Acceptance Criteria for the Corrosion Resistance of Medical Devices: A Statistical Study of the Pitting Susceptibility of Nitinol accounting for the In-Vivo Environment

ASTM F 2129 test method, nor the FDA provides any guidance as to what constitutes an acceptance criterion.  Neither provides any guidance on how many samples to test or how to handle censored data, i.e. data sets where there are only a few tests that breakdown.  The development of both a statistically valid acceptance criterion for corrosion resistance and a method of evaluation would be of significant benefit to the medical device community. 

 

This study, which builds on previous research that was presented at SMST 2007, investigates the effect of long-term exposure to simulated in-vivo environments with differing degrees of aeration.  This was accomplished by pre-exposing electropolished nitinol to phosphate buffered saline (PBS) at 37°C that had been sparged with either UHP nitrogen, lab air, or simulated blood gas.  Immersion times ranged from 1 hour up to 30 days.  A large number of samples were tested at each exposure time in order to obtain a reasonable number of samples with breakdown.  This data allows us to test our statistical model that was presented at SMST 2007.  This model takes into account the probability of breakdown per unit of exposed surface area and, if breakdown occurs, predicts the probability that E_b-E_r is greater than some threshold value.