PLACENTAL REGULATION OF METAL TRANSFER IN PATIENTS WITH METAL-METAL SURFACE REPLACEMENT HIP ARTHROPLASTY

Abstract

Introduction: Metal-metal (MM) surface replacement arthroplasty of the hip is showing promise as an effective and enduring solution in the management of hip arthritis in the young, and is being increasingly used. The main concern is the release of metal ions into the systemic circulation and their long-term effects. There is concern that metal ions cross the placenta in pregnant women with MM bearing arthroplasties with the potential for mutagenic effects in the offspring. The hypothesis is that metal ions pass freely through the placenta and that there is no difference in the maternal and cord metal ion levels.

Methods: This is a controlled cross-sectional study of women of child-bearing age with MM bearing surface replacement arthroplasties. (n= 25, mean age: 32 years, mean duration after resurfacing 60.3 months, 3 bilateral). The control group consisted of 24 subjects who do not have a metallic implant, were not taking any supplements containing cobalt or chromium salts and were registered to undergo an elective LSCS at the regional hospital. Mean age of the control group was 31.3 years. No patient in either group was known to have renal failure. Whole blood specimens were obtained from the mothers just before delivery and before infusion of any fluids, and cord blood specimens were obtained immediately after delivery. High resolution inductively couples mass spectrometry (HRICPMS) was used for metal ion analysis.

Results: Cobalt and chromium were detectable in all specimens in both the study patients and controls. In the study group, mean cord metal ion levels were significantly lower than the maternal cobalt (p < 0.05) and chromium (p < 0.0001) levels thus rejecting the null hypothesis. In the control group, the mean difference between maternal and cord metal ion levels was only 5 to 7% and was statistically not significant (p > 0.5).

The mean difference in cord chromium between the study and control groups is not statistically significant (p > 0.05), although cord cobalt in the study group is significantly higher (p < 0.01) than that in controls. Whilst there is a four-fold elevation of maternal cobalt in the study patients and an almost 7-fold increase in maternal chromium levels as compared to the control group, the elevation in the cord cobalt and chromium in the study group are smaller.

Discussion and Conclusion: The differences between maternal and cord metal ions in the control patients indicate that normally the placenta allows an almost free passage of metal ions. This is understandable when we realize that these elements are also required by the developing foetus for its cellular and metabolic functions. The relative levels of metal ions in the maternal and cord blood in the study group patients reveal that the placenta does exert a modulatory effect on metal transfer. The mean cord levels of cobalt and chromium in these patients are only 57% and 24% of the maternal blood levels respectively. There is a continuing need for efforts to reduce metal ion release from artificial joints and also to monitor metal ion levels in patients with MM devices.