L. E. Eiselstein, D. L. Steffey, N. Corlett, A. Nissan, E. Kus, Exponent, Menlo Park, CA
ASTM F 2129 is a test method that is used to assess the corrosion susceptibility of small metallic implants using cyclic potentiodynamic polarization. The U.S. Food and Drug Administration (FDA) typically require medical device manufacturers to provide data that will demonstrate that their device will not corrode during the expected in-vivo service life. Frequently, medical device manufacturers use ASTM F 2129 to satisfy this FDA requirement. Neither the ASTM F 2129 test method, however, nor the FDA provides any guidance as to what constitutes an acceptance criterion. ASTM F 2129 suggests comparison with a device already approved and on the market in the U.S. This approach can, however, be quite expensive as it requires testing twice the number of experimental units (i.e., a number of candidate devices and predicate devices); moreover, it presumes the existence of a suitable predicate device. 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 is an attempt to establish such a methodology.
Summary: 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 is an attempt to establish such a methodology. We collected experimental data on the breakdown potential (Eb) of replicate surrogate devices and on the variation in rest potential (Er) over time for individual units. Using these data, we conducted simulation studies to investigate the implications for device performance—specifically, the probability that rest potential would exceed breakdown potential during the useful life of the device. The simulation results provided statistical support in defining a threshold value that would represent an acceptable margin between breakdown and rest potential (Eb−Er) and a correspondingly low probability of corrosion-induced failure during the lifetime of the device.
We then implemented a tolerance interval approach for determining, from standard testing of a candidate device, whether there is sufficient confidence that a high proportion of replicate units will attain the specified threshold value. Finally, we developed guidance for medical device manufacturers’ quality assurance program managers in addressing the key practical question of how to determine the number of replicate units required for testing to establish an acceptable lower bound on the value of Eb−Er for manufactured devices.
