The Starr-Edwards Aortic Ball Valve: Flow Characteristics, Thrombus Formation and Tissue Overgrowth
The Starr-Edwards ball valve has been one of the more commonly used aortic valve prostheses. In the study reported here, in vitro velocity, shear-stress and pressure drop measurements were made under steady-flow conditions and used to interpret some of the failure modes of this prosthesis as observed at autopsy. The findings show that some failure modes can be explained by the nature of the values for velocities and shears in the near vicinity of the valve.
Our results indicate that the Starr-Edwards ball valve has major fluid dynamic drawbacks such as: (a) relatively large pressure drop (17.3 to 31.0 mm Hg at a flow rate of 417 cm3 /sec), (b) hydrodynamically unstable poppet, (c) regions of flow separation at the base of each of the three struts, (d) region of flow stagnation at the apex of the cage (~7 to 15 mm in diameter) (e) large wall-shear stresses (~500–2000 dynes/cm2) and bulk turbulent shear stresses (on the order of 100–5000 dynes/cm2) in the immediate downstream vicinity of the valve, and (f) large shear stresses adjacent to the poppet surface and struts (on the order of 102–103 dynes/cm2).
The observed stagnation zone could encourage thrombus formation on the apex of the cage, while the observed regions of flow separation could lead to thrombus formation and tissue overgrowth at the base and upwards along the struts. The observed wall shear could lead to damage of endothelial tissue in the proximal ascending aorta, to hemolysis, and to thrombus formation. In addition, the elevated shears adjacent to the struts and the surface of the poppet could lead to increased hemolysis with those Starr-Edwards ball valves having cloth covered struts.
Examinations have been made at the USC-LA County Medical Center of 13 Starr-Edwards aortic ball valves recovered during autopsy. Thrombus formation and tissue overgrowth were observed at various locations on the recovered valves. The locations of thrombus formation and tissue overgrowth correlate well with those predicted by the in vitro fluid dynamic data. In addition, endothelial damage and tissue proliferation in the proximal ascending aorta were observed in some cases. Similar pathologic findings have also been observed by other investigators.
KeywordsStarch Glycerine Corn Cage Rubber
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