A model of the geometrical changes in aortic valve leaflets in response to leaflet extension and variable boundary conditions

Cyclic loading response of bioprosthetic heart valves: effects of fixation stress state on the collagen fiber architecture

Biologically derived, chemically modified collagenous tissues are being increasingly used to fabricate cardiac valve prostheses and as biomaterials in cardiovascular repair. A stress-free state during chemical modification has been shown to preserve the collagen fiber architecture of the native tissue, potentially preserving native mechanical properties and improving prostheses durability. However, it is not known if the native collagen fiber architecture is stable during long-term in vivo operation. To address this question, we obtained porcine aortic valves chemically treated at (i) 0 mmHg transvalvular pressure (with 40 mmHg aortic pressure) and (ii) 4 mmHg transvalvular pressure, then subjected the valves to 0, 1 x 10(6), 50 x 10(6), and 200 x 10(6) in vitro accelerated wear testing (AWT) cycles. The resulting changes in collagen fiber architecture were quantified using small angle light scattering analysis (SALS). SALS measurements indicated that collagen fibers in the 0 mmHg pressure-fixed leaflets became more aligned between 1 x 10(6) and 50 x 10(6) AWT cycles. In contrast, only minor changes (not statistically significant) in collagen fiber orientation occurred in the 4 mmHg pressure-fixed valvular tissue with cycling. It was also noted that although the 0 mmHg group was fixed without transvalvular pressure, distention of the root induced significant changes in collagen structure of the leaflets. Overall, our observations suggest that the native collagen fiber crimp of the 0 mmHg pressure-fixed leaflets were rapidly lost after only 50 x 10(6) AWT cycles (equivalent to approximately 1.6 patient years) and thus may not be maintained over a sufficient period of time to be clinically beneficial. Further, the collagen structure of the native aortic valve is exquisitely sensitive to dimensional change in the aortic root-independent of the presence of transvalvular pressure. Our findings also suggest that without in vivo remodeling, any collagenous tissue used to fabricate BHV may undergo similar degenerative, irreversible changes in vivo.

Geometry of homograft valve leaflets: effect of dilation of the aorta and the aortic root

With the increasing interest in aortic homografts as either a free-sewn valve or whole-root replacement, the effect of internal pressure and dilation of the aorta and aortic root on valve leaflet geometry has been studied. Seven aortic homograft roots were studied, six that had been stored in antibiotics and one that had been cryopreserved. The diameter of the aorta was determined as a function of internal pressure and correlated with the stress-strain characteristics of the aortic wall. Three-dimensional leaflet surface geometry was measured in the "neutral" position, and the leaflet was characterized by its radius of curvature and angle of inclination, using a cylindrical model. The diameter of the aorta increased by between 30% and 50% as the dilation pressure increased from 0 to 120 mm Hg. This was consistent with the stress-strain data obtained from strips of the aortic wall in the circumferential direction. The angle of inclination of the leaflet increased from 20 to 80 degrees and the radius of curvature increased from 4 to 17 mm as the internal pressure increased from 0 to 80 mm Hg. The open leaflet configuration showed a triangular orifice with low bending strains for a dilated root, but increased bending strains with reduced dilation pressure. These are important considerations when implanting a free-sewn homograft into the aortic root.