A vascular surgeon, Patrick Kelly, MD, of Sanford Health in Sioux Falls, SD, had designed a stent graft and knew that his patients with thoracoabdominal aneurysms were doing better with it, but he wanted a better understanding of the mechanics before testing the device more widely in a clinical trial. (See Figure 1) So, he contacted Associate Professor Stephen Gent in the Mechanical Engineering Department at South Dakota State University for assistance. Gent had done computational fluid dynamics modeling for more than 10 years, but said this was his first experience simulating blood flow.
Setting Up the Model
Gent noted that blood is a pulsed flow. He and a graduate student used a commercially available computational fluid dynamics solver, Star CCM+®, and wrote additional code to model blood flow through five stents. Three were commercially available, while two, including Kelly’s, were new designs.
First, the engineers had to learn medical terminology and then link the engineering results with what mattered to the medical professionals. Gent said it was helpful that Kelly was a structural engineer before becoming a surgeon, so there was a common overlap of language.
To compare the devices, the engineers had to create a geometrically correct model of each graft relative to the same aorta coordinates and positioning in the body trunk and the arteries that feed the organs and extend into the legs. This took quite a while to modify the CAD files to make the comparison as fair as possible.
Simulating Blood Flow
Simplified blood flow resembles a liquid moving through pipes from a fluids analysis perspective but, said graduate student Taylor Suess, who worked on the project with Gent, “the boundary conditions are intricate and the parameters aren’t well-known.”
To capture what was happening to blood flow near the artery walls where atherosclerosis tends to begin, the researchers had to write their own code. They had to consider oscillating shear index, time-averaged wall shear stress, relative residence time, and wall shear stress temporal gradient.
“The longer the blood stays at one site, the greater the chance white blood cells will build up and cause thickening of the artery walls,” Suess said. That narrowing then increases the shear stress on the vessel wall.
Sanford Health biomedical engineer Tyler Remund, who is part of Kelly’s product development team, explained the arteries that feed the kidneys are very prone to clotting. “The challenge that has faced the industry is keeping the renal vessels open,” he added, noting that the short length and small diameter of renal arteries leave them prone to narrowing once stented.
The fluid flow modeling “helped validate that the configuration is delivering more well developed blood flow with the design,” according to Kelly.
“The simulation shows flow behavior next to the artery wall is more ordered, predictable, and moderate with the design. It splits the blood flow upstream and lets it gradually come to the renals,” Kelly said. Gent and Suess confirmed this when they ran the computer simulations.
Only a small number of patients—two per 100,000 a year—are diagnosed with thoracoabdominal aneurysms, according to Kelly. However, most of these patients would not survive traditional surgery, which involves an incision from the chest to the groin. This less invasive approach could give more patients an option, Kelly explained.
The Sanford Health team began a U.S. Food and Drug Administration clinical trial on the device in March with the support of data from the computational fluid dynamics simulation and the patients Kelly has treated. In April, Sanford Health signed a licensing deal with Medtronic that will bring this life-saving device closer to commercialization. The research was supported by a grant from Sanford Frontiers.
For more information visit www.sdstate.edu .