Abstract / Summary
Abstract Background Whole blood cobalt testing is widely used in surveillance of metal-on-metal (MoM) hip arthroplasty, but clinically validated reference values for well-functioning implants are not well defined, and modest elevations are often misinterpreted as pathologic. This study aimed to establish outcome-validated cobalt ranges and evaluate their use in distinguishing adverse wear-related failure (AWRF) from trunnion corrosion failure (TCF). Methods We analyzed early (1 to 5 year) cobalt levels in MoM hip resurfacing arthroplasty (HRA) and total hip arthroplasty (THA) patients with confirmed implant survivorship at 10 and 15 years, and compared these to cases that failed due to AWRF, TCF, and other causes. AWRF and TCF classification was based on intraoperative and imaging findings independent of ion results. The influence of implant design and postoperative activity level was also assessed. Results Mean cobalt in well-functioning unilateral HRA was 1.2 ± 1.2 µg/L; all successful implants had levels below 14 µg/L. All 18 AWRF cases had cobalt levels of 15 µg/L or higher (mean 74.0 ± 47.0 µg/L), a clean separation from successful implants. TCF cases (n = 7) showed only modest cobalt elevation (mean 5.3 ± 2.0 µg/L) that overlapped substantially with successful implants, and a definitive diagnostic threshold for TCF could not be established given the small cohort. Activity level did not significantly influence early ion levels. Conclusion Cobalt levels below 14 µg/L are consistent with normal MoM bearing function, while levels of 15 µg/L or higher strongly suggest AWRF. Ion testing reliably identifies bearing-related wear failure but is not diagnostic for TCF, which requires additional clinical and imaging correlation. These outcome-validated thresholds may help reduce unnecessary revision surgery and improve MoM surveillance.