Biaxial mechanical behavior of excised porcine mitral valve leaflets

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.