Abstract
Introduction: The increasing use of metal-metal bearings in hip arthroplasty raises concerns of elevated systemic metal levels. Lubrication theory and in-vitro studies suggest that larger diameter bearings generate less wear. Does this hold true in real life?
Methods: This is a cross-sectional study of 181 patients with either a Birmingham Hip Resurfacing or a 28mm Metasul THR. Whole blood levels (at 1year) and urinary metal ion output in the early (1 to 3) years and medium term (4 to 6years) in patients with these bearings were compared. None of the patients had other metal devices or compromised renal function. Both devices used a high-carbon cobalt-chrome bearing with no post-casting heat treatment. Cementless porous-ingrowth titanium cups and cemented polished tapered stainless steel stems or cementless porous-ingrowth titanium alloy stems were used in THRs.
Results: Whole blood metal levels at 1-year and daily output of metal ions in the early years and medium term (figure 1) in both bearings were in the same range and without a significant difference.
Discussion: Metal ion monitoring is the best way to estimate bearing wear in vivo and the best measure of device safety in the long term. Total wear over a given time period is best estimated from timed metal ion excretion rates. Blood levels represent a balance between the release of metal from the device and its renal clearance.
An earlier study (Clarke et al JBJS(Br) 2003) suggests that smaller bearings generate less wear. In that study bearings with different metallurgy and wear properties were grouped together, a potential confounding factor. The present study does not suffer from that error and our findings do not support the view that a larger bearing diameter leads to either an increase or decrease in metal ion generation.
Correspondence should be addressed to The Secretary, BHS, c/o BOA, The Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
