Marcelo E. Berli

Numerical analysis of the effects of material parameters on the lubrication mechanism for knee prosthesis

The tibial component of current knee prostheses made of ultra high molecular weight polyethylene (UHMWPE) has a high degree of wear that causes knee inflammation, prosthesis loosening and subsequent replacement in not more than 15 years. In order to know which UHMWPE material properties have more influence on wear, a steady state lubrication model with non-Newtonian synovial fluid has been studied through numerical solution. The results show that UHMWPE has a very high elastic modulus that makes difficult a well lubricated artificial joint and induces the formation of very thin lubricating films between the moving surfaces with the same magnitude of roughness components. This study shows that the use of deformable porous materials in the tibial component could cause the lubricating film thickness to be higher than the average roughness and the pressure levels to be lower than the one predicted for UHMWPE. These two facts imply friction and wear reduction.

Predicción de la reducción del impacto térmico en un edificio con doble pared

At Santa Fe de la Vera Cruz city, Argentina, a building that includes elements of sustainable architecture, energy efficiency and comfort based on the use of natural resources is built. Particularly, a double facade design on the front walls is meant to achieve an air chamber that prevents heat transfer from the outside to the inside in summer and vice versa in winter. In This work, a numerical study is presented for the evaluation of the thermal performance of a cavity (air chamber) interposed in a double facade of the building for different climatic conditions, considering two air chambers alternatives: connected and non connected to the outside. Both cases are energetically compared with the standard facade design without chamber. The results show that for summer conditions, a chamber connected to the outside would be the most efficient design, while for winter, the closed cavity is the best saving-energy alternative.

Numerical Solution of a 2D Lubrication Model with Sommerfeld Boundary Conditions for Hip Prostheses

Prosthetic in-contact surfaces wear is one of the main causes of hip replacements fallure. Lubrication is essen- tial to reduce the friction and consequently the wear. A two- dimensional model based on a modified Reynolds equation, was used to aoalyze the lubricated contact In a hypothetical hip prosthesis. It is assumed that prosthesis is made of a me- tallic component and a low rigidity elastic porous material that can absorb or exude Duid. The Duid pressure and channel Dow shape were simultaneously solved by a double precision computer code based on the rmite element method, Newton’s method and parametric continuation. The Information provided by this model is extremely difficult to obtain by experimental methods. Results show that a great- er deformation capacity of the elastic material promotes a thicker lubrication mm and induces a better load distribution reducing normal stresses; whlle exudation capacity reduces friction and wear. The sub-ambient pressures predicted and the oscillatory solution, show the need of more realistic boundary conditions and fmite element mesh refinement.

Computer Simulation of the Blood Flow in a Planar Configuration for a Pulsatile Ventricular Assist Device

Patients that suffer congestive cardiac insufficiency need to be assisted with assist devices (VAD) as a temporal solution when the cardiac transplantation is not possible. For that reason it is of great interest to know the performance of such devices, especially the potential blood damage that they can produce. In this work, a computational twodimensional simulation of a novel pulsatile VAD is performed. This new design of VAD has a double acting piston driven electromagnetically at 2.1 Hz. In addition, the VAD has four active valves. The flow velocities, the pressure and the shear stress developed in the blood are analyzed in the chambers and inflow and outflow conduits. The results suggest that the developed flow would not be dangerous for the blood.

Spacers in the treatment of hip joint infections: numerical analysis of their durability

Hip spacers are temporary implants having a geometry similar to the femoral component of a hip prosthesis, and they are manufactured with antibiotic-impregnated bone cement. The use of spacers in two stage revisions is the most effective treatment to eradicate infections and to avoid limb shortening. The most frequent complication associated with spacers is fatigue failure, for which doctors recommend patients to stay at rest. In this work, several spacer designs are analyzed in order to determine the feasibility of doing activities like walking, standing up or sitting down while performing the antibiotics treatment. Designs combine both different neck diameters and the presence/absence of an internal, stainless steel reinforcement. By means of computational simulations based on the finite element method, stress fields are calculated for various hip spacer designs under several load and fixing conditions. For this purpose, a 3D model of human femur is generated by processing tomographic images with segmentation techniques and inverse engineering. The results allow us to estimate the life expectancy of each design, by considering the fatigue behavior of the bone cement. Only the introduction of a reinforcement with a proper diameter into the bone cement matrix could assure the integrity of the spacer along the treatment period.

Simulation of the filtration mechanism of hyaluronic acid in total knee prosthesis

Polyethylene (UHMWPE) wear in current knee prosthesis causes prosthesis loosening after no more than 15 years. In this work, a steady state one-dimensional lubrication model with non- Newtonian fluid, porous elastic layer on tibial component, ultra-filtration mechanism of fluid and some features of the surface roughness is studied through a numerical technique based on the Finite Element Method. The results show that the UHMWPE stiffness makes difficult the lubrication mechanism of the artificial joint and promotes abrasive and fatigue wear. Nevertheless, the use of compliant porous materials on the tibial component could reduce friction and wear. Moreover, the ultra-filtration mechanism promotes efficiency on the joint.