Structural study of a polymeric aortic valve prosthesis. Analysis for a hyperelastic material.

This work presents a structural computational simulation of a polymeric aortic valve prosthesis, made with a hyperelastic material (Styrene-Ethylene/Propylene-Styrene). The valve has a suture ring, three pillars placed at 120° and three leaflets. The analysis is based on a modification over previous designs consisting in a fillet concave surface to avoid stress concentration at the junctions between leaflets and pillars. Three shapes were simulated. The first one was used to validate the computational method by comparison of the results with a recent paper. The second shape was designed to show that a fillet or "rounding" can be beneficial to the stress leaflet reduction. The third shape was also designed to show that the reduction of leaflet thickness and intercommissural distance between leaflets at the pillar junctions improves the valve opening and closure. The use of fillet with a 0.5 mm radius, reduced 26.5% the maximum Von Mises stresses for the second shape and 33.9% for the third shape. Additionally, for the latter, the opening area was not affected for the high stiffness due to fillet. The results -mainly for the third shape-are promising and give rise to future studies: further shape optimization, analysis for other materials and valve simulation under pathological loads.

The interplay between BMU activity linked to mechanical stress, specific surface and inhibitory theory dictate bone mass distribution: Predictions from a 3D computational model.

Bone mechanical and biological properties are closely linked to its internal tissue composition and mass distribution, which are in turn governed by the purposeful action of the basic multicellular units (BMUs). The orchestrated action of osteoclasts and osteoblasts, the resorbing and forming tissue cells respectively, in BMUs is responsible for tissue maintenance, repair and adaptation to changing load demands through the phenomenon known as remodelling. In this work, a computational mechano-biological model of bone remodelling based on the inhibitory theory and a new scheme of bone resorption introduced previously in a 2D model, is extended to a 3D model of the real external geometry of a femur under normal walking loads. Starting from a uniform apparent density (ratio of tissue local mass to total local volume) distribution, the BMU action can be shown to lead naturally to an internal density distribution similar to that of a real bone, provided that the initial density value is high enough to avoid unrealistic final mass deposition in zones of high energy density and excessive damage. Physiological internal density values are reached throughout the whole 3D geometry, and at the same time a 'boomerang'-like relationship between apparent and material density (ratio of tissue mass to tissue volume) emerges naturally under the proposed remodelling scheme. It is also shown here that bone-specific surface is a key parameter that determines the intensity of BMU action linked to the mechanical and biological requirements. Finally, by engaging in simulations of bone in disuse, we were able to confirm the appropriate selection of the model parameters. As an example, our results show good agreement with experimental measurements of bone mass on astronauts a fact that strengthens our belief in the insightful nature of our novel 3D computational model.

The Phan-Thien and Tanner Model Applied to the Lubrication of Knee Prostheses.

This work aims to provide a contribution to determine a proper model for the study of fluid film lubrication for the reduction of knee prostheses failure due to polyethylene wear. The Phan-Thien and Tanner (PTT) rheological law and the elastic deformation of the articular surfaces were considered in this modeling. The governing equations were solved numerically for different geometries and different Weissenberg numbers. The lubrication approximation applied to the PTT rheological law leads to an expression for the apparent viscosity similar to the Cross model. The results attest the importance of considering the non-Newtonian behavior of the synovial fluid, the elastic deformation, and the geometrical features of the prostheses to obtain quantitative information.