As has been mentioned the ON-X valve has some design feature to reduce the effects of pannus growth, another issues (which few discuss) is the activation of platelets by pressures created by jet streams at opening and closing of the valves. This is Hemodynamics and is seldom discussed (even though it has significant influence). For instance:
However, for the regurgitant flow phase (Fig. 8(b)), the
SJM exhibits a stronger regurgitant jet, particularly in the hinges
region. Owing to its different open pivot hinge design for the ATS
valve, this jet is broken into two much weaker jets. Regurgitant
jets were previously implicated in mechanical heart valve hemolysis
and platelet activation.
Accordingly, fewer platelets are carried beyond the
activation threshold with the weaker leakage jets of the ATS
valve. This offers a viable explanation to the disparities observed
in the thrombogenic behavior of different types of bileaflet heart
valve designs. A similar explanation was offered when
comparing the hinge flow dynamics (measured with laser Doppler
velocimetry) of a CarboMedics bileaflet MHV to that of the SJM
Regent MHV used in our study and the Medtronic Parallel MHV.
...
Thus, a seemingly small difference may translate into a significant difference in the overall thrombogenic performance of these two valves. The difference between the two values reported here represents an almost
30% increase in the number of platelets reaching full activation during regurgitation in the SJM over the ATS valve.
...
it indicates that overall, the ATS valve may offer a lower thrombogenic potential owing to its different hinge mechanism design.