On Stress Reduction in Bioprosthetic Heart Valve Leaflets by the Use of a Flexible Stent

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Abstract
It has been postulated that flexible stent posts can reduce tensile stress at the commissures of tissue heart valves by about 90% when compared with the same valve mounted on a rigid stent. We have used a detailed computer model to investigate the role of flexible stent posts in reducing stress in the leaflets of three types of bioprosthetic heart valves: the bovine pericardial and the high- and zero-pressure fixed porcine valves. The models use stress/strain data from biaxial experiments to characterize the tissue properties and are subjected to a back pressure of 120 mmHg. We found that strain was reduced linearly with stent post deflection and that this was a purely static process--it did not require the load to be applied impulsively. This finding was in close agreement with earlier experimental studies, which measured the same strain reduction whether the valve was loaded quasi-statically or at physiological rates. In addition, we found that for this mechanism to be effective the valve must have good coaptation at the center and the tissue should be stiff; in other cases, the advantages of strain reduction through the use of a flexible stent are considerably diminished.