Tears in bioprosthetic heart valve leaflets without calcific degeneration.
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BACKGROUND AND AIM OF THE STUDY: The mechanism of structural failure of bioprosthetic valves is still not clearly understood. This study was undertaken to assess pure leaflet tear as a mode of failure in porcine and pericardial bioprostheses. METHODS: Of 246 bioprosthetic valves (109 porcine, 137 pericardial) implanted between 1975 and 1991, 101 had to be explanted and served as the study population. RESULTS: The reasons for valve failure were calcific degeneration in 73, pure tear in 12, and endocarditis in 10. Six other patients had a perivalvar leak. The mean age at operation was 32 years. Freedom from degeneration at 10 years was 45 +/- 7% and from pure tear it was 92 +/- 2%. The hazard functions were strikingly different, as that for degeneration showed a progressive increase while that for pure tear peaked at six years post-implant. The mean age of the patients with pure tear was 41 years and for degeneration it was 24 years (p = 0.00001). The reasons for the difference in hazard function are discussed. The characteristic clinical features of pure tear allow clinical diagnosis in the majority of patients. CONCLUSION: Pure tear is the result of uneven tissue loading with tearing occurring at sites of maximal stress. The four possible mechanisms in pericardial valves are (a) intense stress concentration at the top of the stent post (commissure); (b) compression stress below the top of the post; (c) abrasion stress in tissue mounted outside the frame; and (d) increased bending stresses on leaflet opening. In stent-mounted porcine valves, pure tear is related to incorrect mounting or to increased bending stresses.
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