Abstract
Background: Metal-on-metal bearing hip replacements release between three and nine times more cobalt and chromium ions than a metal on polyethylene bearing hip replacement. We do not fully understand the cause for the variability of ion levels after metal on metal hip replacement. The factors that determine an individual’s levels of metal ions include: firstly, patient factors (renal failure, patient weight, high activity); secondly, manufacture factors (head size (and fluid film lubrication), carbide density, surface finish) and lastly study factors (bilateral implants, time from operation). Biomechanical studies suggest that component position, in particular acetabular inclination, is important for wear rate but there is no published correlation from clinical studies.
Aim: To investigate the relationship between acetabular inclination angle and metal ion levels of patients with Birmingham Hip resurfacings.
Methods: Using standardised radiographs, we measured the inclination angle (using UTHSCSA image tool) of the acetabular components in thirty-one patients (mean age 54 years) who underwent unilateral Birmingham hip resurfacing (mean time post operation of 22 months). We also measured peripheral whole blood chromium and cobalt ion concentrations using inductively coupled mass spectrometry. All components were well fixed.
Results: There was a positive correlation between the inclination angle (range 28 degrees – 55 degrees) of the acetabular component and whole blood concentration of Cobalt (range 2.3 – 7 mcg/L), Chromium (range 0.56 – 4.3 mcg/L) and total metal ion levels (range 3.1 – 10.3 mcg/L). This finding was statistically significant, with a Pearson correlation coefficient of 0.46 (95% CI 0.13-0.70) and a p-value of 0.00398.
Conclusion: Acetabular inclination angle is likely to be a factor in determining an individual’s metal ion levels in patients with metal on metal resurfacing. We also iden-tified a threshold level of 50 degrees inclination, after which the metal ion levels rise dramatically. We describe the possible biomechanical mechanisms to explain these results. We recommend surgeons implant the metal socket at an inclination angle of less than 50 degrees.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
