Abstract
Introduction
Hip simulator studies show that metal-on-metal bearing wear can be reduced by reducing the diametral clearance of the bearing. We present the six-year follow-up results of a prospective clinico-radiological and metal ion study in patients with a low clearance metal-metal surface arthroplasty. The results are compared to published results of similar design bearings with conventional clearance.
Methods
Twentysix male patients (mean age 55 years, mean BMI 26) who received a 50 mm bearing resurfacing (radial clearance 50μm) were included in an ongoing clinico-radiological and metal ion study. Urine/blood specimens were obtained before and periodically after hip resurfacing. Patients were also assessed with Oxford Hip Scores and Harris Hip Score questionnaires. Two hips were excluded during follow-up, one for revision and another for contralateral hip arthroplasty.
Results
Twentyone patients have so far been reviewed at the six-year stage and had excellent hip function (Median Oxford score 13/60 and median Harris Hip Score 89/90). Metal ion data shows encouraging results with mean daily output of cobalt and chromium in urine at 6 years being 3.8 μg/24 hr and 2.2 μg/24 hr respectively. The mean whole blood concentrations for cobalt and chromium were 0.5 μg/l and 1.42 μg/l respectively. Three patients who we reported earlier had radiolucent lines in acetabular zones 1 and 2 continue to have these features, albeit asymptomatic.
Discussion and Conclusion
Metal ion levels in this group are lower compared to earlier published reports with similar design bearings with conventional clearance. However the presence of radiolucencies raises the concern that reduced clearance affects early implant fixation possibly from increased friction. Further investigation is needed to assess the long-term outcomes of low clearance bearings.
