Pathophysiology of chordae tendinae
The CT function to transmit the contractions of the PMs to the leaflets. The first-order CT serve to secure the leaflets to maintain valve closure and prevent MV prolapse. Their section causes acute mitral regurgitation (MR). On the contrary the section of the second-order CT does not produce MR and it is believed that they play a role in maintaining normal LV size and geometry. To perform these functions, the CT must contain a high degree of elasticity, as well as considerable strength and endurance. A study showed that CT are composed of multiple layers of elastic fibers, an inner collagen core, and an outer layer of endothelial cells (fig. 1)4. During stretching, when PM contraction straightens the chords, the collagenous wavy pattern disappears and transfers the peak stress during contraction to the leaflets. The sleeve‐shaped elastic network that surrounds the internal collagen suggests that on relaxation of tension, the elastic fibers would tend to restore the collagen to their wavy configuration.
From the mechanical point of view, the smaller chordae are less extensible than the larger ones5. Then marginal chordae are half as extensible as second-order chordae, over 4 times stiffer in the pre-transitional range, and 1.6 times stiffer in the post-transitional range, than second-order chordae6. The reason is that collagen fibrils are more highly crimped in ticker CT and hence have a smaller period, as supported by direct measurements5.
CT are instrumental for the reciprocal exchange of force between the PMs, the mitral annulus and the fibrous skeleton. The PMs, anatomically and functionally distinct from the LV wall, function as shock absorbers that compensate for the geometric changes of the LV wall while maintaining their tips at a constant distance to their hemi-annulus7. Marginal chordae may be fundamental for leading edge control and for correct leaflet coaptation, but, since their shape and tension change continuously during the cardiac cycle, the mechanism responsible for maintaining constant this annulo-papillary distance relies on the second-order chordae, and in particular on the two, thick strut chordae of the AL, that, for their characteristics, can support this annulo-papillary distance. The AL collagen fibers are oriented from the insertion of the strut chords toward the fibrous trigons, in direction of the load. This suggest that the strut chordae connect the PMs to both trigons under tension. The tension distributed towards the second-order chordae has been demonstrate to be more than three-fold that in the first-order counterpart8. The correct distribution of the tension does not make the mitral annulus distance vary during the cardiac cycle, while the apex to mitral annulus distance changes significantly at the expense of the changes in the apex to papillary muscle tips7.