Date of Graduation

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Biomedical Engineering

Advisor

Morten O. Jensen

Committee Member

Hanna K. Jensen

Second Committee Member

Woodrow Shew

Third Committee Member

Robyn Goforth

Keywords

Aneurysm, Aorta, Rupture

Abstract

Studying the development of abdominal aortic aneurysm (AAA) through quantification of its growth kinetics and rupture criteria is important to decrease the risk of this life-threatening event. Uniaxial and biaxial testing of healthy and time-dependent Type-I collagen degraded aortic specimens from pigs was performed. Stress-strain suitable mathematical models describing the behavior of abdominal aortic tissue were utilized to derive specific tissue properties and parameters. Reduction in Type-I collagen fraction was observed using picrosirius red staining method, bright field microscopy, and MATLAB. Finite Element Modeling (FEM) of healthy and time-dependent Type-I collagen degraded abdominal aortas were performed using ABAQUS finite element software. The experimental tissue parameters were inputted in ABAQUS as tissue material properties. The focus was on finding the values of ultimate tensile strength (σmax), maximum strain at rupture (εmax), elastic modulus (E), and the critical strain (εc), which is identified as the point beyond which high rupture risk is present. These properties vary significantly between healthy tissue and time-dependent Type-I collagen degraded tissue. Significant differences were found in the biomechanical behavior of aortic tissue due to time-dependent Type-I collagen degradation. Tissue compliance increased; however, tissue strength decreased. Also, E, σmax, εmax, and εc values were significantly higher for the healthy tissue group than for time-dependent Type-I collagen degraded tissue groups. Picrosirius red images showed fragmented Type-I collagen fibers, and the observation was linked to the change in biomechanical behavior of the specimens. FEM of healthy and time-dependent Type-I collagen degradation models mimicked “aneurysmal” growth from an initial stage, a finding which will contribute to better assessment of patients’ specific AAA cases. In conclusion, the data indicate that Type-I collagen is important in maintaining abdominal aortic tissue’s structural integrity, and the growth kinetics and rupture risk of AAA increase significantly in the time-dependent Type-I collagen degraded tissue. Thus, quantification of Type-I collagen, the most abundant collagen type in the tissue, along with the quantification of other types of collagen in the tissue, should be included as a rupture criteria for monitoring the growth in AAA and predicating rupture.

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