Actually, there is a difference in materials between the On-X and the St. Jude, as well as in hemodynamics. I'm surprised the surgeon brushed those issue off so readily.
Yes, both - in fact all of the best-known mechanical valves - are now pyrolytic carbon (thus the term "carbon valve"). However, the processes for creating that material are different. The pyrolytic carbon used in the manufacture of On-X valves is the only one at this time that is free of silicon, which is a part of the manufacturing process for other carbons. This makes its surface more slick than standard carbon materials and less permeable than the carbons used for other valves. It's even less brittle, although the value of that is moot, because all carbons used in valves are capable of outlasting their human hosts several times over.
The hemodynamic differences include shaping that creates flow points where momentary stagnations were previously possible, particularly around pivots, which reduce the blood's ability to clot. Along with the slicker surface and reduced permeability, it also interferes with the spontaneous growth of tissue on the valve.
The valve leaflets are also desiged to fall closed more gently, which causes less hemolysis (damage to blood cells - mostly red cells), reducing the chance and amount of anemia from that cause. This also means that fewer byproducts of broken blood cells are floating through the body. The body is set up to react to a significant presence of damaged blood cell components, and they can raise inflammation chemical levels and even cause enlargement of the spleen. Their very presence can form the base for blood clots or add to arterial plaque.
It may also mean less noise from the valve, but that's not important to function. I believe the amount of noise is dependent on body mass, body density, volume of the area inside the rib cage and above the diaphragm, conductive ("bone") hearing of the patient, volume of lungs, size of the heart, position of the heart, heart rate, and how hard the heart beats (pump stroke impact), and probably a few other things as well...
The current warfarin INR recommendations are really based mostly on years of experience with the St. Jude valve. It is by far the most common mechanical valve and has thus been the dominant valve in all carbon valve studies done over the last decade and a half. If the St. Jude could tolerate a lower INR profile, it would likely have surfaced in that time.
The St. Jude is a fine valve that has probably saved more lives than any other single valve of any type from any manufacturer. But the St. Jude and the On-X are not the same valve, and in no way would I agree that the results of the study under discussion would be interchangeable between them.
Best wishes,
