Micromechanics of the fibrosa and the ventricularis in aortic valve leaflets

The elastic response of aortic valve cusps is a summation of its fibrous components. To investigate the micromechanical function of valve leaflet constituents, we separated the fibrosa and the ventricularis from fresh and glutaraldehyde-fixed leaflets and tested them individually. The ventricularis was stiffer circumferentially than radially (7.41 kPa vs 3.68 kPa, p less than 0.00001) and was more extensible radially (62.7% vs 21.8% strain to high modulus phase, p less than 0.00001). The fibrosa was also stiffer circumferentially than radially (13.02 kPa vs 4.65 kPa, p less than 0.0008), but had uniform extensibility. Glutaraldehyde fixation did not affect the circumferential elastic modulus of the fibrosa, but reduced its radial modulus from 4.65 kPa to 2.32 kPa (p less than 0.0078). The elastic modulus of the ventricularis remained unchanged. Fixation also reduced the extensibility of the ventricularis circumferentially (from 21.8% to 15.2% strain, p less than 0.018), but not radially, and increased the radial extensibility of the fibrosa from 27.7% to 46.1% (p less than 0.0048). These data show that while the ventricularis contains a large amount of elastin, the amount of radially oriented collagen is similar to that of the fibrosa. The fibrosa, by itself, has the same extensibility in both directions (about 23% strain), but can extend much more radially when connected to the rest of the leaflet because it is attached to the ventricularis in a highly folded configuration. The two layers therefore complement each other during aortic valve function, and become detrimentally altered by fixation in glutaraldehyde.