An asymptomatic 62-year-old man is reviewed 6 years after total hip replacement with a 36 mm cobalt-chromium head on a titanium trunnion. Routine surveillance shows serum cobalt 14 micrograms per litre with chromium 4 micrograms per litre, disproportionately elevated cobalt. Radiographs show no osteolysis and the acetabular component is a ceramic-lined shell. The source of the metal ions is most likely:

• A Wear of a metal-on-metal acetabular bearing surface
• B Fretting and crevice corrosion at the modular head-neck taper junction ✓
• C Corrosion of the porous coating of the femoral stem
• D Systemic absorption from dietary sources

Explanation

Trunnionosis arises from micromotion fretting and mechanically assisted crevice corrosion at the modular head-neck taper, typically releasing cobalt predominantly over chromium, especially with mixed-metal couples such as a cobalt-chromium head on a titanium trunnion and with large heads. Here the bearing is ceramic-lined, eliminating option A. Management centres on serial metal ion monitoring and imaging for pseudotumour, with revision exchanging the head and taper if levels rise or symptoms develop.

Reference: Miller's Review of Orthopaedics, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.