Abstract
Introduction
A metal ion study was undertaken in patients who had received an articular surface replacement. The design of these components is optimised in line with lubrication theory and produces low levels of wear in hip joint simulators.
Methods
Patients were recruited in four centres. Whole blood samples were analysed for metal ion levels using high resolution ICP-MS (inductively coupled plasma mass spectrophotometry). A total of 75 patients was enrolled into the study and 65 and 47 patients were assessed after 12 and 24 months implantation respectively. Results are included irrespective of clinical outcome.
Results
The 12 and 24 month median ion levels were 1.4 μg/l and 1.6 μg/l respectively for chromium and 1.5(μg/l and 1.6 μg/l respectively for cobalt. The pre-operative levels were 0.38(μg/l and 0.34(μg/l for chromium and cobalt respectively. Twenty-seven percent of patients had a pre-operative chromium level that was higher than the 24 month median level. Similarly the value for cobalt was seven percent.
However close examination of the data shows that it is skewed by 6 outliers with cobalt or chromium levels greater than 10(μg/l after 24 months implantation. This appears to be related to acetabular component placement. Patients with acetabular components implanted at a high inclination angle (>55deg) are more likely to have elevated metal ion levels compared with a standard angle (<55deg). In the 24 month group two patients (both outliers) have been revised.
Furthermore, the high metal ion levels at 24 months could be predicted by intermediate levels after 12 months. Conversely those levels below 10ug/l had already stabilised after 12 months in vivo.
Conclusion
Large diameter metal-on-metal bearing with optimised tolerances can produce very low blood metal ion levels. However, in line with other studies, mal-positioning of components may lead to significantly elevated levels of wear and hence blood metal ions.
