CORR Insights®: Highly Elevated Metal Ion Levels After Tumor Resections and Reconstruction With Megaprostheses Around the Knee

Where Are We Now?

Reconstruction of the femur and tibia is challenging after a large tumor surgery because of bone loss and the often substantial disruption of soft tissues. Raising the stakes are that many patients undergoing these operations are young and active, as was the case with the patients in the study by Dürr et al. [2], published in this issue of Clinical Orthopaedics and Related Research® (median [range] age 28 years [10 to 76]). It is critically important to keep these patients moving as they return to their lives and to provide them with reliable, stable, and responsive implants.

But the articulations in these large metallic implants are necessarily more constrained than are conventional hip or knee implants, and they often have more modular junctions, which raises questions about problems that sometimes have occurred with metal-on-metal hip and knee implants, including immune reactions to wear debris, galvanic corrosion, pseudotumors [3], and metal allergies. Dürr et al. [2] mention both local and systemic concerns associated with cobalt-chromium-molybdenum (CoCrMo) alloys, including implant breakage and weakening, metallosis, as well as cardiac and neurologic toxicity. There is also some concern about potential carcinogenicity from long-term exposure to Co and Cr ions in CoCrMo alloys [1], which could be related to the level of systemic ion exposure.

Dürr and colleagues [2] noted increased blood levels of Co, Cr, and presumably titanium in all of the patients, but they did not see correlations with time since implantation. Blood Cr levels ranged from 0 to 25 μg/L, with median levels being concerningly high based on what we think we know from studies of conventional and metal-on-metal THAs. Several patients with metallosis were noted in the study, but those patients did not appear to have experienced neurologic or cardiac events. Based on the findings of Dürr et al. [2], it seems like the balance between local toxicity and systemic issues may have been different for these megaprostheses than it was for metal-on-metal hip and knee implants (also perhaps because the articulation of these devices was more constrained). While the restoration of function to young patients with such large defects is encouraging, the ion levels are high enough to be quite concerning for long-term consequences. More information is needed to evaluate long-term success. A major challenge is balancing the need for aggressive restoration with the potential to create major long-term health issues in patients who beat their aggressive disease.

Where Do We Need To Go?

Generally, when dealing with articulating prostheses, the presence of elevated metal ions indicates increased wear and potential complications related to local and systemic toxicity. Of particular concern with metal alloys is the potential for corrosion (galvanic, fretting, or crevice) and tissue damage, which can be a contributing factor to the formation of pseudotumors and other severe local tissue reactions. One case series in metal-on-metal hip arthroplasty saw asymptomatic pseudotumors in 32% (10 of 31) of patients [6]. The difference here is that these implants are much larger and have more restricted articulating surfaces, and may therefore achieve similar systemic ion concentrations with less wear at each joint. It appears from the study of Dürr et al. [2] that these larger prostheses are successfully restoring function, but there was a wide range of follow-up (13 to 379 months), and 35% of patients developed mechanical complications. That said, these surgeries are intended to manage major deformities in young patients for whom there are few good options. Follow-up was not correlated with patient activity levels. Given the young age of the patients in this series, it would be very useful to know to what extent these implants allowed them to return to their previous daily activities and sports. There is also concern that long-term exposure to elevated metal ion levels locally or systemically may contribute to the development of metal hypersensitivity reactions like nickel allergies, although patient-specific risk factors are not well understood.

How Do We Get There?

Correlating ion levels with physical activity, as from a fitness tracker, would be an interesting next step and seems like an opportunity that Dürr et al. [2] or others could explore in a follow-up study, likely because of the retrospective nature of their analysis. More patients and additional follow-up will be necessary to better understand the longevity of these implants and the potential contraindications for their use. I agree that the elevated metal ions are concerning, and given the range in follow-up times, the complications that may be coming may simply have not occurred yet. That said, this seems to be a successful procedure for individuals with a very challenging orthopaedic condition, and getting these patients healthy and mobile is a big win for their long-term outlooks. Experience from metal-on-metal hip implants would indicate that continued monitoring of patients with elevated ion levels is advisable.

Read This Next

• This article provides a good summary of the risks of pseudotumors and their complex relationship with metal ion wear [4].

• Read this for a review of what is known in terms of metal hypersensitivity reactions and orthopaedic implants, which can have implications in long-term exposure to metal wear debris [5].