Clots can be formed for several reasons. One possibility is due to Eddy Currents forming in the flow pattern.
On-X is the ONLY mechanical valve I am aware of where the leaflets open a full 90 degrees to prevent turbulence in the output flow pattern. They have a very clever solution to the 'how to close a valve that is open 90 degrees' challenge.
ATS, Carbomedics, and St. Jude Mechanical Valves open less than 90 degrees to catch the 'back-flow' at the end of the 'pumping cycle' to close the leaflets. The downside of this kind of design is Turbulence in the output flow.
EDIT - The ATS Valves only open to 75 degrees according to my contact 'in the biz'.
On-X has some supporting evidence of lower complication rates in non-compliant populations (in South Africa) where they reported lower complication rates than other mechanical valve manufacturers. One of the 'other' mechanical manufacturers tried a No Anticoagulation study in Europe many years ago. It was Terminated abruptly after several patients suffered Strokes. The FDA approved ON-X Low/NO anticoagulation study is still ongoing with (presumably) good results.
Al, I've been intrigued by your comments about how far the mech-valve leaflets open (e.g., from a low of 75 degrees to a high of 90 degrees), and the effect that has on turbulence in the output flow pattern -- which I believe should affect a number of outcomes, including destruction of blood cells, hemodynamics, and maybe also valve noise. So I've responded by doing what I do (when I have too much spare time), which is doing some online research on it!
It seems that the On-X is, indeed, designed to open to 90 degrees, and the ATS is designed to open to 85 degrees. But it seems that both of them may ACTUALLY open LESS than those "design opening angles"! And moreover, there is some evidence that the LESSER openings may actually have some ADVANTAGES(!), despite presenting (obviously) a slightly smaller effective opening area! E.g., it seems to be widely acknowledged (=~ often repeated) in many of the studies I found that the ATS valve is the QUIETEST of the leading mech-valves, and that low noise level may be associated with the smaller "throw" of the leaflets. (One study touting the ATS as the quietest is "Evaluation of valve sound and its effects on ATS prosthetic valves in patients' quality of life" by Sezai A, Shiono M, Orime Y, Hata H, Yagi S, Negishi N, Sezai Y., Ann Thorac Surg. 2000 Feb;69(2):507-12,
http://www.ncbi.nlm.nih.gov/pubmed/10735689 .)
And one study found that at least in the 29mm size, the ATS valve had the best hemodynamics (lowest pressure gradient), either DESPITE the low-angle opening, or BECAUSE of it:
"Influence of valve size on the hydrodynamic performance of the ATS valve" by Zhonggang Feng, Takao Nakamura, Tetsuo Fujimoto and Mitsuo Umezu, Journal of Artificial Organs, Volume 4, Number 4, 303-307, DOI: 10.1007/BF02480022 [2002],
www.springerlink.com/content/l0055q1347046r33/ :
Cineradiography has revealed the presence of the “non-fully-open” phenomenon in patients with the ATS valve. Preliminary in vitro investigations have identified two contributing factors: the expanding space at the outlet of the valve, and the local flow in the pivot area. This further study was performed with the aim of elucidating these factors with respect to different sizes of the ATS valve. Three bileafet valves, ATS, CarboMedics (CM), and St. Jude Medical (SJM), with tissue annulus diameters of 25 and 29 mm, were studied. The hydrodynamic performance of the valves was tested at the mitral position of our own pulse duplicator. The opening angle was measured using a high-speed video camera. All the CM and SJM valves were able to open fully in these tests, whereas
the 25-mm and 29-mm ATS valves opened to 75° and 82°, respectively, despite their design maximum of 85°.
The ATS exhibited the smallest pressure drop of the 29-mm valves, and the SJM the smallest of the 25-mm valves.
The incomplete opening of the ATS valve might be explained by the ability of its leaflets to align themselves with the divergent outlet flow, due to its unique open-pivot design. Such a feature would also exhibit a low pressure drop, as seen in the 29-mm valve. The smaller opening angle in the 25-mm valve, however, could be caused by the additional pivot-flow, which might cause greater deviation of the leaflets in the smaller valve, resulting in a higher relative pressure drop.
The same authors took another crack at this question with more valves, including the On-X, in
"In Vitro Investigation of Opening Behavior and Hydrodynamics of Bileaflet Valves in the Mitral Position", Zhonggang Feng1, Takao Nakamura1, Tetsuo Fujimoto2, Mitsuo Umezu3, Artificial Organs, Volume 26, Issue 1, pages 32–39, January 2002, onlinelibrary.wiley.com/doi/10.1046/j.1525-1594.2002.06833.x/abstract :
The performance of four 25 mm bileaflet valves of different designs was evaluated in the mitral position of our own pulse simulator. With the aid of a high-speed video camera, it was demonstrated that both the St. Jude Medical (SJM) valve (Hemodynamic Plus [HP] Series, St. Jude Medical, Inc., St. Paul, MN, U.S.A.) and the CM valve (CarboMedics, Inc., Austin, TX, U.S.A.) were able to open fully and that the CM valve fluttered much more vigorously at the fully open position than did the SJM HP valve. Conversely, neither the ATS valve (ATS Medical, Inc., Minneapolis, MN, U.S.A.) nor the On-X valve (Medical Carbon Research Institute, Austin, TX, U.S.A.) exhibited movement to a fully open configuration. The overall average opening angles of the ATS and the On-X, on 3, 4, and 5 L/min flow rate for a heart rate of 70 bpm and 5, 6, and 7 L/min for 100 bpm, were 74.8 degrees and 81.6 degrees, respectively, whereas their design opening angles were 85 degrees and 90 degrees. Pressure drops across the CM and the ATS were consistently higher than those of the On-X and the SJM HP. Closing volumes for all the valves were below 8% for a heart rate of 70 bpm. This in vitro investigation yielded the following conclusions: The ATS and On-X valves are not able to open fully in the mitral position, but this does not impair their normal function; both a larger orifice diameter and a large opening angle can decrease the pressure drop; in general, the On-X valve achieves its design goals in this experiment (i.e., it produces a lower pressure drop and lower closing volume by virtue of its large orifice and high-profile design); however, the hinge flow in the non-fully open state should be investigated further.
My recent research also found a number of sources that suggest (as I'd already concluded) that the reported differences we see in
hemodynamics are usually of no practical consequence except in unusually small valve sizes and especially in cases of "donor-prosthesis mismatch". But there are other good reasons to avoid turbulence, besides hemodynamics for its own sake. And at least the first reference above suggests that the angled opening of the ATS valves leaflets may create LESS turbulence than the On-X's "straighter" opening, because it aligns more perfectly with the blood flow, which is angling outwards. It's also possible that the answer is different for Aortic and Mitral valves (and both of those studies simulated MITRAL valves), because the blood-flow angles are different.
A good and ALMOST up-to-date summary of the comparison among most popular mechanical valves, most popular tissue valves, and the comparison between them, is "Are all bileaflet mechanical valves equal?" by Halkos, Michael E; Puskas, John D, Current Opinion in Cardiology: March 2009 - Volume 24 - Issue 2 - p 136-141, doi: 10.1097/HCO.0b013e328324e698, journals.lww.com/co-cardiology/Fulltext/2009/03000/Are_all_bileaflet_mechanical_valves_equal__.8.aspx#P85 . It mentions the ongoing On-X low-ACT studies over and over, always using hopeful "waffle-words" like "may" and "holds the promise" and such. . .
Finally, we've been arguing some about a 2006(?) Quebec study that concluded that the "cutoff" between mech and tissue valves should stay at 65 and not be moved younger.
www.ncbi.nlm.nih.gov/pubmed/20956488 is British study from 2010 arguing the opposite: "Microsimulation and clinical outcomes analysis support a lower age threshold for use of biological valves", by Stoica S, Goldsmith K, Demiris N, Punjabi P, Berg G, Sharples L, Large S. Heart. 2010 Nov;96(21):1730-6. According to their analysis, the cross-over (or "saw-off") "plateau", based on Life Expectancy, is between 56 and 69 for men, and between 58 and 63 for women.
I hope this helps SOMEBODY!!
