A 10-Year Aortic Center Experience with Hybrid Repair of Chronic "Residual" Aortic Dissection After Type A Repair.

PURPOSE: Hybrid aortic arch repair in patients with chronic residual aortic dissection (RAD) is a less invasive alternative to conventional surgical treatment. The aim of this study was to describe the short-term and long-term results of hybrid treatment for RAD after type A repair. METHODS: In this retrospective single-center cohort study, all patients treated for chronic RAD with hybrid aortic arch repair were included. Indications for treatment were rapid aortic growth, aortic diameter > 55 mm, or aortic rupture. RESULTS: Between 2009 and 2020, we performed 29 hybrid treatments for chronic RAD. Twenty-four patients were treated for complete supra-aortic debranching in zones 0 and 5 with left subclavian artery debranching alone in zone 2. There was 1 perioperative death (3.4%): The patient was treated for an aortic rupture. There was no spinal cord ischemia and 1 minor stroke (3.4%). After a median follow-up of 25.4 months (range 3-97 months), the long-term mortality was 10.3% (3/29) with no late aortic-related deaths. Twenty-seven patients (93.1%) developed FL thrombosis of the descending thoracic aorta; the rate of aneurysmal progression on thoraco-abdominal aorta was 41.4% (12/29), and the rate of aortic reintervention was 34.5% (10/29). CONCLUSION: In a high-volume aortic center, hybrid repair of RAD is associated with good anatomical results and a low risk of perioperative morbidity and mortality, including that of patients treated in zone 0. A redo replacement of the ascending aortic segment is sometimes necessary to provide a safer proximal landing zone and reduce the risk of type 1 endoleak after TEVAR.

Examination of factors in type I endoleak development after thoracic endovascular repair.

OBJECTIVE: The objective of this study was to assess the effects of operative indication, anatomy, and stent graft on type I endoleak occurrence after thoracic endovascular aortic repair. METHODS: A retrospective review was conducted of patients admitted for thoracic endovascular aortic repair between 2007 and 2013. All computed tomography angiography imaging was analyzed for the presence of endoleak and measurement of diameters and lengths. Variables studied included underlying disease, emergency, achieved aortic neck length, difference between proximal and distal neck diameters, landing zone 2, and stent graft characteristics (diameter, number, type of device, oversizing degree, and covered aorta length). RESULTS: The study population involved 84 patients (mean age, 56 years; range, 17-94 years) who were treated for thoracic aortic aneurysm (TAA) (n = 29; 34.5%), traumatic aortic rupture (n = 27; 32%), type B aortic dissection (n = 19; 22.5%), intramural hematoma (n = 2; 2%), penetrating aortic ulcer (n = 5; 6%), and aortoesophageal fistula (n = 2; 2%). Of these, 60 patients (71.5%) were treated emergently and 24 (28.5%) electively. Primary type I endoleak was noted in eight patients (9.5%), of which two resolved spontaneously. After a mean follow-up of 32 months (range, 3-76 months), secondary type I endoleak was detected in four patients (4.5%). All of them occurred after emergent TAA treatment. Comparison between emergent and elective groups revealed no significant differences in neck length (19.5 mm vs 26.5 mm; P = .197), oversizing degree (11.1% vs 10.9%; P = .811), or endoleak rates (13.3% vs 8.3%; P = .518). Hemorrhagic shock was not predictive of endoleak (P = .483). Cox regression analysis of the different anatomic and stent graft-related factors revealed short proximal landing zone as the unique independent predictor of type I endoleak (hazard ratio, 0.89; 95% confidence interval, 0.81-0.99; P = .032). CONCLUSIONS: Endoleak risk seems not to be increased by an emergency setting. However, the relatively high rate of late endoleak observed after emergent TAA repair advocates for close follow-up, contrary to traumatic aortic rupture. Furthermore, regardless of the pathologic process, a longer proximal landing zone is likely to guarantee early and late success.

The Value of Aortic Volume and Intraluminal Thrombus Quantification for Predicting Aortic Events after Endovascular Thoracic Aneurysm Repair.

Objectives: To assess the ability of the aortic aneurysm volume (AAV), aneurysmal lumen volume (ALV), and aneurysmal thrombus volume (ATV) to predict the need for aortic reintervention when using the maximal aortic diameter as a reference. Methods: This monocentric retrospective study included 31 consecutive patients who underwent successful thoracic endovascular aortic repair (TEVAR) to treat an atheromatous thoracic aortic aneurysm. All patients underwent clinical and computed tomography angiography (CTA) for 3 years after TEVAR. The patients were categorized into group 0 if no aortic reintervention was required during the follow-up period and categorized into group 1 if they experienced a type I or III endoleak or aneurysm diameter increase requiring intervention. The maximum aneurysm sac diameter and the AAV, ALV, and ATV were calculated using CTA images obtained preoperatively (T0) and at 6-12 months (T1), 24 months (T2), and 36 months (T3) postoperatively, and their changes over time were analyzed. Correlations between diameter and changes in AAV, ALV, and ATV were assessed, and the association between diameter and volume changes and reintervetion was examined. The cutoff values for predicting the need for reintervention was determined using a receiver operating characteristic (ROC) curve. The accuracy of volume change versus diameter change for predicting the need for reintervention was analyzed. Results: There were no significant differences in terms of the mean aneurysm diameter or AAV, ALV or ATV between the groups at preoperative CTA or after one year of follow-up imaging. The mean ATV was higher in group 1 than in group 0 at 2 years (187.6 ± 86.3 mL vs. 114.7 ± 64.7 mL; p = 0.057) and after 3 years (195.0 ± 86.7 mL vs. 82.1 ± 39.9 mL; p = 0.013). The maximal diameter was greater in group 1 than in group 0 at 3 years (67.3 ± 9.5 mm vs. 55.3 ± 12.6 mm; p = 0.044). The rate of AAV change between T0 and T1 was significantly higher in group 1 (7 ± 4.5%) than in group 0 (-6 ± 6.8%; p < 0.001). The rate of ATV change between T1-T3 was significantly higher in group 1 than in group 0 (34 ± 40.9% vs. -13 ± 14.4% (p = 0.041)); similar results were observed for the rate of ATV change between T2 and T3 (27 ± 50.1% for group 1 vs. -8 ± 49.5% in group 0 (p < 0.001)). According to our multivariate analysis, the annual growth rate for AAV between T0 and T1 was the only independent factor that was significantly associated with aortic reintervention (area under the curve (AUC) = 0.84, OR = 1.57, p = 0.025; optimal cutoff +0.4%). An increase in the annual growth rate of the ATV between T0 and T3 was independently associated with the need for aortic reintervention (area under the curve (AUC) = 0.90, OR = 1.11, p = 0.0347; optimal cutoff +10.1%). Conclusions: Aortic volume analysis can predict the need for aortic reintervention more accurately and earlier than maximal aortic diameter.

Volume Analysis to Predict the Long-Term Evolution of Residual Aortic Dissection after Type A Repair.

Background: The aim of this study was to evaluate the aortic diameter and volume during the first year after a type A repair to predict the long-term prognosis of a residual aortic dissection (RAD). Methods: All patients treated in our center for an acute type A dissection with a RAD and follow-up > 3 years were included. We defined two groups: group 1 with dissection-related events (defined as an aneurysmal evolution, distal reintervention, or aortic-related death) and group 2 without dissection-related events. The aortic diameters and volume analysis were evaluated on three postoperative CT scans: pre-discharge (T1), 3−6 months (T2) and 1 year (T3). Results: Between 2009 and 2016, 54 patients were included. Following a mean follow-up of 75.4 months (SD 31.5), the rate of dissection-related events was 62.9% (34/54). The total aortic diameters of the descending thoracic aorta were greater in group 1 at T1, T2 and T3, with greater diameters in the FL (p < 0.01). The aortic diameter evolution at 3 months was not predictive of long-term dissection-related events. The total thoracic aortic volume was significantly greater in group 1 at T1 (p < 0.01), T2 (p < 0.01), and T3 (p < 0.01). At 3 months, the increase in the FL volume was significantly greater in group 1 (p < 0.01) and was predictive for long-term dissection-related events. Conclusion: This study shows that an initial CT scan volume analysis coupled with another at 3 months is predictive for the long-term evolution in a RAD. Based on this finding, more aggressive treatment could be given at an earlier stage.

Stereoscopically observed deformations of a compliant abdominal aortic aneurysm model.

A new experimental setup has been implemented to precisely measure the deformations of an entire model abdominal aortic aneurysm (AAA). This setup addresses a gap between the computational and experimental models of AAA that have aimed at improving the limited understanding of aneurysm development and rupture. The experimental validation of the deformations from computational approaches has been limited by a lack of consideration of the large and varied deformations that AAAs undergo in response to physiologic flow and pressure. To address the issue of experimentally validating these calculated deformations, a stereoscopic imaging system utilizing two cameras was constructed to measure model aneurysm displacement in response to pressurization. The three model shapes, consisting of a healthy aorta, an AAA with bifurcation, and an AAA without bifurcation, were also evaluated with computational solid mechanical modeling using finite elements to assess the impact of differences between material properties and for comparison against the experimental inflations. The device demonstrated adequate accuracy (surface points were located to within 0.07 mm) for capturing local variation while allowing the full length of the aneurysm sac to be observed at once. The experimental model AAA demonstrated realistic aneurysm behavior by having cyclic strains consistent with reported clinical observations between pressures 80 and 120 mm Hg. These strains are 1-2%, and the local spatial variations in experimental strain were less than predicted by the computational models. The three different models demonstrated that the asymmetric bifurcation creates displacement differences but not cyclic strain differences within the aneurysm sac. The technique and device captured regional variations of strain that are unobservable with diameter measures alone. It also allowed the calculation of local strain and removed rigid body motion effects on the strain calculation. The results of the computations show that an asymmetric aortic bifurcation created displacement differences but not cyclic strain differences within the aneurysm sac.

Side-dependent effect in the response of valve endothelial cells to bidirectional shear stress.

Endothelial cells covering the aortic and ventricular sides of the aortic valve leaflets are exposed to different stresses, in particular wall shear stress (WSS). Biomechanical stimuli actively regulate valve tissue structure and induce remodeling events leading to valve dysfunction. Endothelial to mesenchymal transformation (EndMT), for example, has been associated with aortic valve disease. The biomechanical response of cells at different sides of the leaflets has not been clearly characterized. To analyze the mechanical response of valve endothelial cells (VECs) we developed a unique fluid activation device that applies physiologically relevant pulsatile WSS. We characterized the morphology and function of adult porcine aortic VECs derived from the opposite sides of aortic valve leaflets following exposure to different pulsatile WSS. We found that elongation and orientation of cells in response to pulsatile WSS depends on their side of origin. Quantification of gene expression confirms phenotypic differences between aortic and ventricular VECs. Aortic VECs exposed to pulsatile WSS similar to that in vivo at the tip of aortic side of the valve leaflet upregulated pro-EndMT (ACTA2, Snail, TGFß1) and inflammation (ICAM-1, VCAM-1) genes, whereas expression of endothelial markers like PECAM-1 was decreased. Conversely, ventricular-VECs showed strong increase of PECAM-1 expression and no activation of pro-EndMT marker. Finally, we found that stress-induced genes are upregulated in both cell types, at higher levels in ventricular compared to aortic VECs. Application of physiological shear stress levels using a fluid activation device therefore reveals functional differences in VECs derived from opposite sides of the aortic valve leaflets.

Results of a prospective follow-up study after type A aortic dissection repair: a high rate of distal aneurysmal evolution and reinterventions.

OBJECTIVES: We investigated the anatomical evolution of residual aortic dissection after type A repair and factors associated with poor prognosis at a high-volume aortic centre. METHODS: Between 2017 and 2019, all type A aortic dissections were included for prospective follow-up. Patients without follow-up computed tomography (CT) scan available for radiological analysis and patients without residual aortic dissection were excluded from this study. The primary end point was a composite end point defined as dissection-related events including aneurysmal evolution (increased diameter > 5 mm/year), aortic reintervention for malperfusion syndrome, aortic diameter >55 mm, rapid aortic growth >10 mm/year or aortic rupture and death. The secondary end points were risk factors for dissection-related events and reintervention analysis. All immediate and last postoperative CT scans were analysed. RESULTS: Among 104 patients, after a mean follow-up of 20.4 months (8-41), the risk of dissection-related events was 46.1% (48/104) and the risk of distal reintervention was 17.3% (18/104). Marfan syndrome (P < 0.01), aortic bicuspid valve (P = 0.038), innominate artery debranching (P = 0.025), short aortic cross-clamp time (P = 0.011), initial aortic diameter >40 mm (P < 0.01) and absence of resection of the primary entry tear (P = 0.015) were associated with an increased risk of dissection-related events or reintervention during follow-up. CONCLUSIONS: Residual aortic dissection is a serious disease requiring close follow-up at an expert centre. This study shows higher reintervention and aneurysmal development rates than currently published. To improve long-term outcomes, the early demographic and anatomic poor prognostic factors identified may be used for more aggressive treatment at an early phase.

Nutrient distribution and metabolism in the intervertebral disc in the unloaded state: a parametric study.

A 2-D finite element model for the intervertebral disc in which quadriphasic theory is coupled to the transport of solutes involved in cellular nutrition was developed for investigating the main factors contributing to disc degeneration. Degeneration is generally considered to result from chronic disc cell nutrition insufficiency, which prevents the cells from renewing the extracellular matrix and thus leads to the loss of proteoglycans. Hence, the osmotic power of the disc is decreased, causing osmomechanical impairments. Cellular metabolism depends strongly on the oxygen, lactate and glucose concentrations and on pH in the disc. To study the diffusion of these solutes in a mechanically or osmotically loaded disc, the osmomechanical and diffusive effects have to be coupled. The intervertebral disc is modeled here using a plane strain formulation at the equilibrium state under physiological conditions after a long rest period (called unloaded state). The correlations between solute distribution and various properties of healthy and degenerated discs are investigated. The numerical simulation shows that solute distribution in the disc depends very little on the elastic modulus or the proteoglycan concentration but greatly on the porosity, diffusion coefficient and endplate diffusion area. This coupled model therefore opens new perspectives for investigating intervertebral disc degeneration mechanisms.

Assessment of intervertebral disc degeneration-related properties using finite element models based on [Formula: see text]-weighted MRI data.

Quantitative magnetic resonance imaging (MRI) provides useful information about intervertebral disc (IVD) biomechanical properties, especially those in relation to the fluid phase. These properties may improve IVD finite element (FE) models using data closer to physiological reality. The aim of this study is to investigate IVD degeneration-related properties using a coupling between MRI and FE modeling. To this end, proton density ([Formula: see text])-weighted MRI sequences of a porcine lumbar IVD were carried out to develop two biphasic swelling models with hyperelastic extracellular matrix behavior. The first model is isotropic, and the second one is anisotropic and takes into account the role of collagen fibers in the mechanical behavior of the IVD. MRI sequences permitted to determine the geometry and the real porosity mapping within the disc. The differentiation between disc components (nucleus pulposus, annulus fibrosus and cartilaginous end plates) was taken into account using spatial continuous distributions of the mechanical properties. The validation of the FE models was performed through two steps: the identification of the model's mechanical properties using relaxation compressive test and the comparison between the MRI after load porosity distributions and those numerically obtained using the set of identified properties. The results confirmed that the two developed FE models were able to predict the mechanical response of uniaxial time-dependent compressive test and the redistribution of porosity after load. A slight difference between the measured and the numerical local bulges of the disc was found. This study suggests that from the coupling between MRI imaging in different state of load and finite element modeling we can deduce relevant information that can be used in the assessment of the early intervertebral disc degeneration changes.

Mechanical characterisation of human ascending aorta dissection.

Mechanical characteristics of both the healthy ascending aorta and acute type A aortic dissection were investigated using in vitro biaxial tensile tests, in vivo measurements via transoesophageal echocardiography and histological characterisations. This combination of analysis at tissular, structural and microstructural levels highlighted the following: (i) a linear mechanical response for the dissected intimomedial flap and, conversely, nonlinear behaviour for both healthy and dissected ascending aorta; all showed anisotropy; (ii) a stiffer mechanical response in the longitudinal than in the circumferential direction for the healthy ascending aorta, consistent with the histological quantification of collagen and elastin fibre density; (iii) a link between dissection and ascending aorta stiffening, as revealed by biaxial tensile tests. This result was corroborated by in vivo measurements with stiffness index, ß, and Peterson modulus, Ep, higher for patients with dissection than for control patients. It was consistent with histological analysis on dissected samples showing elastin fibre dislocations, reduced elastin density and increased collagen density. To our knowledge, this is the first study to report biaxial tensile tests on the dissected intimomedial flap and in vivo stiffness measurements of acute type A dissection in humans.

New insights into the assessment of the prosthetic valve performance in the presence of subaortic stenosis through a fluid-structure interaction model.

The aim of this study was to contribute to improving the accuracy of clinical assessments of valve performance in situations involving the concomitant presence of a prosthetic valve and subaortic stenosis (SAS). Physiological flow in a two-dimensional model for a bileaflet mechanical heart valve was investigated numerically in terms of the fluid-structure interactions. The fluid dynamics in a model with SAS of the left ventricle outflow tract were compared with those given by a healthy model. The results show that in the model with SAS, one leaflet did not close during the observed systolic phase, whereas the other one showed similar behaviour to that of the leaflet in the healthy model. In addition, the main flow did not occur along the central axis and a deviated jet was set up between leaflets, contrary to what occurred in the model without SAS. Current clinical diagnostic indices, which are mainly based on the central jet flow velocities, are therefore unsuitable for use in this pathological situation and should be used with great caution.

Flow behaviour in an asymmetric compliant experimental model for abdominal aortic aneurysm.

An experimental study was carried out on asymmetrical abdominal aortic aneurysm (AAA) to analyse the physiological flows involved. Velocity measurements were performed using particle image velocimetry. Resting and exercise flow rates were investigated in models with rigid and compliant walls to assess the parameters affecting the flow behaviour. The secondary flow patterns, and especially the evolution of the vortices within the AAA, were found to be highly dependent on both the flow waveforms and the wall behaviour. Vortices impacts on the distal walls of the AAA occur in the compliant model and can increase the local pressure on the AAA walls and thus increase the wall stresses; AAA wall stresses are one of the most important factors contributing to ruptured aneurysm.

Biaxial tensile tests of the porcine ascending aorta.

One of the aims of this work is to develop an original custom built biaxial set-up to assess mechanical behavior of soft tissues. Stretch controlled biaxial tensile tests are performed and stereoscopic digital image correlation (SDIC) is implemented to measure the 3D components of the generated displacements. Using this experimental device, the main goal is to investigate the mechanical behavior of porcine ascending aorta in the more general context of human ascending aorta pathologies. The results highlight that (i) SDIC arrangement allows accurate assessment of displacements and so stress strain curves, (ii) porcine ascending aorta has a nearly linear and anisotropic mechanical behavior until 30% of strain, (iii) porcine ascending aorta is stiffer in the circumferential direction than in the longitudinal one, (iv) the material coefficient representing the interaction between the two loading directions is thickness dependent, (v) taking into account the variability of the samples the stress values are independent of the stretch rate in the range of values from 10(-3) to 10(-1)s(-1) and finally, (vi) unlike other segments of the aorta, 4-month-old pigs ascending aorta is definitely not a relevant model to investigate the mechanical behavior of the human ascending aorta.

Flow of a blood analogue fluid in a compliant abdominal aortic aneurysm model: experimental modelling.

The aim of this work is to develop a unique in vitro set-up in order to analyse the influence of the shear thinning fluid-properties on the flow dynamics within the bulge of an abdominal aortic aneurysm (AAA). From an experimental point of view, the goals are to elaborate an analogue shear thinning fluid mimicking the macroscopic blood behaviour, to characterise its rheology at low shear rates and to propose an experimental device able to manage such an analogue fluid without altering its feature while reproducing physiological flow rate and pressure, through compliant AAA. Once these experimental prerequisites achieved, the results obtained in the present work show that the flow dynamics is highly dependent on the fluid rheology. The main results point out that the propagation of the vortex ring, generated in the AAA bulge, is slower for shear thinning fluids inducing a smaller travelled distance by the vortex ring so that it never impacts the anterior wall in the distal region, in opposition to Newtonian fluids. Moreover, scalar shear rate values are globally lower for shear thinning fluids inducing higher maximum stress values than those for the Newtonian fluids. Consequently, this work highlights that a Newtonian fluid model is finally inadequate to obtain a reliable prediction of the flow dynamics within AAA.

CCM proteins control endothelial ß1 integrin dependent response to shear stress.

Hemodynamic shear stress from blood flow on the endothelium critically regulates vascular function in many physiological and pathological situations. Endothelial cells adapt to shear stress by remodeling their cytoskeletal components and subsequently by changing their shape and orientation. We demonstrate that ß1 integrin activation is critically controlled during the mechanoresponse of endothelial cells to shear stress. Indeed, we show that overexpression of the CCM complex, an inhibitor of ß1 integrin activation, blocks endothelial actin rearrangement and cell reorientation in response to shear stress similarly to ß1 integrin silencing. Conversely, depletion of CCM2 protein leads to an elongated "shear-stress-like" phenotype even in the absence of flow. Taken together, our findings reveal the existence of a balance between positive extracellular and negative intracellular signals, i.e. shear stress and CCM complex, for the control of ß1 integrin activation and subsequent adaptation of vascular endothelial cells to mechanostimulation by fluid shear stress.

New insights into the understanding of flow dynamics in an in vitro model for abdominal aortic aneurysms.

An in vitro dynamics set-up of the flow in a compliant abdominal aortic aneurysm (AAA) model with an anterior posterior asymmetry, aorto-iliac bifurcation, and physiological inlet flow rate and outlet pressure waveforms was developed. The aims were first to show that the structural mechanical behavior of the used material to mimic the AAA wall was similar to this of patients with AAA and then to study the influence of the aorto-iliac bifurcation presence and to study the influence of the imbalanced flow rate in the iliac branches on the AAA flow field. 3D visualizations, never performed in the literature, have clearly put into evidence the development of a vortex ring generated at the AAA proximal neck during the decelerating phase of flow rate, which detaches and progresses downstream during the cardiac cycle, impinges on the anterior wall in the distal AAA region, breaks up, and separates into two vortices of which one rolls on upstream along the anterior wall. 2D particle image velocimetry measurements, swirling strength and enstrophy calculations allowed quantification of the vorticity, vortex trajectory and energy for the different geometrical and hydrodynamical conditions. The main results show that the instant and the intensity of the vortex ring impingement depend on the presence of the aorto-iliac bifurcation, with higher intensity, by about 90%, for an AAA without bifurcation. The imbalance of the flow rates into the iliac branches induces different propagation velocities of the vortex ring and lowers the intensity of the vortex impact by about 60%. The potential influence of the AAA dynamics is discussed in terms of AAA remodeling and rupture.

Numerical and experimental study of blood flow through a patient-specific arteriovenous fistula used for hemodialysis.

Arteriovenous fistula (AVF) pathologies related to blood flow necessitate valid calculation tools for local velocity and wall shear stress determination to overcome the clinical diagnostic limits. To illustrate this issue, a reconstructed patient-specific AVF was investigated, using computational fluid dynamics (CFDs) and particle image velocimetry (PIV). The aim of this study was to validate the methodology from medical images to numerical simulations of an AVF by comparing numerical and experimental data. Two numerical grids were presented with a refinement difference of a factor of four. A mold of the same volume was created and mounted on an experimental bench with similar boundary conditions. The patient's acquired echo D006Fppler flow waveform was injected at the arterial inlet. Experimental and numerical velocity vector cartography qualitatively produced similar flow fields. Quantification with a point-to-point approach thoroughly investigated the velocity profiles using the mean difference between both results. The finest mesh generated CFD results with a mean percentage of the difference in velocity magnitude, taking the PIV as reference, did not exceed 10%. At specific zones, the coarse mesh required adaptive meshing to improve fitting with experimental data. Meshing refinement was necessary to improve velocity accuracy at wide diameters and wall shear stress at narrow diameters. Provided that these criteria were properly respected, we show through this difficult example the validity of using CFD to properly describe flow patterns in image-based reconstructed blood vessels.

Validation of a numerical 3-D fluid-structure interaction model for a prosthetic valve based on experimental PIV measurements.

A numerical 3-D fluid-structure interaction (FSI) model of a prosthetic aortic valve was developed, based on a commercial computational fluid dynamics (CFD) software program using an Arbitrary Eulerian Lagrangian (ALE) formulation. To make sure of the validity of this numerical model, an equivalent experimental model accounting for both the geometrical features and the hydrodynamic conditions was also developed. The leaflet and the flow behaviours around the bileaflet valve were investigated numerically and experimentally by performing particle image velocimetry (PIV) measurements. Through quantitative and qualitative comparisons, it was shown that the leaflet behaviour and the velocity fields were similar in both models. The present study allows the validation of a fully coupled 3-D FSI numerical model. The promising numerical tool could be therefore used to investigate clinical issues involving the aortic valve.

Analysis of blood flow behaviour in custom stent grafts.

Endovascular aneurysm repair (EVAR) is an attractive alternative to open surgery for treating abdominal aortic aneurysms (AAAs). However, the implantation of stent grafts into AAAs can result in post-operative complications such as stent graft migration, rupture or endoleak. EVAR has therefore been carried out only on selected patients. Stent grafts are usually standard commercial stent grafts (CSGs); however, custom made stent grafts (cmSGs) of various shapes and sizes are sometimes used to fit patients' anatomies. In the present study, the cmSGs were specially designed and fabricated by the surgeons at the Pitié-Salpétriére hospital in Paris. Two patients carrying cmSGs with unfavourable geometries showing tortuous shapes, angulation, widening, narrowing, curvature and kinking and one patient with a cmSG with a more favourable geometry resembling a straight tube were examined. These three clinical cases were investigated using three-dimensional numerical simulations, and the results showed that even when the cmSG geometries are unfavourable, the drag forces to which they are subjected are of a similar magnitude to those exerted on CSGs, or even smaller. The hemodynamic analysis carried out on the two unfavourable cmSGs showed the occurrence of low velocity values in the main trunk of the cmSGs, high velocities linked to recirculation areas downstream from kinking and strong distal narrowing. These flow patterns are liable to induce thrombus. However, since cmSG implantation can save the lives of patients for whom neither classical stent grafts nor open surgery are indicated, it can be concluded that these devices are useful in some cases.

Influence of wall compliance on hemodynamics in models of abdominal aortic aneurysm.

PURPOSE: To determine complementary criteria to existing morphological criteria, which are not reliable but are used to justify surgical intervention to treat abdominal aortic aneurysm (AAA). METHODS: An experimental study was conducted in which 2 models of AAA, 1 rigid and 1 soft, were used to study the influence of compliance on aneurysm dynamics. The heart rate was 70 beats per minute, and the mean flow rate was 1.02 L/min. Velocity measurements were made with particle image velocimetry in 2 planes parallel to flow (1 vertical and 1 horizontal). RESULTS: The general flow patterns generated in the rigid AAA model were in agreement with the literature. In both models, a vortex occurred at the beginning of systolic deceleration in the proximal part of the AAA, near the anterior wall. The vortex remained confined to the proximal part during the entire cycle in the rigid model, whereas in the soft model, the vortex migrated to the distal segment during the cycle and impacted the AAA walls. This impact generated a local pressure increase on the wall. In the soft model, another vortex was created near the posterior wall. These vortices eroded and weakened the walls of the distal segment, which can cause rupture. CONCLUSION: Compliance of the aneurysm wall might become another criterion to justify surgical intervention.