Metal ion levels in the blood of patients after hip resurfacing: a comparison between twenty-eight and thirty-six-millimeter-head metal-on-metal prostheses.

BACKGROUND: Metal ion toxicity, metal hypersensitivity, and metal carcinogenicity are causes for concern for patients with metal-on-metal hip replacements. Thus, understanding the biological fate of metal ions, and consequently their long-term systemic effects, is of great interest to patients and surgeons alike.
METHODS: Inductively coupled plasma mass spectrometry was used to measure the levels of cobalt, chromium, and molybdenum ions in the blood of control patients (preoperative control pre-resurfacing patients), patients with a metal-on-polyethylene total hip prosthesis, patients with a metal-on-metal total hip prosthesis with either a 28 or 36-mm femoral head, and patients with a hip resurfacing prosthesis. Since cobalt and chromium ions have the potential to induce oxidative stress through irreversible biochemical damage to macromolecules, the levels of ions were correlated to the concentration of three oxidative stress markers in the plasma of these patients.
RESULTS: The median cobalt level was significantly lower (p < 0.001) in the 36-mm metal-on-metal total hip arthroplasty group (1.8 parts per billion [1.8 microg/L]) compared with the 28-mm metal-on-metal total hip arthroplasty group (2.5 parts per billion [2.5 microg/L]) and the hip resurfacing group (2.3 parts per billion [2.3 microg/L]) at six months postoperatively. The median chromium level was also significantly lower (p < 0.01) in the 36-mm metal-on-metal total hip arthroplasty group (0.25 parts per billion [0.25 microg/L]) compared with the 28-mm metal-on-metal total hip arthroplasty group (0.35 parts per billion [0.35 microg/L]) and the hip resurfacing group (0.50 parts per billion [0.50 microg/L]) at six months postoperatively. However, neither the median cobalt levels nor the median chromium levels were significantly different among the three metal-on-metal groups at one year. The median levels of molybdenum were not significantly different among the three groups at either six months or one year. In addition, there was no significant difference in the plasma concentration of oxidative stress markers in patients with metal-on-metal bearings compared with that in control patients.
CONCLUSIONS: The blood metal ion levels in the hip resurfacing group were similar to those in the 28 and 36-mm-head metal-on-metal total hip arthroplasty groups. This study suggests that the increased metal ion levels had no effect on oxidative stress markers in the blood of these patients.