Effects of Fatigue Testing on Nickel Release in Nitinol Stents

Medical implants such as cardiovascular stents can corrode over time resulting in a release of metal ions, which in severe cases may lead to patient complications such as nickel toxicity/sensitization. The FDA’s guidance document “Non-Clinical Engineering Tests and Recommended Labeling for Intravascular Stents and Associated Delivery Systems” recommends tests to assess corrosion resistance, including nickel leach testing. This testing is typically performed under static loading conditions; however, stents are subjected to repetitive mechanical forces in-vivo, which may crack the protective oxide layer and increase metal ion release into surrounding tissues. Therefore, the objective of this study was to understand whether fatigue testing accelerates uniform corrosion in-vitro.

Nitinol stents were assigned to the following groups (n=5/group): (1) static pressure (120 mmHg) in a straight tube (2) static pressure (120 mmHg) on a static bend (15mm radius of curvature - ROC) (3) dynamic pressure (80-160 mmHg) on a static bend (15mm ROC) and (4) dynamic bending (15-30mm ROC) with static pressure (120 mmHg). Immersion fluid was collected at day 1, 2, 3, 4, 8, 16, and 32 and analyzed with mass spectrometry.  At day 8, stents under dynamic bending (Group 4) had significantly higher nickel release (p<0.01) compared to all other groups. Mean nickel release was an order of magnitude higher for dynamic pressure (Group 3) compared to static pressure groups (Groups 1 and 2), but not significant (p>0.17). These findings demonstrate that nickel release increases during fatigue testing. Further testing is ongoing to understand longer term consequences of dynamic testing.