Abstract
Introduction
Wear and corrosion of metal-on-metal (MM) bearings releases (a) soluble metal ions which collect locally and pass into the systemic circulation and (b) insoluble particles which undergo local deposition and lymphoreticular dissemination. Debris-related failures from osteolysis, metallosis and pseudotumours warrants revision of these MM bearing devices to non-MM bearing arthroplasties with the expectation that both the systemic and local effects will be reversed with time since the source of metal ion release is removed.
The purpose of the present study is to determine (a) whether metal ion levels in blood and urine decrease after revision of a MM bearing arthroplasty to a non-MM bearing device and (b) the rate at which this decrease is effected.
Methods
Blood and urine levels of cobalt and chromium ions are studied prospectively over two years in 15 patients whose MM resurfacings were revised to cross-linked polyethylene containing total hip replacements (THRs). Specimen collection was started before and periodically after the revision at 2, 4 and 6 days and 2 months, 6, 12 and 24 months after operation. None of the patients had other MM devices or compromised renal function.
Results
Pre-revision mean urine-cobalt was 676 nmol/day (range 84 to 1691 nmol/day) and urine-chromium 348 nmol/day (range 68 to 804 nmol/day). As anticipated these are higher than the expected levels in patients with well-functioning metal-on-metal bearings. Post-revision metal levels decrease in a biphasic manner. Over the first week there is a rapid decline to 235 nmol/day cobalt and 217 nmol/day chromium followed by a period of slow decrease over the next 24 months to 10 nmol/day cobalt and 44 nmol/day chromium. Blood cobalt also shows a similar biphasic decrease from a preoperative level of 151 nmol/L to 79 nmol/L during the first week and further reduction to 13 nmol/L over the following 2 years and with blood chromium the reduction is from 103 nmol/L preoperative to 64 in the first week and on to 14 nmol/L at 2 years.
Discussion and Conclusion
A limitation of the study is the small cohort of 15 patients with a wide range of pre-revision blood and urine levels. However the same biphasic trend as seen in the group is evidenced in each individual patient which gives us confidence that the trend will not be different in much larger groups of patients. The early steep reduction following revision supports the reported short half-life of cobalt. The later protracted trend is explainable through progressive corrosion from deposited particles. This trend is also not sustained indefinitely and tends to approach control levels at 2 years.
