Thesis
Investigation of discrepancy in the uniaxial versus biaxial response of the abdominal aortic aneurysm tissue
University of Iowa
Master of Science (MS), University of Iowa
Spring 2021
DOI: 10.17077/etd.006112
Abstract
Abdominal Aortic Aneurysm (AAA) is the permanent enlargement of a segment of the abdominal aorta due to the weakening of its wall. When left untreated, patients are exposed to a fatal rupture of the aortic wall. Physicians seek a patient-based method that could help them accurately predict the AAA rupture, and consequently decide confidently which patient to elect for surgery. They currently rely solely on three inefficient metrics that are the maximum diameter of the AAA (Raghavan et al., 1996), the annual growth rate of the AAA (typically when it grows at a rate of more than 1cm/year), and the symptomatic state of the patient (Vitásek et al., 2021). Understanding the biomechanical behavior of AAA tissues may be useful in accurately predicting their rupture. Their stress-strain behavior has been studied and reported upon through uniaxial extensions tests by some researchers and through biaxial extension tests by others. In comparing these studies, it was found that the mechanical response of AAA tissues under uniaxial tensile load was stiffer than their mechanical response under biaxial tensile load. Theory suggests the opposite.To investigate the source of such discrepancy, uniaxial and biaxial tensile tests were performed on porcine aortic tissue as a substitute to human abdominal aortic aneurysmal tissue. The following nine potential reasons were postulated for the observed discrepancy: population diversity, experimental parameters, instrumentation diversity, specimen procurement and storage, specimen location, data analysis procedure, gripping method, fiber re-alignment, and preconditioning/non-preconditioning. The first six postulated reasons were controlled for during the experiments leaving the last three as the likely sources if the discrepancy was successfully replicated. Further, to evaluate the effect of gripping method (clamp grips versus hook grips) on measured stress-strain behavior, finite element simulations of AAA tissue under uniaxial extension tests were conducted and assessed.
Results of the experiments showed that the observed discrepancy was replicable, with the mechanical response of cardiovascular soft tissue behaving significantly stiffer under uniaxial loading as compared to biaxial loading.
Results of the finite element simulations of the gripping method showed a higher stiffness response of the material under uniaxial clamp grips than uniaxial hooks grips, but difference was not large enough to explain the observed discrepancy.
The findings of the study suggest that the observed discrepancy between reported uniaxial and biaxial data for AAA is attributable to differences in preconditioning protocols between these approaches and/or due to hitherto poorly understood microstructured fiber realignment that may be taking place under uniaxial versus biaxial extension. However, one major limitation of this study is the use of very high preloads in the uniaxial tests that may render these findings questionable. Further studies with appropriate negligible preloads at zero stretch should be conducted before drawing conclusions.
Details
- Title: Subtitle
- Investigation of discrepancy in the uniaxial versus biaxial response of the abdominal aortic aneurysm tissue
- Creators
- Didier Ninsemon Omer Gossiho
- Contributors
- Suresh Raghavan (Advisor)Sarah C Vigmostad (Committee Member)Seth I Dillard (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Biomedical Engineering
- Date degree season
- Spring 2021
- DOI
- 10.17077/etd.006112
- Publisher
- University of Iowa
- Number of pages
- xviii, 100 pages
- Copyright
- Copyright 2021 Didier Ninsemon Omer Gossiho
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 69-72).
- Public Abstract (ETD)
Abdominal Aortic Aneurysm (AAA) is the permanent enlargement of a segment of the abdominal aorta due to the weakening of the vessel’s wall. When left untreated, the blood vessel can rupture unexpectedly and cause an instantaneous death due to internal bleeding. This disease is the thirteenth leading cause of death in the United States. As of now, physicians are lacking an efficient patient-based method to predict its rupture to elect patients for surgery.
Knowing how the blood vessel tissues behave is a key to their rupture prediction. The goal of this study is to conduct tensile testing experiments with pig’s abdominal aortic tissue as a substitute of that of humans to better learn the properties (behavior) of these vessels.
The present study is an investigation into the mechanical behavior of aneurysm tissue focused on identifying possible explanations for an unexpected observation during meta-analysis of reported studies in AAA tissue biomechanics literature. To better understand the source(s) of the observed discrepancy in reported studies, we conducted experiments with porcine aortic tissue under controlled conditions and performed computer simulations of biomechanical tissue behavior under those test conditions. These experiments and simulations helped shed light on potential reasons for the discrepancy and likely contribute to more controlled, reliable research in the mechanical behavior of aneurysm tissues. Improved research will likely lead to important insights that may aid in the management of patients with this deadly disease.
- Academic Unit
- Roy J. Carver Department of Biomedical Engineering
- Record Identifier
- 9984097169702771
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