METAL ION LEVELS FOLLOWING HIP RESURFACING ARTHROPLASTY

Abstract

Metal on metal hip articulations are associated with the release of cobalt and chromium ions. There are few published reports of these levels following hip resurfacing arthroplasty. This study was undertaken to determine cobalt and chromium ion levels following hip resurfacing arthroplasty.

Ninety-one patients were prospectively entered in a study assessing ion levels following hip resurfacing arthroplasty. Serum, erythrocyte and urine cobalt and chromium levels were measured. Samples were collected preoperatively then at three months, six months and yearly thereafter. Each sample was analyzed with an ICPMS assay machine.

Preoperatively the median values for serum, erythrocyte and urine cobalt and chromium levels were all within the normal range. Median serum cobalt levels increased from baseline (0.1 ug/l) to 1.1 ug/l at one year and 1.6 ug/l at two years. Median serum chromium levels increased from baseline (0.2 ug/l) to 1.9 ug/l at one year and 3.3 ug/l at two years. Median erythrocyte cobalt levels increased from baseline (0.1 ug/l) to 1.0 ug/l at one year and 1.3 ug/l at two years. Median erythrocyte chromium levels increased slightly from baseline (1.0 ug/l) to 1.2 ug/l at both one and two year follow-up. No correlation was seen between ion levels and patient activity, implant position, clinical scores or BMI. No clinical effects of elevated ions were observed. Outliers of either cobalt or chromium were seen in twelve patients (13%) in the series.

Hip resurfacing arthroplasty is associated with significant elevations of cobalt and chromium ions. An increase in serum levels of both cobalt and chromium has been noted at two years. This trend also occurs with erythrocyte cobalt but not with erythrocyte chromium. There is a close correlation between serum and erythrocyte cobalt levels but not between serum and erythrocyte chromium levels. We recommend using serum cobalt and serum chromium measurements as a standardised method of ion analysis following metal on metal hip replacement. This allows for capture of maximal levels of both ions in vivo. Further study is required to determine if any clinical effect will be seen as a result of these elevated ions.