Stress Characteristics of Non-Axisymmetric Synthetic Abdominal Aortic Aneurysm Models: How Real are They?
Abdominal aortic aneurysms (AAAs) are abnormal aortic dilatations that are prone to rupture, with fatal consequences. Synthetic aneurysm models are being used to assess in vivo stress characteristics of aneurysms before and after surgical reinforcement. This study seeks to assess peak wall stress characteristics in a latex life- like model. A life-like non-axisymmetric latex AAA model, constructed from a 3D computed tomographic reconstruction of a real AAA, was incorporated into a pulsatile flow unit (PFU) to simulate the cardiac output. Strain gauges were placed at the neck (n= 2 x 3), inflection point (the junction of neck and sac, n=4 x 3) and maximum anteroposterior diameter (n=4 x 3). The arterial pressure settings used were 130/90 and 140/100mmHg, termed the low and high setting respectively. Strain readings were obtained at 10Hz over 30 seconds using a data logger. Stress was derived using the relationship between stress and Young’s modulus (E= 5.151872 Nmm-2). Peak wall stresses were statistically analysed over the two pressure settings using ANOVA in Minitab 13. The highest stresses were noted at the inflection point and not at the maximum diameter, as might have been expected. Peak inflection point stress anteriorly measured 394.69 (SD 218.1) x10-4 N/cm2 in the low setting, increasing to 715.39(SD 230.32) in the high setting (p<0.001). Posteriorly, peak wall stress measured as high as 373.61(SD207.24) x10-4 Ncm-2 in the low setting, and increased to 1053.32 (SD 347.01) x10-4 Ncm-2 in the high setting (p<0.001). High posterior stress conforms to in vivo studies. Peak wall stresses were not as high in the sac (range 35.08-204.98 x 10-4 Ncm-2 in the low setting and 54.66- 322.73 x 10-4 Ncm-2 in the high setting). An increase in blood pressure caused an increase only in the anterior and lateral, but not the posterior aspect of the sac (p<0.05). Abdominal aortic wall stress is highest at the inflection point, and is affected by blood pressure, which suggests that it is the area most likely to rupture and is critical to reinforcement of the AAA. These readings are lower than stress noticed in vivo due to the lower E of latex as compared to aneurysmal aorta, which structurally is primarily a multilaminate of elastin and collagen; however, the trends themselves may parallel those that occur in AAAs before and after endovascular or open grafting, and therefore justify artificial stress modelling of AAAs.
A. Chaudhuri et al., "Stress Characteristics of Non-Axisymmetric Synthetic Abdominal Aortic Aneurysm Models: How Real are They?", Applied Mechanics and Materials, Vols. 1-2, pp. 245-250, 2004