Abstract
Introduction
Tribological studies have described a characteristic wear pattern of metal-on-metal hip resurfacings (MoMHRAs) with a run-in period followed by a ‘bedding-in’ phase minimising wear or by an increasing wear patch with edge loading. The use of metal ions as surrogate markers of in-vivo wear is now recommended as a screening tool for the performance of MoMHRAs. The aims of this retrospective, single-surgeon study were to measure ion levels in unilateral MoMHRAs at different stages during the steady-state in order to study the evolution of wear and factors affecting it.
Methods
218 consecutive patients with minimum two serum ion measurements were included. The mean age at surgery was 52.3 years, the first assessment was made at a mean of 2.5 years (11 months–8 years) and the last assessment at a mean of 4.6 years post resurfacing (2– 12 years). Ion level change was defined as Ion level at last assessment minus Ion level at first assessment. Ten different resurfacing designs were implanted, the majority being BHR (n=104), Conserve plus(n=55) and ASR (n=25). The median femoral component size was 50 mm (38–59mm). Radiological assessment of acetabular component orientation was made with EBRA.
Results
For the whole cohort a significant reduction in Chromium (Cr) levels between initial [2.6 μg/ml (SD: 6.8)] and last assessment [1.9 μg/ml (SD: 8.3)] was found (p= 0.004). Cobalt (Co) levels were similar at initial [1.7 (SD: 7.7)] and last [1.8 (SD: 10.6)] follow-up (p=0.78). Cr levels increased in 31% and Co increased in 46% of patients. There was a significant ion level increase in 5% of patients. There was no significant difference between genders (p= 0.4) or implant designs (p = 0.07), but a trend of higher ions at last follow up with the ASR implants in comparison to BHR and C+. Neither component size (p=0.4) nor acetabular orientation (p=0.46) correlated with change in ion levels. However, a CPR distance (contact patch-rim) of <10mm was associated with an increase in Cr levels over time (p= 0.042). Patients with increasing ion levels had significantly lower Harris Hip Scores (p=0.038).
Discussion
The analysis of the evolution of ion levels in unilateral hip resurfacing after the run-in phase demonstrates an overall decrease of Cr levels but no significant change in Co levels. This in vivo finding is consistent with tribocorrosion studies showing the formation of a passive protective film on the articulating surfaces after the initial wear-in, preventing further corrosion. From that point on, provided there is no edge loading causing increased surface wear, ions are mainly formed by corrosion of the particulate debris generated during the run-in phase. In vitro studies have shown a higher dissolution of the more soluble Co from the debris while Cr remains in solid form with less ion formation. In a number of cases in our study, ion levels continued to increase, indicating ongoing surface wear. These increasing ion levels were correlated with a lower coverage angle more prone to edge loading and with the development of clinical symptoms.
