Estimation of mechanical stresses on closed cusps of porcine bioprosthetic valves: effects of stiffening, focal calcium and focal thinning.

The magnitude and distribution of mechanical stresses acting on the closed cusps of porcine bioprosthetic valves (PBVs) were estimated using a finite element model. The effects of leaflet stiffening, focal calcium and focal thinning on leaflet stresses were determined. In a normal closed PBV leaflet, stresses increased as pressure was increased. At a pressure of 80 mm Hg, the maximal normal principal stresses were 11 g/mm2 near the center of the leaflet and increased to 19 g/mm2 at a pressure of 160 mm Hg. These observations suggest that the closed valve in the mitral position would experience higher mechanical stresses than the closed valve in the aortic position. Tissue stiffening increased stresses throughout the leaflet and introduced a site of stress concentration near the center of the leaflet. At a pressure of 80 mm Hg, the maximal principal normal stress increased 55% when the leaflet was stiff in comparison to the normal leaflet. Focal calcium and focal thinning caused marked gradients of stress between the sites of calcium or thinning and the immediate surrounding tissue. The magnitude of these stress gradients increased with increasing pressure. These sites of mechanical stress concentration or stress gradients appear to be compatible with sites of leaflet calcification or disruption. Such stresses may contribute to spontaneous degeneration of PBVs.