Design, modeling and characterization of lattice structures for orthopedic implant applications

Resumen

Interest in lattice structures has soared in recent years thanks to the advances in the field of additive manufacturing, which has led to increasingly complex designs and the production of parts which was impossible up to not long ago. These advances enabled to create lattice structures that mimic the cellular solids of nature, which attracted broad attention due to its applicability in the aerospace and biomedical industries. These structures can be designed to have predefined stiffness and strength values, which enables the production of parts with engineered mechanical properties. One problem of traditional implants made of monolithic parts of Ti6Al4V, CoCr, pure Ti, etc., is the mismatch between the stiffness of the host bone and the metallic implant, which creates the so-called stress shielding. Stress shielding occurs when the bone adjacent to the implant does not have to withstand the main physiological loads because the much stiffer implant bears them in its place. Bone is a living tissue, which is created or resorbed (diluted in blood) depending on the loads to optimize its functionality. Thus, the stiffness mismatch between bone and the implant leads to the bone resorption due to the lack of mechanical stimuli on the host bone. The bone surrounding the implant loses density and weakens, which causes pain to the patient, affects implant stability, and may lead to the loosening of the implant. Lattice structures offer the possibility to create porous implants with tailored mechanical properties to match the stiffness of the surrounding bone, thus avoiding stress shielding and subsequent bone loss. Furthermore, lattice structures form porous parts with high surface to volume ratios, and this porosity enables the bone ingrowth within the implant, improving its fixation and long-term stability. This work is devoted to the development of tools to design lattice structures with controlled mechanical properties, as well as to deepen into the factors that affect such properties. Thus, the main purpose of this dissertation is to create lattice structures that mimic bone stiffness and could be implemented in orthopedic implants to avoid the stress shielding. In addition, orthopedic implants must withstand millions of load cycles throughout the lifetime of the patients, thus the fatigue behavior of the structures was also studied in this thesis. Finally, the small feature sizes required to implement such structures in orthopedic implants requires to reach the manufacturing limits of current additive manufacturing technologies, which induces important deviations from the actually designed geometry and in turn the mechanical properties. Another goal of this work is to understand the impact of such manufacturing deviations on the stiffness of the structures. The obtained results show that there are different possibilities to design structures with stiffness levels comparable to bone. The developed analytical or semi-analytical models predict and enable to design the mechanical properties of the structures for different topologies. These models can be used with personalized bone data to mimic the bone stiffness of each patient. Furthermore, the anisotropy of the structures can also be controlled to adapt it to the complex loads that arise in various anatomical sites. Regarding dynamic loads, fatigue life prediction tools in literature were compared and adapted to improve their applicability, and a fatigue failure surface was developed to easily predict the fatigue life of the structures. Moreover, it was concluded that hot isostatic pressing enhanced the fatigue strength of the structures. Finally, the manufacturing deviations were studied, developing a methodology to consider the proximity to the nodes in the analysis of the imperfection level, and to include such imperfections in a numerical model that predicts the change of anisotropy in the structure.