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.
Statistical Analysis
The data reported are the means for each group for each of the metal ion levels analyzed. Intergroup comparisons were made with the Mann-Whitney-Wilcoxon test using SPSS statistics software (SPSS Science Inc, Chicago, Illinois) to compare implant and control groups in regards to serum metal ion levels. Intragroup comparisons were made using the Friedman test with significance set at P < .05. Spearman rank-order correlation tests were used to investigate relationships between maternal and infant serum metal ion levels.
Results
The Table shows the mean serum metal ion levels of chromium, cobalt, titanium, and nickel for both groups. The implant-group mothers had significantly higher chromium and cobalt levels than the control-group mothers, with mean chromium levels of 1.87 ng/mL vs 0.16 ng/mL (P = .01) and mean cobalt levels of 0.97 ng/mL vs 0.20 ng/mL (P = .01), respectively. All control-group maternal chromium and cobalt levels were lower than the implant group. There were no significant differences detected between the implant-group and control-group mothers with respect to serum titanium or nickel levels.
The implant-group cord blood samples also had significantly higher chromium and cobalt levels than did the control-group infants with mean chromium levels of 0.29 ng/mL vs 0.10 ng/mL (P = .03) and mean cobalt levels of 0.49 ng/mL vs 0.16 ng/mL (P = .01), respectively. All but 1 of the control-group infants had chromium levels that were lower than the implant-group infant chromium levels. All of the control-group infant cobalt levels were lower than the implant-group infant cobalt levels. In the mother (I-1) who had a titanium-containing implant, her titanium level was 1.77 ng/mL and her cord blood level was 0.78 ng/mL. In contrast, the other 2 patients did not have titanium-containing implants and had corresponding metal levels of 0.10 ng/mL and cord blood levels either below the detection limit or just slightly above it. No statistically significant differences were found between the implant- and control-group infants with respect to serum titanium or nickel levels.
Considering the implant and control groups separately, we found no statistically significant differences between the maternal and infant titanium levels or the maternal and infant nickel levels. In the implant group, the mother’s chromium level was always higher than her infant’s chromium level, and the mother’s cobalt level was always higher than her infant’s cobalt level (P= .08). In the control group, there was no correlation between the maternal and infant chromium levels, or between the maternal and infant cobalt levels. In the implant group, the maternal and infant chromium levels were highly correlated (r = 1), as were the maternal and infant cobalt levels (r = 1).
When infants’ chromium levels were expressed as a percentage of their mothers’ chromium levels, the mean was 15.4% (range, 12.3%-18%) for the implant group and 58.9% (range, 13.5%-165.7%) for the control group (P = .05). The mean infant cobalt level, expressed as percentage of maternal cobalt level, was 50% (range, 45.8%-53.1%) for the implant group and 76.6% (range, 59.1%-150.8%) for the control group (P = .01).
Discussion
Cobalt and chromium serum metal ion levels obtained from mothers with MOM implants and umbilical cord samples were significantly elevated in comparison with controls. There was also a strong correlation between implant-group maternal cobalt and chromium levels and implant-group infant cobalt and chromium levels; however, no such correlation existed in the control group. Our results suggest that cobalt and chromium cross the placental barrier. Implant mothers had higher chromium and cobalt levels compared with implant babies, and these babies had approximately 15% of the level of chromium and 50% of the level of cobalt when compared to implant maternal levels. This finding suggests that the placenta modulates the transfer of chromium and cobalt to the fetus.
Two studies have reported chromium and cobalt levels in maternal and umbilical cord sera after MOM THA.5,24 Brodner and colleagues5 determined the maternal serum levels of cobalt and chromium in 3 women 3.8 years after MOM THA and compared those to cobalt and chromium levels obtained from umbilical cord blood. At the time of delivery, the maternal chromium concentrations in the 3 patients were 1.6 ng/mL, 0.5 ng/mL, and 0.9 ng/mL, and the cobalt concentrations were 1 ng/mL in 1 patient and below the detection limit in the other 2 patients. Cobalt and chromium concentrations of the 3 umbilical cord sera were below the detection limit. The authors concluded that cobalt and chromium did not cross the placenta based on their laboratory detection limits. Metal ion levels were measured using atomic absorption spectrometry with relatively high detection limits of 0.3 ng/mL, which is not as sensitive as the HR-SF-ICPMS technique used in the present study that has detection limits of 0.04 ng/mL for cobalt and 0.015 ng/mL for chromium. The relatively high detection limits of atomic absorption spectrometry were likely responsible for the authors’ inability to detect elevated chromium and cobalt levels in umbilical cord sera.
