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Abdominal aortic aneurysm inception and evolution - A computational model
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.ORCID iD: 0000-0002-2749-3381
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Abdominal aortic aneurysm (AAA) is characterized by a bulge in the abdominal aorta. AAA development is mostly asymptomatic, but such a bulge may suddenly rupture, which is associated with a high mortality rate. Unfortunately, there is no medication that can prevent AAA from expanding or rupturing. Therefore, patients with detected AAA are monitored until treatment indication, such as maximum AAA diameter of 55 mm or expansion rate of 1 cm/year. Models of AAA development may help to understand the disease progression and to inform decision-making on a patient-specific basis. AAA growth and remodeling (G&R) models are rather complex, and before the challenge is undertaken, sound clinical validation is required.

In Paper A, an existing thick-walled model of growth and remodeling of one layer of an AAA slice has been extended to a two-layered model, which better reflects the layered structure of the vessel wall. A parameter study was performed to investigate the influence of mechanical properties and G&R parameters of such a model on the aneurysm growth.

In Paper B, the model from Paper A was extended to an organ level model of AAA growth. Furthermore, the model was incorporated into a Fluid-Solid-Growth (FSG) framework. A patient-specific geometry of the abdominal aorta is used to illustrate the model capabilities.

In Paper C, the evolution of the patient-specific biomechanical characteristics of the AAA was investigated. Four patients with five to eight Computed Tomography-Angiography (CT-A) scans at different time points were analyzed. Several non-trivial statistical correlations were found between the analyzed parameters.

In Paper D, the effect of different growth kinematics on AAA growth was investigated. The transverse isotropic in-thickness growth was the most suitable AAA growth assumption, while fully isotropic growth and transverse isotropic in-plane growth produced unrealistic results. In addition, modeling of the tissue volume change improved the wall thickness prediction, but still overestimated thinning of the wall during aneurysm expansion.

Abstract [sv]

Bukaortaaneurysm (AAA) kännetecknas av en utbuktning hos aortaväggen i buken. Tillväxt av en AAA är oftast asymtomatisk, men en sådan utbuktning kan plö̈tsligt brista, vilket har hög dödlighet. Tyvärr finns det inga mediciner som kan förhindra AAA från att expandera eller brista. Patienter med upptä̈ckt AAA hålls därför under uppsikt tills operationskrav är uppnådda, såsom maximal AAA-diameter på 55 mm eller expansionstakt på 1 cm/år. Modeller för AAA-tillväxt kan bidra till att öka förståelsen för sjukdomsförloppet och till att förbättra beslutsunderlaget på en patientspecifik basis. AAA modeller för tillväxt och strukturförändring (G&R) är ganska komplicerade och innan man tar sig an denna utmaning krävs de god klinisk validering.

I Artikel A har en befintlig tjockväggig modell för tillväxt av ett skikt av en AAA-skiva utö̈kats till en två-skiktsmodell. Denna modell återspeglar bättre den skiktade strukturen hos kärlväggen. Genom en parameterstudie undersö̈ktes påverkan av mekaniska egenskaper och G&R-parametrar hos en sådan modell för AAA-tillväxt.

I Artikel B utvidgades modellen från Artikel A till en organnivå-modell för AAA-tillväxt. Vidare inkorporerades modellen i ett “Fluid–Solid–Growth” (FSG) ramverk. En patientspecifik geometri hos bukaortan användes för att illustrera möjligheterna med modellen.

I Artikel C undersöktes utvecklingen av patientspecifika biomekaniska egenskaper hos AAA. Fyra patienter som skannats fem till åtta gånger med “Computed Tomography-Angiography” (CT-A) vid olika tillfällen analyserades. Flera icke triviala statistiska samband konstaterades mellan de analyserade parametrarna.

I Artikel D undersöktes effekten av olika tillväxt-kinematik för AAA tillväxt. En modell med transversellt-isotrop-i-tjockleken-tillväxt var den bäst lämpade för AAA tillväxt, medans antagandet om fullt-isotrop-tillväxt och transversellt-isotrop-i-planet-tillväxt producerade orimliga resultat. Dessutom gav modellering av vävnadsvolymsförändring ett förbättrat väggtjockleks resultat men en fortsatt överskattning av väggförtunningen under AAA-expansionen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 24 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0605
Keyword [en]
Aorta, Aneurysm, AAA, Blood Flow, Wall Shear Stress, Growth and Remodeling, Mixture Model, Growth Kinematics, Fluid-Solid-Growth
Keyword [sv]
Aorta, Aneurysm, AAA, Blodflöde, Vägg Skjuvspänning, Tillväxt och Strukturförändring, Blandning Modell, Tillväxt Kinematik
National Category
Biomaterials Science Other Materials Engineering
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-197289ISBN: 978-91-7729-216-6OAI: oai:DiVA.org:kth-197289DiVA: diva2:1051142
Public defence
2016-12-20, F3, Lindstedtsvägen 22, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20161201

Available from: 2016-12-01 Created: 2016-12-01 Last updated: 2016-12-01Bibliographically approved
List of papers
1. Influence of differing material properties in media and adventitia on arterial adaptation: application to aneurysm formation and rupture
Open this publication in new window or tab >>Influence of differing material properties in media and adventitia on arterial adaptation: application to aneurysm formation and rupture
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2013 (English)In: Computer Methods in Biomechanics and Biomedical Engineering, ISSN 1025-5842, Vol. 16, no 1, 33-53 p.Article in journal (Refereed) Published
Abstract [en]

Experimental and computational studies suggest a substantial variation in the mechanical responses and collagen fibre orientations of the two structurally important layers of the arterial wall. Some observe the adventitia to be an order of magnitude stiffer than the media whilst others claim the opposite. Furthermore, studies show that molecular metabolisms may differ substantially in each layer. Following a literature review that juxtaposes the differing layer-specific results we create a range of different hypothetical arteries: (1) with different elastic responses, (2) different fibre orientations, and (3) different metabolic activities during adaptation. We use a finite element model to investigate the effects of those on: (1) the stress response in homeostasis; (2) the time course of arterial adaptation; and (3) an acute increase in luminal pressure due to a stressful event and its influence on the likelihood of aneurysm rupture. Interestingly, for all hypothetical cases considered, we observe that the adventitia acts to protect the wall against rupture by keeping stresses in the media and adventitia below experimentally observed ultimate strength values. Significantly, this conclusion holds true in pathological conditions.

Keyword
media, adventitia, layer-specific, growth, arterial adaptation, abdominal aneurysm
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-142646 (URN)10.1080/10255842.2011.603309 (DOI)000316056100004 ()2-s2.0-84872580188 (ScopusID)
Note

QC 20140311

Available from: 2014-03-10 Created: 2014-03-10 Last updated: 2016-12-01Bibliographically approved
2. A Thick-Walled Fluid-Solid-Growth Model of Abdominal Aortic Aneurysm Evolution: Application to a Patient-Specific Geometry
Open this publication in new window or tab >>A Thick-Walled Fluid-Solid-Growth Model of Abdominal Aortic Aneurysm Evolution: Application to a Patient-Specific Geometry
2015 (English)In: Journal of Biomechanical Engineering, ISSN 0148-0731, Vol. 137, no 3, 031008Article in journal (Refereed) Published
Abstract [en]

We propose a novel thick-walled fluid-solid-growth (FSG) computational framework for modeling vascular disease evolution. The arterial wall is modeled as a thick-walled nonlinearly elastic cylindrical tube consisting of two layers corresponding to the mediaintima and adventitia, where each layer is treated as a fiber-reinforced material with the fibers corresponding to the collagenous component. Blood is modeled as a Newtonian fluid with constant density and viscosity; no slip and no-flux conditions are applied at the arterial wall. Disease progression is simulated by growth and remodeling (G&R) of the load bearing constituents of the wall. Adaptions of the natural reference configurations and mass densities of constituents are driven by deviations of mechanical stimuli from homeostatic levels. We apply the novel framework to model abdominal aortic aneurysm (AAA) evolution. Elastin degradation is initially prescribed to create a perturbation to the geometry which results in a local decrease in wall shear stress (WSS). Subsequent degradation of elastin is driven by low WSS and an aneurysm evolves as the elastin degrades and the collagen adapts. The influence of transmural G&R of constituents on the aneurysm development is analyzed. We observe that elastin and collagen strains evolve to be transmurally heterogeneous and this may facilitate the development of tortuosity. This multiphysics framework provides the basis for exploring the influence of transmural metabolic activity on the progression of vascular disease.

Keyword
aneurysm, three-dimensional, elastin degradation, growth, remodeling, fluid-solid-growth model
National Category
Biophysics
Identifiers
urn:nbn:se:kth:diva-163462 (URN)10.1115/1.4029279 (DOI)000350572600009 ()
Funder
Wellcome trust, WT 088877/Z/09/Z
Note

QC 20150408

Available from: 2015-04-08 Created: 2015-04-07 Last updated: 2016-12-01Bibliographically approved
3. Biomechanical changes during abdominal aortic aneurysm growth
Open this publication in new window or tab >>Biomechanical changes during abdominal aortic aneurysm growth
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2016 (English)Report (Refereed)
Abstract [en]

The biomechanics-based Abdominal Aortic Aneurysm (AAA) rupture risk assessment has gainedconsiderable scientific and clinical momentum. However, such studies have mainly focused oninformation at a single time point, and little is known about how AAA properties change over time.Consequently, the present study explored how geometry, wall stress-related and blood flow-relatedbiomechanical properties change during AAA expansion. Four patients with a total of 23 ComputedTomography-Angiography (CT-A) scans at different time points were analyzed. At each time point,patient-specific properties were extracted from (i) the reconstructed geometry, (ii) the computedwall stress at Mean Arterial Pressure (MAP), and (iii) the computed blood flow velocity atstandardized in and out flow conditions. Testing correlations between these parameters identifiedseveral non-intuitive dependencies. Most interestingly, the Peak Wall Rupture Index (PWRI) and themaximum Wall Shear Stress (WSS) independently predicted AAA volume growth. Similarly, Intra-luminal Thrombus (ILT) volume growth depended on both the maximum WSS and the ILT volumeitself. In addition, ILT volume, ILT volume growth and maximum ILT layer thickness correlated withPWRI as well as AAA volume growth. Consequently, a large ILT volume as well as fast increase of ILTvolume over time may be a risk factor for AAA rupture. However, tailored clinical studies would berequired to test this hypothesis and to clarify whether monitoring ILT development has any clinicalbenefit.

18 p.
Keyword
Aorta, AAA, Rupture Risk, Blood Flow, Wall Stress, Thrombus, ILT, Wall Shear Stress, Oscillatory Shear Index
National Category
Biomaterials Science
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-197288 (URN)
Available from: 2016-12-01 Created: 2016-12-01 Last updated: 2016-12-01
4. Growth description for vessel wall adaptation: a thick-walled mixture model of abdominal aortic aneurysm evolution
Open this publication in new window or tab >>Growth description for vessel wall adaptation: a thick-walled mixture model of abdominal aortic aneurysm evolution
2016 (English)Report (Other academic)
Abstract [en]

Modeling the soft tissue volumetric growth has received considerable attention in the literature.However, due to the lack of experimental observations, the growth kinematics, that are reported in the literature, are based on a number of assumptions.The present study tested the plausibility of different growth descriptions when applied to the abdominal aortic aneurysm (AAA) evolution.

A structurally motivated material model and the multi-constituent tissue growth descriptions were utilized. The mass increment of the individual constituents preserved either the density or the volume.Four different growth descriptions were tested, namely isotropic (IVG), in-plane (PVG), in-thickness (TVG) growth and no volume growth (NVG) models.

Based on the model sensitivity to the increased collagen deposition, TVG and NVG models were found to be plausible scenarios, while IVG and PVG were found to be implausible. In addition, TVG and NVG models were less sensitive to the initial constituent volume fractions, than IVG and PVG models.In conclusion, the choice of the growth kinematics is of crucial importance when modeling the AAA growth and remodeling, and,probably, also for other soft biological tissues.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. 22 p.
Keyword
soft tissue, mixture model, growth deformation, growth description, volume growth, vascular adaptation, abdominal aortic aneurysm, AAA
National Category
Biomaterials Science
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-197286 (URN)
Note

QC 20161201

Available from: 2016-12-01 Created: 2016-12-01 Last updated: 2016-12-01Bibliographically approved

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