THE EFFECT OF OPERATIVE FACTORS ON OUTLIER ION LEVELS IN PATIENTS WITH METAL ON METAL HIP IMPLANTS

Abstract

Introduction: Metal on metal hip implants continue to be successful alternatives to conventional bearings in younger patients with osteoarthritis. Levels of metal ions such as cobalt (Co) and chromium (Cr) increase in patients with metal bearing hip replacements and resurfacings. These particles are cytotoxic, induce bone loss, and lead to malignant tumors in rats. A subset of these patients are considered outliers as they have unusually high levels of Co and Cr ions. Given the increasing prevalence of metal bearings and the potential for cellular toxicity, we attempted to determine whether patient or surgical factors could account for abnormally elevated ion levels.

Methods: We analyzed the Co and Cr levels from whole blood in 661 patients with metal on metal hip bearings. Patient outliers were defined as those who had ion levels ≥ three-fold the mean value. Twenty-four patients (3.6%) had abnormally high metal ion levels, which included 15 patients that underwent total hip replacements and 9 patients following hip resurfacings. These patients were followed prospectively with the Harris Hip Score (HHS) and the University of California Los Angeles (UCLA) activity score. Serial radiographs and ion levels were analyzed at regular intervals. Oxidative stress markers (total anti-oxidants, peroxide, and nitro-tyrosine) were also measured from whole blood to determine if these correlated with an increase ion levels in outlier patients.

Results: Post-operative HHS and UCLA activity scores improved significantly compared to pre-operative values. There was no statistical correlation between outlier ion levels, patient demographics, HHS and UCLA activity scores. Radiologic parameters such as cup inclination and femoral component neck-shaft angle could not account for higher ion levels in these outliers. Oxidative stress markers were similar to the levels observed in the control patients with normal ion values following with metal on metal hip implants.

Conclusion: We could not identify any patient or surgical factors that could explain the abnormally high metal ion levels in the outlier patients. This suggests that the cause of ion level increase is multifactorial. The clinical relevance of such high levels of ions remains unknown given that there was no increase in serum oxidative stress markers. Further studies are necessary to better understand the effect of abnormal elevations in metal ions given the recent concerns of pseudotumours following metal on metal hip implants.