What are the normal metal ion levels after total hip arthroplasty? A serologic analysis of four bearing surfaces.
- B.T. Barlow, P.A. Ortiz, J.W. Boles, Y.-Y. Lee, . D.E. Padgett, G.H. Westrich
Division of Adult Reconstruction and Joint Replacement Surgery, Department of Orthopaedic Surgery and Epidemiology and Biostatistics Core, Hospital for Special Surgery, New York, USA
- J Arthroplasty. 2016 Dec 3. pii: S0883-5403(16)30824-5. doi: 10.1016/j.arth.2016.11.024. [Epub ahead of print].
- Level of Evidence:
- Level II -Prospective non-randomized comparative study.
The authors asked what should be considered “normal” levels of metal ions after THA. They investigated serum Co, Cr and Ti levels in a prospective, non-randomized case series of unilateral THA patients with 4 different bearings in order to prove or disprove their hypothesis that there is no difference in serum Co, Cr, and Ti levels between 4 well-functioning bearings from a single manufacturer: Metal-on-Polyethylene (MoP), Ceramic-on-Polyethylene (CoP), Ceramic-on-Ceramic (CoC), and Dual Mobility (DM). Patient-reported outcome measures (PROMs) were compared between 4 bearing surfaces to identify potential correlation between metal ion levels and outcome scores.
Twenty patients were recruited for each group (eighty patients total). Patients were asked to refrain from diet and nutritional supplements before the blood test. Serum metal ion levels were taken at one year following THA. The laboratory dependent detection limits for metal ions were 1.0μg/L (ppb) for Co and Cr and 10μg/L for Ti.
There were significant differences between the 4 bearing surfaces regarding implantation time, mean age and head size. Patients with MoP bearings were significantly older than the others and patients with CoC bearings had a significantly longer follow up time. No significant difference in Co and Cr serum levels were found between the 4 different bearing surfaces. However, when patients were grouped according to femoral head material, the metal head group showed significantly higher serum Co levels. Mean Co and Cr levels for ceramic heads were 0.59μg/L and 0.6μg/L, respectively and most patients with CoC bearings had concentrations below the detection limit. For metal heads the Co and Cr concentrations were 1.05μg/L and 0.7μg/L, respectively.
PROMs did not show significant difference between the bearing groups.
Barlow et al. conclude that their study showed no significant differences in Co and Cr serum levels in well-functioning THA with various bearings. The absolute serum metal ion levels were low and did not seem to be clinically significant as shown by PROMs.
- Small sample size, not based on power analysis.
- No pre-operative metal ion levels.
- Some CoCr-alloy stems in the MoP groups
- All implants were produced by a single manufacturer and may not be generalizable across different manufacturers.
- Detection limit for Ti was ten times higher than for Co and Cr and may have resulted in „false negatives“.
- High variability of implantation time.
- Most patients with CoC bearings had Co and Cr levels below the detection limit and non-detectable Ti ions.
- Patients with metal heads had significantly higher Co serum levels compared to patients with ceramic femoral heads.
- In well-functioning THA ceramic components, the metal ion levels were low.
The finding of Co and Cr ions in patients without any implant component made from Co or Cr containing alloy is surprising, but no baseline measurements were taken. Although this study has its limitations, it showed that THAs with CoC bearings have Co, Cr and Ti ion levels below the detection limit, suggesting that there is negligible release of metal ions from the taper junction.