Case reporting and metal reaction: the big picture.

Cobalt alloys have been used successfully as bearings since the earliest days of arthroplasty, now seeing use in the overwhelming majority of knees and hips. Although reports of fracture, corrosion, and other unexpected events leading to revision are sporadically found throughout the literature over the last 3 decades, failure reports involving cobalt components have historically been dwarfed by failure reports of polyethylene oxidation, osteolysis, and poor fixation, among others. The current issue is a reminder that the human body and its permanently implanted devices constitute a fragile system, and even small perturbations of this system from cobalt alloys can result in untoward events with devastating outcomes. Through a thematic selection of case reports, the importance of good science and learning from the patient is underscored.

Over the last decade, we have seen an uptick in metal-related failure reports secondary to wear of metal-on-metal (MoM) articulations. Much of the science aimed at understanding the root problem has been outstanding and exceptionally informative for future implementation of any bearing couple, not just MoM. The in vitro focus has necessarily been on discrete contributors to the problem such as inclination angle, cup opening, materials, head size, patient selection, and so forth. Due in large part to these studies and registry-related reports of patient outcomes, MoM usage is now only indicated for a small segment of the population, and many manufacturers no longer market a hard-on-hard bearing couple.

We are nonetheless cautioned against losing sight of the bigger picture for both new and old problems. The case reports in this issue document important considerations relative to systemic introduction of cobalt alloys. The first should be read as a cautionary statement. Harris et al describe a situation in which a fractured ceramic hip was revised to metal-on-polyethylene (MoP), which subsequently wore at a very high rate. Although this is not an isolated incident, toxicity from this type of wear merits reporting because no systematic investigations have been performed to document this occurrence. It is worth noting that nearly a dozen other case reports reaching back to the mid-1990s can be found that chronicle this phenomenon. Although this does not constitute level I evidence, one must also consider that (1) ceramic bearings have been a small minority of implanted bearing surfaces since the 1970s; (2) only a small percentage of those have fractured (typically a fraction of a percent [1]); and (3) only a fraction of those failures are revised to MoP. Therefore, although reported numbers are low, the high wear rate of the revision cobalt head is to be expected from an engineering standpoint. From a practice standpoint, the revising surgeon and the patient are faced with the potentially unpalatable decision of implanting another ceramic device (the failure of which put them in this conundrum) or a metal device, which could possibly experience accelerated wear. In this situation, level V case reporting is likely the very best mechanism to exact change in clinical practice and direct users and consumers toward the best care decisions.

Even when the ceramic stays intact, there are challenges for the bearing couple and hence the patient. The article by Pulley et al is the first to show that the alternative hard-on-hard couple of ceramic-on-metal (CoM) is not immune from wear-related failure. Because the only source of cobalt in this system is the acetabular liner, elevated systemic cobalt levels are likely to have come from articular wear of the liner or corrosion on the nonarticular side of the liner. CoM components never gained a market foothold, but this case report provides an extraordinary “n = 1” validation of the clinically derived hypothesis from a plurality of prior authors that wear of the concave bearing is one of the primary contributors to the biological response of the host.

The next case report reminds us that the patient and their caregivers are the ones who bear the burden of adverse events. A devastating result is observed in the case by Martin et al, in which elevated systemic cobalt and chromium levels are implicated in cardiac toxicity. Although this case may represent the first documented fatality secondary to metal debris, it is an indication that one cannot focus on solely the local tissue response to a globally distributed insult to the biological system.

Each of the aforementioned case reports documents adverse biological reaction given the failure of a device to behave in its intended way. Such reports of reaction secondary to metal ion presence are becoming more prevalent in the literature and likely a more prevalent observation in the clinic. Case reporting is the earliest public indication of a potential failure mode, and the results should not be taken lightly. That said, generalization from case reports should only be done with caution.

We are thus left in a quandary when reviewing the final case report by Moskal and Stover. Their report implicates corrosion at the modular interface in the adverse local response of the joint. In light of two of the previous case reports in this issue, the MoM bearing in their report must be suspect. We are nonetheless faced with the question of how much the taper corrosion contributed to the failure. To provide context, other case reports may be found in the literature including those wherein the intended articulation (MoP) is highly unlikely to be the source of metal ions 2, 3. The only remaining source is the modular interface.

Because modular interfaces have been around for >30 years, the reader must consider whether we have become hypervigilant in the post-MoM era of patient assessment, or whether we are beginning to see outcomes related to relatively recent changes in taper design such as length of taper engagement, microgrooving of the male taper, and, in some cases, modification of the taper angle. The number of variables between and within manufacturers makes definitive conclusions challenging without large numbers. We find ourselves at the potential tip of an iceberg with respect to corrosion of modular devices but with limited options to control an experiment to prove any hypotheses.

The momentous addition of the American Joint Replacement Registry annual report to Arthroplasty Today will allow us to potentially map these single case reports to clinical results using large data sets. Moreover, the American Joint Replacement Registry allows us to see how recent science impacts clinical decision-making. For instance, the report in this issue documents a recent increase in use of ceramic heads. This increase may be a function of documented lower wear rates [4], or it could be an indicator of surgeon preference to avoid corrosion of the modular interface with a CoCr head, as described in this issue's technique guide by Leibiger and McGrory.

In the face of multiple and possibly conflicting information sources, the challenges to our science become those of careful reporting, good statistics, thorough documentation of causation, and addressing design deficiencies. Dr. Philip Noble has elegantly noted that the ability of a registry to document an increased failure rate is only as strong as the rate of participation in the program and sufficient time to collect enough data to generate robust statistics [5]. Thus, we cannot depend on registry data alone for the most recent indications of failure. If the earliest, but least statistically robust warnings come from case reports, and the later, but statistically powerful population statistics come from a registry, the gap in early warning systems can be filled by explant analysis accompanied by good basic science. In aggregate, these three information sources represent a powerful triad of sentinels that can establish causation and provide a pathway to ameliorate future patient harm through improved design, watchful waiting, or, in extreme cases, a recall.

Congratulations to Arthroplasty Today on completion of the first year of high-quality, fast turnaround reporting. My colleagues and I look forward to many stimulating, high-impact years to come.