Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement
R. De Haan, MD, Resident in Orthopaedic Surgery1; C. Pattyn, MD, Orthopaedic Surgeon2; H. S. Gill, DPhil, University Lecturer in Orthopaedic Engineering3; D. W. Murray, MD, FRCS(Orth), Professor of Orthopaedic Surgery3; P. A. Campbell, PhD, Associate Professor4; and K. De Smet, MD, Consultant Orthopaedic Surgeon5
1 University Hospital Brussels, Laarbeeklaan 101, 1090 Brussels, Belgium. 2 Ghent University Hospital, De Pintelaan 185, 9000 Ghent, Belgium. 3 Nuffield Department of Orthopaedic Surgery, University of Oxford, Botnar Research Centre, Nuffield Orthopaedic Centre, Oxford OX3 7LD, UK. 4 J Vernon Luck Orthopaedic Research Center, Orthopaedic Hospital/UCLA, 2400 S Flower St, Los Angeles, California 90007, USA. 5 ANCA Medical Centre, Krijgslaan 181, 9000 Ghent, Belgium.
We examined the relationships between the serum levels of chromium and cobalt ions and the inclination angle of the acetabular component and the level of activity in 214 patients implanted with a metal-on-metal resurfacing hip replacement. Each patient had a single resurfacing and no other metal in their body. All serum measurements were performed at a minimum of one year after operation. The inclination of the acetabular component was considered to be steep if the abduction angle was greater than 55°.
There were significantly higher levels of metal ions in patients with steeply-inclined components (p = 0.002 for chromium, p = 0.003 for cobalt), but no correlation was found between the level of activity and the concentration of metal ions. A highly significant (p < 0.001) correlation with the arc of cover was found. Arcs of cover of less than 10 mm were correlated with a greater risk of high concentrations of serum metal ions. The arc of coverage was also related to the design of the component and to size as well as to the abduction angle of the acetabular component. Steeply-inclined acetabular components, with abduction angles greater than 55°, combined with a small size of component are likely to give rise to higher serum levels of cobalt and chromium ions. This is probably due to a greater risk of edge-loading.