The Björk-Shiley aortic prosthesis: flow characteristics, thrombus formation and tissue overgrowth.

Thrombus formation and tissue overgrowth were observed in nine Björk-Shiley aortic prostheses recovered six months or longer after implantation. These pathologic findings may be attributed to the flow characteristics of the prosthesis. The open disc of the valve separates the flow into two unequal regions. Varying degrees of thrombus formation were observed in the minor outflow region, including the depression in the aortic face of the disc and the metal strut bridging this area. Tissue overgrowth was noted along the perimeter of the prosthesis adjacent to the minor outflow region. That overgrowth further reduced the available cross section for flow in this already constrained area. In vitro velocity measurements with a laser-Doppler anemometer identified a zone of stagnation about 20 mm wide near the aortic face of the disc. The average velocities in the major and minor outflow regions were around 100 and 25 cm/sec, respectively, and the corresponding peak-shear stresses were approximately 700 and 150 dynes/cm2. There is reason, then, to attribute the thrombus formation and tissue overgrowth to the stagnation zone and the low shear in the minor outflow region.