Fatigue strength testing of hip stems with statistical analysis

Component fatigue testing, the final step in the development of total joint replacements, is performed to validate the safety of these components against fatigue failure before clinical use. Fatigue test prediction can aid the design of an efficient fatigue-testing program. The objective of this study was to perform an efficient and accurate statistical analysis of component fatigue test results, for the validation of future fatigue test predictions. Testing was performed with two aims: first, to determine the local component stress-force relationship using strain gauges; and second, to provide a statistical description of the fatigue test results. Forty-nine hip stems, in three sizes, were tested in a series of static and fatigue tests. Through effective planning and analysis, a statistical description of the component fatigue test results was determined including, 3-parameter Weibull distributions of life at two stress levels and log-Normal distributions of fatigue strength at various lives up to 5 million cycles.