Influence of Active Af on the Fatigue Performance of Peripheral Stents Subjected to Physiological Loading Conditions

The temperature difference between Active A_f and the operating temperature has been reported in literature to have a negative effect on the fatigue lifetime of a pseudoelastic device. Clinical studies showed a correlation between strut fractures and in-stent restenosis, pointing out the practical significance of this study. The effect of reduced fatigue life described in literature is not considering the manufacturing process, device design and geometry of state-of-the-art stents, and physiological loading conditions. It is unknown, which temperature difference between Active A_f and 37°C is acceptable, before a statistically significant effect on the number of strut fractures and fatigue life of Nitinol stents occurs. In this study, generic peripheral stents were manufactured and measured using the bend-and-free-recovery method based on ASTM F2082/F2082M. The stents were preconditioned and deployed in mock vessels mounted in an axial fatigue setup. The samples were subjected to an axial compression of 3 % simulating walking for 10 million load cycles + 6% simulating stair climbing for 650,000 load cycles. Following fatigue testing, the stents were inspected using optical microscopy to document the location and number of strut fractures. The variations in the heat treatments resulted in three Active Af temperatures of AAf = 20±2°C, BAf = 15±2°C, and CAf = 10±2°C, respectively. This study contributes to a better understanding of the importance of the temperature difference between Active Af and 37°C for Nitinol stents regarding fatigue life. It provides guidance on the effect of Active Af on the safety of medical devices.