Abstract
Introduction
Large diameter metal-metal total hip replacements (MM THRs) offer the advantages of low wear and low dislocation risk and are being increasingly used in high-demand patients whose bone quality rules out the possibility of a hip resurfacing. However suggests that large headed MM devices may result in greater systemic metal exposure compared to small diameter bearings. This raises fresh concerns of elevated systemic metal levels.
Methods
Whole blood concentrations and daily output of cobalt and chromium in 28 patients with unilateral large diameter MM THRs (42 to 54mm bearings) were studied at 1-year follow-up. These were compared with the whole blood levels in 20 patients at 1 year and daily output of metal ions in 28 patients with 28mm MM THRs at 1 to 3 years. Both bearings are made of high carbon cobalt-chrome alloy, the larger bearing is as-cast alloy and the smaller is wrought alloy. High resolution inductively coupled plasma mass spectrometry was used for analysis. None of the patients had other metal devices or compromised renal function. They had either a cemented polished tapered stainless steel stem or a cementless porous ingrowth titanium alloy stem.
Results
Mean whole blood levels of cobalt at 1 year were 1.7 and 2.3 μg/l and chromium were 1.7 and 1.4 μg/l in the small and large diameter THR cohorts respectively. The mean differences were not statistically significant for either metal. The mean daily output of cobalt and chromium in the early years in the small and large diameter THRs, which were 11.6 and 12.3 μg/24 hrs; and 3.7 and 4.1 μg/24 hrs respectively, were also without a statistically significant difference.
Discussion
This study shows that whole blood concentrations of cobalt and chromium; and daily output of metal ions in urine are similar in small and large diameter bearing MM THRs.
