Abstract
Introduction
There are increasing reports of total hip replacement (THR) failure due to corrosion within modular taper junctions, and subsequent adverse local tissue reactions (ALTRs) to corrosion products. Modular junction corrosion is a multifactorial problem that depends on material, design, patient and surgical factors. However, the influence of alloy microstructure on corrosion has not been studied sufficiently. Especially for cast CoCrMo, there are concerns regarding microstructure variability with respect to grain size and hard-phase volume fraction. Therefore, it was the goal of this study to (1) identify different types of microstructures in contemporary implants, and (2) determine implications of alloy microstructure on the occurring corrosion modes.
Methods
Fifteen surgically retrieved femoral stems made from cast CoCrMo alloy were analyzed for this study. Damage on the taper surfaces was investigated by scanning electron microscopy (SEM) and damage was assessed with the Goldberg Score. The alloy microstructure was evaluated by standard metallographic techniques. Alloy samples were sectioned off the femoral stem, and microstructural features were visualized by chemical etching. Cyclic potentio-dynamic polarization tests were carried out with alloy samples from two implants with different commonly occurring types of microstructures. Both had a similar grain size, but type 1 had no hard-phases, where as type 2 exhibited hard-phases along the grain boundaries, as well as intra-granular hard-phase clusters. Tests were performed in bovine serum at 37°C with a saturate calomel reference electrode and a graphite counter electrode. In vitro generated corrosion damage was then compared to in vivo generated damage features on the taper surfaces of the corresponding implants.
Results
Tapers with high damage scores exhibited varying degrees of grain and phase boundary corrosion, along with fretting and pitting corrosion. In several cases thick chromium oxide films were observed. The metallographic analysis showed that nominally identical alloys (ASTM F75) exhibited a broad variability in grain size (250 micrometers to several millimeters), hard-phase volume fraction (0–6%), and hard-phase type (carbides and intermetallic phases). The corrosion tests revealed that the alloy without hard-phases (type 1) had a significantly higher pitting potential (p=0.001) than type 2 alloy without hard-phases. After testing, both alloys exhibited grain boundary corrosion. However, type 2 had a higher degree of material loss due to hard-phase detachment. Additionally, type 2 exhibited pitting within the grains around hard-phases, along with the formation of thick oxide films which was consistent with the lower pitting potential. The results also corresponded with the damage features on the corresponding tapers, where type 1 exhibited only mild damage features, and type 2 underwent severe grain and phase boundary corrosion along with thick oxide films (Figure 3).
Discussion
It appears that the alloy microstructure drives local modes of corrosion. Additional phase boundaries due to hard-phase content promote corrosion. The fact that the same alloy can differ broadly even within the same design shows that material standards are currently not sufficient. Optimizing implant alloys will help to reduce in vivo corrosion processes, and subsequently the risk of implant failure due to ALTRs.
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