PITTSBURGH, July 6, 1998 — Researchers at the University of Pittsburgh Medical Center (UPMC) are using CT scans and state-of-the-art computer modeling to construct "virtual aneurysms" which may aid surgeons in determining when to operate on an abdominal aortic aneurysm (AAA) before it ruptures.
Some 15,000 Americans die each year from AAAs, which are a ballooning or bulging of the aorta, the main artery that carries blood to organs and the lower extremities. Left untreated, the aneurysm will continue to expand until it ruptures unexpectedly and bleeds, causing death in up to 80 percent of patients.
"We are combining medical imaging techniques with engineering analyses to develop a non-invasive method of estimating AAA severity," said David Vorp, Ph.D., assistant professor of surgery and mechanical engineering and director of the Vascular Biomechanics and Vascular Research Lab at the University of Pittsburgh.
"By utilizing these models and estimating biomechanical stresses within the aneurysm wall, clinicians may be better able to assess a specific aneurysm’s propensity to rupture and make a sound judgment on whether surgical intervention is warranted. Ideally, surgery is performed only when the associated risks are exceeded by the risk of an AAA rupture. Making this decision is presently very difficult since AAAs afflict mainly those over 60 years of age."
The standard surgical procedure to repair an AAA involves a large abdominal incision. The aneurysm is repaired by suturing a Teflon/polyester graft into place inside the AAA.
Recently, vascular surgeons at UPMC have used a minimally invasive procedure called endovascular surgery in which the aneurysm is repaired from inside the aorta using a catheter. The catheter contains a collapsed polyester tube which is inserted into the patient’s femoral artery and moved to the site of the aneurysm. Once inside the aneurysm, a spring-type attachment system hooks the tube to the inside walls of the artery on either end of the aneurysm and is anchored into place. Blood then flows through the implant, effectively depressurizing the aneurysm.
Endovascular surgery is still a new, exploratory field and only a small percentage of patients are eligible for this technique. Therefore, the only option for most patients is to undergo traditional surgical repair, according to Dr. Vorp.
"The decision to repair an aneurysm is usually based on its maximum diameter," he said. "Surgeons typically will repair an AAA when it exceeds 5 cm (2- 1/2 inches) in diameter but this does not take into account important characteristics of individual aneurysms."
Some AAAs may have the same maximum diameter, but may have differences in shape, wall thickness, or mechanical properties which could have an effect on their growth and potential to rupture.
"From an engineering perspective, the proper definition of ‘critical state’ for an AAA is when its capacity to withstand forces is about to be breached," Dr. Vorp said. "That is, a more sound indicator of the severity of a specific aneurysm is to compare the acting wall stresses to the tensile strength of the wall tissue, since AAA rupture occurs when the wall stresses exceed the strength."
Dr. Vorp and his team have developed a technique to noninvasively assess the wall stresses acting in individual AAAs based on the patient’s blood pressure and the virtual AAA, a three-dimensional reconstruction of CT scans taken prior to surgery. The scans provide cross-sectional images of the AAA which are then processed and refined by computer analyses into a virtual aneurysm.
In his research, Dr. Vorp also has tested the tensile strength of 150 tissue samples taken at the time of AAA surgery repair and found a 50 percent decrease in the strength of the AAA wall compared to healthy aorta tissue.
"Our next step is to develop a method to noninvasively determine the tensile strength of an individual aneurysm, rather than a patient population, perhaps based on clinical parameters such as the patient’s age, gender, body size, etc.," Dr. Vorp said. "Once this becomes a possibility, we may move this technology to the clinical setting. With the ability to estimate both wall stress and wall strength for an individual patient, we no longer would need to rely on a general rule of thumb to help guide the surgical decision for AAA patients. Instead, the decision of whether to surgically intervene may be customized for each particular patient based on his or her aneurysm."
For additional information on Dr. Vorp’s research, please access his website at http://www.surgery.upmc.edu/vorplab.