Ann Thorac Surg 2005;80:788
© 2005 The Society of Thoracic Surgeons
Correspondence
Mechanical Bioprosthesis
Didier M. Lapeyre, MD
Triflo Medical, Inc, 30 Rue Val du Puits, Chaignes, 27/20 France
(Email: didier-lapeyre{at}wanadoo.fr).
To the Editor:
As inventor of the concept of the mechanical bioprosthesis developed by Triflo Medical, Inc (Irvine, CA) [1], I have some comments with reference to the article by Gregoric and coauthors [2]. The rationale for the concept of mechanical bioprosthesis is to eliminate the need of long-term anticoagulation. Areas that pivot are the Achilles’ heel of mechanical heart valves in terms of thrombogenicity. "Windows" were introduced into the valve housing to prevent platelet deposits at the commissural post by means of thorough flushing during the cardiac cycle. To ensure better protection, we added small triangular openings, called clefts, through the housing at the bottom of the windows. In this way, the hinges could be washed through the windows by backflow when the valve is closed and by forward flow from the inside out through the clefts when the valve is open.
In the mitral position, the windows and clefts worked well in calves for more than a year without any platelet deposits in the hinges, but paradoxically, in the less thrombogenic aortic position, the clefts induced acute platelet thrombosis within the first few postoperative days. The clefts, not the windows as stated by Gregoric and colleagues, were responsible for the white platelet deposits. Using computational fluid dynamics and digital particle image velocimetry, my co-workers and I were able to retrospectively detect high shear micro-jets, whose peak shear stress was measured at values much higher than the value recognized as critical for platelet activation, through the clefts in the aortic position.
After this conclusive first round of micro-flow analysis, mechanical trileaflet valves without clefts and unencumbered space in the commissural posts were implanted for the long term in the aortic position in calves, and there were no any thrombotic problems in the hinges or elsewhere. This experience illustrates the importance of design verification by laboratory micro-flow risk analysis before costly and time-consuming animal testing for regulatory approval.
The devastating clinical experiences with prosthetic heart valves that met traditional standards must be kept in mind [3]. The International Standards Organization (ISO) has now recognized that the 1996 document ISO 5840 on prosthetic heart valves needs to be updated [4]. Human blood is drastically different from the blood of animals [5]. Although conventional animal testing remains helpful for detecting some defects, advanced laboratory techniques are today mandatory to design a mechanical substitute for the bioprosthesis.
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References
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- Lapeyre DM, Frazier OH, Conges JL, et al. In vivo evaluation of a trileaflet mechanical heart valve ASAIO J 1994;40:M707-M713.[Medline]
- Gregoric I, Conges JL, Reul H, et al. Preclinical assessment of a trileaflet mechanical valve in the mitral position in a calf model Ann Thorac Surg 2004;77:196-202.[Abstract/Free Full Text]
- Yoganathan AP, Ellis JT, Healy TM, Chatzimavroudis GP. Fluid dynamic studies for the year 2000 J Heart Valve Dis 1998;7:130-139.[Medline]
- PR NF EN ISO 5840, April 2003. Cardiac valve prostheses: draft revision of 3rd ed (1996)..
- Goodman SL. Sheep, pig, and human platelet-material interactions with model cardiovascular biomaterials J Biomed Mater Res 1999;45:240-250.[Medline]
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