Biaxial mechanical behavior of excised porcine mitral valve leaflets

Abstract

Anterior and posterior leaflets from excised porcine mitral valves were mechanically tested under cyclic equibiaxial and strip biaxial stretch protocols at a strain rate of 4-12%/s after preconditioning. Cauchy stress and Lagrangian strain were calculated for both membrane and three-dimensional cases. The leaflets exhibited nonlinearly elastic, anisotropic behavior. Both anterior and posterior leaflets were less extensible in the circumferential than in the radial direction under equibiaxial stretch, with stress ratios of 5.7 and 4.3, respectively. The posterior leaflets exhibited greater extensibility in both directions and lower circumferential posttransitional moduli (ranges 690-820 vs. 2,500-3,200 N/m for anterior). This larger posterior extensibility may be due to the greater number of chordal attachments, which provide additional mechanical stability to this structure. Coupling of radial and circumferential mechanical behavior was evidenced by the response to different stretch protocols, indicating a complex microstructural coupling between individual collagen fibers or bundles. These are the first biaxial data for mitral valves and are a foundation for the development of a more detailed quantitative material description.

Active stiffening of mitral valve leaflets in the beating heart

“…Figure 4 provides a conceptual model consistent with the material behavior observed in the present in vivo study, as well as the findings from previous ex vivo studies (3,14,22,27). The passive collagen fibers in the leaflet are represented by coiled springs, and the finding that circumferential stiffness is greater than radial stiffness in isolated leaflets (3,14,22,27) results from a model-postulated greater density of circumferentially oriented than radially oriented collagen fibers. Contractile elements (red arrows, possibly VICs), in close proximity to these collagen fibers (40) and developing force in vivo, but not ex vivo, greatly increase all collagen spring constants in vivo relative to in vitro.…”

“…Although circumferential and radial elements are illustrated as independent entities in Fig. 4, they may also be coupled, as discussed by May-Newman and Yin (27) and supported by the parallel circumferential and radial response to the ESML and STIM interventions in the present study.…”

“…MV leaflets have extensive passive collagen fiber networks (5,11,12,15,18), and the stress-strain relationship of isolated leaflets (3,14,22,27) is consistent with the material properties of passive collagen fibers. Our recent study (21), however, suggested that leaflet stiffness in the beating heart (slope of the leaflet stress-strain curve) was much greater than that of passive tissue, and studies of freshly isolated, oxygenated, stimulated leaflets in myographs (39) and in open hearts on cardiopulmonary bypass (CBP) (7,32) have suggested that the mechanical properties of MV leaflets may be altered actively.…”

Mitral Valve Leaflet Remodeling After Myocardial Infarction

“…Leaflets can even stretch, probably contributing to effective valve closure. 20,21 Tethering force, closing force, leaflet pathology, and function…, there are several confounding factors based on the heterogeneous underlining ischemic condition in each individual. Although the study of Beaudoin et al 13 has made an important contribution, there is still much to be learned before we have a complete understanding of the pathophysiology of ischemic mitral regurgitation.…”

A new approach for the in-vivo characterization of the biomechanical behavior of the breast and the cornea

“…The measured force can be used to infer tissue properties like elasticity or compressibility. However, uniaxial tensile tests are not accurate enough to completely estimate the anisotropic behavior of the soft tissue [Fung, 1974;Chew et al, 1986;May-Newman and Yin, 1995;Billiar and Sacks, 2000;Zhao et al, 2011].…”