Tissue damage and calcification may be independent mechanisms of bioprosthetic heart valve failure

Background and aim of the study: Porcine bioprosthetic valves have excellen t hemodynamics and do not require anticoagulation, but have limited durabil ity. Cusp tearing is a major cause of bioprosthetic valve failure. It has b een suggested that the mechanism of bioprosthetic valve failure is stiffeni ng by calcification, which leads to elevated stresses and secondary collage n fiber damage and leaflet tearing. This thesis was tested in explanted por cine bioprostheses. Methods: A total of 60 explanted porcine bioprosthetic valves was tested me chanically, and 15 explanted valves were examined grossly and histologicall y. Circumferentially and radially oriented samples of cusp tissue were test ed uniaxially in a materials testing machine and compared with five control s. Results: Mean (+/- SD) duration of implantation was 10.9 +/- 5.6 years. Cir cumferential specimens from explants were less extensible than controls (11 .0 +/- 5.5% versus 24.5 +/- 2.8% strain, p <0.001), and failed at lower ten sions (973 <plus/minus> 733 versus 3075 +/- 911 N/m, p = 0.001) and at lowe r strains (21.2 +/- 8.1% versus 47.3 +/- 7.1% strain, p <0.001). Radial spe cimens from explants were less extensible (28.7 <plus/minus> 6.8% versus 39 .2 +/- 5.9% strain, p = 0.002) and failed at lower strains (60.3 +/- 17.3% versus 112.2 +/- 24.9% strain, p <0.001) than the controls. The stiffness o f the explants was unchanged from controls in both circumferential and radi al samples. There were no differences between explants and controls in radi al and circumferential stiffness, and in radial failure strength. Calcifica tion was mild and diffuse in most of the tested samples. Tears were found i n areas without calcific deposits, along with breaks in collagen fiber bund les. Conclusion: These results do not support the thesis that calcification stif fens glutaraldehyde-fixed porcine bioprostheses, except when the entire cus p is transformed into a solid mass of mineral. Rather, leaflet tears may de velop as a result of accumulated mechanical damage that is independent of c alcification.