Metal Ion Levels in Maternal and Placental Blood After Metal-on-Metal Total Hip Arthroplasty
There is concern regarding elevated metal ion levels in the blood during pregnancy and the potential fetal effects in women with metal-on-metal (MOM) implants. We obtained maternal and umbilical cord blood samples from 3 patients with a MOM hip arthroplasty and 7 control subjects without any metallic implants. Serum metal ion levels including chromium, cobalt, titanium, and nickel were tested using high-resolution sector-field inductively-coupled plasma-mass spectrometry. Mothers with MOM-bearing implants had significantly elevated levels of serum cobalt and chromium compared with control-group mothers, and umbilical cord blood from mothers with MOM implants also had significantly higher serum metal ion levels compared with control-group mothers.
The results of this study show that circulating serum levels of metal ion degradation products from MOM bearings cross the placenta and expose the fetus to metal ions. However, the placenta exerts a modulatory effect on cord blood, resulting in decreased levels compared with maternal samples (approximately 15% of maternal chromium and 50% of maternal cobalt). Physicians and women of child-bearing age should be aware of this potential effect when considering the use of MOM-bearing implants.
It is important to recognize that rodent data are limited and may not provide accurate translational insight into the effects of metal ions in human maternal and fetal subjects. Mammalian species have significant heterogeneity in the structure and function of their placentas. Rurak31 has shown that rodents have an additional persisting yolk sac placenta that allows the transfer of maternal immunoglobulins to the fetus. Humans, on the other hand, have a yolk sac placenta that regresses early in pregnancy. Differing placental biologic function makes it difficult to extrapolate the effects of metal ions in rodents to human subjects.
It is also important to note that serum levels of cobalt, chromium, and titanium can remain persistently elevated in well-functioning metal-on-polyethylene THA for several years and that elevated metal ion levels are not confined to MOM bearings.2 Levine and colleagues4 reported that serum levels of cobalt, chromium, and titanium remain persistently elevated after 10 years in a cohort of 27 well-functioning primary metal-on-polyethylene THA (hybrid, cobalt-chrome, titanium). Cobalt concentrations were elevated in all implants compared with controls at all follow-up periods through 10 years with absolute values less than 1 ng/mL. The authors noted that metal release at the modular femoral head-neck junctions was likely the dominant source of serum cobalt and chromium rather than passive dissolution. Hsu and colleagues32 have also shown that patients undergoing a second metal-on-polyethylene THA after primary THA have elevated serum metal ion levels (cobalt, chromium, titanium) up to 6 years after second surgery. Reported cobalt concentrations in patients with unilateral THA reached a maximum of 0.5 ng/mL during the follow-up course compared with 1.5 ng/mL for patients with bilateral THA. It is unknown what the potential metal ion transfer load would be in mothers with metal-on-polyethylene THA and associated taper corrosion to their infants.
Conclusion
Mothers with MOM-bearing implants and their children have higher cobalt and chromium levels than control subjects, demonstrating that the placenta is not a complete barrier to metal ion transport, although it seems to have a modulating effect. Physicians and women of child-bearing age should be aware of these findings when considering the use of MOM-bearing couples for THA. The effects of metal ions on long-term maternal and fetal health require research through serial clinical exams and metal ion level testing in prospective studies of different THA-bearing surfaces.
