A structural basis for the size-related mechanical properties of mitral valve chordae tendineae

It has been reported previously that the mechanical properties of mitral valve chordae tendineae vary with chordal size and type. The popularity of mitral valve repair and chordal transposition warrant a better understanding of this phenomenon. The objectives of this study were to characterize the size- and type-related variations in chordal mechanics and explain them from the ultra-structural viewpoint. A total of 52 porcine mitral valve chordae from eight hearts were mechanically tested. We found that thicker chordae were more extensible than thinner chordae (4.2+/-1.5%, 8.1+/-2.5%, 15.7+/-3.9% and 18.4+/-2.8% strain corresponding to chordae with cross-sectional areas of 0.1-0.5, 0.5-1.0, 1.0-2.0, and 2.0-3.0mm(2), respectively), and had lower moduli (90.1+/-22.3, 83.7+/-18.5, 66.3+/-13.5 and 61.7+/-13.3 MPa corresponding to the same chordae groups). Polarized light microscopy was used to measure collagen fibril crimp. Thicker chordae had smaller crimp period than thinner chordae (11.3+/-1.4 microm vs. 14.8+/-3.0 microm), and were thus more highly crimped. Thicker chordae could therefore extend to greater strain before lock-up. Transmission electron microscopy (TEM) was used to measure choral fibril ultra-structure. Thinner chordae had lower average fibril diameter than thicker chordae but greater average fibril density. The cross-sectional area occupied by fibrils, however, was found to be constant at 49+/-2% regardless of chordal size or type. The difference in moduli between thick and thin chordae can therefore be explained by differences in fibril packaging and hence fibril-to-fibril interactions. According to a simple fibril interaction model, chordae with smaller diameter fibrils will have a greater number of fibril-to-fibril interactions, and hence a greater modulus.