Automated Simulations Reveal Blood Flow Effects of Aortic Grafts

When a surgeon replaces a section of the body’s largest artery with a tube of woven polyester, the operation is meant to be a definitive fix. For tens of thousands of people each year diagnosed with an enlarged ascending aorta, open surgical repair with a synthetic Dacron graft remains the gold standard, a proven way to prevent the aortic wall from tearing catastrophically. Yet a growing body of clinical evidence suggests the surgery may leave a hidden signature far downstream of the stitches: blood flowing through the remaining native aorta behaves differently after grafting, and the changes may help explain why some patients later develop dissection in the descending aorta, the very segment the operation never touched. A new study from a multidisciplinary team of bioengineers and cardiac surgeons, published in the Annals of Biomedical Engineering, has now built a fully automated computational engine capable of simulating blood flow and aortic wall mechanics in individual patients before and after surgery — and its first results point a striking finger at a narrow segment of the aorta known as the isthmus.

The clinical backdrop is sobering. Ascending thoracic aortic aneurysms, marked by abnormal enlargement of the aortic lumen, affect an estimated 5.3 per 100,000 people annually and arise from progressive degeneration that weakens the arterial wall. Surgical guidelines recommend replacing the ascending aorta once its diameter exceeds 5.5 centimeters in men and 5 centimeters in women, because enlargement raises the risk of rupture or dissection. The replacement graft, typically made of Dacron, is prized for its biocompatibility and durability, but it is far stiffer and less compliant than living aortic tissue. Surgeons have long reported post-operative complications, including type-B dissection of the descending aorta, distal to the implanted graft, with the aortic isthmus — the narrow segment just beyond the origin of the left subclavian artery, where the arch meets the descending aorta — recognized as the most vulnerable site. What has remained unclear is whether the increased risk stems from altered hemodynamic loading caused by the stiff proximal graft, or simply from the fact that extending a patient’s lifespan gives more time for other regions to deteriorate.

The new study does not answer that question definitively, but it equips the field with the tool needed to try. The research team, led by Ione Ianniruberto, Davide Astori, Emiliano Votta, and Alberto Redaelli at Politecnico di Milano together with collaborators at Weill Cornell Medicine, Yale University, and elsewhere, developed a fully automated patient-specific fluid–structure interaction (FSI) pipeline. FSI simulation is the computational gold standard for capturing how flowing blood deforms a vessel wall and how, in turn, the moving wall reshapes the flow. Earlier attempts by the same group and others relied on simplified surrogate approaches that treated the aortic wall as a thin linear elastic membrane, or on fully coupled simulations that were so labor-intensive they could only be applied to a single patient. The new framework, implemented in the open-source software SimVascular, automates nearly every step, transforming what once took weeks of manual model building into a standardized, repeatable workflow.

The machinery behind the pipeline is a carefully orchestrated marriage of clinical imaging and computational mechanics. High-resolution magnetic resonance angiography provides the three-dimensional anatomy of each patient’s thoracic aorta, which is automatically segmented using an artificial-intelligence-based tool called TotalSegmentator and converted into a luminal surface model. Cine-MRI, a functional imaging modality that captures the aorta pulsing through the cardiac cycle in forty frames, supplies regional measurements of wall thickness and systole-to-diastole area change — data that encode how much the vessel expands with each heartbeat. Four-dimensional flow MRI, which measures the full velocity vector of blood throughout the aortic volume at every point in the cardiac cycle, provides the inlet velocity profiles that drive the simulations. The algorithm even corrects the raw 4D flow data for eddy-current-induced errors and aliasing artifacts, and coregisters it with the anatomy using a computed phase-contrast angiography image, ensuring that velocities land in the right place in the reconstructed geometry.

Once the imaging data are harvested, the pipeline assigns mechanical properties to the aortic wall through an approach the authors describe as “patient-informed.” Because living tissue cannot simply be pulled apart to measure its stiffness before surgery, the team built a database of more than eighty candidate stress–strain curves drawn from published equi-biaxial tensile tests of human aortic tissue, classified by patient age group and underlying etiology — whether the aneurysm is degenerative or heritable, as in Marfan and Loeys–Dietz syndromes. For each patient, the algorithm selects the literature curve that best reproduces the area change actually measured from that patient’s Cine-MRI, using the Laplace equation to estimate circumferential wall stress from pressure, radius, and thickness. The selected curves are then fitted to the incompressible, hyperelastic, anisotropic Holzapfel–Gasser–Ogden constitutive model, which mathematically describes how two families of collagen fibers embedded in the aortic wall resist stretching — a crucial refinement over earlier isotropic models that ignored the tissue’s directional architecture.

Blood itself is modeled as an incompressible Newtonian fluid with a density of 1060 kilograms per cubic meter and a viscosity of 4 centipoise, a standard and well-justified simplification for the high-shear environment of large arteries. At the aortic inlet, the measured velocity field is applied directly, with velocity forced to zero along the wall edges to satisfy the no-slip condition. At the outlets — the three supra-aortic branches and the descending aorta — the pipeline attaches lumped-parameter Windkessel models, electrical-circuit-like representations of the downstream vasculature whose resistances and compliances are automatically tuned by an optimization algorithm until the simulated systolic and diastolic pressures match the patient’s measured brachial blood pressure and the simulated descending-aortic flow matches the 4D flow MRI waveform. The surrounding tissue’s mechanical support is captured through a Robin boundary condition with an external stiffness of 10,000 pascals per millimeter.

The full simulation sequence unfolds in three stages. First, a computational fluid dynamics simulation on a rigid-wall model computes the pressure field acting on the luminal surface at mean arterial pressure. Second, a structural analysis applies that pressure to the aortic wall to estimate its prestress state — the internal tension the vessel carries even at rest, which must be known to initialize realistic FSI simulations. Third, the fully coupled FSI simulation brings fluid and solid together, initialized with the computed pressure and prestress, and runs two complete cardiac cycles with a time step of just 0.1 milliseconds. The team applied this workflow to five patients with ascending aortic aneurysms treated at New York-Presbyterian Hospital between October 2023 and May 2025, each imaged within a month before surgery and again within six months after, yielding ten patient-specific simulations that systematically compared the diseased pre-operative state against the grafted post-operative one.

The results, while preliminary in a cohort of five, are remarkably consistent — and they land precisely where clinicians worry most. After graft implantation, the simulations showed increased flow-induced wall shear stress, the frictional force that blood exerts on the endothelial lining, along with increases in the time-averaged wall shear stress (TAWSS) and in the oscillatory shear index (OSI), a measure of how chaotically the direction of wall friction reverses over the cardiac cycle. All three indices rose consistently in the aortic isthmus, the region most commonly associated with post-operative dilation. Because endothelial cells are exquisitely sensitive sensors of shear stress, sustained elevation and oscillation of these forces are known triggers of adverse biological responses, including inflammatory signaling and structural degeneration of the vessel wall. In effect, the simulations suggest that stiffening the proximal aorta may redirect hemodynamic stress onto the one segment least equipped to handle it.

The study went further than scalar indices, adding a topological analysis of the wall shear stress field that tracked so-called fixed points — locations where the surface shear pattern has critical points, akin to stagnation zones in the flow landscape. Before surgery, these fixed points clustered in the ascending aorta; after grafting, they redistributed toward the isthmus region, a migration the authors interpret as a signature of localized hemodynamic disturbance that may correlate with adverse remodeling. The statistical framework was deliberately conservative: field variables were sampled in percentile distributions across a standardized isthmus region of interest, verified for non-normality with a Shapiro–Wilk test, and compared with paired non-parametric Wilcoxon signed-rank tests, with a ninety-ninth percentile threshold used to quantify near-maximum values while ruling out numerical artifacts.

What makes the work resonate beyond the aortic community is its automation. Previous state-of-the-art FSI studies validated their frameworks on single patients precisely because model generation was too manual to scale. By contrast, this pipeline — from DICOM images to simulation-ready input files — runs end-to-end through a single Python script, opening the door to longitudinal studies that could follow large cohorts of grafted patients and correlate early hemodynamic changes with later aortic remodeling. The authors are candid that five patients constitute a preliminary cohort, and the framework still depends on literature-derived stress–strain curves rather than direct tissue measurements, but the infrastructure now exists to test the compliance-mismatch hypothesis at scale. If larger studies confirm that rigid grafts systematically elevate shear stress in the isthmus, the implication for device engineering is direct: next-generation aortic grafts should be designed with controlled compliance to soften the hemodynamic impact downstream — turning a simulation pipeline into a blueprint for the operating room.