Is zero-pressure fixation of bioprosthetic valves truly stress free?

Zero-pressure fixation has often been referred to as stress-free fixation, which implies that no leaflet stresses exist during the fixation process. The two aortic valve cusp layers, the fibrosa and the ventricularis, however, are believed to produce mutually opposing forces within the valve cusps. Residual stresses may therefore exist even during zero-pressure fixation. We first verified the presence of such internal stresses by separating the layers of pig aortic valve leaflets and measuring dimensional changes. In the 11 specimens examined, the fibrosa expanded radially by 30% +/- 13% (mean +/- standard deviation), whereas the ventricularis contracted by 12% +/- 4%. The ventricularis also contracted circumferentially by 13% +/- 3%. We measured the extensibility of 120 fresh and glutaraldehyde-fixed fibrosa and ventricularis components to investigate the mechanical effects of glutaraldehyde fixation under such internal stresses. We also tested the layers from leaflets that were fixed whole. We compared the extensibility of the fibrosa and the ventricularis, each fixed in the presence and absence of residual stresses, and found that, in the radial directions, the ventricularis from valve cusps that were fixed whole was less extensible than fresh ventricularis (35.4% versus 63.7% strain to high-modulus phase, p < 0.00001). The fibrosa from cusps that were fixed whole, however, was more extensible than fresh fibrosa (39.2% versus 29.5% strain, p < 0.0122 radially; 12.8% versus 8.2% strain, p < 0.0001 circumferentially). The ventricularis became less extensible because it was fixed under tension, and the fibrosa became more extensible because it was fixed under compression. This study therefore demonstrates the presence of residual tensile and compressive stresses in the ventricularis and fibrosa, even when the leaflets are relaxed. Zero-pressure fixation cannot therefore be considered truly stress free, in the engineering sense, because residual internal stresses affect collagen fiber crimp and change the extensibility of the fibrosa and the ventricularis.