Computational mechanics of Nitinol stent grafts

A finite element analysis of tubular, diamond-shaped stent grafts under representative cyclic loading conditions for abdominal aortic aneurysm (AAA) repair is presented. Commercial software was employed to study the mechanical behavior and fatigue performance of different materials found in commercially available stent-graft systems. Specifically, the effects of crimping, deployment, and cyclic pressure loading on stent-graft fatigue life, radial force, and wall compliances were simulated and analyzed for two types of realistic but different Nitinol materials (NITI-1 and NITI-2) and grafts (expanded polytetrafluoroethylene-ePTFE and polyethylene therephthalate-PET). The results show that NITI-1 stent has a better crimping performance than NITI-2. Under representative cyclic pressure loading, both NITI-1 and NITI-2 sealing stents are located in the safe zone of the fatigue-life diagram; however, the fatigue resistance of an NITI-1 stent is better than that of an NITI-2 stent. It was found that the two types of sealing stents do not damage a healthy neck artery. In the aneurysm section, the NITI-1&ePTFE, NITI-1&PET, and NITI-2&PET combinations were free of fatigue fracture when subjected to conditions of radial stress between 50 and 150mmHg. In contrast, the safety factor for the NITI-2&ePFTE combination was only 0.67, which is not acceptable for proper AAA stent-graft design. In summary, a Nitinol stent with PET graft may greatly improve fatigue life, while its compliance is much lower than the NITI-ePTFE combination.