Failure properties of intraluminal thrombus in abdominal aortic aneurysm under static and pulsating mechanical loads

ObjectivesIt has been suggested that mechanical failure of intraluminal thrombus (ILT) could play a key role in the rupture of abdominal aortic aneurysms (AAAs), and in the present study, this hypothesis has been investigated. An in vitro experimental approach has been proposed, which provides layer-specific failure data of ILT tissue under static and pulsatile mechanical loads.MethodsIn total, 112 bone-shaped test specimens are prepared from luminal, medial, and abluminal layers of eight ILTs harvested during open elective AAA repair. Three different types of mechanical experiments, denoted as control test, ultimate strength test, and fatigue test were performed in Dulbecco's modified eagle's medium (DMEM) supplemented with fetal calf serum, L-ascorbic acid, and antibiotics at 37°C and pH 7.0. In detail, fatigue tests, which are experiments, where the ILT tissue is loaded in pulsatile manner, were carried out at three different load levels with a natural frequency of 1.0 Hz.ResultsILT's ultimate strength (156.5 kPa, 92.0 kPa, and 47.7 kPa for luminal, medial, and abluminal layers, respectively) and referential stiffness (62.88 kPa, 47.52 kPa, and 41.52 kPa, for luminal, medial, and abluminal layers, respectively) continuously decrease from the inside to the outside. ILT tissue failed within less than 1 hour under pulsatile loading at a load level of 60% ultimate strength, while a load level of about 40% ultimate strength did not cause failure within 13.9 hours.ConclusionsILT tissue is vulnerable against fatigue failure and shows significant decreasing strength with respect to the number of load cycles. Hence, after a reasonable time of pulsating loading ILT's strength is far below its ultimate strength, and when compared with stress predictions from finite element (FE) studies, this indicates the likelihood of fatigue failure in vivo. Failure within the ILT could propagate towards the weakened vessel wall behind it and could initialize AAA failure thereafter.

Clinical RelevanceRupture of an abdominal aortic aneurysm is the 10th leading cause of death in men above 60 years of age. Diameter of the aneurysm, the only accepted parameter to determine its rupture risk, is unreliable, and there is need for others. Aneurysm rupture has been related to growth of the intraluminal thrombus and failure of it might indicate a rupture risk. We demonstrate here that the strength of thrombus tissue decreases significantly under pulsating mechanical loads, a material characteristic to be known as fatigue. The derived data highlight the biomechanical role of the thrombus in abdominal aortic aneurysms, and therefore, improved risk prediction criteria could be drawn from that finding.