The Mechanical Properties of Native Porcine Aortic and Pulmonary Heart Valve Leaflets

Aortic heart valves and their replacements fail in vivo for reasons that are not fullyunderstood. Mechanical evaluation and simulations of the function of native aorticvalves and their replacements have been limited to tensile and biaxial tests that seek toquantify the behavior of leaflet tissues as a homogenous whole. However, it is widelyunderstood that valvular tissues are multi-layered structures composed of collagen,elastin, and glycosaminoglycans. The mechanical behavior of these layers within intactvalve leaflet tissues and their interactions are unknown. In addition, pulmonary valveshave been used as substitutes for diseased aortic valves without any real understanding ofthe mechanical differences between the aortic and pulmonary valves. The pulmonaryvalve operates in an environment significantly different than that of the aortic valve and,thus, mechanical behavioral differences between the two valve leaflets may exist. In thisstudy, we sought to determine the mechanical properties of the porcine aortic andpulmonary valves in flexure, and to determine the mechanical relationship between theleaflet layers: the fibrosa, spongiosa, and ventricularis. This was accomplished bydeveloping a novel flexure mechanical testing device that allowed for the determinationof the flexural stiffness of the leaflet tissue was determined using Bernoulli-Eulerbending. Moreover, transmural strains were quantified and used to determine thelocation of the neutral axis to determine if differences existed in the layer properties ofthe fibrosa and ventricularis. To contrast the flexural studies, biaxial experiments werealso performed on the aortic and pulmonary valves to determine the mechanicaldifferences in the tensile behavior between the two leaflets.Results indicated that the pulmonary valve is stiffer than the aortic valve inflexure but less compliant than the aortic valve in biaxial tensile tests. The interactionsbetween the layers of the leaflets suggest an isotropic mechanical response in flexure, butdo so through mechanisms that are not fully understood. For heart valve leafletreplacement therapy, this study illustrates the biomechanical differences between theaortic and pulmonary valve leaflets and emphasizes the need to fully characterize the twoas separate but related entities. Understanding the interactions of microscopic structuressuch as collagen and elastin fibers is critical to understanding the response of the tissue asa whole and how all these elements combine to provide a functioning component of theorgan system