On the characterisation of abdominal aortic tissues

Abdominal aortic aneurysm (AAA) is the gradual and irreversible expansion of the distal region of the aorta. It can be defined as a balloon like dilation 1.5 times the diameter of the normal abdominal aorta. The prevalence of AAA is high, in particular amongst men over the age of 65. AAA is primarily asymptomatic and therefore, can continue to grow undetected until rupture. Rupture of AAA is a devastating event responsible for millions of deaths worldwide. Currently, the decision to perform preventative surgery is primarily based on the maximum diameter criterion, with AAAs larger than 5 – 5.5 cm regarded as at risk of rupture. However, the reliability of maximum diameter to predict rupture on a patient-to-patient basis has been the source of continued debate and has led to the proposal of a number of alternative techniques. AAA is known to alter the tissue’s structural integrity and rupture occurs when the blood pressure induced wall stress becomes greater than the local wall strength. Therefore, knowledge of the mechanical properties of AAA tissues is critically important to furthering the understanding of AAA development and assessing the risk of rupture. The primary aim of this research was to characterise AAA tissues, using refined experimental test procedures, to elucidate their role in AAA rupture risk. The measurement soft tissue thickness and the gripping method employed in biaxial tests can potentially impact the evaluation of experimentally derived tensile stress. The influence of these variables were measured and it was found that commonly employed measurement techniques induced a measurement related error of varying magnitude in the evaluation of tensile stress, however the thickness gauge and micrometer performed the best for structured and unstructured tissue, respectively. On average, the central 25% of the biaxial test specimen area was found to have a uniform strain field and thus strain measurement taken from this area can be considered free from edge effects. Furthermore, storage of tissue until commencement of experimental analyses is often necessary due to logistics and the often unpredictable nature of the surgical environment. Therefore, the impact of freezing tissue in isotonic saline at -20°C was assessed for up to one year and it was found that freezing the tissue for extended periods of time does not affect the evaluation of gross biaxial mechanical properties in the physiological range. Following these experimental investigations, the biaxial properties of the intraluminal thrombus (ILT) and AAA wall tissue were evaluated. In general, the ILT’s properties were found to be isotropic, inhomogeneous and somewhat regionally dependent, and that differences in the properties could be linked to differences in ILT morphologies. The AAA wall tissue was found to be anisotropic and the properties were not influenced by patient-specific factors such as age, sex, AAA diameter or status, i.e. elective or emergency repair. In addition, the uniaxial failure properties of AAA wall tissue in the presence of calcification were assessed and compared to predominantly fibrous tissue. It was found that the presence of calcification reduced the tissue’s failure properties. Scanning Electron Microscopy (SEM) in conjunction with Electron Dispersive X-ray spectroscopy (EDS) confirmed the presence of calcification at the site of failure and indicated that the boundary between the calcification and the surrounding fibrous tissue is vulnerable to rupture. The results and conclusions presented throughout this thesis may increase our knowledge of AAA biomechanics and rupture potential, and may aid in the development of more reliable AAA rupture risk assessment methods and ultimately improve clinical management of the disease.