Eight RCTs (669 patients) were included in the review. Follow-up ranged from two to 10 years.
Cobalt: At two years patients who received metal-on-metal implants had significantly higher postoperative versus pre-operative serum cobalt concentrations (WMD 0.67, 95% CI 0.48 to 0.86) and significantly higher postoperative levels compared to metal-on-polyethylene (WMD 0.64, 95% CI 0.49 to 0.79). Pre-operative and postoperative levels were not significantly different in metal-on-polyethylene patients. Similar patterns were reported for urine and erythrocyte cobalt concentrations.
Chromium: Metal-on-metal implants significantly increased postoperative (at two years) serum (WMD 0.59, 95% CI 0.44 to 0.74), urine (WMD 1.91, 95% CI 1.49 to 2.34) and erythrocyte (WMD 0.46, 95% CI 0.15 to 0.77) chromium concentrations compared to pre-operative levels. There were no statistically significant differences in pre-operative and postoperative levels in patients who received metal-on-polyethylene implants for serum or urine. All concentrations were significantly higher in metal-on-metal groups compared to metal-on-polyethylene at two years for serum (WMD 0.58, 95% CI 0.34 to 0.82), erythrocyte (WMD 0.38, 95% CI 0.03 to 0.73) and urine (WMD 1.88, 95% CI 1.46 to 2.30).
Titanium: At two years there were no significant differences in erythrocyte titanium concentrations for any comparison. Both metal-on-metal and metal-on-polyethylene showed significant differences between pre-operative (WMD 0.20, 95% CI 0.08 to 0.32) and postoperative urine titanium concentrations (WMD 0.18, 95% CI 0.09 to 0.27). There were no significant differences in postoperative concentration between implant groups (p=0.56).
There were no significant differences in complication rates (four RCTs), reoperation rates (six RCTs), function and health-related quality of life (seven RCTs) or radiographic evaluation (three RCTs).