Metal ion measurement as a diagnostic tool to identify problems with metal-on-metal hip resurfacing.

Introduction etal-on-metal bearings for hip replacement are gaining popularity as an alternative to the most widely used bearing couple of metal on polyethylene. The recent resurgence of hip resurfacing with the socalled third and fourth generations of hip resurfacing arthroplasty devices has played a substantial role in the wider use of metal-on-metal bearings. Concerns about the wear of polyethylene and the role of polyethylene wear debris in the process of aseptic loosening have also increased the use of hardon-hard bearing couples. However, wear still takes place with metal-on-metal bearings, and there is concern about the relatively high levels of metallic ions released into the body as a consequence. Cobalt-chromium alloy is the metal used for metal-on-metal implants and, while these elements are required for normal biological function, high concentrations of cobalt and chromium are toxic and are known to interfere with a number of biological processes. The metal ion levels typically seen in patients with well-functioning metal-on-metal implants are not thought to be close to toxic levels and in many instances are not much higher than those in patients with metal-onpolyethylene implants. However, there is a potential for much higher concentrations of metal wear products in the joint fluid in the vicinity of the bearing surfaces of poorly functioning metal-on-metal implants. It is these very high levels that are thought to be responsible for adverse reactions such as osteolysis and the formation of soft-tissue masses and that give rise to metallosis (defined as gray discoloration of the hip joint). It is not clear whether the levels of metal ions measured in blood serum are indicative of the metal ion concentrations in the joint fluid. Perhaps more importantly, the serum concentrations that are indicative of problems occurring with the bearing and high levels in the joint fluid and tissues have not been defined. Detection of wear in metal-on-polyethylene couples is possible with use of radiographic methods to measure the penetration of the head into the polyethylene. This is not possible with metal-on-metal bearings. It is important to know if excessive wear is occurring, and this will play a key role in the management of patients who have a hip replacement. While it is accepted that metal ion measurements are important for monitoring metal-on-metal hip replacements, a relationship between serum metal ion levels and wear has not been defined. In the current study, we attempted to determine if measurement of serum ion levels could be used as a diagnostic tool by correlating blood and joint fluid metal-ion measurements with findings of metallosis and component wear at revision arthroplasty.