A 52-year-old man with a ceramic-on-polyethylene total hip replacement presents at 7 years with groin pain. Serum metal ions show cobalt markedly elevated at 11 micrograms per litre while chromium remains low at 1.5 micrograms per litre. There is no metal-metal bearing. The most likely source of the isolated cobalt release is:
- A Wear of the polyethylene liner producing dissolved cobalt
- B Corrosion at the head-neck trunnion junction beneath the ceramic head ✓
- C Systemic absorption from the titanium acetabular shell
- D Laboratory contamination of the chromium sample only
Correct answer: B. Corrosion at the head-neck trunnion junction beneath the ceramic head
Explanation
Even with a ceramic bearing surface, the metallic head-neck taper (trunnion) remains vulnerable to mechanically assisted crevice corrosion, releasing cobalt predominantly. A disproportionately high cobalt relative to chromium in a patient without a metal-on-metal articulation points to trunnionosis rather than bearing wear. Titanium implants corrode to inert titanium dioxide particles and do not raise serum cobalt, which eliminates option C.
Reference: Miller's Review of Orthopaedics, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.