The biocompatibility and unique mechanical behaviors of Nitinol such as shape memory and superelasticity have revolutionized the medicine.  In past decades, guidewires, stents, filters and many minimumly invasive devices and implants are made of this material and they proved to be very successful.

However, the fatigue behavior of the alloy has not been well understood.  As a consequences, many stent fractures have been observed in-vivo.  Below is a list of misconcepts that may contribute to the widely observed in-vivo fractures on Nitinol stents:

1.  The safety margin of stent is low.  For historical reasons, calculation of the safety factor was not properly established at the time the device was designed.

2.  Processing change when going from prototype devices to massive manufacturing.  Alloy is highly thermal-mechanical coupled.  Both heattreatment and surface finishing can change the fatigue resistance of the alloy.

Above all the possible causes on designs and material processes, there is a lack of understanding of the anatomy, let alone the interactions of the devices and the anatomy.  Many stents were design only to survive under a straight pulsatile fatigue environment, however, the in-vivo deformation, especially in the peripheral arteries, is multiaxial.  Furthermore, our knowledge on the mechanical properties of the artery, the motions of the artery, and the interaction of the artery and the stent are extremely limited.

To short, design engineers in stent industry are in desparate need for knowledge of material fatigue, loading conditions and how the stent is interacting with the anatomy to modernize the stent design.

These motivate the formation of this forum so that multiple topics and opinions can be seen here from basic fundamental material knowledge to complex human anatomy motion and the interaction of the device and the body.

As Dr. Michael Mitchell contantly reminded me that "To run, you must walk first".  I will start this forum by posting an old fatigue article, please be crucial to criticize and beging to post your opinions.