Examination of cavitation-induced surface erosion pitting of a mechanical heart valve using closing velocities

Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures that occur in implanted mechanical heart valves. Several factors, including peak dp/dt of the ventricular pressure, maximum closing velocity of the leaflet, and squeeze flow, have been studied as indices of the cavitation threshold. In the present study, cavitation erosion on the surface of a mechanical valve was examined by focusing on squeeze flow and the water-hammer phenomenon during the closing period of the valve. A simple solenoid-actuated test device that can directly control the valve closing velocity was developed, and opening-closing tests of 3000 and 40000 cycles were performed at various closing velocities. The results showed that there was a closing velocity threshold above which erosion pitting was induced and that the threshold was about 0.4m/s in the valves used in this study. Cavitation-induced erosion pits were observed only in regions where squeeze flow occurred immediately before valve closure. On the other hand, the number of the pits was found to be closely related to the area of the water hammer-induced pressure were below the critical pressure defined by water vapor pressure. Therefore, it was concluded that cavitation is initiated and augmented by the two pressure drops due to squeeze flow and the water-hammer phenomenon, respectively.[PUBLICATION ABSTRACT]