Effect of cavitation on pyrolytic carbon in vitro.

It has long been known that mechanical heart valves, when tested for durability using non-physiologic conditions common in accelerated testers, would cavitate. Until recently, cavitation was never observed in vivo. The discovery that a small number of Edwards-Duromedics heart valve explants indicated signs of cavitation erosion prompted a reassessment of the cavitation erosion potential of pyrolytic carbon (PyC). Analyses of the explanted valves indicated that cavitation may be accentuated by porous regions in the pyrolytic carbon coating from which most mechanical heart valves are constructed. Early studies have shown that for PyC (a) the resistance to cavitation erosion is comparable to that of aluminum, (b) the resistance to cavitation erosion is high initially, but with time the erosion rate accelerates, and (c) the cavitation erosion resistance is somewhat variable. In this study, similar experiments were performed utilizing polished pyrolytic carbon as well as microporous surfaces since microporous surfaces have been implicated as accelerating erosion. Within the accuracy of the measurement, we found no contributing acceleration due to the microporous nature of the pyrolytic carbon surfaces tested when compared to the polished surfaces. Examination of cross sections of samples exposed to cavitation conditions revealed the presence of extensive microcracking even without the presence of substantial surface erosion.