Study: No ‘safe’ blood metal ion level helps to predict function in MoM hips

Issue: April 2013

CHICAGO — In a large, single-center cohort study using one type of metal-on-metal hip bearing, researchers from the United Kingdom and Finland concluded there is no ideal blood metal ion cut-off level to predict hip function.

“Cobalt ions were significantly raised in the poor-functioning group and that applied to both resurfacing and large head [total hip replacement] THR. Cobalt is a useful biomarker, and more so than chromium,” Alister Hart, FRCS, said during his presentation at the American Academy of Orthopedic Surgeons Annual Meeting, here.

“There was no cut-off level that had ideal test characteristics, and therefore, there is no safe level. We continue to use prevention assessment supplemented by special tests including metal ions, but none in isolation, provide the key. Metal ions are useful, but they are not a substitute for clinical imaging and evaluation,” Hart said.

Hart and colleagues studied 597 patients who underwent hip resurfacing or large head THR using the ASR design (DePuy, Warsaw, Ind., USA) and had reached at least 1 year post-surgery. They categorized hips as poor functioning if the patient was on a wait list to be revised, had undergone a revision or had an Oxford Hip Score (OHS) of less than 30 points. Hips in which the OHS was greater than this were considered well-functioning. One hundred and seventy-three cases met their inclusion criteria.

For hip resurfacings, the investigators found upper outlier limits of 4.6 mcg/L for both chromium and cobalt. According to the study abstract, these limits for the THR group were 5.1 mcg/L for chromium and 11.9 mcg/L for cobalt.

The study findings showed significantly higher blood metal ion levels of cobalt in poor functioning hips than in the well-functioning group.

“This was not true for chromium levels, and this held for both hip resurfacing and large head THRs,” Hart said.

Using receiver operating characteristic curves to find the discriminate capability of blood metal ions showed that cobalt was more useful than chromium in predicting function. Using the Medicines and Healthcare Products Regulatory Agency threshold of 7 ppb, the researchers found 53.2% sensitivity and 87.7% specificity for all cases. These rates changed to 63% sensitivity and 81.2% specificity when 5 ppb was used as a threshold, according to the study abstract.

“The multivariate analysis showed that for a small increase in blood metal ion levels, it had a much greater effect in placing that patient in the poor-functioning group if they were a large head THR or resurfacing,” Hart said.

Reference:

Hart A. Paper #66. Presented at: American Academy of Orthopaedic Surgeons Annual Meeting; March 19-23, 2013; Chicago.

Disclosure: Hart is a paid consultant for DePuy and has received research or institutional support as a principal investigator from Biomet, Corin USA, DePuy, Finsbury, Mathys Ltd., Smith & Nephew, Zimmer and Stryker.