Fatigue and fracture of pyrolytic carbon: a damage- tolerant approach to structural integrity and life prediction in "ceramic" heart valve prostheses.

The fracture and fatigue properties of Si-alloyed LTI pyrolytic carbon and pyrolytic carbon-coated graphite are described as a framework for establishing damage-tolerant analyses for maintaining structural integrity and for predicting the lifetimes of mechanical heart valve prostheses fabricated from these materials.The analyses are based on fracture-mechanics concepts and provide conservative (worst-case) estimates of the time, or number of loading cycles, before the valve will fail, or more precisely for pre-existing defects in valve components to grow subcritically to critical size under elevated physiologic loading and environmental conditions.For structural life in excess of patient life-times, a minimum required detectable defect size is computed which must be detected by quality-control procedures prior to the device entering service; this defect size is typically of the order of tens of microns for such "ceramic" valves, compared to sizes in the hundreds of microns for corresponding metal valves.It is concluded that in light of the brittle nature of pyrolytic carbon and the unacceptable cost of mechanical valve failures, the use of such analyses should be regarded as essential in order to provide maximum assurance of patient safety.