Nitinol self-expanding
stents are effective in treating
peripheral artery disease, including the superficial femoral, carotid, and renal arteries. However, fracture occurrences of up to 50% have been reported in some
stents after one year. These
stent fractures are likely due to in vivo cyclic displacements. As such, the cyclic
fatigue and durability properties of
Nitinol-based endovascular
stents are discussed in terms of an engineering-based experimental testing program. In this paper, the combined effects of cardiac pulsatile
fatigue and
stent-vessel oversizing are evaluated for application to both
stents and
stent subcomponents. In particular, displacement-controlled
fatigue tests were performed on
stent-like specimens processed from
Nitinol microtubing.
Fatigue data were collected with combinations of simulated oversizing conditions and pulsatile cycles that were identified by computer modeling of the
stent that mimic in vivo deformation conditions. These data are analyzed with non-linear finite
element computations and are illustrated with strain-life and strain-based constant-life diagrams. The utility of this approach is demonstrated in conjunction with 10 million cycle pulsatile
fatigue tests of Cordis SMART Control((R))
Nitinol self-expanding
stents to calculate
fatigue safety factors and thereby predict in vivo
fatigue resistance. These results demonstrate the non-linear constant
fatigue-life response of
Nitinol stents, whereby, contrary to conventional engineering materials, the
fatigue life of
Nitinol is observed to increase with increasing mean strain.