Reliability of Structural Components In Heart Valves Subjected to Very High Cycle Fatigue

Characterization and reliability prediction of materials subjected to very high cycle fatigue is an increasingly critical aspect of life cycle design and management for structural components. This problem is exacerbated for components in medical devices that are implanted into humans because of the high safety expectation. The primary structural component of a commercially available heart valve consists of highly-resilient, cold-worked Elgiloy® wire, which is a cobalt-chromium based alloy. In order to be assured of sufficiently high reliability for the component, an extensive high cycle fatigue testing program was conducted. The accelerated test conditions simulated extreme hypertension conditions. For human safety, the structural component is expected to have a fatigue life in excess of 600 million cycles under all possible physiological conditions for all potential patients. Using basic principles for fatigue life evaluation, a constant lifeline for mean stress versus stress amplitude has been statistically established. The fundamental model for the analysis is a generalized Weibull distribution for which the parameters are stress dependent. The model has been confirmed by maximum likelihood methods and chi-squared goodness-of-fit. The methodology used for this investigation, well established in fatigue engineering, is shown to be equally effective when applied to life science problems and results in great benefit to patients. This procedure can be extended to fatigue reliability analysis for other medical device components where very high fatigue and high confidence reliability is required. Salient features of this analysis methodology will be presented.