Wall stress analysis: A new way to measure aneurysm rupture risk
A team of researchers at Dartmouth has developed a new way of determining which aneurysms are the deadliest. The rupture of an aneurysma ballooning in the wall of an arteryis usually fatal. But it can be difficult to determine which aneurysms are in most danger of rupturing, so that risk can be accurately weighed against the risk of surgically repairing the weakened blood vessel wall.
Method: For over 40 years, the standard method of predicting rupture risk has been by measuring the diameter of the aneurysm. But researchers at DMS and Dartmouth's Thayer School of Engineering have developed a new, noninvasive method, called "wall stress analysis," to assess the likelihood that an abdominal aortic aneurysm (AAA) will rupture. A study of the new method, published in the Journal of Vascular Surgery, showed wall stress to be much more accurate than diameter at predicting an aneurysm's risk of rupture. That's an important piece of information in deciding which patients with AAAs should have surgery and which should merely be observed.
More patients may be able to avoid the risk of surgery thanks to a new way of analyzing an aneurysm's risk of rupture.
The strategy for deciding if an AAA requires surgery encompasses "the risk of rupture and the risk of repair in the context of the patient's life expectancy," explains Mark Fillinger, M.D., an associate professor of surgery at DMS and the lead author of the study. "Generally we have a reasonably good idea of a patient's life expectancy, and we have a fairly good idea of the risks of the various procedures." But, he says, "rupture risk is still nebulous."
For AAAs up to 5.5 centimeters in diameter, the belief has been that the risk of rupture is low. Therefore, the usual strategy has been to forego surgery and closely observe the patient. And for AAAs larger than 5.5 centimeters, it has been common to operate under the premise that the risk of rupture outweighs the risk of surgery.
"For the majority of patients, I would say our current methods work pretty well. They've been refined over the last 40 to 50 years," says Fillinger. But an aneurysm's size is not the only determinant of whether it will rupture. Some small AAAs rupture, while some large ones never rupture. And since most patients with AAAs are between the ages of 55 and 70, the risk of surgery is often not trivial.
So Fillinger teamed up with fellow vascular surgeon Jack Cronenwett, M.D., and several engineering colleagues to develop a way of measuring stress within the wall of an aneurysm. Their study comparing the new and old ways of assessing rupture risk included more than 100 patients at DHMC who were under observation for AAAs.
The new method involves processing CT scans of AAAs through a series of computer programs, including an engineering process called finite element analysis. Finite element analysis segments a structure into thousands of small elements, enabling a computer to calculate the wall stress in all areas of a three dimensional shape. The end result is a stress map displaying gradations of stress across the surface of the aneurysm.
Stress: The method's ability to predict an aneurysm's risk of rupture was much more accurate than measuring the aneurysm's diametershowing a 25-fold increase in rupture risk for patients with high wall stress, and only a nine-fold increase in rupture risk for patients with large-diameter AAAs. Further, in those patients whose AAAs did rupture, if the location of the rupture could be identified it correlated exactly with the location of the highest level of wall stress.
Fillinger and his colleagues are currently planning a multicenter study to further test this approach to assessing AAAs. Among other issues, they hope to explore the effects of gender and of the presence of arterial plaque on rupture risk.