Are Metal Ion Levels Elevated After Dual Mobility Acetabular Systems Minimum Five-Year Analysis

Dual mobility acetabular systems for total hip arthroplasty (THA) have been shown to have decreased dislocation rates and reduced revision rates, but there is controversy over the potential release of sufficient levels of metal ions into the blood to cause adverse local tissue reactions. However, there is a lack of long-term studies analyzing these levels of cobalt and chromium. Therefore, the purpose of this study was to investigate the levels these metal ions at a minimum 5-year follow-up after dual mobility implantation. Specifically, we analyzed: 1. overall blood and urine cobalt levels; 2. overall blood and urine chromium levels; 3. cobalt levels stratified by length of follow-up and various implant-related metrics (i.e., offset, cup size, stem, and neck angle); as well as 4. chromium levels stratified by length of follow-up and these various implant-related metrics.

A total of 41 patients who underwent THA with modular dual mobility acetabular systems between January 1, 2011, and December 31, 2016, were identified and followed for a mean time of 6 years (range: 5 to 10 years). All patients had well-functioning hips (Harris Hip Scores greater than 90 points (range: 90 to 100 points) and had no evidence of impending radiographic failure or progressive radiolucencies. Cobalt and chromium serum and plasma, blood, as well as urine levels were obtained at final followup. Additional parameters analyzed included: head material and size, stem offset, cup size, as well as stem-neck angle.

Concentrations of cobalt were low as the mean blood and urine levels for all patients were 0.6 ± 0.5 μg/L (normal < 1.8 μg/L) and 0.8 ± 0.8 μg/L (normal < 2.8 μg/L), respectively. Only one patient had a minimally elevated blood cobalt level by 0.1 μg/L. These levels were not substantially different when subgroup analyses were performed for ceramic and cobalt-chrome heads. The mean chromium levels in blood and urine were also found to be low for all patients as values were 0.8 ± 0.2 μg/L (normal: < 1.2 μg/L) and 1.2 ± 0.5 ng/milliliter (normal: < 2 ng/L), respectively. Similarly, only one patient had a very slightly elevated blood chromium level of 1.3 μg/L. Additionally, analyses of ceramic or cobalt-chrome heads separately did not demonstrate differences in blood or urine levels. Blood cobalt or chromium concentrations had minimal changes with longer lengths of follow-ups, and with different stem offsets, cup sizes, stems, or neck angles.

Dual mobility acetabular systems when combined with the two stems studied produced low levels of blood as well as urine cobalt and chromium levels at a minimum follow-up of 5 years (mean: 6 years; range: 5 to 10 years). These results remained below the threshold of normal and clinically insignificant regardless of length of follow-up, head material, or various implant measurements. To the best of our knowledge, this is the first study to demonstrate low levels of metal ions at longer than 4-year follow-up. These data may be of importance to surgeons deciding on the appropriate implants to use for their high-risk patients.