in 10 patients intraoperatively. Ankle-brachial index (ABI) values preoperatively were compared to those after the procedure for all patients to assess the clinical impact of this phenomenon. RESULTS: Median internal cross-sectional area at the site of the stenosis was significantly reduced compared to the area just proximal to the O-rings [57% reduction: 123 mm(2) (range 28-254) vs. 283 mm(2) (range 177-531), respectively; p<0.001]. The same was observed for the radius [6.5 mm (range 3-9) vs. 9.5 mm (range 7.5-13), respectively; p<0.001]. Based on the median 15 mm length of the stenosis (range 13-17) observed in the study population, a median pressure drop of 0.13 mmHg (range 0-0.25) along the stenosis was calculated. Invasive blood pressure measurements indicated a non-significant pressure change along the stenosis (e.g., 0.7 mmHg between the proximal level and halfway inside the stenosis). ABI remained practically unchanged postoperatively. CONCLUSION: The advantages of the Ovation device's unique sealing mechanism come at the expense of a median area inflow stenosis of ∼ 60%. This stenosis does not cause a hemodynamically significant pressure drop. Future modification of the graft ring design may be needed in order to reduce this stenosis.

Changes in geometric configuration and biomechanical parameters of a rapidly growing abdominal aortic aneurysm may provide insight in aneurysms natural history and rupture risk.

BACKGROUND: Abdominal aortic aneurysms (AAA) are currently being treated based on the maximum diameter criterion which has often been proven insufficient to determine rupture risk in case of every AAA. We analyzed a rare case of an AAA which presented an extremely fast growth focusing on biomechanical determinants that may indicate a high risk profile. The examination of such a case is expected to motivate future research towards patient-specific rupture risk estimations. METHODS: An initially small AAA (maximum diameter: 4.5 cm) was followed-up and presented a growth of 1 cm in only 6-months of surveillance becoming suitable for surgical repair. Changes of morphometric characteristics regarding AAA, thrombus and lumen volumes, cross-sectional areas, thrombus maximum thickness and eccentricity, and maximum centerline curvature were recorded. Moreover biomechanical variables concerning Peak Wall Stress, AAA surface area exposed to high stress and redistribution of stress during follow-up were also assessed. RESULTS: Total aneurysm volume increased from 85 to 120 ml which regarded thrombus deposition since lumen volume remained stable. Thrombus deposition was eccentric regarding anterior AAA segment while its thickness increased from 0.3 cm to 1.6 cm. Moreover there was an anterior bulging over time as depicted by an increase in maximum centerline curvature from 0.4 cm-1 to 0.5 cm-1. Peak Wall Stress (PWS) exerted on aneurysm wall did not change significantly over time, slightly decreasing from 22 N/cm2 to 21 N/cm2. At the same time the area under high wall stress remained practically constant (9.9 cm2 at initial vs 9.7 cm2 at final examination) but there was a marked redistribution of wall stress against the posterior aneurysmal wall over time. CONCLUSION: Aneurysm area under high stress and redistribution of stress against the posterior wall due to changes in geometric configuration and thrombus deposition over time may have implications to aneurysms natural history and rupture risk.

Variability of computational fluid dynamics solutions for pressure and flow in a giant aneurysm: the ASME 2012 Summer Bioengineering Conference CFD Challenge.

Stimulated by a recent controversy regarding pressure drops predicted in a giant aneurysm with a proximal stenosis, the present study sought to assess variability in the prediction of pressures and flow by a wide variety of research groups. In phase I, lumen geometry, flow rates, and fluid properties were specified, leaving each research group to choose their solver, discretization, and solution strategies. Variability was assessed by having each group interpolate their results onto a standardized mesh and centerline. For phase II, a physical model of the geometry was constructed, from which pressure and flow rates were measured. Groups repeated their simulations using a geometry reconstructed from a micro-computed tomography (CT) scan of the physical model with the measured flow rates and fluid properties. Phase I results from 25 groups demonstrated remarkable consistency in the pressure patterns, with the majority predicting peak systolic pressure drops within 8% of each other. Aneurysm sac flow patterns were more variable with only a few groups reporting peak systolic flow instabilities owing to their use of high temporal resolutions. Variability for phase II was comparable, and the median predicted pressure drops were within a few millimeters of mercury of the measured values but only after accounting for submillimeter errors in the reconstruction of the life-sized flow model from micro-CT. In summary, pressure can be predicted with consistency by CFD across a wide range of solvers and solution strategies, but this may not hold true for specific flow patterns or derived quantities. Future challenges are needed and should focus on hemodynamic quantities thought to be of clinical interest.

An adaptive semi-implicit finite element solver for brain cancer progression modeling.

Glioblastoma is the most aggressive and infiltrative glioma, classified as Grade IV, with the poorest survival rate among patients. Accurate and rigorously tested mechanistic in silico modeling offers great value to understand and quantify the progression of primary brain tumors. This paper presents a continuum-based finite element framework that is built on high performance computing, open-source libraries to simulate glioblastoma progression. We adopt the established proliferation invasion hypoxia necrosis angiogenesis model in our framework to realize scalable simulations of cancer, and has demonstrated to produce accurate and efficient solutions in both two- and three-dimensional brain models. The in silico solver can successfully implement arbitrary order discretization schemes and adaptive remeshing algorithms. A model sensitivity analysis is conducted to test the impact of vascular density, cancer cell invasiveness and aggressiveness, the phenotypic transition potential, including that of necrosis, and the effect of tumor-induced angiogenesis in the evolution of glioblastoma. Additionally, individualized simulations of brain cancer progression are carried out using pertinent magnetic resonance imaging data, where the in silico model is used to investigate the complex dynamics of the disease. We conclude by arguing how the proposed framework can deliver patient-specific simulations of cancer prognosis and how it could bridge clinical imaging with modeling.

Automated ascending aorta delineation from ECG-gated computed tomography images.

The instrumental role of comprehensive geometrical quantification in contemporary, effective descriptions of aortic growth and disease is well established. General or specific purpose algorithms are being developed to provide automatic landmark detection and high accuracy measurements. In the present study, an objective method for automated delineation of the ascending aorta is introduced, based on geometrical properties of the aortic wall. In the proximal ascending aorta, the method identifies the sinotubular junction by tracing the mean surface curvature transition region from the origins of the coronary arteries to the location where the aorta acquires its tubular shape. In the distal ascending aorta, the brachiocephalic artery origin is defined by a split centreline cross section within the brachiocephalic artery-aortic arch bifurcation region. The method's accuracy of detection was quantified against the manual border identification performed by two experienced observers on 3D aortic reconstructions of 44 computed tomography examinations. Median (method, observer) distance and inclination measurements ranged from 0.89 [1.02] mm and 4.66 [5.07]°, respectively, in the proximal border, to 2.18 [2.39] mm and 7.13 [4.77]° in the distal. Accuracy of border detection was found to be high compared to interobserver variability and relevant automatic and manual methodology results previously reported in literature. Delineation of the ascending aorta on a three-dimensional aortic reconstruction with automated identification of the sinotubular junction (proximal border) and of the origin of the brachiocephalic artery (distal border).

Computational evaluation of aortic aneurysm rupture risk: what have we learned so far?

In current clinical practice, aneurysm diameter is one of the primary criteria used to decide when to treat a patient with an abdominal aortic aneurysm (AAA). It has been shown that simple association of aneurysm diameter with the probability of rupture is not sufficient, and other parameters may also play a role in causing or predisposing to AAA rupture. Peak wall stress (PWS), intraluminal thrombus (ILT), and AAA wall mechanics are the factors most implicated with rupture risk and have been studied by computational risk evaluation techniques. The objective of this review is to examine these factors that have been found to influence AAA rupture. The prediction rate of rupture among computational models depends on the level of model complexity and the predictive value of the biomechanical parameters used to assess risk, such as PWS, distribution of ILT, wall strength, and the site of rupture. There is a need for simpler geometric analogues, including geometric parameters (e.g., lumen tortuosity and neck length and angulation) that correlate well with PWS, conjugated with clinical risk factors for constructing rupture risk predictive models. Such models should be supported by novel imaging techniques to provide the required patient-specific data and validated through large, prospective clinical trials.

Combined effect of shear stress and laser-patterned topography on Schwann cell outgrowth: synergistic or antagonistic?

Although the peripheral nervous system exhibits a higher rate of regeneration than that of the central nervous system through a spontaneous regeneration after injury, the functional recovery is fairly infrequent and misdirected. Thus, the development of successful methods to guide neuronal outgrowth, in vitro, is of great importance. In this study, a precise flow controlled microfluidic system with specific custom-designed chambers, incorporating laser-microstructured polyethylene terephthalate (PET) substrates comprising microgrooves, was fabricated to assess the combined effect of shear stress and topography on Schwann cells' behavior. The microgrooves were positioned either parallel or perpendicular to the direction of the flow inside the chambers. Additionally, the cell culture results were combined with computational flow simulations to calculate accurately the shear stress values. Our results demonstrated that wall shear stress gradients may be acting either synergistically or antagonistically depending on the substrate groove orientation relative to the flow direction. The ability to control cell alignment in vitro could potentially be used in the fields of neural tissue engineering and regenerative medicine.

A decoupled fluid structure approach for estimating wall stress in abdominal aortic aneurysms.

Abdominal aortic aneurysm (AAA) is a localized dilatation of the aortic wall. The lack of an accurate AAA rupture risk index remains an important problem in the clinical management of the disease. To accurately estimate AAA rupture risk, detailed information on patient-specific wall stress distribution and aortic wall tissue yield stress is required. A complete fluid structure interaction (FSI) study is currently impractical and thus of limited clinical value. On the other hand, isolated static structural stress analysis based on a uniform wall loading is a widely used approach for AAA rupture risk estimation that, however, neglects the flow-induced wall stress variation. The aim of this study was to assess the merit of a decoupled fluid structure analysis of AAA wall stress. Anatomically correct, patient specific AAA wall models were created by 3D reconstruction of computed tomography images. Flow simulations were carried out with inflow and outflow boundary conditions obtained from patient extracted data. Static structural stress analysis was performed applying both a uniform pressure wall loading and a flow induced non-uniform pressure distribution obtained during early systolic deceleration. For the structural analysis, a hyperelastic arterial wall model and an elastic intraluminal thrombus model were assumed. The results of this study demonstrate that although the isolated static structural stress analysis approach captures the gross features of the stress distribution it underestimates the magnitude of the peak wall stress by as much as 12.5% compared to the proposed decoupled fluid structure approach. Furthermore, the decoupled approach provides potentially useful information on the nature of the aneurysmal sac flow.

A novel approach for local abdominal aortic aneurysm growth quantification.

Although aneurysm size still remains the most accepted predictor of rupture risk, abdominal aortic aneurysms (AAAs) with maximum diameter smaller than 5 cm may also rupture. Growth rate is an additional marker for rupture risk as it potentially reflects an undesirable wall remodeling that leads to fast regional growth. Currently, an indication for surgery is an expansion rate >10 mm/year, measured as change in maximum diameter over time. However, as AAA expansion is non-uniform, it is questionable whether measurement of maximum diameter change over time can capture increased localized remodeling activity. A method for estimating AAA surface area growth is introduced, providing a better measure of local wall deformation. The proposed approach is based on the non-rigid iterative closest point algorithm. Optimization and validation is performed using 12 patient-specific AAA geometries artificially deformed to produce a target surface with known nodal displacements. Mesh density sensitivity, range of uncertainty, and method limitations are discussed. Application to ten AAA patient-specific follow-ups suggested that maximum diameter growth does not correlate strongly with the maximum surface growth (R 2 = 0.614), which is not always colocated with maximum diameter, or uniformly distributed. Surface growth quantification could reinforce the quality of aneurysm surveillance programs.

Deformation and distensibility distribution along the abdominal aorta in the presence of aneurysmal dilatation.

BACKGROUND: In order to evaluate the elastic behavior of the abdominal aortic aneurysm (AAA), the distribution of aortic deformation during the cardiac cycle is measured. Moreover, the distensibility of the AAA composite structure consisting of the AAA wall and the intraluminal thrombus (ILT), as well as that of the adjacent non-aneurysmal aortic segment (NAA), are calculated. METHODS: Ten patients underwent electrocardiographically-gated computed tomography. 3D-surfaces of aortic wall and lumen were reconstructed during peak-systole and end-diastole and cross-sections perpendicular to the centerline were extracted 1 mm apart. Comparison of cross-sectional areas between peak-systole and end-diastole provided the relative area change (RAC). Mean values were calculated for NAA (RACNAA), aneurysmal wall (RACWall), and aneurysmal lumen (RACLumen). Distensibility of aneurysmal and unaffected aorta was calculated using brachial blood pressure measurements (DAAA and DNAA respectively). Normalized distensibility (DNORM) of the AAA was calculated with respect to normal aortic segment distensibility and related to aneurysm size and thrombus content. RESULTS: A map of aortic deformation during the cardiac cycle was obtained. Differences between RACWall (median=0.7%, range=0.3-2.1%) and both RACNAA (median=2.8%, range=0.9-4.8%) and RACLumen (median=1.8%, range=0.5-3.4%) were statistically significant. DAAA (median=0.30∙10-5 Pa-1, range=0.05-0.64∙10-5 Pa-1) was lower than DNAA (median=0.43∙10-5 Pa-1, range=0.16-0.83∙10-5 Pa-1) but difference was not statistically significant. Median DNORM was 0.73 (range=0.1-3.1) and presented a significant positive correlation with AAA size and thrombus content. CONCLUSIONS: Aneurysmal wall deforms significantly less than non-aneurysmal wall and aneurysmal lumen, due to altered elastic properties and reduced loading. In large AAAs with larger amounts of ILT, the lumen deformation is comparable or even exceeds that of NAA and subsequently so does the distensibility of the Wall-ILT composite, an observation suggesting a thrombus cushioning effect. DNORM may provide insight in the estimation of AAA evolution and assist in rupture risk assessment.

A robust approach for exploring hemodynamics and thrombus growth associations in abdominal aortic aneurysms.

Longitudinal studies of vascular diseases often need to establish correspondence between follow-up images, as the diseased regions may change shape over time. In addition, spatial data structures should be taken into account in the statistical analyses to avoid inferential errors. This study investigates the association between hemodynamics and thrombus growth in abdominal aortic aneurysms (AAAs) while emphasizing on the abovementioned methodological issues. Six AAA surfaces and their follow-ups were three-dimensionally reconstructed from computed-tomography images. AAA surfaces were mapped onto a rectangular grid which allowed identification of corresponding regions between follow-ups. Local thrombus thickness was measured at initial and follow-up surfaces and computational fluid dynamic simulations provided time-average wall shear stress (TAWSS), oscillatory shear index (OSI), and relative residence time. Six Bayesian regression models, which account for spatially correlated measurements, were employed to explore associations between hemodynamics and thrombus growth. Results suggest that spatial regression models based on TAWSS and OSI offer superior predictive performance for thrombus growth relative to alternative specifications. Ignoring the spatial data structure may lead to improper assessment with regard to predictor significance.

Hemodynamic impact of abdominal aortic aneurysm stent-graft implantation-induced stenosis.

The current study aims to computationally evaluate the hemodynamic impact of a novel sealing mechanism employed by a recently developed endograft (Ovation TriVascular Stent-Graft System) for endovascular aneurysm repair. The exploitation of two inflatable O-rings to achieve sealing may be advantageous in terms of accommodating challenging anatomies, but comes at a price of a marked inflow stenosis. Here, four representative patient cases of inflow stenosis ranging from 30 to 80 % were analyzed. Lumen surface models were constructed from 1 month post-operative computed tomography images and then used to numerically compute the complex endograft flow field. Our results highlight coexistence of stenotic wall regions exposed to high shear rate and post-stenotic recirculation zones. These conditions may implicate platelet activation and predispose thrombus formation and thromboembolic complications. A clinically insignificant cycle-averaged pressure drop along the inflow stenosis and further in the endograft main body legs was predicted (range 0.01-1.72 mmHg) which was, however, notable at peak systole (range 3.52-19.73 mmHg). Although the functional impact of the endograft stenosis at rest flow conditions may appear insignificant, increased flow rate during exercise is expected to strongly accentuate the observed effects. Pressure drop in the endograft legs was attributed to suboptimal, based on Murray's scaling law, cross-sectional area ratio between trunk and legs of the device.

The influence of intraluminal thrombus on noninvasive abdominal aortic aneurysm wall distensibility measurement.

Abdominal aortic aneurysm wall distensibility can be estimated by measuring pulse pressure and the corresponding sac volume change, which can be obtained by measuring wall displacement. This approach, however, may introduce error if the role of thrombus in assisting the wall in bearing the pulse pressure loading is neglected. Our aim was to introduce a methodology for evaluating and potentially correcting this error in estimating distensibility. Electrocardiogram-gated computed tomography images of eleven patients were obtained, and the volume change between diastole and systole was measured. Using finite element procedures, we determined the equivalent pulse pressure loading that should be applied to the wall of a model where thrombus was digitally removed, to yield the same sac volumetric increase caused by applying the luminal pulse pressure to the model with thrombus. The equivalent instead of the measured pulse pressure was used in the distensibility expression. For a relative volumetric thrombus deposition (V ILT) of 50 %, a 62 % distensibility underestimation resulted when thrombus role was neglected. A strong linear correlation was observed between distensibility underestimation and V ILT. To assess the potential value of noninvasive wall distensibility measurement in rupture risk stratification, the role of thrombus on wall loading should be further investigated.

Advancements in identifying biomechanical determinants for abdominal aortic aneurysm rupture.

Abdominal aortic aneurysms are a common health problem and currently the need for surgical intervention is determined based on maximum diameter and growth rate criteria. Since these universal variables often fail to predict accurately every abdominal aortic aneurysms evolution, there is a considerable effort in the literature for other markers to be identified towards individualized rupture risk estimations and growth rate predictions. To this effort, biomechanical tools have been extensively used since abdominal aortic aneurysm rupture is in fact a material failure of the diseased arterial wall to compensate the stress acting on it. The peak wall stress, the role of the unique geometry of every individual abdominal aortic aneurysm as well as the mechanical properties and the local strength of the degenerated aneurysmal wall, all confer to rupture risk. In this review article, the assessment of these variables through mechanical testing, advanced imaging and computational modeling is reviewed and the clinical perspective is discussed.

The geometry of unstented and stented pig common carotid artery bypass grafts.

The long-term success of arterial bypass grafting with autologous saphenous veins is limited by neointimal hyperplasia (NIH), which seemingly develops preferentially at sites where hydrodynamic wall shear is low. Placement of a loose-fitting, porous stent around end-to-end, or end-to-side, autologous saphenous vein grafts on the porcine common carotid artery has been found significantly to reduce NIH, but the mechanism is unclear. In a preliminary study, we implanted autologous saphenous vein grafts bilaterally on the common carotid arteries of pigs, placing a stent around one graft and leaving the contralateral graft unstented. At sacrifice 1 month post implantation, the grafts were pressure fixed in situ and resin casts were made. Unstented graft geometry was highly irregular, with non-uniform dilatation, substantial axial lengthening, curvature, kinking, and possible long-pitch helical distortion. In contrast, stented grafts showed no major dilatation, lengthening or curvature, but there was commonly fine corrugation, occasional slight kinking or narrowing of segments, and possible long-pitch helical distortion. Axial growth of grafts against effectively tethered anastomoses could account for these changes. CFD studies are planned, using 3D MR reconstructions, on the effects of graft geometry on the flow. Abnormality of the flow could favour the development of vascular pathology, including NIH.