Timothy M. McGloughlin

Vessel asymmetry as an additional diagnostic tool in the assessment of abdominal aortic aneurysms

OBJECTIVE Abdominal aortic aneurysm (AAA) rupture is believed to occur when the local mechanical stress exceeds the local mechanical strength of the wall tissue. On the basis of this hypothesis, the knowledge of the stress acting on the wall of an unruptured aneurysm could be useful in determining the risk of rupture. The role of asymmetry has previously been identified in idealized AAA models and is now studied using realistic AAAs in the current work. METHODS Fifteen patient-specific AAAs were studied to estimate the relationship between wall stress and geometrical parameters. Three-dimensional AAA models were reconstructed from computed tomography scan data. The stress distribution on the AAA wall was evaluated by the finite element method, and peak wall stress was compared with both diameter and centerline asymmetry. A simple method of determining asymmetry was adapted and developed. Statistical analyses were performed to determine potential significance of results. RESULTS Mean von Mises peak wall stress +/- standard deviation was 0.4505 +/- 0.14 MPa (range, 0.3157-0.9048 MPa). Posterior wall stress increases with anterior centerline asymmetry. Peak stress increased by 48% and posterior wall stress by 38% when asymmetry was introduced into a realistic AAA model. CONCLUSION The relationship between posterior wall stress and AAA asymmetry showed that excessive bulging of one surface results in elevated wall stress on the opposite surface. Assessing the degree of bulging and asymmetry that is experienced in an individual AAA may be of benefit to surgeons in the decision-making process and may provide a useful adjunct to diameter as a surgical intervention guide.

Fibrin: A Natural Biodegradable Scaffold in Vascular Tissue Engineering

Arterial occlusive disease remains a major health issue in the developed world and a rapidly growing problem in the developing world. Although a growing number of patients are now being effectively treated with minimally invasive techniques, there remains a tremendous pressure on the vascular community to develop a synthetic small-diameter vascular graft with improved long-term patency rates. The field of tissue engineering offers an exciting alternative in the search for living organ replacement structures. Several methodologies have emerged for constructing blood vessel replacements with biological functionality. Common strategies include cell-seeded biodegradable synthetic scaffolds, cell self-assembly, cell-seeded gels and xenogeneic acellular materials. A wide range of materials are being investigated as potential scaffolds for vascular tissue engineering applications. Some are commercialised and others are still in development. Recently, researchers have studied the role of fibrin gel as a three-dimensional scaffold in vascular tissue engineering. This overview describes the properties of fibrin gel in vascular tissue engineering and highlights some recent progress and difficulties encountered in the development of cell fibrin scaffold technology.

3-D Numerical Simulation of Blood Flow Through Models of the Human Aorta

A Spiral Computerized Tomography (CT) scan of the aorta were obtained from a single subject and three model variations were examined. Computational fluid dynamics modeling of all three models showed variations in the velocity contours along the aortic arch with differences in the boundary layer growth and recirculation regions. Further down-stream, all three models showed very similar velocity profiles during maximum velocity with differences occurring in the decelerating part of the pulse. Flow patterns obtained from transient 3-D computational fluid dynamics are influenced by different reconstruction methods and the pulsatility of the flow. Caution is required when analyzing models based on CT scans.

A Computational Study of the Magnitude and Direction of Migration Forces in Patient-specific Abdominal Aortic Aneurysm Stent-Grafts

OBJECTIVES Endovascular aneurysm repair for abdominal aortic aneurysm (AAA) is now a widely adopted treatment. Several complications remain to be fully resolved and perhaps the most significant of these is graft migration. Haemodynamic drag forces are believed to be partly responsible for migration of the device. The objective of this work was to investigate the drag forces in patient-specific AAA stent-grafts. METHODS CT scan data was obtained from 10 post-operative AAA patients treated with stent-grafts. 3D models of the aneurysm, intraluminal thrombus and stent-graft were created. The drag forces were determined by fluid-structure interaction simulations. A worst case scenario was investigated by altering the aortic waveforms. RESULTS The median resultant drag force was 5.46 N (range: 2.53-10.84). An increase in proximal neck angulation resulted in an increase in the resultant drag force (p = 0.009). The primary force vector was found to act in an anterior caudal direction for most patients. The worst case scenario simulation resulted in a greatest drag force of 16 N. CONCLUSIONS Numerical methods can be used to determine patient-specific drag forces which may help determine the likelihood of stent-graft migration. Anterior-posterior neck angulation appears to be the greatest determinant of drag force magnitude. Graft dislodgement may occur anteriorally as well as caudally.

ECM-based materials in cardiovascular applications: Inherent healing potential and augmentation of native regenerative processes.

The in vivo healing process of vascular grafts involves the interaction of many contributing factors. The ability of vascular grafts to provide an environment which allows successful accomplishment of this process is extremely difficult. Poor endothelisation, inflammation, infection, occlusion, thrombosis, hyperplasia and pseudoaneurysms are common issues with synthetic grafts in vivo. Advanced materials composed of decellularised extracellular matrices (ECM) have been shown to promote the healing process via modulation of the host immune response, resistance to bacterial infections, allowing re-innervation and reestablishing homeostasis in the healing region. The physiological balance within the newly developed vascular tissue is maintained via the recreation of correct biorheology and mechanotransduction factors including host immune response, infection control, homing and the attraction of progenitor cells and infiltration by host tissue. Here, we review the progress in this tissue engineering approach, the enhancement potential of ECM materials and future prospects to reach the clinical environment.

Fluid-structure interaction of a patient-specific abdominal aortic aneurysm treated with an endovascular stent-graft.

BackgroundAbdominal aortic aneurysms (AAA) are local dilatations of the infrarenal aorta. If left untreated they may rupture and lead to death. One form of treatment is the minimally invasive insertion of a stent-graft into the aneurysm. Despite this effective treatment aneurysms may occasionally continue to expand and this may eventually result in post-operative rupture of the aneurysm. Fluid-structure interaction (FSI) is a particularly useful tool for investigating aneurysm biomechanics as both the wall stresses and fluid forces can be examined.MethodsPre-op, Post-op and Follow-up models were reconstructed from CT scans of a single patient and FSI simulations were performed on each model. The FSI approach involved coupling Abaqus and Fluent via a third-party software - MpCCI. Aneurysm wall stress and compliance were investigated as well as the drag force acting on the stent-graft.ResultsAneurysm wall stress was reduced from 0.38 MPa before surgery to a value of 0.03 MPa after insertion of the stent-graft. Higher stresses were seen in the aneurysm neck and iliac legs post-operatively. The compliance of the aneurysm was also reduced post-operatively. The peak Post-op axial drag force was found to be 4.85 N. This increased to 6.37 N in the Follow-up model.ConclusionIn a patient-specific case peak aneurysm wall stress was reduced by 92%. Such a reduction in aneurysm wall stress may lead to shrinkage of the aneurysm over time. Hence, post-operative stress patterns may help in determining the likelihood of aneurysm shrinkage post EVAR. Post-operative remodelling of the aneurysm may lead to increased drag forces.

3D Reconstruction and Manufacture of Real Abdominal Aortic Aneurysms: From CT Scan to Silicone Model

Abdominal aortic aneurysm (AAA) can be defined as a permanent and irreversible dilation of the infrarenal aorta. AAAs are often considered to be an aorta with a diameter 1.5 times the normal infrarenal aorta diameter. This paper describes a technique to manufacture realistic silicone AAA models for use with experimental studies. This paper is concerned with the reconstruction and manufacturing process of patient-specific AAAs. 3D reconstruction from computed tomography scan data allows the AAA to be created. Mould sets are then designed for these AAA models utilizing computer aided designcomputer aided manufacture techniques and combined with the injection-moulding method. Silicone rubber forms the basis of the resulting AAA model. Assessment of wall thickness and overall percentage difference from the final silicone model to that of the computer-generated model was performed. In these realistic AAA models, wall thickness was found to vary by an average of 9.21%. The percentage difference in wall thickness recorded can be attributed to the contraction of the casting wax and the expansion of the silicone during model manufacture. This method may be used in conjunction with wall stress studies using the photoelastic method or in fluid dynamic studies using a laser-Doppler anemometry. In conclusion, these patient-specific rubber AAA models can be used in experimental investigations, but should be assessed for wall thickness variability once manufactured.

Identification of Rupture Locations in Patient-Specific Abdominal Aortic Aneurysms Using Experimental and Computational Techniques

In the event of abdominal aortic aneurysm (AAA) rupture, the outcome is often death. This paper aims to experimentally identify the rupture locations of in vitro AAA models and validate these rupture sites using finite element analysis (FEA). Silicone rubber AAA models were manufactured using two different materials (Sylgard 160 and Sylgard 170, Dow Corning) and imaged using computed tomography (CT). Experimental models were inflated until rupture with high speed photography used to capture the site of rupture. 3D reconstructions from CT scans and subsequent FEA of these models enabled the wall stress and wall thickness to be determined for each of the geometries. Experimental models ruptured at regions of inflection, not at regions of maximum diameter. Rupture pressures (mean+/-SD) for the Sylgard 160 and Sylgard 170 models were 650.6+/-195.1mmHg and 410.7+/-159.9mmHg, respectively. Computational models accurately predicted the locations of rupture. Peak wall stress for the Sylgard 160 and Sylgard 170 models was 2.15+/-0.26MPa at an internal pressure of 650mmHg and 1.69+/-0.38MPa at an internal pressure of 410mmHg, respectively. Mean wall thickness of all models was 2.19+/-0.40mm, with a mean wall thickness at the location of rupture of 1.85+/-0.33 and 1.71+/-0.29mm for the Sylgard 160 and Sylgard 170 materials, respectively. Rupture occurred at the location of peak stress in 80% (16/20) of cases and at high stress regions but not peak stress in 10% (2/20) of cases. 10% (2/20) of models had defects in the AAA wall which moved the rupture location away from regions of elevated stress. The results presented may further contribute to the understanding of AAA biomechanics and ultimately AAA rupture prediction.

A Finite Element Analysis Rupture Index (FEARI) as an Additional Tool for Abdominal Aortic Aneurysm Rupture Prediction

Currently, abdominal aortic aneurysms (AAAs), which are a permanent dilation of the aorta, are treated surgi- cally when the maximum transverse diameter surpasses 5.5cm. AAA rupture occurs when the locally acting wall stress exceeds the locally acting wall strength. There is a need to review the current diameter-based criterion, and so it may be clinically useful to develop an additional tool to aid the surgical decision-making process. A Finite Element Analysis Rup- ture Index (FEARI) was developed. Ten patient-specific AAAs were reconstructed, and the corresponding wall stress computed. Previous experimental work on determination of ultimate tensile strengths (UTS) from AAA tissue samples was implemented in this study. By com- bining peak wall stress along with average regional UTS, a new approach to the estimation of patient-specific rupture risk has been developed. Ten cases were studied, all of which were awaiting or had previously undergone surgical AAA repair. A detailed exami- nation of these ten cases utilising the FEARI analysis suggested that there was a possibility that some of the AAAs may have been less prone to rupture than previously considered. It is proposed that FEARI, used alongside other rupture risk factors, may improve the current surgical decision-making process. The use of FEARI as an additional tool for rupture prediction may provide a useful adjunct to the diameter-based approach in surgical decision-making.

Geometrical enhancements for abdominal aortic stent-grafts.

Purpose: To compare the function of 2 stent-graft designs for endovascular abdominal aortic aneurysm repair. Methods: Computational fluid dynamics was used to investigate the performance of a conventional stent-graft versus one with a novel tapered configuration (equal area ratios at the inlet and bifurcation). Idealized geometries (uniplanar) were formed first for both devices. To mimic the clinical setting with pulsatile blood flow, a realistic model (multiplanar) was created for the conventional stent-graft based on computed tomography scans from 3 patients with different aortic geometries. A similar model was created for the tapered stent-graft by mimicking the deployment of the conventional stent-graft through its centerline. Results: The tapered stent-graft model demonstrated reduced secondary flow vortices and wall shear stresses in the iliac limbs compared to the conventional graft in the idealized scenario. The drag forces in the idealized models were similar for both designs, though the tapered stent-graft showed a 4% reduction. Flow was split more evenly between the tapered stent-graft limbs in the realistic scenario. Conclusion: The novel tapered design reduced flow velocities and secondary flows due to its smooth trunk-to-limb transition, while also splitting the flow between the iliac limbs more evenly. In multiplanar models, the out-of-plane curvature was the greatest cause of skewed flow, which reduced the benefits of the tapered stent-graft.