Ion levels poor indicator for hip arthroplasty debris

Aug 17 2012

By Lynda Williams, Senior MedWire Reporter

Research shows that plasma metal ion analysis is only moderately effective at identifying patients with adverse reaction to metal debris (ARMD) following metal-on-metal (MoM) hip arthroplasty.

Metal artifact reduction sequence-magnetic resonance imaging (MARS-MRI) screening detected ARMD in 40% of 209 patients with unilateral symptomatic MoM hip arthroplasties an average of 1.9 years after surgery.

Analysis showed that patients with MARS-MRI ARMD had significantly higher levels of both chromium (5.9 vs 4.3 µg/L) and cobalt (8.5 vs 3.5 µg/L) than those without the condition. There was a significant and positive correlation between levels of the two ions.

But compared with MARS-MRI results, a plasma cobalt or chromium level above 7 µg/L was just 65% specific and 57% sensitive for identifying ARMD, with positive and negative prediction values of 52% and 69%, respectively, report I Malek (Wales Deanery, Cardiff, UK) and co-authors

When the threshold for ARMD was reduced to a cobalt or chromium level of 3.5 µg/L or above, the sensitivity increased to 86% but specificity fell to just 27%. While a 2.0 µg/L threshold for either ion would give 90% sensitivity, specificity at this threshold would fall to 25% for cobalt and 14% for chromium.

"Plasma levels of cobalt and chromium cannot therefore be used as a sole screening test for the presence of ARMD and should be used in combination with symptoms, plain radiological findings and cross sectional imaging," the researchers write in the British edition of the Journal of Bone and Joint Surgery.

Recommending that clinicians have a low threshold for use of cross-sectional imaging in patients with MoM hip arthroplasty, even in the absence of high plasma ion levels, they conclude: "We welcome the updated [Medicines and Healthcare products Regulatory Agency] guidance that recommends MARS-MRI or ultrasound in all symptomatic patients with a MoM arthroplasty."

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