Stress Corrosion Cracking in Björk-Shiley Convexo-Concave Prosthetic Heart Valves Due to Random in vivo Electrochemical Pulsing

Abstract

Welded downstream struts of Björk-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® 25 alloy ring, which is electrochemically machined from solid with the upstream retaining struts. The weld area is de-alloyed, with residual porosity and carbide inclusions. The valve becomes a short-circuited electrochemical cell when fully open or closed. It is in an aggressive chloride electrolyte, whose high pulsed flow (2 m/s) ensures that supply of oxygen-rich cathode reactant is not mass-transport-limited. During the flight of the occluder, the cell is randomly at open circuit. A random current pulse is applied to the metal parts on circuit closure. Failure is not from simple mechanical fatigue, but from stress-corrosion-cracking and erosion of the less noble weld area caused by these pulses. All welded valves of this type may be susceptible to ultimate in vivo failure.