Fatigue of isotropic pyrolytic carbon used in mechanical heart valves.

This study shows that the strength of silicon-alloyed pyrolytic carbon is not affected by long-life cyclic stress up to 6 x 10(8) cycles at stress levels which are two, four and eight times higher than the estimated peak service stress of a mechanical heart valve. It is demonstrated that there is clearly a threshold size for cyclic fatigue crack propagation below which the cracks do not grow at all when cycled up to 6 x 10(8) cycles at strain levels that are two and four times higher than the estimated peak service stress of the heart valve. The large sample size of the fatigue test gives a high statistical confidence in the intrinsic fatigue behavior of pyrolytic carbon, which has been demonstrated in over three million implants of the material in actual devices with an accumulated experience of over ten million patient years. Nevertheless, mechanical heart valve designers are cautioned to consider the effects of extrinsically introduced flaws.