Blood Metal Ion Levels May Predict Hip Implant Problems

Janis C. Kelly

March 13, 2013

Blood cobalt (Co) concentration was a significant risk factor for joint failure, according to a report published online March 12 in BMJ Open.

Current guidance from US and UK regulatory authorities against routine blood metal ion testing should be reconsidered based on these data, argue study author David J. Langton, MRCS, from the Joint Replacement Unit, University Hospital of North Tees, Middlesbrough, United Kingdom, and colleagues.

"It would be reasonable to conclude that the current [Medicines and Healthcare Regulatory Agency] guidance on follow-up blood metal ion testing needs to be reconsidered, as we have been saying for quite some time. The really important point is that these rules apply for resurfacings only," Dr. Langton told Medscape Medical News.

"Anything above a blood cobalt level of 2 μg/L, I personally consider a threshold," Dr. Langton said. He would begin tighter follow-up for patients with metal-on-metal (MoM) hip resurfacing whose blood cobalt level is 4 or 5 μg/L.

"Blood cobalt above 20 μg/L is a real concern," Dr. Langton said.

Both the US Food and Drug Administration and the UK Medicines and Healthcare Regulatory Agency currently advise against routine blood metal ion testing in asymptomatic patients with MoM hip resurfacing arthroplasties.

After the initial concerns about MoM joint debris, Dr. Langton and colleagues initiated a blood metal ion screening program at their clinic in 2007. In the current study, they reviewed those screening data and outcomes for 278 patients with articular surface replacement (ASR) and Birmingham MoM hip resurfacings. The researchers conducted a retrospective analysis using mixed-effect modeling to investigate the timing of blood Co levels and survival analysis to investigate the role of demographics and blood Co levels as risk factors for subsequent failure secondary to adverse reactions to metal debris.

"Our hypothesis was that blood Co levels would increase with time since the operation, would differ between males and females and be independent of patient age. We hypothesised that the size of the implant would also impact on the levels of Co because smaller implants are known to wear (and hence release Co) more readily than larger ones," the authors write.

They found that blood Co concentration was a positive and significant risk factor for joint failure, that Birmingham hip resurfacing was significantly less likely than ASR to require revision, and that men had a 66% lower risk for joint failure than women.

Co concentrations greater than 20 μg/L were frequently associated with metal staining of tissues and with osteolysis. "Development of soft tissue damage appears to be more complex[,] with females and patients with ASR devices seemingly more at risk when exposed to equivalent doses of metal debris," the authors note.

Revision Surgery Offered

They conclude, "Grossly elevated ion concentrations indicate the risk of early prosthetic failure and can be used to direct further investigations or implement closer follow-up. At our unit, which acts as a referral centre for the treatment of failed MoM joints, in total 40 patients with blood Co concentrations greater than 20 μg/l have undergone revision of their hip resurfacing so far. All were found to have macroscopic metal staining of the local tissues and 35 were found to have some degree of bone loss. In light of these findings, at our unit patients with grossly elevated metal ion concentrations are now offered revision surgery in the absence of symptoms."

Dr. Langton said that more completely answering the question of when routine blood metal ion testing would be useful would require prospective longitudinal studies in which control patients are routinely scanned, but that such studies are unlikely to be done. "Unfortunately, the horse is pretty far away from the stable door," he said.

"David Langton's article confirms our findings and follow-up strategy, which we have presented for many years now at many conferences and published last year," Catherine Van Der Straeten, MD, told Medscape Medical News. Dr. Van Der Straeten, director of research, Department of Orthopaedics and Traumatology, Ghent University Hospital, Belgium, was not involved in the Langton study. Her group's article on the interpretation of metal ion levels in unilateral and bilateral hip resurfacing won the 2012 Otto Aufranc Award, presented at the Hip Society for innovative research encompassing important advances in the management of hip disorders.

"Since 2006, we have used blood metal ion testing for the monitoring of all patients (over 4000 now) at our clinic with a MoM hip, regardless of their symptoms (ie, even asymptomatic patients), and have thus picked up problems in some patients. We do not check metal ion levels in total hip arthroplasty with a different bearing couple (usually ceramic–ceramic or ceramic on polyethylene at our centre) because it has been shown Co and Cr ions are not relevantly elevated in those cases," Dr. Van Der Straeten said.

Dr. Langton and one coauthor received travel funding from Zimmer, Smith and Nephew, and Depuy/Finsbury to attend educational orthopedic conferences. Dr. Langton works as an unpaid consultant to Wright Medical. Three coauthors have served as expert witnesses in litigation proceedings with regard to failed MoM joints. One of these coauthors also worked formerly as a consultant for DePuy. Dr. Van Der Straeten has disclosed no relevant financial relationships.

BMJ Open. Published online March 11, 2013. Abstract