| Background: Metal on Metal implants was first introduced in1950s.However, these implants went out of favour due to manufacturing anddesign deficiencies, which led to poor performance and early failures. Metalon metal was almost rejected with the invention of low friction bearings withpolyethylene cup inserts. Second generation metal on metal bearings werereintroduced with better metallurgy, improved tribology and reduced wearrates, since metal on poly ethylene bearings led to osteolysis and asepticloosening caused by polyethylene wear debris.While the second generation metal on metal implants are more promising,they also pose many concerns regarding the biological effects of long termexposure to metal ions, such as hypersensitivity, carcinogenicity and trans-placental transfer to the fetus.Objectives: Goal of this exercise was to evaluate serum metal ion levelsand the clinical outcome in large diameter head metal on metal total hipreplacement.Methods: Thirty patients who underwent large diameter head metal onmetal, primary, unilateral total hip arthroplasty were registered for the study.An equal number of matched control group was also registered.Questionnaires with the inclusion of Harris hip score, and University ofCalifornia Los Angeles activity score were duly completed by the candidates.Phlebotomy was performed with an intra venous cannula. All precautionswere taken to minimize metal contamination during the procedure.5ml ofblood was drawn in to deionized plastic tubes containing heparin. Sampleswere then centrifuged at1500rpm for10minutes and frozen at-20C. Levelsof Cobalt, Chromium and Titanium were measured by inductively-coupledplasma spectrometry.Results: Significant increase in Cobalt, Chromium and Titanium wereobserved at all post operative evaluations compared to the mean controlvalue. All three metal ion levels increased up to1year. Mean values ofCobalt, Chromium and Titanium ions were0.914±0.186SD,1.176±0.609SD,0.861±0.313SD (μg/L) at3months and2.018±1.103SD,1.7711±0.970SD, 2.009±1.425SD (μg/L) at1year respectively. No significant difference wasnoted between1year and2years follow up levels. There was no significantcorrelation between the ion concentrations, age, sex, body mass index,University of California Los Angeles activity score and Harris hip score.The mean preoperative Harris Hip score was39.5. Mean Harris Hip score at1year follow-up was94.74±6.78SD. Mean pre-operative University ofCalifornia Los Angeles activity score2.5(0.82SD) it improved to6.93(1.02SD) at1year. These values show a remarkable improvement at oneyear. Result did not change much at2years follo-up. There were noradiological or clinical evidence of osteolysis and implant loosening at allpost-operative evaluations. There were no revisions done. These resultsindicate excellent clinical results and100%survivorship at2years.Conclusion: Significant elevations in Cobalt, Chromium and Titanium ionlevels were observed although not much of evidence is reported of itsharmful effects. Metal ion concentrations on postoperative follow up ofChinese patients are similar to that of internationally published data. Largediameter heads providing a greater surface area for wear and corrosion,modular double taper sleeve and the open head design in large diameter headtotal Hip Arthroplasty may be the causes for high metal ion concentrations.Further passive corrosion leads to release of titanium. Long term monitoringof post operative metal ion levels and its harmful effects should be continued,while tribolgy and design of the implants needs further review andimprovements.Large diameter head metal on metal total Hip replacement remains afavoured implant in young and active patients due to its long durability andincreased range of motion despite the risk posed by increased metal ionlevels. |
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