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Correlation seen between metal-ion levels and pseudotumors in asymptomatic, symptomatic THA patients
Results from this study demonstrated the presence of pseudotumors in both symptomatic patients and asymptomatic patients correlated with elevated serum levels of cobalt and cobalt/chromium.
Researchers performed a retrospective cross-sectional study of 97 patients who underwent total hip arthroplasty with a dual-taper modular femoral stem. Of these patients, 83 were categorized as either symptomatic or asymptomatic. Investigators evaluated these patients with metal artifact reduction sequence-MRI, serum metal-ion level measurements and the University of California, Los Angeles (UCLA) functional hip score.
Results showed a 15% prevalence of pseudotumors in asymptomatic patients. Investigators noted patients with a pseudotumor compared with those without a pseudotumor had a significantly higher median serum cobalt level and cobalt/chromium ratio. Researchers found no significant difference between symptomatic patients with pseudotumors and asymptomatic patients with pseudotumors regarding the serum cobalt level and the cobalt/chromium ratio. UCLA scores were similar in asymptomatic patients with a pseudotumor compared to patients without a pseudotumor. ‒ by Monica Jaramillo
Disclosures: Kwon reports he receives research support from Biomet, Stryker and Zimmer. Please see the full study for a list of all other authors’ relevant financial disclosures.
Perspective
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This study further emphasizes the importance of modular junctions in the development of adverse reactions following hip arthroplasty. Clearly a pseudotumor rate of 36% at mean range follow-up of 22 months (16-38 months) is catastrophic and demonstrates the point that classic adverse reactions to metal can occur in the absence of a metal-on-metal (MoM) bearing.
This study also provides further evidence that soft tissue lesions can develop in asymptomatic patients. This was shown a number of years ago by Wynn-Jones and colleagues in their MRI-based study of patients with ASR prostheses (DePuy). Similar to large-diameter MoM total hip replacements, surgeons must therefore keep patients with these devices under close surveillance, even if they appear to have had a successful result initially.
Metal ion levels in patients with modular junctions can be difficult to interpret. My personal experience is that cobalt (Co) to chromium (Cr) ratios can be misleading, particularly at lower concentrations, due to variations in measurement of Cr concentrations between labs. It is of note, however, there were no pseudotumors with a Co concentration below 2.0 µg/L. This indicates all the implants with an associated pseudotumor were malfunctioning in tribological terms.
While the authors discuss corrosion at length, it is perhaps worthwhile to consider the relatively complex geometries of the press-fit components used in the Rejuvenate system (Stryker Orthopaedics). It is logical that any (inevitable) manufacturing variation in these modular neck components may lead to instability, particularly when subjected to the large lever arm brought about by the location of the modular junction. Matters are further complicated by the use of a titanium alloy that is more flexible than the standard Ti6Al4V.
Finally, while there remains a focus on Co and Cr metal ions, it is unclear whether particulate debris released from the TMZF alloy used in the system may play an additive role in the subsequent development of negative host immune responses.
References:
Pandit H, et al. J Bone Joint Surg Br. 2008;doi:10.1302/0301-620X.90B7.20213.
Wynn-Jones H, et al. Acta Orthop. 2011;doi:10.3109/17453674.2011.579518.