STRESS-CORROSION CRACKING IN BJORK-SHILEY CONVEXO-CONCAVE PROSTHETIC HEART-VALVES DUE TO RANDOM IN-VIVO ELECTROCHEMICAL PULSING

Welded downstream struts of Bjork-Shiley Convexo-Concave heart valves show failure in vivo, but not in in vitro testing. A pyrolytic carbon pivoting disk occluder closes against a Haynes(R) 25 alloy ring, which is electrochemically machined from solid with the upstream retaining struts. The weld area is de-alloyed, with residual porosity and carbid e inclusions. The valve becomes a short-circuited electrochemical cell when fully open or closed. It is in an aggressive chloride electrolyt e, whose high pulsed flow (2 m/s) ensures that supply of oxygen-rich c athode reactant is not mass-transport-limited. During the flight of th e occluder, the cell is randomly at open circuit. A random current pul se is applied to the metal parts on circuit closure. Failure is not fr om simple mechanical fatigue, but from stress-corrosion-cracking and e rosion of the loss noble weld area caused by these pulses. All welded valves of this type may be susceptible to ultimate in vivo failure.