Abdominal aortic aneurysm rupture ranks among the most common causes of death in western countries. In Germany 1 to 3% of deaths of men over 65 are caused by this incident. If left untreated about 30% of all abdominal aortic aneurysms rupture which in most cases causes death. However, operative aneurysm repair procedures also carry significant mortality risks. Thus, clinicians are faced with the decision weather the risk of repair is justified by the risk of aneurysm rupture.
Currently, this decision is taken with respect to a single "maximum diameter'' criterion stating that treatment is required if the maximum aneurysm diameter reaches a size of about 5 or 5.5 cm. In con-trast in healthy individuals the aorta exhibits a diameter of 1.3 up to 2.7 cm.
This project aims at the development of more individual criteria to asses the patient specific risk of aneurysm rupture. Rupture is a mechanical incident as it takes place when and where the stress ex-ceeds the resistance of the aortic wall. However, both the actual stresses as well as the load carrying capacity of the arterial wall are governed by biological conditions. The wall stress distribution highly depends upon the individual aneurysm geometry. Aneurysm geometries show a significant variation between patients. Additionally an intraluminal thrombus develops within most aneurysms as a result of locally decreased flow speed. Presumably this thrombus reduces the wall stresses by playing a me-chanically protective role. It has been found that the thrombus material may be assumed incompressi-ble, nonlinearly elastic, isotropic and inhomogeneous. On the other hand evidence has been found that the presence of a thrombus reduces the strength of the arterial wall which might be due to the thrombus acting as a barrier to oxygen flux from the blood to the inner layers of the aortic wall.
Overall progress in understanding of the cause of aneurysm rupture requires combine clinical experi-ence with mechanical considerations. In order to access a particular aneurysm numerically a geometry has to be deduced from medical imaging. Here again the intraluminal thrombus poses a major chal-lenge. While the blood filled volume (the lumen) can easily be segmented almost automatically the thrombus tends to share the grey scale of the surrounding tissue. The development of improved seg-mentation tools is thus a significant issue within this project.
Finally, finite element simulations of the behavior of aneurysms are performed. These simulations on patient specific geometries and with boundary conditions deduced form in vivo measurements provide additional insight to assess the individual risk of rupture of a specific aneurysm.