To the Editor:
We read with interest the article by Back et al1 in the September 2005 issue of Clinical Orthopaedic and Related Research entitled “How do serum cobalt and chromium levels change after metal-on-metal hip resurfacing?” They report 16 patients with metal-on-metal hip resurfacing (Birmingham Hip Resurfacing, Smith and Nephew) in which they measured serum chrome and cobalt metal ions. Because this is the first published paper that included preoperative and up to 2-year measures in such a patient cohort, we congratulate the authors for their scientific efforts.
However, we suspect a misprint in the ion concentration unit. The results were presented in μmol/L which is likely an error according to the data expected for metal-on-metal articulations (nmol/L is probably the unit of measure intended). One micromole per liter (μmol/L) represents 1000 nanomole per liter (nmol/L). Studies reporting metal ion levels are difficult to compare as different measurement methods, units (μg/L, μmol/L, ppb, etc) and scales are used.3 It would be easier if authors reported data in the same units. Because key studies on this topic have results reported in μg/L, we believe further investigations should use the same unit.2,4
When evaluating the performance of metal-on-metal implants in terms of metal ion production, we suspect average and median values might be less important than data set range and variance (number of outliers). Reporting the number of subjects with extremely high ion levels (outliers) and their specific results is crucial because they probably represent the subgroup of patients with the greater risk of long-term metal ion exposure in comparison to subjects with approximately average levels. We believe the median should never be presented alone in such a paper and the results should always contain minimum and maximum values and the standard deviation from the mean.
Pascal-A Vendittoli, MD, MSc, FRCS;
Martin Lavigne, MD, FRCS;
Alain-Guy Roy, MD, FRCS
Maisonneuve-Rosemont Hospital Montréal, Québec, Canada
1. Back DL, Young DA, Shimmin AJ. How do serum cobalt and chromium levels change after metal-on-metal hip resurfacing? Clin Orthop Relat Res
2. Brodner W, Bitzan P, Meisinger V, Kaider A, Gottsauner-Wolf F, Kotz R. Serum cobalt levels after metal-on-metal total hip arthroplasty. J Bone Joint Surg Am
3. MacDonald SJ, Brodner W, Jacobs JJ. A consensus paper on metal ions in metal-on-metal hip arthroplasties. J Arthroplasty
. 2004;19 (suppl 3):12-16.
4. MacDonald SJ, McCalden RW, Chess DG, Bourne RB, Rorabeck CH, Cleland D, Leung F. Metal-on-metal versus polyethylene in hip arthroplasty: a randomized clinical trial. Clin Orthop Relat Res