Early Post-operative Rises in Serum Metal Ion Levels in Total Hip Arthroplasty

Background: Serum Cobalt (Co) and Chromium (Cr) forms part of the diagnostic process for metallosis following Total Hip Arthroplasty (THA). While knowledge exists on longer term metal ion levels, expected early post-operative rises in serum Co and Cr in Metal-on-Polyethylene (MoP) THAs are currently unknown. This study aims to describe early rises in serum Co and Cr at 6 months postoperatively. Methods: A prospective cohort study of 84 consecutive patients with an uncemented titanium stem from a single THA manufacturer was performed. Patients had either a metal (n=43) or ceramic (n=41) head articulating with a highly cross-linked polyethylene. Serum Co and Cr levels were measured six months post-operatively. Analysis compared mean values between groups and to determined baseline levels. Subgroup analysis investigated the effect of femoral head size and offset on metal ion levels. Results: A mean difference of 0.002259 ppb (95% CI 0.000449-0.004069 ppb; p=0.015) was found when comparing 6-month serum Co in the metal head group compared to baseline. No significant differences were found in serum Cr (p=0.943) at six months post-surgery compared to baseline. Mean serum Co levels were higher in the MoP group compared to the CoP (Ceramic-on-Polyethylene) and auxiliary control group (p=0.012). There were no differences in serum Cr (p=0.976) between the MoP and CoP groups at 6 months post-surgery. Variations in femoral head size and offset did not impact metal ion levels. Conclusion: At six months post-surgery, a higher magnitude of serum Co exists in metal heads when compared to baseline (p=0.015) and to ceramic heads (p=0.012). Further study is required to determine whether serum concentrations of metal ions will continue to increase over time which might leads to implant failure and revision.