Comparison of 3D printed trabecular structure with porous plasma spray: A method based on mapping the local modulus of elasticity

The surface of orthopaedic bone implants is in most cases formed by porous plasma spray. The introduction of 3D printing into the production of implants has made it possible to replace plasma spray with a trabecular bone-like structure that can be fabricated by 3D printing. The principle is to create a suitable surface porosity for anchoring the implant in the bone and adapting the modulus of elasticity to the bone properties. The elastic behaviour of both the structures can be compared by means of a compression test, but it is not possible to compare the modulus of elasticity at the local sites. The aim of the work was to verify the possibility to use the nanoindentation method for the local testing of the modulus of elasticity. The selected method made it possible to map the so-called reduced modulus of elasticity at the precisely defined places. Specifically, the 3D printed trabecular structure of Ti6Al4V ELI was compared with titanium plasma spraying. The printed structure with a bulk porosity of 77% showed the local modulus of elasticity in the range of 57-97 GPa depending on the test site. In contrast, for the plasma spray with a volume porosity of 28%, the modulus of elasticity was around the average value of 54 GPa. At the same time, the microstructure and chemical composition were analysed at the critical points of the structure crossover or the transition from the bulk material to the trabecular structure. No change in the microstructure or chemical composition was observed. Both structures bring the modulus of elasticity closer to the bone modulus of elasticity, but 3D printing offers more possibilities to modify the elastic behaviour and the shape and pore size as well.