Background and aim of the study: Most bileaflet mechanical heart valves (BMHVs) incorporate some retrograde flow through their hinge mechanism to prevent flow stasis and inhibit microthrombus formation. This reverse flow is characterized by high velocities and shear stresses, thereby promoting platelet activation and hemolysis inside the hinge region. In the present study, the thromboembolic potential of three 27-mm BMHVs with varying hinge gap widths was assessed via in-vitro characterization of the hinge microflow structures.
Methods: Three 27-mm BMHV prototypes with different hinge gap widths (50, 100, and 200 mm) were provided by St. Jude Medical Inc. The valves were mounted in the mitral position of a left heart flow simulator, and two-dimensional laser Doppler velocimetry was used to measure the hinge velocity fields.
Results: All three valve prototypes revealed Reynolds

shear stress (RSS) levels above 2,000 dynes/cm2, which exceeded the threshold for platelet activation and hemolysis. The hinge flow fields were characterized by leakage jets during systole, and a strong vortical flow during diastole. The leakage jet size and corresponding RSS levels were found to increase with the hinge gap width. All three gap widths had RSS >4,000 dynes/cm2 (range: 5,640 to 13,315 dynes/cm2). The hinge with the smallest gap width registered the highest jet velocity magnitude (2.08 m/s) during systole.
Conclusion: The study results showed that the hinge gap width influences washout and RSS levels inside the hinge recess. The 100-mm hinge gap width provided optimum fluid dynamic performance. In contrast, the two valves with large and small hinge gap widths may have higher thromboembolic potential.

The Journal of Heart Valve Disease 2006;15:800-808