Bio-prosthetic Heart Valve Stress Analysis: Impacts of Leaflet Properties and Stent Tip Deflection Caitlin Martin and Wei Sun Tissue Mechanics Laboratory Biomedical Engineering and Mechanical Engineering University of Connecticut, Storrs, USA ABSTRACT Bio-prosthetic heart valves (BHV) have complex geometry, and their normal function involves interactions between the leaflets and the supporting stent, thus, the alteration of tissue mechanical properties can have a confounding influence on the stress distribution throughout the valve. Recently, structural models have been proposed to capture the nonlinear, anisotropic mechanical properties of native and engineered tissues. This anisotropic behavior arises as a consequence of inherent structural heterogeneity present in tissue arising from regional variations in fiber orientation and density. In this study, we utilize a nonlinear fiber-based structural model to evaluate the impact of the leaflet properties on the stress distribution of a pericardial BHV. Valve deformations under 120 mmHg hydrostatic pressure are simulated with various perturbations of the tissue properties including the collagenous fiber orientation, as well as the fiber and matrix stiffness, in addition to perturbations of the stent elastic modulus. The maximum principal stress of the valve leaflets under each condition was normalized with respect to that of a 45 degree fiber orientation and the original un-perturbed material constants. The results indicated that the valve leaflet peak stress can be reduced, provided the proper manipulation of the structure and property of tissue constituents. These results can provide guidelines to engineer and design leaflet tissues to improve valve durability. INTRODUCTION Artificial heart valves have been used for over five decades as replacements for diseased heart valves and have saved millions of lives. Over 50 different valve designs have been developed, yet despite their widespread use, there remains no optimal design. Currently, bioprosthetic heart valves (BHVs) made from biologically-derived, glutaraldehyde-treated bovine pericardium or porcine aortic leaflets, continue to be the dominant replacement valve modality due to their superior hemodynamic function. However, BHVs are only recommended for patients who are 65 years or older, due to concerns of long-term durability 1. The main factors contributing to the limited durability of BHV have been identified as: 2,3 1) cuspal mineralization and calcification, causing cuspal stiffening, and 2) non-calcific cuspal damage, including mechanical fatigue and possible proteolytic degradation of the collagenous extracellular matrix. Studies have shown that mechanical fatigue damage alone can be a major clinical cause of BHV failure 4. The regions of tearing and calcification in BHVs correlate with the regions of high tensile and bending stresses 5,6. Therefore, stress concentrations within the leaflet can either directly accelerate tissue structural fatigue damage, or initiate calcification by causing structural disintegration, thus enabling multiple calcification pathways that lead to valve failure 3,7. Proceedings of the SEM Annual Conference Mohegan Sun, Uncasville, Connecticut, USA, June 13 - 16, 2011 ©2011 Society for Experimental Mechanics T. Proulx (ed.), Mechanics of Biological Systems and Materials, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 9999, DOI 10.1007/978-1-4614-0219-0_9, © The Society for Experimental Mechanics, Inc. 2011 73
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