Comparison of the fatigue characteristics of centrifuged and uncentrifuged Simplex P bone cement

Fatigue test specimens of Simplex P bone cement (Howmedica Inc., Rutherford, NJ) prepared according to the manufacturer's instructions and specimens of Simplex P prepared by centrifuging the cement immediately after mixing were subjected to fully reversed tension-compression fatigue tests at initial strain levels ranging from 0.01 to 0.001. S:N curves for both cement preparations were generated and compared on the basis of regression analysis, Weibull analysis, and Student's t tests. Centrifuged Simplex P was able to withstand significantly more fatigue cycles than uncentrifuged Simplex P at all strain levels tested. Importantly, at the more physiologic strain levels of 2,000 and 1,000 microstrain, the centrifuged cement demonstrated superior fatigue life. Specifically, at the 1,000-microstrain level eight of the 11 uncentrifuged specimens fractured before undergoing 10 million cycles. Of those that failed, the average number of cycles to failure was 1.8 million with a range of 560,000 to 4 million cycles. In contrast, all 11 centrifuged specimens tested at 1,000 microstrain remained intact at 10 million cycles.