Natural preload of aortic valve leaflet components during glutaraldehyde fixation: effects on tissue mechanics

The mechanics of glutaraldehyde-fixed aortic valve leaflets depend largely on the amount of stress present during fixation. Our previous work has suggested that even when the aortic valve is flaccid, the leaflet components are preloaded. We have, therefore, hypothesized that fixing valve leaflets in this naturally preloaded state will affect the function of their components, the fibrosa and the ventricularis. We have compared the elastic response of fibrosa and ventricularis fixed under 'low' and 'zero' tensile and compressive preload by testing 120 of these layers: (i) fresh, (ii) glutaraldehyde-fixed, and (iii) isolated from whole porcine aortic valve leaflets fixed while intact. In both the radial and circumferential directions, the fibrosa from intact-fixed valves was more extensible than the fresh (39.2 vs 29.2% strain to high modulus phase at p < 0.0122, and 12.7 vs 8.1% strain, at p < 0.0003, respectively). The ventricularis from intact-fixed valves, however, was less extensible than when fresh (35.4 vs 63.7% strain, at p < 0.00001 in the radial direction). The fibrosa must have, therefore, been fixed under compression and the ventricularis under tension, when fixed together in the intact aortic valve cusp. The tensile stresses in the intact-fixed ventricularis produced a greater circumferential elastic modulus than in separately fixed tissue (9.62 vs 4.65 MPa, at p < 0.00001), likely through a fibre recruitment process. Compressive stresses in the fibrosa produced a decrease in the elastic modulus both radially and circumferentially (from 3.79 to 2.26 MPa at p < 0.0023, and from 9.55 to 4.65 MPa at p < 0.00001, respectively). Fixing porcine aortic valves at even minimal tensile and compressive preload, such as that which occurs naturally, significantly alters both the extensibility and the elastic modulus of the valve leaflet components.