Chromium and Cobalt to Assess Metal Implant Wear
Metal on Metal (MoM) hip implants were developed with the expectation that they would have increased durability, less chance of hip dislocation and less wear, allowing them to be used in younger, more active individuals. While most recipients benefit from joint replacement, some patients have experienced adverse effects due to excessive wear of the implant and deposition of metal particles into surrounding tissue.
Orthopedic implants contain several metal alloys. The acetabular cup is made of porous titanium which allows bone to grow into the implant, strengthening the bond between bone and implant. The ball and socket joint where motion occurs, is made from a hard alloy that facilitates smooth motion and weight bearing. The most commonly used alloy is a combination of cobalt and chromium. Continuous motion at the MoM surfaces releases microparticles of chromium and cobalt into the surrounding tissue and metal ions into the circulation.
In symptomatic patients, measurement of serum chromium and cobalt concentrations is helpful in assessing the degree of MoM implant wear. All patients with MoM implants have approximately 10 times higher levels of these two metals than unexposed individuals. Modest increases in serum chromium (3-6 ng/mL) and in serum cobalt (4-10 ng/mL) concentrations are likely to be associated with a prosthetic device in good condition. Clinically important implant wear is indicated when serum chromium exceeds 15 ng/mL and cobalt exceeds 10 ng/mL. Elevated chromium and cobalt concentrations may indicate implant wear, but not systemic toxicity.
Increased serum chromium and cobalt concentrations in the absence of symptoms such as joint pain do not, independently, indicate implant wear. Serum cobalt and chromium are highest in the first year after implant and slowly decline in subsequent years, reaching a steady state around 3 years after implant. For this reason, it is advisable to repeat serum cobalt and chromium at 6 months and yearly to see if concentrations have dropped before recommending revision. Reference range for chromium is 0-1.0 ng/mL and for cobalt is 0–1.0 ng/mL.
Proper specimen collection is essential for accurate interpretation of results. Most specimen collection products contain chromium in the rubber stopper. Specimens must be collected in vacutainer tubes with a royal blue-top (EDTA anticoagulant), which are free of trace elements. Blood should not be drawn into plastic syringes with black rubber plunger seals because they contain high concentrations of both cobalt and chromium. Specimens should not be collected for at least 96 hours after receiving iodine or gadolinium contrast media because they can interfere with most metal tests. Testing of joint fluid is not recommended.
