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OBJECTIVES: Current size-based decisions for AAA repair can misrepresent individual risk of rupture. A specific patient’s rupture risk is related to tissue mechanics relative to wall stress created by AAA geometry and hemodynamics. Our hypothesis is that patient-specific AAA failure can be modeled in vitro using a novel molding technique using tissue mimicking material, Polyvinyl Alcohol Cryogel (PVA-C), and physiologic pulsatile hemodynamic stress.
METHODS: 3-dimensionial stereolithography (STL) CAD models are rendered from patient CT-angiograms using Mimics® image processing software (Figure 1A). Hoop stress is calculated in Matlab® from the model geometry during peak systolic loading with a heat map to show points of relative maximum stress (Figure 1B). The model is printed as solid plastic form, using a Dimension® 3D printer (Figure 1C). Separate silicon casts are created of both the inner and outer vessel wall lumens. A low temperature wax model is created from the inner lumen cast and assembled into the outer lumen cast to create the final mold. The elastance of the PVA-C material is programmed through a sequence of cyclic freeze-thaw cycles. The model is mounted onto an In vitro hemodynamic simulator and tested with physiologic pressure and flow until failure (Figure 1D). A combination of high-speed video (500 frames/s), 2D ultrasound, and UV-fluorescence intensity imaging are used to determine the exact location of failure and wall thickness and aneurysmal radii at that time.
RESULTS: PVA-C models dimensions of radii and wall thickness accurately mimicked patient geometries as determined by 2D ultrasound with +/- 10% tolerance. UV-fluorescence intensity imaging corresponded to relative vessel wall thickness when compared with STL CAD model wall thickness. Hoop stress heat maps accurately predicted the location of PVA-C model failure, during pulsatile physiologic stress, as determined by high-speed video analysis.
CONCLUSIONS: We have created a novel technique for the creation of a low cost in vitro model of patient specific aneurysmal geometries. This work will form the basis for testing new modalities of rupture risk assessment; including the use of transcutaneous ultrasound stain analysis for the determination of patient specific rupture risk to determine the need for operative intervention.