To verify that the metal-on-metal (MOM) surfaces produce metal ions and to ascertain the differences between large and small femoral head diameters with regard to the release of metal ions.

Forty two cases of MOM hip resurfacing arthroplasty (mean head diameter: 47.1 mm) and 39 cases of conventional total hip replacement arthroplasty (head diameter: 28 mm) were performed at our hospital from June 1998 to April 2003. All the cases were followed-up using the serum cobalt and chromium ion levels. The serum cobalt and chromium ion levels were compared in the 21 cases in each group, which were matched by age, weight and follow-up period. The control group involved 21 patients who had undergone an alumina-alumina total hip arthroplasty. The patient's serum metal ion concentration was measured from the serum, which had been separated from the whole blood and diluted using inductively-coupled plasma mass spectrometry (Agilent 4500, Rhode Island, USA).

All MOM bearing implants produced higher serum levels of cobalt and chromium ions than the control. The serum concentration of cobalt with MOM resurfacing was higher than that observed with a 28 mm MOM total hip arthroplasty (p=0.025), but the chromium levels were not significantly different (p=0.501).

The MOM bearing implants in this study produced elevated serum levels of cobalt and chromium ions. In addition, large diameter heads resulted in greater systemic exposure of cobalt ions than the small diameter heads. However, a long-term follow-up will be needed to determine the clinical significance.