Blood Metal Ion Levels Can Identify Hip Replacement Patients at Low Risk of ARMD
Patients with metal-on-metal (MoM) artificial hips are at risk of complications caused by adverse reactions to metal debris (ARMD). A study in the September 20, 2017 issue of the Journal of Bone & Joint Surgery confirms that blood metal ion levels specific to the type of hip implant used can help predict patients who are at low risk of ARMD.
Using implant-specific cutoff points resulted in fewer missed cases of ARMD compared with using fixed thresholds currently recommended by US and UK regulatory authorities, according to the report by researchers from the University of Oxford in the UK.
Their findings offer a refined approach to predicting the risk of ARMD in patients with 2 widely used types of MoM hip replacements: the Birmingham Hip Resurfacing (BHR) implant and the Corail-Pinnacle implant. The research was funded by Arthritis Research UK (grant reference number 21006).
Measuring levels of chromium and cobalt ions in the blood – reflecting the presence of small particles from the worn implant surfaces – is recommended as part of patient follow-up for early detection of ARMD. However, there is no agreement as to the specific blood metal ion levels that should raise concern after MoM hip replacement. Previous studies by the Oxford researchers suggested that using implant-specific thresholds could improve the ability to detect patients at low risk of developing ARMD.
To confirm this approach, the researchers performed an external validation study including 710 patients (803 hips) who underwent MoM hip replacement with either the BHR or the Corail-Pinnacle implant at 3 European hospitals. Implant-specific versus fixed regulatory authority cutoff points for blood metal ion levels were tested for their ability to correlate with clinically diagnosed ARMD, rates of which were 12% after single-hip BHR replacement, 18% after double-hip BHR replacement, and 7% after single-hip Corail-Pinnacle replacement.
The implant-specific cutoff points performed well in distinguishing between patients with and without ARMD: Sensitivity ranged from 65% to 79% in the 3 groups, and the negative predictive value ranged from 93% to 97%.
Overall, the implant-specific cutoff points identified all but 2.8% percent of ARMD cases. In contrast, the fixed regulatory authority cutoff points missed up to 6.5% of cases.
Early identification of ARMD is important, especially for some patients in whom early revision (implant removal and replacement) can reduce complications and provide a better outcome. The new findings confirm the value of implant-specific blood metal levels in helping assess the risk of ARMD after MoM hip replacement.
Using these cutoff points helps to pinpoint patients who should be monitored more closely for possible ARMD, while also minimizing the use of additional testing in patients unlikely to have or develop ARMD. But the researchers emphasize that no single test can determine whether a patient actually has ARMD or what treatment is needed, writing that, “blood metal ion levels should be only a part of the complete clinical assessment.”
Gulraj M, Berryman F, Judge A, et al. Blood metal ion thresholds to identify patients with metal-on-metal hip implants at risk of adverse reactions to metal debris: an external multicenter validation study of Birmingham hip resurfacing and Corail-Pinnacle implants. J Bone Joint Surg Am. 2017 September 20;99(18):1532-9. doi: 10.2106/JBJS.16.01568