BETTER - New biocompatible, low stiffness, BETa-Ti ankle EndopRostheses manufactured by Laser Powder Bed fusion with functionally graded lattice (FGL) structure

Abstract:

The project aims at applying an innovative beta-Ti alloy (Ti-Nb-Zr) for the design and manufacturing of a novel talar component (NTC) for total ankle replacement (TAR) fabricated by Laser-based Powder Bed Fusion (LPBF). TAR is increasingly preferred to othertreatments and the application of LPBF and beta-Ti alloys to TAR could overcome current limitations, like poor bone adaptation leading to long-term implant instability. The beta-Ti alloy is considered in substitution of the Ti6Al4V alloy, a gold standard orthopaedic implant material widely used for its strength, corrosion resistance, and biocompatibility (used as benchmark in the present proposal). A limitation of the Ti6Al4V alloy, in fact, is the mismatch between its elastic modulus (100–110 GPa) and that of the bone (10–30 GPa), leading to the stress-shielding phenomenon which, in the long term, causes bone resorption and implant loosening. Moreover, it has been recently reported thatelements such as V and Al have potentially cytotoxic effects.Extensive studies have been performed to develop novel beta-Ti alloys suitable for biomedical applications, characterized by lower elastic modulus (up to 50 GPa) and non-toxic alloying elements, thus ensuring improved biocompatibility properties. Moreover, by focusing on the personalised design and manufacturing, the reduction of stress shielding issues can be enhanced using functionally graded lattice structures (FGL), with tailored porosity gradients, fabricated by AM technologies, like LPBF.

The development of the Project (Fig.1) will be achieved by:

• Optimization of the LPBF manufacturing parameters and post process heat treatment by preliminary characterization of beta-Ti alloy (composition, density, hardness, microstructure).
• Characterization of samples produced under optimized process conditions, including tensile, wear and corrosion tests, as well as microstructural and fractographic analyses. The LPBF Ti6Al4V alloy will be used as reference alloy.
• Medical imaging of three ankle specimens which will undergo TAR, to design the NTC and validate the overall customizationprocedure.
• Customization of the TAR positioning and of the cutting jigs. The positioning of a commercial endoprosthesis will be optimized on the specimens, using a personalized ankle model.
• Customization of the NTC internal structure. Topological Optimization (TO) of a finite element method (FEM) model will beperformed, defining FGL that replicates stiffness of the talus.
• Production by LPBF of customized implant produced with the innovative beta-Ti alloy. Fabrication and test of the NTC. The component is printed, finished and heat treated and then geometrically and mechanically tested to validate the feasibility andreliability of the entire production cycle.
• Surgical implant of the TAR on the three ankle specimens to validate the overall customization procedure.