SUMMARY

We report a prospective study of clinical data collected pre, intra and post operation to remove both cup and head components of 118 failed, current generation metal on metal (MOM) hips. Whilst component position was important, the majority were unexplained failures and of these the majority (63%) had cup inclination angles of less than 55 degrees. Poor biocompatibility of the wear debris may explain many of the failures.

Introduction: Local problems of metal on metal (MOM) hip arthroplasty such as pseudotumours, neck thinning and osteolysis maybe related to concentrations of cobalt and chromium ions in the synovial fluid. There is little reported on these values. Our aim was to determine the range of metal ion levels in synovial fluid, and to investigate the relationship between these samples and simultaneous blood samples.

Methods: Synovial fluid and whole blood samples were taken from 30 consecutive patients at the time of revision surgery for a painful MOM hip. Aspirated fluid was not visibly contaminated with blood. Impants were in situ for a mean period of 31 months. All had normal renal function. Samples were analysed using ICP mass spectrometry and compared with 10 samples from patients without implants.

Results: The mean (and range) of synovial fluid metal ion levels were 1965 ug/l (30 to 13618) and 6265 ug/l (11 to 81630) for Cobalt and Chromium respectively. There was a good correlation between synovial and blood levels for both cobalt (R=0.65, p = 0.0001) and chromium (R = 0.59, p = 0.006).

Discussion and Conclusion: Metal ions in synovial fluid from MOM hips are generated from wear of the bearing surfaces, the correlation with blood metal ion levels, shown in this study, suggest that blood levels may be used as surrogate marker for hip wear rate. Our range of synovial fluid metal ion levels may be useful for those conducting in vitro studies on the biocompatibility of MOM hips.

Blood metal ions have been widely used to investigate metal-on-metal hip replacements, but their ability to discriminate between well-functioning and failed hips is not known. The Medicines and Healthcare products Regulatory Agency (MHRA) has suggested a cut-off level of 7 parts per billion (ppb).

We performed a pair-matched, case-control study to investigate the sensitivity and specificity of blood metal ion levels for diagnosing failure in 176 patients with a unilateral metal-on-metal hip replacement. We recruited 88 cases with a pre-revision, unexplained failed hip and an equal number of matching controls with a well-functioning hip. We investigated the 7 ppb cut-off level for the maximum of cobalt or chromium and determined optimal mathematical cut-off levels from receiver-operating characteristic curves.

The 7 ppb cut-off level for the maximum of cobalt or chromium had a specificity of 89% and sensitivity 52% for detecting a pre-operative unexplained failed metal on metal hip replacement. The optimal cut-off level for the maximum of cobalt or chromium was 4.97 ppb and had sensitivity 63% and specificity 86%.

Blood metal ions had good discriminant ability to separate failed from well-functioning hip replacements. The MHRA cut-off level of 7 ppb provides a specific test but has poor sensitivity.

We retrospectively analysed concentrations of chromium and cobalt ions in samples of synovial fluid and whole blood taken from a group of 92 patients with failed current-generation metal-on-metal hip replacements. We applied acid oxidative digestion to our trace metal analysis protocol, which found significantly higher levels of metal ion concentrations in blood and synovial fluid than a non-digestive method. Patients were subcategorised by mode of failure as either ‘unexplained pain’ or ‘defined causes’. Using this classification, chromium and cobalt ion levels were present over a wider range in synovial fluid and not as strongly correlated with blood ion levels as previously reported. There was no significant difference between metal ion concentrations and manufacturer of the implant, nor femoral head size below or above 50 mm. There was a moderately positive correlation between metal ion levels and acetabular component inclination angle as measured on three-dimensional CT imaging.

Our results suggest that acid digestion of samples of synovial fluid samples is necessary to determine metal ion concentrations accurately so that meaningful comparisons can be made between studies.

Introduction: Approximately 0.5 % of patients with metal on metal hip replacements develop post operative pain which is thought to be due to an immune reaction to metal wear particles, known as Aseptic Lymphocyte Dominated Vasculitis Associated Lesion (ALVAL). Treatment usually requires revision to a non metal on metal hip.. Is the development of ALVAL more likely in those patients with high wear rates?

Methods: Retrieved Metal on Metal (MOM) hip implants; periprosthetic tissue and blood samples were obtained from patients (n = 18) undergoing revision for unexplained pain at a mean of 2 years post operatively. The following variables were measured:

linear wear rate (depth of the femoral head and acetabular socket wear patch/time from operation);

All implants and tissue samples were analysed against control samples from patients undergoing revision of MOM hips for fractured femoral neck or impingement.

Results: Linear wear rates of retrieved implants, and blood levels of cobalt and chromium from patients with unexplained plain were greater than from control patients. Histolopathological analysis of tissue showed dense inflammatory infiltrates with healthy looking endothelial cells in all vessels from both patient groups.

Discussion and Conclusion: A painful MOM hip was associated with high wear rates and blood metal levels. The local inflammatory response was similar to “ALVAL”, ie lymphocyte dominated, but not exclusive to those patients with unexplained pain. We question whether ALVAL represents a vasculitis, or merely a classical lymphocyte driven inflammatory tissue response to metal debris particles.

We carried out metal artefact-reduction MRI, three-dimensional CT measurement of the position of the component and inductively-coupled plasma mass spectrometry analysis of cobalt and chromium levels in whole blood on 26 patients with unexplained pain following metal-on-metal resurfacing arthroplasty.

MRI showed periprosthetic lesions around 16 hips, with 14 collections of fluid and two soft-tissue masses. The lesions were seen in both men and women and in symptomatic and asymptomatic hips. Using three-dimensional CT, the median inclination of the acetabular component was found to be 55° and its positioning was outside the Lewinnek safe zone in 13 of 16 cases. Using inductively-coupled plasma mass spectrometry, the levels of blood metal ions tended to be higher in painful compared with well-functioning metal-on-metal hips.

These three clinically useful investigations can help to determine the cause of failure of the implant, predict the need for future revision and aid the choice of revision prostheses.