Ti-6Al-4V is the most common alloy used for orthopaedic implants. Its popularity is due to low density, superior corrosion resistance, good osseointegration and lower elastic modulus when compared to other commonly used alloys such as CoCrMo and stainless steel. In fact, the use of Ti64 has even further increased lately since recent controversy around adverse local tissue reactions and implant failure related to taper corrosion of CoCrMo alloy. However, implants made from Ti64 can fail in some cases due to fatigue fracture, sometimes related to oxide induced stress corrosion cracking or hydrogen embrittlement, or preferential corrosion of the beta phase. Studies performed with Ti-6Al-4V do often not consider that the alloy itself may have a range of characteristics that can vary and could significantly impact the implant properties. These variations are related to the material microstructure which depends not only on chemical composition, but also the manufacturing process and subsequent heat treatments. Different microstructures can occur in implants made form wrought alloys, cast alloys, and more recently, additive manufactured (AM) alloys. Implant alloy microstructure drives mechanical and electrochemical properties. Therefore, this study aims to analyse the microstructure of Ti-6Al-4V alloy of additive manufactured and conventional retrieved orthopaedic implants such as acetabular cups, tibial trays, femoral stem and modular neck by means of electron backscatter diffraction (EBSD). Microstructural features of interest include grains shape and size, phase content and distribution, preferred grain orientation (texture), alloying elements distribution (homogenization) and presence of impurities. Additionally, we demonstrate the direct impact of different microstructural features on hardness. We analysed 17 conventional devices from 6 different manufacturers, 3 additive manufactured devices from 2 different manufactures and 1 control alloy (bar stock). The preliminary results showed that even though all implants have the same chemical composition, their microstructural characteristics vary broadly. Ti64 microstructure of conventional alloys could be categorized in 3 groups: equiaxed grains alloys (Fine and Coarse), bimodal alloys and dendritic alloys. The additive manufactured implants were classified in an additional group on its own which consists of a needle-like microstructures - similar to Widmanstätten patterns, Fig. 1, with a network of β phase along α phase grains. Furthermore, AM alloys exhibited residual grain boundaries from the original β grains from the early stage of the solidification process, Fig. 2. These characteristics may have implication on the fatigue and corrosion behaviour. In addition, it we observed inhomogeneous alloying element distribution in some cases, Fig. 3, especially for the additive manufactured alloys, which also may have consequences on corrosion behaviour. Finally, the hardness testing revealed that the implants with large grain size, such as AM alloys, exhibit low hardness values, as expected, but also the amount of beta phase correlated positively with lower hardness. Grain aspect ratio and beta phase grain size correlated positively with higher hardness. In summary, we found that common Ti64 implants can exhibit a broad variety of different alloy microstructures and the advent of AM alloys introduces an entirely new category. It is imperative to determine the ideal microstructure for specific applications.

Fretting corrosion of taper junctions is long known and of great concern, because of metal ion and particle release and their related adverse local and systemic effects on the human body (1–3). Orthopedic taper junctions are often comprised of CoCr29Mo6/TiAl6V4 pairings. Beside others the imprinting of the TiAlV-machining marks into the CoCrMo-taper is of clinical interest (4, 5). Thus, the multifactorial details and their interdependencies on the macro-, micro, and nanoscale are still a matter of research (6). This contribution presents the mechanisms of imprinting found in an in-vitro fretting corrosion test. The worn surfaces, the lubricant as well as its remains were analyzed after test and the findings brought into relation to the characteristic wear sub-mechanisms. The fretting tests were conducted by means of a cylinder-on-pin set-up. All details about the test and the sequence of analyses can be found in (7, 8). A marked tribofilm of C-rich organic matter and oxidized wear particles of both bodies was generated at the TiAlV/CoCrMo contact area (Figure 1a, c). After removing the tribofilm chemically, extremely fine scratches of sub-µm depth became visible on the CoCrMo body (Figure 1b). The TiAlV body showed shallow shelves leaving troughs filled with grainy debris (Figure 1d) mainly of Ti-oxide wear particles. The shelves stick to the surfaces and, therefore, move relatively to the counterbody. In combination with the grainy debris this brings about “Microploughing” on the CoCrMo surfaces. Microploughing is known for destroying any passive film resulting in “Tribocorrosion”. The question remains how the shelves are formed. From the surface analyses one could conclude that they point towards “Delamination”. But this would also mean that they would not stick rigidly to the surfaces but be ejected from the contact area. Focused Ion Beam (FIB) cuts were done in order to investigate the near- and subsurface structure of the shelves in order to clarify the governing mechanisms (Figure 2). Below the platinum protection layer appears a laminated structure of highly deformed nanocrystalline and amorphous areas. EDS confirmed that the lighter intermediate layers consist mainly of Ti-oxide. This microstructure is supposedly formed by severe plastic deformation and the generation of shear bands, which under fretting pile up on top of each other. This cannot be connected to “Delamination”. We therefore propose to categorize the formation mechanism of these shelves as a specific form of microploughing. Thus, imprinting is neither driven by any galvanic effects (9) nor by hardness differences of TiO₂ and Cr₂O₃ (10) but by microploughing on the TiAlV-body leading to tribocorrosion at specific sites of CoCrMo what imprints the surface grooves of the softer TiAlV into the harder CoCrMo.

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With its high wear and corrosion resistance, CoCrMo alloy has been widely used for metal-on-metal total hip replacements (THRs). However, the use of the metal-on-metal implants has dropped substantially as a result of several alerts issued by the Medicines and Healthcare products Regulatory Agency (MHRA) due to concern on metal ion release [1]. However, some of the first generation of metal-on-metal THRs have lasted for more than 20 years [2]. It is far from clear why some MoM joints have survived, while other failed. It is known that dynamic changes occur at the metal surface during articulation. For example, a nanocrystalline layer has been reported on the topmost surface of both in vivo and in vitro CoCrMo THRs [3, 4] but it is not known whether this layer is beneficial or detrimental.

The current work focuses on the sub-surface damage evolution of explanted MoM hips, which is compared to in vitro tested CoCrMo hip prostheses. Site-specific TEM cross-section of both in vivo and in vitro CoCrMo samples were prepared by focused ion beam (FIB) in situ lift-out method (Quanta 200 3D with Omniprobe, FEI, the Netherlands). TEM of the FIB specimens was performed on various microscopes. Routine bright field imaging was performed on a Tecnai 20 (FEI, the Netherland) operating at 200 kV, while high resolution transmission electron microscopy (HRTEM) of the nanocrystalline layer and other surface species was undertaken on a Jeol 2010F (Jeol, Japan) operating at 200 kV.

A nanocrystalline layer (which was not present on the starting surfaces) was observed on both explanted in vivo and in vitro tested materials. For the explanted joints, the nanocrystalline layer was thin (a few 100 nm) and the extent did not appear to correlate with the local wear rate. For in vitro samples, the nanocrystalline layer was thicker (up to micron). HRTEM from this layer are shown in Fig. 1 and Fig. 2. The nanocrystallite size was ∼5 nm and appeared to be a mixture of face centred cubic and hexagonal close packed phases. The formation of the nanocrystalline layer and its correlation with wear behaviour are discussed.