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

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.