CORR Insights®: Are Damage Modes Related to Microstructure and Material Loss in Severely Damaged CoCrMo Femoral Heads?

Where Are We Now?

Mechanically assisted crevice corrosion (MACC) of cobalt-chrome-molybdenum (CoCrMo) alloy heads within the modular taper junctions of total hip replacements (THRs) has gained increased clinical and patient attention due to its association with soft tissue pathologies known as adverse local tissue reaction (ALTR) or adverse reaction to metal debris. While these soft tissue reactions were first clearly identified with CoCrMo metal-on-metal bearing THRs in the mid-2000s, more recent studies of metal-on-polyethylene bearing systems with modular taper junctions have documented an association of taper corrosion with similar soft tissue reactions [1] that was not well understood previously. Thus, because modularity is a near-universally used design concept and there are major clinical concerns associated with these tapers, this remains a worthy topic of study in orthopaedics.

In the current study, McCarthy et al. [4] report on severe corrosion of CoCrMo head tapers in retrieved modular junctions that interact with both CoCrMo and Ti-6Al-4V stems. They identify a dichotomy in the nature of the damage on the CoCrMo head tapers: either mechanically driven or more chemically driven. These observations show corrosion within modular tapers is not solely the result of mechanical wear processes, but that conditions within the taper environment can lead to crevice corrosion reactions that are not well understood in these alloys and have not been duplicated in the laboratory.

One aspect of the electrochemically-driven degradation of CoCrMo alloys, on which this paper focuses, is the metallurgical state of the wrought high-carbon and low-carbon CoCrMo head alloys used in THRs. The authors have identified conditions of the alloy microstructure of the wrought low- and high-carbon CoCrMo alloy that relate to a chemical heterogeneity within the alloy, resulting in bands of local variation in alloy composition. Specifically, there are spatially oscillating Mo and Cr concentrations of a few percent aligned in the wrought bar axial direction that appear to be the result of alloy fabrication (the extrusion of bar stock from which the heads are machined) and heat treatment conditions. It should be noted that metallurgical heat treatments are often used after fabrication of the bar stock to affect the alloy structure and properties that include homogenizing the chemistry, relieving internal stresses, and affecting the grain size and deformation state. Not all wrought CoCrMo alloy exhibits such banding in chemistry, and this periodic compositional variation may be the result of incomplete homogenization annealing of the alloy after thermomechanical treatment (bar extrusion). It is unclear at present how and why some alloy bar stock exhibits such inhomogeneity and others do not.

The authors assessed how this inhomogeneity in the microstructure may have affected material loss in vivo from a set of retrieved, severely corroded CoCrMo heads. Different degradation modes were defined as “chemical” (really, electrochemical) and “mechanical,” and it was found that the most severely damaged head tapers (those with the most material loss) were associated with the chemical damage mode that led to column damage associated with the chemical inhomogeneity. This result clearly identifies that MACC is not just a fretting process, but that corrosion-dominated processes can develop within the body leading to the most severe damage.

Where Do We Need To Go?

It is important for clinicians, scientists, and engineers to better understand the biological, chemical, and metallurgical basis for such electrochemically-driven events and the interplay between these factors, which impact the local solution environment both outside and within the crevice region. For example, are there specific immune cells associated with the more electrochemically-driven corrosion processes that may contribute to the local conditions driving electrochemically-based taper corrosion? Are there signs of immune cells proximal to tapers or within taper crevices that may be attracted by the corrosion or affect the processes of corrosion?

Examples of MACC in vivo were described in the early 1990s for modular tapers, and with each passing year, we learn more about it. Still, this complex degradation mode is dependent on a wide range of patient, surgeon, implant design, and alloy variables that are not yet clearly understood. Questions remain regarding MACC, including the nature of the chemical environment within the crevice, the role of implant electrode potential and its variation with patient activity, the stability of the passive oxide films that form on these alloys, and the progression of damage mechanisms (fretting in a crevice resulting in the initiation of ongoing crevice corrosion) that may be at play. In addition, its association with ALTR in some patients and not others remains unclear.

Additionally, this paper raises important questions regarding the relative importance of underlying electrochemical, mechanical, and biological factors that affect the incidence and severity of modular taper corrosion and its impact on adverse biological reactions that may arise and interact with the corroding implant. A continuing focus on only mechanical aspects of fretting corrosion in tapers will not address these important additional factors related to the degradation of tapers. Since the first study on MACC in 1993 [2], the scientific community has been predominantly focused on the mechanical (fretting/wear) aspect of the process. Indeed, it is often the case that papers refer to “taper wear” and only focus on mechanical factors as contributors. This study points out the importance of the “crevice” and the “corrosion” aspects of the overarching MACC mechanism, along with the biological contributions to the local solution chemistry that remain poorly understood and little studied. Additional studies in these areas will need to address the ongoing degradation seen in taper junctions and the clinical consequences observed.

Recent work has shown, for example, that MACC is not the whole story and that crevice corrosion processes (even without ongoing fretting) can develop and propagate [13]. But we have not figured out how this works for Ti or CoCr alloys in vivo. More interdisciplinary work between clinicians, metallurgists, corrosion scientists, and immunologists is needed to understand the mechanisms at play and to develop strategies, materials, and/or designs that minimize the occurrence of fretting-initiated crevice corrosion.

Such “chemically” driven modes of alloy degradation in vivo are highly dependent on the local solution chemistry in (or near to) the crevice of the taper in vivo, the electrode potentials possible in these devices, and/or the biological species that may impact on corrosion. We know very little about the local joint solution chemistry (as it pertains to corrosion of CoCrMo or Ti-6Al-4V) or its modification within the taper crevice. What chemical or biological species are arising within the joint space in patients with possible ongoing immune and/or inflammatory processes that may impact corrosion? Electrode potential, such as potential drops across the metal-solution interface that develop when metals are placed into ionic solution, impacts the corrosion processes and is affected by them as well. These potentials are highly dependent on the mechanical disruption of the passive oxide films that arises during fretting and can change by as much as -1 V in these alloys [3, 9], giving rise to a complex feedback interaction on oxide film behavior. All of these factors remain poorly understood in terms of how and when the electrochemically driven modes of corrosion develop in vivo and point to a number of gaps in our understanding of MACC and its clinical consequences.

How Do We Get There?

The complex body environment is highly variable, where changes may arise due to the inflammatory state of the patients’ joint [5] as well as the possible chemical additions that may arise from electrode reactions at the alloy surfaces [10, 12]. These local environmental conditions may be critical in determining when a chemically dominated degradation process may arise. Indeed, recent work has shown that an electrochemical attack outside of modular taper regions can arise in, for example, acetabular taper junctions [8] remote from where fretting may or may not be taking place. Reduction reactions, which must occur in equal proportion to the oxidation reactions, have been shown to generate reactive oxygen species (ROS), which can, themselves, degrade and damage the passive oxide film. Oxidation products (metal ions) may also alter the local crevice solution chemistry and cause a loss of the passivity of the oxide film. In addition, the generation and presence of biologically derived oxidizing agents like ROS and other proteins, enzymes, and biochemical species may play a role in affecting alloy surfaces and their corrosion resistance.

Corrosion of these alloys in vivo, across a range of medical device applications, will continue to be of concern until we are able to reproduce the crevice corrosion conditions needed to drive degradation and use such knowledge to develop materials and design strategies that minimize corrosion-based damage.

Not all CoCrMo alloys are the same. Orthopaedic device manufacturers and corrosion scientists need to engage the suppliers of CoCrMo alloys in the bar stock used to make CoCrMo heads. The goal would be to examine the various processing-structure-property relationships that are linked to the banded microstructural heterogeneity and the thermomechanical and post-processing heat treatments used to make the starting alloys.

Correlating processes to structure and structure to electrochemical behavior will be critical. To achieve this, a combined, interdisciplinary team approach is needed. A clinical study of the possible joint fluid conditions within the patient and, critically, within the modular tapers of retrieved implants will help us better understand the electrochemically dominated degradation modes. Collaborations between clinicians, biologists, and corrosion experts are needed to design human joint fluid electrochemical tests to recapitulate the observed in vivo corrosion processes. Future studies should also develop simulated inflammatory joint fluid conditions [11]. Researchers can then test the corrosion characteristics of both CoCrMo and Ti-6Al-4V.

There are possible materials-driven design approaches [6, 7] that may address fundamental elements of the MACC process in modular tapers, and these alternatives may help to alleviate, minimize, or eliminate this challenging and complex problem. Only with deep knowledge of the complex mechanisms and the approaches that may address these can we find ways to further mitigate modular taper corrosion.