Cobalt-Ion Thresholds Could Ease Follow-Up Burden on Some MoM Patients

Cobalt.gif In the mid-2000s, before the orthopaedic community was aware of the potentially catastrophic shortcomings of metal-on-metal (MoM) hip implants, nearly half of all hip replacements used MoM components. With so many people walking around today on those prostheses, orthopaedists are looking for rational and effective ways to monitor these patients for adverse reactions to metal debris.

Findings from a diagnostic Level II study by Matharu et al. in the April 20 edition of The Journal of Bone & Joint Surgery will help identify patients who are at low risk of metal-debris adverse reactions, specifically among those who have Birmingham Hip Resurfacing (BHR) and Corail-Pinnacle MoM implants. The authors found that when measuring cobalt ions in whole blood, the optimal threshold for identifying adverse reactions to metal debris was 2.15 µg/L for BHR patients and 3.57 µg/L for Corail-Pinnacle patients. These implant-specific thresholds are lower than the non-implant-specific thresholds proposed by US and UK regulatory agencies, and in this study the cutoffs had good sensitivities and specificities, and their negative predictive values exceeded 98%.

The authors say identifying low-risk individuals will allow orthopaedists to focus surveillance efforts on patients who are at higher risk of adverse reactions to metal debris. They even suggest that patients below the implant-specific thresholds for cobalt could be excluded from regular follow-up regimens, provided they have normal clinical examinations and radiographs.

While lauding Matharu et al. for prospectively determining these thresholds, commentator Lawrence Dorr, MD says these findings need “to be tested at multiple centers to validate [their] superiority,” and he emphasizes that better methods to identify asymptomatic patients who are at high risk of adverse reactions to metal debris still must be developed.

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