Although the use of Nitinol in the design of vascular devices has been extensive and well documented, the literature abounds with reports of fatigue related fractures.  The possible causes of these fractures may be attributable to a limited understanding of the vascular environment but some of these fractures may be also be due to the poor understanding of the fatigue properties of the material, specifically the effects of processing, design, and the cyclic environment on the device fatigue resistance.

 Method

The results of a study to characterize the design and material fatigue properties of an Endovascular stent graft are summarized.  In summary, a fatigue life diagram was developed based on the design and processing of the Talent Endovascular Stent Graft springs in order to calculate the design safety factor.  The fatigue life diagram of a wire stent spring along with wire tensile data was incorporated into a finite element analysis (FEA) and used to ensure the device strains exhibited appropriate safety factors.  Correlation of FEA and fatigue life diagram results are compared against bench top in vitro testing.

 Results

The endurance life diagram of the Nitinol wire specimens was similar to those reported by previous investigators.  At low mean strains (i.e. <2% mean strains), the endurance limit was identified at ~0.6% alternating strain.  Higher mean strains (~4% mean strain) resulted in the endurance limit of ~0.8% alternating strain.  The increase in the endurance limit, relative to previously published data, may be attributed to improved surface finish as compared to laser-cut models for which previous life-diagrams were developed.

 Conclusion

The fatigue endurance limits of the Talent Endovascular Stent Graft are discussed.  The incorporation of these results and other Nitinol material characterization data into the design of the Talent Stent Graft are discussed.