Abstract
Introduction
Metal-on-metal hip resurfacings (MoMHRAs) have a characteristic wear pattern initially characterised by a run-in period, followed by a lower-wear steady-state. The use of metal ions as surrogate markers of in-vivo wear is now recommended as a screening tool for the in-vivo performance of MoMHRAs. The aims of this retrospective study were to measure ion levels in MoMHRAs at different stages during the steady-state in order to study the evolution of wear at minimum 10 years postoperatively and describe factors that affect it.
Materials and methods
A retrospective study was conducted to investigate the minimum 10-year survivorship of a single-surgeon Birmingham Hip Resurfacing (BHR) series, and the evolution of metal ion levels. Implant survival, Harris Hip Scores (HHS), radiographs and serum metal ion levels were assessed. The evolution of metal ion levels was evaluated in 80 patients for whom at least two ion measurements were available at more than 12 months postoperatively, i.e. past the run-in phase. Ion level change (Delta Cr; Delta Co) was defined as Cr or Co level at last assessment minus Cr or Co level at initial assessment. Sub-analysis was performed by gender, diagnosis, age, femoral component size and cup inclination angle.
Results
Overall Cr and Co levels were low (Cr: 1.3 μg/L; Co: 1.0 μg/L for unilateral and Cr: 3.2 μg/L; Co: 2.3 μg/L for bilateral resurfacings) and decreased significantly (p < 0.001) from the initial assessment at 4 to 8 years postoperatively (median 6 years) to the last assessment at 10 to 13 years (median 11 years) with a mean reduction of 1.24 μg/L for Cr (Median: −0.80; SD: 1.948; Range: −11.8 to +2.0 μg/L) (p < 0.001) and 0.88 μg/L for Co (Median: −0.75; SD: 1.645; Range: −7.5 to +5.3 μg/L) (p < 0.001). Three patients (4%) had an increase of Co ions >2.5 μg/L, associated with head sizes £50 mm and with clinical symptoms and high cup inclination angles. There was no significant difference in the change of ion levels between genders (Cr: p = 0.845; Co: p = 0.310) although Cr levels at initial and last assessments were higher in females (p = 0.008). Component size did not correlate with change in ion levels (Cr: p = 0.505; Co: p = 0.370). Patients with increased ion levels at follow-up had lower Harris Hip Scores (p = 0.038).
Discussion
Low serum Cr and Co ion levels were found in patients with well-functioning unilateral and bilateral MoMHRA. In well-functioning MoMHRA ion levels continue to be low even after 310 years in situ. In this series, there were no symptomatic patients with low metal ion levels. Increasing metal ion levels >10 μg/l were invariably associated with poorly functioning MoMHRA. The analysis of consecutive ion levels demonstrated a statistically significant overall decrease of Cr and Co levels with time in well-functioning BHR. In 25% of patients ion levels were undetectable at 310 years postoperatively. Our data are in accordance with tribocorrosion studies indicating a lower-wear bedding-in phase after the initial running-in phase of higher wear. An increase in ion levels after the run-in phase is an indication of continued surface wear which may eventually lead to implant failure potentially associated with soft tissue and bone destruction.
