Highly Elevated Metal Ion Levels After Tumor Resections and Reconstruction With Megaprostheses Around the Knee

Tumor resections around the knee leave defects that are often reconstructed with megaprostheses. Because megaprostheses have a large surface area, modular design, and constrained coupling mechanism, they are prone to metal wear, which can result in elevated levels of metal ions, particularly cobalt (Co) and chromium (Cr). Currently, there are limited published data on elevated ion levels in megaprostheses but not in a homogeneous, well-defined patient population with respect to location and type of prosthesis.

What metal ion levels were observed in patients who underwent distal femoral or proximal tibial megaprostheses at a minimum follow-up time of 1 year?

Between May 1992 and November 2023, we performed 81 total knee megaprostheses as part of tumor reconstructions. Of those, we considered 65 surviving patients as potentially eligible. Based on that, 34% (22) were excluded because they did not respond and were lost to follow-up or refused to participate, leaving 66% (43) for analysis in this study at a median (range) follow-up time of 88 months (13 to 379). Whole blood samples were obtained and analyzed for Co and Cr. Medical records were reviewed, and signs of metallosis were assessed.

Elevated levels of Co and Cr were found in 95% (41 of 43) of patients. The levels of both ions were found to correlate with each other. For distal femoral and proximal tibial replacement, the median (range) Co levels were 13.7 µg/L (1.0 to 91.5) and 7.3 µg/L (0.8 to 19.9), respectively. For Cr, the values were 9.6 µg/L (0.5 to 22.3) and 7.2 µg/L (0.5 to 7.2), respectively (p = 0.07). The length of resection was not associated with ion concentrations. Metallosis was observed in 26% (11 of 43) of patients either visible through the skin at presentation or noted at the time of a revision procedure. Mechanical failure (such as breakage of the joint mechanism and loosening of the stems), increased BMI, and adjuvant therapies were not associated with elevated blood ions. Both Co and Cr levels were lower in patients measured at longer follow-up compared with those measured earlier after arthroplasty.

Based on our results, elevated ion levels are common in patients with megaprostheses. In addition, we note that 21% (9 of 43) of patients had Co levels above 20 µg/L, which is worrisome. Considering that the patients in this and other studies showed no clinical symptoms even with elevated ion levels, care must be taken not to alarm patients. We were not able to identify any factors associated with having higher ion level concentrations that could be used to define groups that could benefit from ion measurements. Currently, as we do not have good thresholds for when apparently high ion levels are worrisome, in patients with megaprostheses who develop unexplained cardiac or neurologic abnormalities, one might consider testing metal ion levels as part of the diagnostic process.

Level IV, therapeutic study.