Wall stress studies of abdominal aortic aneurysm in a clinical model

To estimate when an abdominal aortic aneurysm (AAA) may rupture, it is nece ssary to understand the forces responsible for this event. We investigated the wall stresses in an AAA in a clinical model. Using CT scans of the AAA, the diameter and wall thickness were measured and the model of the aneurys m was created. The wall stresses were determined using a finite element ana lysis in which the aorta was considered isotropic with linear material prop erties and was loaded with a pressure of 120 mmHg. The AAA was eccentric wi th a length of 10.5 cm, a diameter of 2.5 to 5.9 cm, and a wall thickness o f 1.0 to 2.0 mm. The aneurysm had specific areas of high stress. On the inn er surface the highest stress was 0.4 N/mm(2) and occurred along two circum ferentially oriented belts-one at the bulb and the other just below. The st ress was longitudinal at the anterior region of the bulb and circumferentia l elsewhere, suggesting that a rupture caused by this stress will result in a circumferential tear at the anterior portion of the bulb and a longitudi nal tear elsewhere. In the mid-surface the highest stress was 0.37 N/mm(2) and occurred at two locations: the posterior region of the bulb and anterio rly just below. The stress was circumferential, suggesting that the rupture caused by this stress will produce a longitudinal tear. The location and o rientation of the maximum stress were influenced more by the tethering forc e than by the wall thickness, luminal pressure, or wall stiffness. In concl usion, the rupture of an AAA is most likely to occur on the inner surface a t the bulb. Such analytical approaches could lead to a better understanding of the aneurysm rupture and may be instrumental in planning surgical inter ventions.