Background and aims of the study: 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.
Methods 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.
Results 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.
Conclusion 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.
