Stress/strain characteristics of porcine mitral valve tissue: parallel versus perpendicular collagen orientation

Mitral valve tissue was analyzed in uniaxial tension testing. Rectangular strips were excised from fresh, whole, porcine mitral valve leaflets, with the long axis in the following orientation: perpendicular to the annulus (posterior [PPERP] and anterior [APERP]), parallel to the annulus (posterior [PPAR] and anterior [CAPAR]), and parallel to the annulus and involving chordal insertions (anterior [MAPAR]). Basal and marginal chordae were also tested. These samples were tested in uniaxial tension (INSTRON Model 1000) at deformation rates of 5 and 10 mm/min, the load applied along the long axis of the strip. The specimens were preconditioned by cyclically loading from 0 to 4.0 g for five cycles, and then applying a final 50-g load. Whole excised porcine mitral valves were previously examined by small angle light scattering, polarized light microscopy, and routine histologic examination in order to ascertain the collagen fiber orientation throughout the valve. Groups tested in uniaxial tension with collagen fibers parallel to the applied stress are significantly stiffer than those with perpendicular fibers (p less than 0.001). Collagen fiber density is greater in the chordae than in the leaflets, and a corresponding increase in stiffness is demonstrated. This indicates that the mitral valve tissue behaves as a classic fiber reinforced composite, i.e., increasing mechanical stiffness (modulus) is related to density and direction of fibers. This information can be applied to the design of biosynthetic valve substitutes with a similar fiber reinforced composite structure.