We thank Drs Venditolli, Lavigne, and Roy for their comments regarding our article.1
They have correctly pointed out the unit error we unfortunately failed to note in the editing process. The units should have been nmol/L. We agree it is difficult to compare much of the literature on serum ion levels because of the vast methods of collection, analysis, and units used. It may be prudent for those involved in this type of research to agree to a set unit of measurement for published material, therefore stopping the reader from having to make complex calculations to ensure correct comparisons.
We also agree including the standard deviations from the mean would be useful and probably should have been included. However, we are uncertain high levels indicate increased risk of metal ion-related complications and are unaware of any evidence supporting this view. Some of our patients did start with higher than expected preoperative levels and these patients subsequently had some of the highest postoperative levels, but we have no evidence at 4 years that this is a cause for concern. However, with a small sample size, we must be careful not to extrapolate the data too far. A much larger prospective study incorporating, different head sizes, weights, gender, and metallurgy is required before we can safely answer questions like this.
D. L. Back, FRCS Ed
Guy's and St Thomas' Hospital Worcester, United Kingdom;
D. A. Young, FAOrthA
Melbourne Orthopaedic Group Melbourne, Australia; and
A. Shimmin, FAOrthA
Melbourne Orthopaedic Group Melbourne, Australia
1. Back DL, Young DA, Shimmin AJ. How do serum and chromium levels change after metal-on-metal hip resurfacing? Clin Orthop Relat Res