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HipFurther Opinion

The immunobiology of cobalt: demonstration of a potential aetiology for inflammatory pseudotumours after metal-on-metal replacement of the hip

H Lawrence, D Deehan, J. Holland, J. Kirby, A. Tyson-Capper
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Bone Joint J 2014;96-B:1172-1177.

Orthopaedic surgery is an ever-changing field that requires the combination of impeccable patient care and cutting-edge research.  For most orthopaedic surgeons, the most familiar research is clinical.  However, as surgeons, we cannot forget the value of basic science and translational research.  While other specialties often have difficulty translating bench-top research into improved patient care, orthopaedic surgery is a leader in the direct translation of laboratory findings into clinical practice.  The sub-specialty of hip surgery has become particularly enlightened by basic science research, especially when the discussion addresses bearing surfaces, and specifically metal-on-metal (MOM) bearings. 

In the September 2014 edition of The Bone and Joint Journal (BJJ), Lawrence et al provide a rather thought-provoking basic science study that looks at the possible effects of cobalt ions on toll-like receptor 4 (TLR4), and thus CXCL8 (IL-8) and CXCL10.  Such a study is essential as it can be directly translated to patients who have MOM bearing surfaces.  While it is clear that bacterial lipopolysaccharides (LPSs) activate TLR4, and thus the inflammatory cascade, the influence of cobalt on the pathway is less understood.  As such, the clinical basis of the study, which makes this a translational investigation, is that many patients with MOM bearing surfaces developed pseudotumours, which may be partially explained by the aforementioned pathway.

 The authors treated human macrophage cells with cobalt ions, and measured the mRNA and protein expressions of both CXCL8 and CXCL10.  The investigators found that both the mRNA and protein expressions of CXCL8 and CXCL10 were increased by the administration of cobalt.  Since both can be increased by cell damage, the investigator also blocked TLR4 and found that the increase was lost, indicating that such an increase was directly attributable to the TLR4 pathway.

Some may ask how this is relevant to clinical practice.  Foremost, by laboratory testing, as well as the work of joint replacement registries, we can identify early failure and analyse long-term success.  Second, many entities cannot, and should not, be studied in humans.  As such, translational studies such as the current one are essential.  However, there are certainly limitations to the current study.  Most importantly, it does not take into account the individual genetic host variation.  There are undoubtedly patient factors that influence the patient response.  In addition, laboratory work cannot always be directly translated into patient care.

In summary, the study is well-designed, well-executed, and well-written, even for those readers who have minimal background in immunobiology.  It is also a very timely topic.  Not only does this study have the ability to shed light on MOM tissue reactions, but also those seen with dual-modular necks.  Given the timely nature of the subject matter, the solid study design and execution, and enlightening findings, this study should be of interest to practicing arthroplasty surgeons.



Matthew P. Abdel, M.D.
Mayo Clinic, Rochester, Minnesota, USA