Mechanical, chemical and biological damage modes within head-neck tapers of CoCrMo and Ti6Al4V contemporary hip replacements

Abstract

Total hip replacement (THR) failure due to mechanically assisted crevice corrosion within modular head-neck taper junctions remains a major concern. Several processes leading to the generation of detrimental corrosion products have been reported in first generation modular devices. Contemporary junctions differ in their geometries, surface finishes, and head alloy. This study specifically provides an overview for CoCrMo/CoCrMo and CoCrMo/Ti6Al4V head-neck contemporary junctions. A retrieval study of 364 retrieved THRs was conducted which included visual examination and determination of damage scores, as well as the examination of damage features using scanning electron microscopy. Different separately occurring or overlapping damage modes were identified that appeared to be either mechanically or chemically dominated. Mechanically dominated damage features included plastic deformation, fretting, and material transfer, whereas chemically dominate damage included pitting corrosion, etching, intergranular corrosion, phase boundary corrosion, and column damage. Etching associated cellular activity was also observed. Furthermore, fretting corrosion, formation of thick oxide films, and imprinting were observed which appeared to be the result of both mechanical and chemical processes. The occurrence and extent of damage caused by different modes was shown to depend on the material, the material couple, and alloy microstructure. In order to minimize THR failure due to material degradation within modular junctions, it is important to distinguish different damage modes, determine their cause, and identify appropriate counter measures, which may differ depending on the material, specific microstructural alloy features, and design factors such as surface topography. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1672-1685, 2018.

Keywords: corrosion; electron microscopy; fretting; implant retrieval; joint replacement.

Figure 1

Bar diagrams illustrating the prevalence of minimally, mildly, moderately and severely damaged stem (A) and head (B) tapers for Co/Co couples and for Co/Ti couples.

Figure 2

Bar graphs illustrating the prevalence of specific damage modes on moderately to severely damaged stem tapers (A), and head tapers (B). Numbers are based on the subset group that was analyzed by SEM, except for cases marks with (*) which include all moderately to severely damaged tapers.

Figure 3

SEM images of deformed machining marks with arrows indicating the following features: A) Flattened machining mark peak on a CoCrMo alloy head coupled with a CoCrMo alloy stem, B) Flattened machining mark peaks with groves in proximal/distal direction on a CoCrMo head coupled with a Ti6Al4V stem, C) Flattened machining mark peak on a Ti-alloy stem taper. The flattened area stretches over 100 micrometers and exhibits deep grooves in the proximal distal direction, D) SEM image of a severely deformed area that was subjected to toggling damage. The inset photograph of the entire stem taper shows the extent of the damage mark, which can be seen by eye. For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 4

A) Back scatter electron (BSE) SEM image of a Co/Ti head taper surface. Grey areas (arrows) represent flakes of Ti6Al4V that appear to have transferred onto the head taper surface. The EDS spectrum (inset) exhibits all three alloying elements (Ti, Al, V) indicating that adhesion took place. B) 3D reconstruction of the same head taper surface based on optical CMM data. It can be seen that a considerably large area is affected by Ti6Al4V alloy material transfer (darker area, arrows). C) BSE image of a Co/TI head taper surface exhibiting large amounts of transferred material (arrows). The EDS spectrum indicates the presence of Cr, Mo, Ti and oxygen, and little to evidence of Al and V. Based on chemical composition and appearance, it appears an accumulation of oxidized particles and corrosion products took place rather than material transfer due to adhesion. D) Surface of a Ti6Al4V stem taper which exhibits material breakouts due to material transfer (arrows). For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 5

Fretting and fretting corrosion damage on CoCrMo alloy stem tapers (taper axis is vertical in all images) : A) Typical damage features of mechanically induced fretting wear. The lower half of the image shows fine ridges (arrow and higher magnification inset) that are usually located in areas of former machining marks and oriented perpendicular to the taper axis. B) SEM image of randomly oriented mechanically induced fretting marks after complete disruption of the initial machining mark topography. C) SEM image showing elongated pits, a typical damage feature of electrochemically-dominated fretting marks. D) Example of overlapping fretting damage features that were caused by mechanical and electrochemical fretting processes. For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 6

A) SEM image of a CoCrMo stem taper exhibiting pitting. Pitting was characterized by areas covered with fine round pits. It appeared that pits occurred first within in troughs of the machining mark topography, then expanded to other areas as damage progressed. B) Close up image of a pitted area of a CoCrMo stem taper. C) Pitting also occurred in the non-contact area of femoral heads. Due to the shallower machining mark topography, pitting was independent of machining mark troughs. D) Close-up image of a head taper surface that underwent complete disruption of the initial surface due to pitting corrosion. For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 7

A) Typical damage feature on CoCrMo head taper associated with etching with visible fine grain structure (grain size of ~8μm), twin boundaries, and slip planes. B) In some cases, the head taper surface exhibited a higher density of slip planes, fine pits and the presence of a thin carbon rich film associated with etching. C) Similar etching features were also found on CoCrMo stem tapers that were made from wrought CoCrMo alloy, however, this appeared to have been the case for a only a few THR designs based on the etching damage patterns observed. D) Most stem tapers were made from cast CoCrMo alloy, and in cast CoCrMo alloys etching is less obvious due to the large grain size. Yet, etching did occur as shown by the visualization of slip planes in this image. Also, there appeared to be a slight step between grains indicating different corrosion rates depending on grain orientation. This image also shows the result of intergranular corrosion on cast alloy samples resulting in wide trenches that are partially or entirely filled with carbon rich organic residue (arrow).

Figure 8

A) 3D reconstruction of a severely damaged CoCrMo alloy stem taper. The surface was affected by intergranular corrosion over nearly the entire contact area (arrows). B) SEM image of an area affected by intergranular corrosion. C) Close up image of a grain boundary that underwent severe material dissolution. Locally hard phase fragments can be detected that are located at the grain boundaries (white arrows). In this case, there was clear evidence of fretting marks on the bordering grains (black arrows). D) SEM image of a mixed hard phase located within the grain of a cast CoCrMo alloy stem taper. It can be seen that there was considerable material loss around the hard phase due to phase boundary corrosion (white arrows). In the upper left corner holes and voids can be seen that are the result of phase boundary corrosion and subsequent hard phase detachment from the surface. Those holes have partly refilled with organic residue and mixed metal oxides (black arrows). Areas around hard phases were also affected by pitting corrosion. For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 9

A) SEM image of thick deposit films covering an area of a CoCrMo stem taper that was affected by pitting corrosion. The EDS spectrum exhibited a prominent oxygen peak along with chromium and molybdenum, thus indicating that the deposits consisted mainly of mixed oxides. Cobalt was not present within the film. B) Another typical deposit (here on a CoCrMo head taper) that is usually located toward the distal end of the stem taper and around the opening of the head taper. The EDS spectrum revealed a prominent peak of phosphorus along with chromium and oxygen, thus confirming the presence of chromium phosphate. C) A Ti-alloy stem taper with another type of deposit, which was a carbonaceous- rich film that also exhibited a molybdenum peak, but no other metal. Often these deposits accumulated within the troughs of the stem taper topography. For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 10

SEM images showing the imprint of the deeper machining mark topography of the stem taper (black arrows) into the smoother surface of a head taper (white arrows) for A) a Co/Co, and B) a Co/Ti modular junction. In both cases the imprinted surface on the head taper was fairly clean with only minimal amounts of deposits or organic residue. For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 11

Unwrapped 3D reconstructions of severely damage head tapers that exhibit column damage. Column damage was characterized by a banded structure that was strongly oriented in proximal-distal direction, parallel to the taper axis. A) On this head taper it can be see that column damage (marked area) started out on the proximal end of the taper (top of image A) and progress towards the middle in the distal direction. The column damage pattern did not cover the entire contact area between head and stem taper. B) In this case almost the entire head taper surface was affected by column damage (marked area). It appears that it even reached slightly outside of the contact area on the proximal end (top of image B). For all images, the tapers are oriented proximal (top) to distal (bottom).

Figure 12

A) SEM micrograph of column damage on a femoral head taper that is characterized by deep grooves or troughs in proximal distal direction. B) Close up image on column damage showing the etched appearance with in the troughs of the damage pattern. Locally, organic residue has accumulated within the troughs. C). Close up image of shallow troughs within the column damage pattern of a severely damaged femoral head. Organic material remains within the troughs with a morphology and size that compares well to cells such as macrophages. D) In some cases, an etching trail follows these cell-like features (arrow). For all images, the tapers are oriented proximal (top) to distal (bottom).