Patient-specific in silico endovascular repair of abdominal aortic aneurysms: application and validation

Biomechanics and Modeling in Mechanobiology - Non-negligible postinterventional complication rates after endovascular aneurysm repair (EVAR) leave room for further improvements. Since the potential...     

Access sites to vascular system for endovascular abdominal aortic aneurysms repair

The authors describe their experience with access sites for endovascular abdominal aortic aneurysm repair in a group of 165 patients treated over a 10-year period.     

Conversion to open surgery after endovascular abdominal aortic aneurysms repair

The authors describe experience with conversions to open surgery after endovascular abdominal aneurysm repair and evaluate the frequency, causes and results of a total of 7 cases in their series of 165 patients treated over a 10-year period.     

Flow patterns in an endovascular stent-graft for abdominal aortic aneurysm repair

Endovascular exclusion of the abdominal aortic aneurysm (AAA) has been carried out in selected patients during the past decade. The deployment of a complex multicomponent endovascular device in an aneurysmal aorta may alter the local haemodynamics and lead to thrombosis and intimal hyperplasia development. The aim of this in vitro study was to investigate the flow patterns using flow visualisation and laser Doppler anemometry in a commercial bifurcated stent-graft. Two configurations of the stent-graft, endo-stent and exo-stent, were investigated in an idealised planar AAA model. The flow structures in the main trunk in both configurations of the stent-graft are three-dimensional with complex secondary structures. However, these flow structures were not entirely caused by the stent-graft. The stent struts in the endo-stent configuration cause localised alteration in the flow pattern but the overall flow structures were not significantly affected. Low velocity regions in the main trunk and flow separation in the stump region and the curved segment of the iliac limbs were observed. These areas are associated with thrombosis in the clinical situation. Improvements in the design of endovascular devices may remove these areas of unfavourable flow patterns and lead to better clinical performance.     

Ten-year follow-up after endovascular repair of traumatic abdominal aortic rupture

Blunt abdominal aortic injury is often associated with bowel injury that precludes operative repair because of the risk of graft infection. Endovascular repair has been reported but with limited follow-up. We present a case of a 15-year-old boy who underwent endovascular repair of blunt abdominal aortic rupture and whom we were able to follow up over a decade. Our experience with this case and three others, as well as the experience reported in the literature, suggests that endovascular repair is a reasonable option in the setting of concomitant bowel injury. The risk of over sizing, collapse, and migration may be less than that described for thoracic aortic injuries because there is no need to deploy the endograft across an angle.     

Intrasac pressure changes and vascular remodeling after endovascular repair of abdominal aortic aneurysms: review and biomechanical model simulation

In this paper, we review existing clinical research data on post-endovascular repair (EVAR) intrasac pressure and relation with abdominal aortic aneurysm (AAA) size changes. Based on the review, we hypothesize that intrasac pressure has a significant impact on post-EVAR AAA size changes, and post-EVAR remodeling depends also on how the pressure has changed over a period of time. The previously developed model of an AAA based on a constrained mixture approach is extended to include vascular adaptation after EVAR using an idealized geometry. Computational simulation shows that the same mechanism of collagen stress-mediated remodeling in AAA expansion induces the aneurysm wall to shrink in a reduced sac-pressure after post-EVAR. Computational simulation suggests that the intrasac pressure of 60 mm?Hg is a critical value. At this value, the AAA remains stable, while values above cause the AAA to expand and values below cause the AAA to shrink. There are, however, variations between individuals due to different cellular sensitivities in stress-mediated adaptation. Computer simulation also indicates that an initial decrease in intrasac pressure helps the AAA shrink even if the pressure increases after some time. The presented study suggests that biomechanics has a major effect on initial adaptation after EVAR and also illustrates the utility of a computational model of vascular growth and remodeling in predicting diameter changes during the progression and after the treatment of AAAs.     

Model reduction methodology for computational simulations of endovascular repair

Endovascular aneurysm repair (EVAR) is a current alternative treatment for thoracic and abdominal aortic aneurysms, but is still sometimes compromised by possible complications such as device migration or endoleaks. In order to assist clinicians in preventing these complications, finite element analysis (FEA) is a promising tool. However, the strong material and geometrical nonlinearities added to the complex multiple contacts result in costly finite-element models. To reduce this computational cost, we establish here an alternative and systematic methodology to simplify the computational simulations of stent-grafts (SG) based on FEA. The model reduction methodology relies on equivalent shell models with appropriate geometrical and mechanical parameters. It simplifies significantly the contact interactions but still shows very good agreement with a complete reference finite-element model. Finally, the computational time for EVAR simulations is reduced of a factor 6–10. An application is shown for the deployment of a SG during thoracic endovascular repair, showing that the developed methodology is both effective and accurate to determine the final position of the deployed SG inside the aneurysm.     

Radiation diagnosis of abdominal aortic aneurysms

Four hundred and forty seven patients with aneurysms of the abdominal aorta (AAA), including 238 patients with aneurysmal rupture, were admitted to the Research Institute of Emergency Care in 1990 to 2000. The results of studies in 225 patients (ultrasonography in 197, computed tomography in 59, and angiography in 104), including 155 patients with aneurysmal rupture were analyzed. Computed tomography (CT) has proved to be the most accurate technique in the detection and estimation of the size of aneurysms, as well as in the identification of ruptures (83.9%) and inferior to angiography (AG) in the study of involvement of the branches of the abdominal aorta. Ultrasound study (US) ranks below CT in its accuracy (US detects ruptures in 67.8%); however, US surpasses CT and AS in screening, particularly valuable at an admission unit and an intensive care unit, which permits repeated studies. AG has turned out to be the most valid method in identifying the involvement of renal and iliac arteries in aneurysm and in detecting aortocaval anastomoses; yet it is inferior to US and CT (the former revealed rupture and dissection in 18.6% of cases) in solving other diagnostic tasks. Based on the analysis, the optimal sequence of studies in the patients is US, CT, and AG.     

Surgical corrections of endovascular aneurysms: repair complications

The authors describe their experience with the use of 21 open surgical corrections after endovascular abdominal aneurysm repair, reporting the frequency, type and outcome of these procedures in their group of 165 patients treated during a 10-year period.     

A comparison of modelling techniques for computing wall stress in abdominal aortic aneurysms

Background  

Aneurysms, in particular abdominal aortic aneurysms (AAA), form a significant portion of cardiovascular related deaths. There is much debate as to the most suitable tool for rupture prediction and interventional surgery of AAAs, and currently maximum diameter is used clinically as the determining factor for surgical intervention. Stress analysis techniques, such as finite element analysis (FEA) to compute the wall stress in patient-specific AAAs, have been regarded by some authors to be more clinically important than the use of a "one-size-fits-all" maximum diameter criterion, since some small AAAs have been shown to have higher wall stress than larger AAAs and have been known to rupture.     

Indications for surgical treatment of abdominal aortic aneurysms]

The results of the surgical treatment of 115 patients with the abdominal aortal aneurysms are presented. Indications to surgery depending on the stage of aneurysm, way of classification, and coexisting cardiological disorders have been discussed. Excellent and favorable results of surgery have been achieved in 70 patients (60.9%). Overall hospital mortality rate was 39.1%. Out of patients who underwent elective surgery 11.6% died, in the group operated urgently--43.3%, and in emergency situation 89.4% of the operated patients died. Percentage of patients with coexisting cardiological disorders amounted to 54%, 62%, and 80% respectively. A strict correlation of the results of surgery and duration of the disease, and consequently classification to the treatment, has been noted. Indications to the elective surgery are related to the size of aneurysm, rate of its enlargement, patient's age, and general health.     

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.     

Porohyperelastic finite element modeling of abdominal aortic aneurysms

Abdominal aortic aneurysm (AAA) is the gradual weakening and dilation of the infrarenal aorta. This disease is progressive, asymptomatic, and can eventually lead to rupture--a catastrophic event leading to massive internal bleeding and possibly death. The mechanical environment present in AAA is currently thought to be important in disease initiation, progression, and diagnosis. In this study, we utilize porohyperelastic (PHE) finite element models (FEMs) to investigate how such modeling can be used to better understand the local biomechanical environment in AAA. A 3D hypothetical AAA was constructed with a preferential anterior bulge assuming both the intraluminal thrombus (ILT) and the AAA wall act as porous materials. A parametric study was performed to investigate how physiologically meaningful variations in AAA wall and ILT hydraulic permeabilities affect luminal interstitial fluid velocities and wall stresses within an AAA. A corresponding hyperelastic (HE) simulation was also run in order to be able to compare stress values between PHE and HE simulations. The effect of AAA size on local interstitial fluid velocity was also investigated by simulating maximum diameters (5.5 cm, 4.5 cm, and 3.5 cm) at the baseline values of ILT and AAA wall permeability. Finally, a cyclic PHE simulation was utilized to study the variation in local fluid velocities as a result of a physiologic pulsatile blood pressure. While the ILT hydraulic permeability was found to have minimal affect on interstitial velocities, our simulations demonstrated a 28% increase and a 20% decrease in luminal interstitial fluid velocity as a result of a 1 standard deviation increase and decrease in AAA wall hydraulic permeability, respectively. Peak interstitial velocities in all simulations occurred on the luminal surface adjacent to the region of maximum diameter. These values increased with increasing AAA size. PHE simulations resulted in 19.4%, 40.1%, and 81.0% increases in peak maximum principal wall stresses in comparison to HE simulations for maximum diameters of 35 mm, 45 mm, and 55 mm, respectively. The pulsatile AAA PHE FEM demonstrated a complex interstitial fluid velocity field the direction of which alternated in to and out of the luminal layer of the ILT. The biomechanical environment within both the aneurysmal wall and the ILT is involved in AAA pathogenesis and rupture. Assuming these tissues to be porohyperelastic materials may provide additional insight into the complex solid and fluid forces acting on the cells responsible for aneurysmal remodeling and weakening.