Design optimization of thoracic endovascular stent graft (EVSG)

Abstract

AIM: EVSG, a tube composed of fabric supported by stent, is inserted percutaneously using a catheter to treat thoracic aortic aneurysm (TAA).The study seeks to improve the mechanical performance of EVSG by design optimization by Finite Element Analysis (FEA). METHODS: A 36 mm tubular device composed of ePTFE (Elastic modulus = 45Mpa, and Poisson’s ratio = 0.49) and supported by nitinol rings (Elastic modulus = 27Gpa, and Poisson’s ratio = 0.3, maximum superelastic strain = 0.08) was modelled. The parameters used to define the EVSG geometry included the number of struts, height, thickness and spacing of the nitinol rings, and the thickness of the ePTFE. The analysis was performed using ANSYS 12.0. The determination of radial force was done by compressing the stent radially to 9 mm and measuring the hoop forces at different diameters during re-expansion. The load that initiates longitudinal buckling and crippling was determined through Eigen Buckling Analysis. The durbaility of the EVSG was assed by simulating 400 million cycles of minimum (50 mmHG) and the maximum (150 mmHG) blood pressure loading. RESULTS: Based on the analysis, a ring with thickness of 0.35mm, 8 struts and 7.5mm height provided for an adequate radial force of 0.9N and 1.75N when deployed in a 34mm and a 32mm artery respectively. A graft thickness, when compared to the spacing of the rings, was major determinent of the buckling load. A 0.25mm ePTFE with 10 mm spacing between the rings provided for adequate buckling strength and could withstand the blood pressure load. The EVSG of ePTFE graft supported by nitinol rings demonstrated adequate durability at 400 million cycles when deployed in a 32mm artery. CONCLUSION: An EVSG with optimal properties could be designed and validated using FEA. Further in-vitro testing and animal trials would validate this analytical study.