Original Research

Metal Ion Levels in Maternal and Placental Blood After Metal-on-Metal Total Hip Arthroplasty

Am J Orthop. 2014 December;43(12):E304-E308

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.

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Serum metal ion levels are increased after primary total hip arthroplasty (THA) with all types of head-liner bearing surfaces.^1-4 In recent years, there has been increasing concern regarding elevated metal ion levels and adverse local and systemic effects, in particular, from metal-on-metal (MOM) implants.⁵ There have been reports of adverse local tissue reactions (ALTR) and systemic effects associated with elevated metal ion levels from MOM THA.^6-10 MOM hip resurfacings have been advocated in the literature for use in select patient populations, such as young, active individuals.^11,12 Purported benefits of MOM-bearing surfaces in this patient population include reduced wear and the prevention of osteolysis.¹³ While the incidence of ALTR has been reported to be approximately 1% within 5 years after MOM hip resurfacing, the prevalence of ALTR at long-term follow-up is unknown.¹⁴ Adverse local tissue reactions in hip resurfacing are related in part to femoral head coverage by the acetabular cup, as well as component design and orientation. The risk for ALTR with MOM THA has been reported to correlate with the level of serum metal ion levels because of the bearing surface, along with taper corrosion and corrosion secondary to the large surface area of the femoral head.^13-15 The overall clinical and prognostic value of metal ion levels in cases of MOM bearings remains controversial and without clear cut-off values.¹⁵

The long-term physiologic response to elevated serum metal ion levels from THA implants remains unknown and is of particular concern in pregnant women because of potential transplacental transfer. Several studies have shown that DNA (deoxyribonucleic acid) and chromosomal changes can occur in patients with both MOM and non-MOM hip implants.^16-18 Techniques to accurately measure the levels of metal ions, including cobalt, chromium, and titanium in the serum, have improved substantially in recent years and have been used successfully in clinical applications with low detection limits.^2,4,19,20 Evidence shows that pregnancy-related complications in women with well-functioning cemented, hybrid, or uncemented metal-on-polyethylene THA are not different from those in healthy women.²¹ However, it is unclear if metal ions can cross the placenta and negatively affect the development of a fetus in mothers with MOM-bearing THAs.²² We conducted this study to determine if metal ions can traverse the placenta by measuring serum metal ion concentrations in mothers with and without MOM THA and the corresponding levels in umbilical cord blood samples.

Materials and Methods

Ten patients were prospectively enrolled in this study, which was approved by the institutional review boards at all 3 participating hospitals prior to initiation. All patients provided informed consent and agreed to maternal and umbilical cord blood sampling. Seven of the 10 patients did not have any metallic implants and served as controls. The remaining 3 patients had either a MOM unilateral primary THA (1 patient) or unilateral MOM hip-resurfacing arthroplasty (2 patients) with no other metal implants in the body. For all subjects, maternal and umbilical cord blood was obtained at the time of delivery.

Group Demographics

The 3 women in the implant group had a mean age of 32.3 years (range, 23-39 years) at time of delivery. The first patient had posttraumatic osteoarthritis and underwent right THA using a modular MOM 60-mm acetabular component, a 36-mm cobalt chrome head, and a cementless, titanium proximally porous-coated femoral component (Pinnacle Total Hip System and Summit Total Hip System; DePuy Synthes, Warsaw, Indiana). Her infant was born 2 years after index THA. The second patient had a diagnosis of degenerative osteoarthritis secondary to developmental dysplasia of the hip and underwent a hybrid MOM hip resurfacing with a monoblock 50-mm cup and 40-mm cobalt chrome head (Conserve Plus; Wright Medical Technology, Arlington, Tennessee). She gave birth to her infant 6 years after her hip resurfacing. The third patient also had a diagnosis of degenerative osteoarthritis secondary to developmental dysplasia of the hip and underwent hip resurfacing with a 54-mm monoblock cup and 44-mm cobalt chrome head (Conserve Plus). Her infant was born 4 years after her resurfacing arthroplasty. All of the infants were born healthy, and the deliveries were uneventful and without complications. Seven women with a mean age of 32.1 years (range, 24-37 years) and their infants served as controls at the time of delivery. None of the women in the control group had a history of renal impairment, inherited genetic disorders, or metal implants.

Serum Metal Ion Analysis

Blood samples were collected using S-Monovette polypropylene syringes (Sarstedt, Princeton, New Jersey), a multi-adapter, and infusion set following an established technique.¹ All vessels and utensils used for specimen collection were verified to be free of metal contamination. Three 10-mL syringes were drawn, and each syringe was labeled to indicate the sequence of collection. The first 10 mL were drawn to rinse the needle and adapter. Blood was then allowed to clot naturally and centrifuged at 1850 rpm for 30 minutes, separating samples into cell and serum fractions that were stored in labeled vials at -80ºC. All specimen manipulations after collection were carried out in a class-100 environment using a SterilGARD Biological Safety Cabinet (Baker, Sanford, Maine) and class-100 gloves (Oak Technical, Ravenna, Ohio) to minimize atmospheric and manual contamination. Maternal and umbilical cord blood metal ion levels were tested using high-resolution sector-field inductively-coupled plasma-mass spectrometry (HR-SF-ICPMS) (Element 2; Thermo Fisher Scientific, Bremen, Germany) following the method of additions as described previously.²³ The HR-SF-ICPMS machine was equipped with an SC-E2 autosampler, Teflon nebulizer and spray chamber, sapphire injector (Elemental Scientific, Omaha, Nebraska) and platinum cones. All calibration and internal standard solutions were prepared by gradual dilutions of single-element standard solutions (1000 μg/mL from High Purity Standards, Charleston, South Carolina). The certified reference material, Seronorm Trace Elements Serum (SERO, Billingstad, Norway), were routinely analyzed with samples. The serum concentrations of cobalt, chromium, titanium, and nickel were measured with detection limits in ng/mL (parts per billion) of 0.04 for cobalt, 0.015 for chromium, 0.2 for titanium, and 0.17 for nickel.⁴ Concentrations below the detection limit were approximated as one-half of the detection limit by convention to calculate means.