MECHANICAL WALL STRESS IN ABDOMINAL AORTIC-ANEURYSM - INFLUENCE OF DIAMETER AND ASYMMETRY

Purpose: Risk for rupture of an abdominal aortic aneurysm is widely be lieved to be related to its maximum diameter. From a biomechanical sta ndpoint, however, risk is probably more precisely related to mechanica l wall stress. Many abdominal aortic aneurysms are asymmetric (for exa mple because of anterior bulging with posterior expansion limited by t he vertebral column). The purpose of this work was to investigate the effect of maximum diameter and asymmetric bulge on wall stress. Method s: Three-dimensional computer models of abdominal aortic aneurysms wer e generated. In one protocol, maximum diameter was held constant while bulge shape factor was varied. The shape factor took into account the asymmetric shape of the bulge. In a second protocol, the shape of the aneurysmal wall was held constant while maximum diameter was varied. Wall stress was computed in each instance with a commercial software p ackage and assumption of physiologic intraluminal pressure. Results: B oth maximum diameter and the shape factor were found to have substanti al influence on the distribution of wall stress within the aneurysm. I n some instances the maximum stress occurred at the midsection, and in others it occurred elsewhere. The magnitude of peak stress acting on the aneurysm increased nonlinearly with increasing maximum diameter or increasing asymmetry. Conclusions: Our computer models showed that th e stress within the wall of an abdominal aortic aneurysm and possibly the potential for rupture are as dependent on aneurysm shape as they a re on maximum diameter. This information may be important in determini ng severity of individual abdominal aortic aneurysms and in improving understanding of the natural history of the disease.