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O'Leary S.A.,University of Limerick | Kavanagh E.G.,HSE Midwestern Regional Hospital | Grace P.A.,HSE Midwestern Regional Hospital | McGloughlin T.M.,University of Limerick | And 3 more authors.
Journal of Biomechanics | Year: 2014

Intraluminal thrombus (ILT) is present in 75% of clinically-relevant abdominal aortic aneurysms (AAAs) yet, despite much research effort, its role in AAA biomechanics remains unclear. The aim of this work is to further evaluate the biomechanics of ILT and determine if different ILT morphologies have varying mechanical properties.Biaxial mechanical tests were performed on ILT samples harvested from 19 patients undergoing open surgical repair. ILT were separated into luminal, medial and medial/abluminal layers. A total of 356 tests were performed and the Cauchy stress (σ) and tangential modulus (TM) at a stretch ratio (λ) of 1.14 were recorded for each test in both the circumferential (θ) and longitudinal (L) directions.Our data revealed three distinct types of ILT morphologies, each with a unique set of mechanical properties. All ILT layers were found to be isotropic and inhomogeneous. Type 1 (n=10) was a multi-layered ILT (thick medial/abluminal layer) whose strength and stiffness decreased gradually from the luminal to the medial/abluminal layer. Type 2 (n=6) was a multi-layered ILT (thin/highly degraded medial/abluminal layer) whose strength and stiffness decreased abruptly between the luminal and medial/abluminal layer and Type 3 (n=3) is a single layered ILT with a lower strength and stiffness than Types 1 and 2. In a sub-study, we found the luminal layer to be stronger and stiffer in the posterior than the anterior region.This work provides further insights to the biomechanical behaviour of ILT and the use of our ILT classification may be useful in future studies. © 2014 Elsevier Ltd.

Doyle B.J.,University of Limerick | Coyle P.,HSE Midwestern Regional Hospital | Kavanagh E.G.,HSE Midwestern Regional Hospital | Grace P.A.,HSE Midwestern Regional Hospital | McGloughlin T.M.,University of Limerick
IFMBE Proceedings | Year: 2010

Clinicians routinely use the maximum transverse diameter of an abdominal aortic aneurysm (AAA) to help gauge the severity of the condition, with AAAs that reach or exceed 5.5 cm deemed a rupture-risk. The effectiveness of the maximum diameter criterion has been questioned and novel techniques to predict the rupture threat of AAAs have recently emerged, including the methodology reported in this study. Preliminary work in a previous publication by the authors highlighted that the FEARI may be a useful additional tool to help assess the rupture threat of AAAs. In this study, 42 electively repaired AAAs and 10 symptomatic/ruptured AAAs were analysed using the FEARI approach. Results (mean ± standard deviation) show that diameter, peak wall stress and FEARI were all higher in the symptomatic group compared to the electively repaired group (diameter = 75.5 ± 13.3 mm v 64.8 ± 12.4 mm, peak wall stress = 0.86 ± 0.36 v 0.55 ± 0.23 MPa, FEARI = 1.01 ± 0.43 v 0.66 ± 0.3). Various geometrical comparisons were also compared between the two groups and results showed that the ILT volume, total AAA volume, total surface area, AAA length, ratio of diameter to length and the ratio of maximum diameter to infrarenal diameter (ROD) were all higher in the symptomatic group. The percentage volume of ILT was lower in the symptomatic group (40% ± 15 v 51% ± 20). The results of this study suggest that numerical modeling may help contribute to the clinical decision-making process in AAA repair and that useful information can be obtained using this approach. © 2010 International Federation for Medical and Biological Engineering.

Carroll G.T.,University of Limerick | McGloughlin T.M.,University of Limerick | Burke P.E.,HSE Midwestern Regional Hospital | Egan M.,HSE Midwestern Regional Hospital | And 2 more authors.
Journal of Biomechanical Engineering | Year: 2011

Maintaining vascular access (VA) patency continues to be the greatest challenge for dialysis patients. VA dysfunction, primarily due to venous neointimal hyperplasia development and stenotic lesion formation, is mainly attributed to complex hemodynamics within the arteriovenous fistula (AVF). The effect of VA creation and the subsequent geometrical remodeling on the hemodynamics and shear forces within a mature patient-specific AVF is investigated. A 3D reconstructed geometry of a healthy vein and a fully mature patient-specific AVF was developed from a series of 2D magnetic resonance image scans. A previously validated thresholding technique for region segmentation and lumen cross section contour creation was conducted in MIMICS 10.01, allowing for the creation of a 3D reconstructed geometry. The healthy vein and AVF computational models were built, subdivided, and meshed in GAMBIT 2.3. The computational fluid dynamic (CFD) code FLUENT 6.3.2 (Fluent Inc., Lebanon, NH) was employed as the finite volume solver to determine the hemodynamics and shear forces within the healthy vein and patient-specific AVF. Geometrical alterations were evaluated and a CFD analysis was conducted. Substantial geometrical remodeling was observed, following VA creation with an increase in cross-sectional area, out of plane curvature (maximum angle of curvature in AVF=30 deg), and angle of blood flow entry. The mean flow velocity entering the vein of the AVF is dramatically increased. These factors result in complex three-dimensional hemodynamics within VA junction (VAJ) and efferent vein of the AVF. Complex flow patterns were observed and the maximum and mean wall shear stress (WSS) magnitudes are significantly elevated. Flow reversal was found within the VAJ and efferent vein. Extensive geometrical remodeling during AVF maturation does not restore physiological hemodynamics to the VAJ and venous conduit of the AVF, and high WSS and WSS gradients, and flow reversal persist. It is theorized that the vessel remodelling and the continued non-physiological hemodynamics within the AVF compound to result in stenotic lesion development. © 2011 American Society of Mechanical Engineers.

Doyle B.J.,University of Limerick | Doyle B.J.,University of Edinburgh | Callanan A.,University of Limerick | Grace P.A.,HSE Midwestern Regional Hospital | Kavanagh E.G.,HSE Midwestern Regional Hospital
International Journal for Numerical Methods in Biomedical Engineering | Year: 2013

Patient-specific modelling of abdominal aortic aneurysm has been shown to have clinical potential. This paper examines a large ruptured abdominal aortic aneurysm where the tissue from the diseased wall and the intraluminal thrombus was excised during open surgical repair and experimentally characterised. The mechanical data were used to develop material parameters that were incorporated into finite element models with measured nonuniform wall thickness. Implementation of the material data into the numerical model increased peak wall stress by 67%, wall strain by 320% and displacement by 177%, when compared with simulations based on material properties available in the literature. Distributions of numerical results were similar for both material data. Magnitudes of numerical results can differ significantly when using patient-specific material properties and therefore, care should be taken when interpreting numerical results derived from population-based data. © 2012 John Wiley & Sons, Ltd.

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