Abstract
Purpose
Elevated blood metal ions are associated with the early failure of the Hip Resurfacing Arthroplasty. The aim of this study was to analyse our prospective database of Hip Resurfacing Arthroplasty patients, to independently review the outliers with elevated blood metal ions and to determine whether a screening program would be of value at our institution.
Method
In 2004 a ten year prospective longitudinal study was set up to evaluate the clinical effectiveness and safety of Metal on Metal Hip Resurfacings in young, active adults with degenerative hip disease.
Six hundred and four patients have enrolled in this multi-surgeon prospective study with strict inclusion criteria for Hip Resurfacing Arthroplasty. All have received the same implant design. All have completed validated functional outcome questionnaires at baseline, three and six months, then annually. A sub-cohort of 196 patients underwent whole blood chromium and cobalt analysis at the same time periods.
Metal on metal bearings have a running in period of a minimum of six months before a steady state wear pattern is attained. We chose five parts per billion for Cobalt or Chromium as our threshold value. This value corresponds to the workplace exposure limit in the United Kingdom to Cobalt in whole blood. Therefore patients with ion levels greater than five parts per billion after six months were recalled for independent review, including further metal ion analysis.
Results
Twenty two patients were recalled. Twenty one patients (32 Hip Resurfacing Arthroplasties) were reviewed. At latest review 11 patients (15 Hip Resurfacing Arthroplasties; eight females) had levels greater than five parts per billion. Mean follow up was 59.8 months (47–78). Mean age at surgery was 48.7 years (37–55). Median femoral component size was 50 millimetres (42–54). Mean acetabular anteversion was 18.3 degrees (−5.2 43.0). Mean acetabular inclination was 46.1 degrees (33.1–57.1). Mean cobalt and chromium levels were 8.82 parts per billion (3.49 18.42) and 9.15 parts per billion (3.79 24.33). Patients with ion levels greater than five parts per billion were associated with inferior functional scores (p= 0.018), inferior hip flexion (p=0.01) and mal-positioned acetabular components (p=0.023). All symptomatic patients were female.
Conclusion
It is reassuring that the majority do not have elevated metal ions (185/196; 94.4%). That said, blood metal ion screening of Hip Resurfacing Arthroplasties aids in the early detection of problematic cases. Comprehensive clinical review should follow as patient safety is paramount. The early detection of problematic cases is advantageous to the surgeon and patient. Revision surgery for an established pseudotumour has been found to be technically challenging, often with a poor outcome.
