A Master of Science thesis in Biomedical Engineering by Mohammad Zubeir Allum Saib entitled, “Nitinol Stents: A Finite Element Analysis of their Interaction with the Wall of Blood Vessels”, submitted in December 2023. Thesis advisor is Dr. Farid Abed. Soft copy is available (Thesis, Completion Certificate, Approval Signatures, and AUS Archives Consent Form).
Abstract
The recent decades have seen huge improvements in stent implantation in arteries affected by atherosclerosis. The latter entails the partial blocking or complete occlusion of the lumen. Self-expandable stents are being commonly utilized alongside traditional stents to provide scaffolding to stenosed arteries. Nitinol alloys are being widely used in the medical industry to produce such stents. However, a significant limitation hampering their efficacy is restenosis, triggered by neointimal hyperplasia and resulting in the loss of gain in lumen size, post-intervention. In this study, a nonlinear finite element model was developed to simulate stent deployment and its interaction with the surrounding vessel. The main aim was to determine contact pressures, forces, and shear stresses induced in an artery wall with plaque. This was followed by a parametric study of Nitinol superelastic properties as well as artery & plaque composition and thickness. The results demonstrate the drawbacks of plaque calcification, which triggered a sharp contact pressure surge at the interface, potentially leading to rupture and restenosis. A regression line was established to relate hypocellular to calcified plaques. Regarding the directionality of wall properties, contact pressure observations were not significantly different between isotropic and anisotropic arteries. Furthermore, the model having a thinner plaque experienced a lower peak contact pressure and radial force at the contact interface. In terms of frictional shear stresses, the device with the lowest martensite modulus was determined to be mechanically safer. It exhibited the lowest interfacial shear stress at the contact area and was deemed less likely to induce vascular injury, neointimal hyperplasia, and in-stent restenosis.