Atomic Scale Origin of Metal Ion Release from Hip Implant Taper Junctions

Abstract

Millions worldwide suffer from arthritis of the hips, and total hip replacement is a clinically successful treatment for end-stage arthritis patients. Typical hip implants incorporate a cobalt alloy (Co-Cr-Mo) femoral head fixed on a titanium alloy (Ti-6Al-4V) femoral stem via a Morse taper junction. However, fretting and corrosion at this junction can cause release of wear particles and metal ions from the metallic implant, leading to local and systemic toxicity in patients. This study is a multiscale structural-chemical investigation, ranging from the micrometer down to the atomic scale, of the underlying mechanisms leading to metal ion release from such taper junctions. Correlative transmission electron microscopy and atom probe tomography reveals microstructural and compositional alterations in the subsurface of the titanium alloy subjected to in vitro gross-slip fretting against the cobalt alloy. Even though the cobalt alloy is comparatively more wear-resistant, changes in the titanium alloy promote tribocorrosion and subsequent degradation of the cobalt alloy. These observations regarding the concurrent occurrence of electrochemical and tribological phenomena are vital to further improve the design and performance of taper junctions in similar environments.

Keywords: Morse taper junctions; biomedical titanium alloys; cobalt–chromium–molybdenum alloys; total hip replacement; tribocorrosion.

Figure 1

Initial microstructure and composition of the titanium (Ti‐6Al‐4V) and cobalt (Co–Cr–Mo) alloys before fretting. A) SEM‐BSE image of the cross‐section of the titanium alloy showing the α (area fraction ≈ 90%) and β phases. B) APT reconstruction showing the distribution of elements in the subsurface. C) 1D composition profile along the dashed arrow in (B), with the composition of the α and β phases indicated. D) ECCI image of the cobalt alloy surface. E) APT reconstruction of the cobalt alloy subsurface. F) 1D composition profile along the dashed arrow in (E), showing the homogenous composition. G) Schematic representation of the in vivo fretting location in a modular hip implant and the in vitro fretting test.

Figure 2

Microstructure of the wear surface of the titanium alloy after fretting. A) SEM‐BSE image of the prominent features on the wear surface: raised shelves and troughs filled with grainy debris. B) TEM‐BF image of the cross‐section of a shelf. Oxides are present on the surface and the subsurface consists of zone 1 (refined matrix phase) and zone 2 (ultrafine equiaxed grains). The diffraction pattern from zone 1 (dotted circle) indicates a partially amorphous matrix. C) TEM‐BF image of cusping and subgrain formation inside the α ribbons seen in the yellow square in B). D) STEM‐BF image of the subsurface oxide seen in the blue square in (B). Nanobeam diffraction patterns indicate that the oxide is crystalline and the surrounding matrix is partially amorphous.

Figure 3

Subsurface of a shelf found on the titanium alloy wear surface after fretting. A) STEM‐HAADF image of the APT specimen from the shelf subsurface. B) corresponding APT reconstruction of the shelf surface showing the surface and subsurface oxides, fragmented α particles and the diffuse interface between the subsurface oxide and β matrix. The approximate position of the APT specimen is marked in Figure 2B. The fragmented particles are isosurfaces generated from a defined range of concentrations. Due to the slight composition variation between the fragments, some features of the STEM image (Figure 3a) are not highlighted in the APT reconstruction (Figure 3b). The regions of interest (ROI 1 to 4) marked by dashed arrows are locations of the 1D composition profiles that will be depicted in Figure 4.

Figure 4

1D composition profiles of the subsurface features in the titanium alloy after fretting. The profiles are extracted from the regions of interest marked in the APT reconstruction in Figure 3B. A) ROI 1: surface oxide and its interface with the matrix, B) ROI 2: subsurface oxide band. C) ROI 2: Magnified view of (B) showing the constituents of the tribomaterial and cobalt alloy present in the subsurface oxide band. D) ROI 3: diffuse interface, between the subsurface oxide band and the matrix, that is enriched in oxygen. E) Titanium rich fragmented α particles in the matrix.

Figure 5

Microstructure of the wear surface of the cobalt alloy after fretting. A) SEM‐BSE image of the prominent features on the wear surface of the cobalt alloy: fine and coarse grooves. B) STEM‐BF image of the subsurface of a groove. C) STEM‐BF image of the APT specimen and D) corresponding APT reconstruction of the cobalt alloy subsurface. E) 1D composition profile along the dashed arrow shown in (D).

Figure 6

Schematic representation of the mechanism of fretting corrosion and metal ion release in the titanium–cobalt alloy couple. A) initial state of the tribosystem, with the loading and fretting directions marked. B) early stage where the titanium oxide wear debris ploughs the titanium alloy surface to form a chip that remains on the surface due to a folding mechanism. The debris and chips formed on the titanium alloy surface can microplough the cobalt alloy surface, thus damaging the passivation layer of the cobalt alloy. C) final stage of the fretting test where the chips formed on the titanium alloy surface pile up on each other during successive fretting strokes and are laminated together to form raised shelves. The shelves contribute to microploughing of the cobalt alloy surface. When the passivation oxide layer of the cobalt alloy is compromised, dissolution of cobalt, chromium, and molybdenum into the interfacial medium takes place. The metal ions from the corroding cobalt alloy, the oxide‐particle laden liquid fraction of the tribomaterial formed between the surfaces as well as refined grains from the titanium alloy are all incorporated into the shelf during the folding process.