Corroded nitinol wires in explanted aortic endografts: An important mechanism of failure?

Purpose: To analyze surface alterations and fractures observed in the nitin ol stent wires of explanted endovascular grafts used for treatment of abdom inal aortic aneurysm. Methods: Twenty-one explanted Stentor devices and 1 Cragg stent were receiv ed from investigators in Germany and France. After macroscopy and photograp hy, the explants were cleaned and the polyester coating removed. The frame was examined completely by stereomicroscopy, and irregularities were assaye d by scanning electron microscopy and energy dispersive x-ray analysis (EDA X). The observed alterations were classified according to stereomicroscopic and electron microscopic morphology. Results: The mean implantation interval for the endografts was 29.1 +/- 13. 2 months (range 5-46). All examined explants, even those retrieved after on ly a few months in situ, showed pitlike surface damage 10-25 mum in diamete r. Larger, irregularly shaped surface alterations were observed in similar to 70% of the explants. Older explants (age >32 months) presented vast regi ons of decay, with bending of the wire and stress cracks in some areas. EDA X examination revealed decreased nickel concentration in the corroded regio ns. Conclusions: Corrosion of the nitinol wire in endovascular grafts is confir med. Presumably, the observed pitting and irregularly shaped corrosion defe cts are the precursors of material failure. They weaken the thin wire, whic h leads to stress cracks and eventually fracture of the stent wire under ci rculatory pulsation. Cell-induced electrochemical corrosion and active cell ular destruction of surfaces are well-known mechanisms that must be investi gated for their possible roles in the corrosion of stent metals.