Resumen de Contribution to the manufacture of porous structures for prostheses by means of additive manufacturing extrusion processes
In recent years, the use of prostheses has widely increased due to their ability to replace defective parts of the skeleton. Among artificial implants, hip joint plays a critical role in the human body since it allows to walk, run, and jump.
Hip prostheses have a hemispherical shape in which the outer surface should be rough enough to favour cell ingrowth and proliferation. In contrast, inner surface should be smooth to enable movement between femur head and inner surface of acetabulum. In this thesis, study and analysis of the properties of printed scaffolds that can be used in the external part of the acetabulum is performed. The ultimate goal of the research is to print ceramic prostheses, which are strong and inert, and release low debris. However, in the present study the specimens were printed in plastic material, whose printing technique by extrusion is far more developed than that for ceramics.
First, in order to mimic the outer surface, different mesostructures such as Voronoi, honeycomb, grid, imitation of the trabecular bone structure or bone-like, and octet-truss were designed and printed. The samples were designed using specific drawing software such as Rhinoceros. In order to check the possibility to use each structure, samples were printed utilizing the fused filament fabrication (FFF) technique, using PLA (polylactic acid), which is more cost-effective for testing and final printing than ceramics. Grid, trabecular and octet-truss were the most appropriate mesostructures, because in these cases porosity and pore size are controllable, with interconnected porosity, and these two factors are critical for bone growth in osseointegrated prostheses.
With the purpose of obtaining required pore size and porosity of the structures, two geometrical models were defined for both the grid and the bone-like structures respectively. By help of tailored geometries and use of Matlab, proper pore size and porosity for cell growth have been analyzed and generated, which is between 0.1~0.5 mm and 55~75%, respectively.
In order to assess the influence of printing parameters on dimensional error, roughness and porosity, design of experiments (DOE) was carried out. The grid structure was selected among the different ones in this case as an example. A factorial design and later multiobjective optimization were used to select the most appropriate values for the printing variables. Four parameters, namely layer height, speed, temperature, and flow rate were considered for printing the samples. Results of obtained dimensional error indicated that the layer height and flow rate are the most significant parameters. Roughness depends mainly on layer height. Porosity is mainly influenced by layer height and interaction between layer height and flow rate.
Afterwards, compression tests for both the octet and the bone-like structure, were carried out. Prismatic samples were printed with a width to height ratio W/H of 2 and 4, for the determination of compression strength and elastic modulus respectively. FEM analyse were also performed using the ANSYS software. The results showed that for both octet and bone-like structure, increasing the radius of the struts, increases the stiffness and, therefore, the modulus of elasticity of the structures. The results for H/W = 4 were more similar to the simulated ones, than the results for H/W=2. Furthermore, for the bone-like structure, it was concluded that, for a greater strut radius, the experimental results of Young’s modulus were lower than that predicted by FEM.
The present thesis will help to select proper structures for the outer surface of the hip prostheses, considering both their porosity and their mechanical properties that are essential factors for the creation of the porous structures.
Future research could apply the structures analysed in this thesis to the ceramic impression, in order to advance in the manufacture of customized hip prostheses.
