In 2010, Depuy, Johnson and Johnson recalled the metal-on-metal hip replacement system (articulating surfaces made of chrome cobalt steel with anatomically sized femoral head) because the five year failure rate of this product was approximately 13% which is unusually high. The recall letter recommended that cobalt and chromium blood concentrations be obtained. When the hip prosthesis becomes misaligned, there is more grinding of metal upon metal and subsequent release of chromium and cobalt into the blood. Thus the recommendations to obtain blood metal concentrations were to provide an estimate of the misalignment of the hip prosthesis, not to look for metal toxicity. These metal concentrations are frequently read as “high” by the pathologist. However, there is no “normal” range for chromium or cobalt when the source of the metal is internal hip prosthesis. The only ranges in the literature are derived from occupational exposures (primarily inhalation) to these metals.
Cobalt and chromium are skin sensitizers and sensitization of the tissues can occur around hip prosthesis. There is no relationship between blood concentrations of either of these metals and skin sensitization. Inhaled chromium is associated with lung cancer, but there is no evidence that chromium can migrate from the hip prosthesis to the lung and produce lung cancer. Potential carcinogenicity of these metals has a long lag time after exposure (10-30 years).
Arthroprosthetic cobaltism has been reported in a very few patients. Patients present with deafness, cognitive decline, headaches, optic atrophy, seizures, fatigue, peripheral neuropathy, heart failure or hypothyroidism (cobalt blocks thyroid uptake of iodine). All had serum cobalt levels > 60 micrograms/L.
Patients with this hip prosthesis have been referred to our Vanderbilt Toxicology Clinic because metal concentrations have been obtained and there is concern about metal toxicity. The majority of the blood metal concentrations are in the single digits. The patients have no symptoms that can be related to cobalt or chromium. In these patients, the decision to remove the hip prosthesis should be made based on the function of the hip prosthesis, not due to the concern of metal toxicity. A reasonable approach to patients who have this hip prosthesis is to annually check thyroid function as well as chromium and cobalt blood concentrations.
This question prepared by: Donna Seger, MD Medical Toxicologist
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Donna Seger, MD
Tennessee Poison Center
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