Computational design of functionally graded hip implants by means of additively manufactured porous materials

摘要

Two of the main mechanically related problems of hip implants are the bone resorption due to the reduction of the stresses in the bone caused by the presence of the implant (stress shielding), and the interfacial failure caused by inadequate stress distribution at the bone-implant interface. While stiff implants increase stress shielding, flexible implants cause higher contact interfacial stresses. A compromise is thus needed, and two design stratégies have been suggested: 1) optimize the hip implant shape, or 2) optimize the functional gradation of mechanical properties of the implant. Additive manufacturing (AM) allows for the fabrication of implant shapes with almost no restrictions, and the production of porous materials whose mechanical properties are dependent on their geometrical parameters, which can be controlled. Therefore, AM allows for the production of implants having optimized shape and optimized functional gradation of mechanical properties. In addition, AM allows for quick design modifications and the possibility of patient-specific implants.ududThis thesis aims at taking advantage of the capabilities offered by AM for the design of the shape and functional gradation of the mechanical properties of hip stems in order to improve the mechanical compatibility with the bone (i.e. reduce the stress shielding and generate adequate interfacial stresses). To this end, finite element (FE) models are used to predict the mechanical behavior of the porous materials when designing hip stems. In a first step, the cost-effectiveness of the FE modeling approach for porous materials (e.g. beam or solid finite elements and sample size choice) was evaluated. In a second step, the irregularities that arise during the manufacturing process (e.g. strut inclination, strut diameter variation and strut elimination) were included in the FE model in order to enhance the correlation with experimental data. In the third part of this work, the shape and distribution of mechanical properties of a hip stem were optimized in order to reduce the bone loss and generate adequate interfacial stresses. The FE model of the porous material developed in the previous steps was then used to obtain the optimized distribution of mechanical properties of the stem.ududThe present work provides a methodology for obtaining accurate predictions of the mechanical behavior of AM porous materials. Furthermore, it provides a framework to conceive optimized hip implants having enhanced mechanical compatibility with the bone and produced with AM. To this end, optimized shape is combined with porous materials to achieve a functional gradation of the mechanical properties of the implants.