A 62-year-old man with a well-functioning 36 mm metal-on-highly-crosslinked-polyethylene total hip replacement presents with groin pain at 7 years. Serum cobalt is elevated although there is no significant polyethylene wear on radiographs. CT shows a small fluid collection at the joint. Which mechanism best explains these findings?

• A Wear-through of the polyethylene liner exposing the metal shell
• B Fretting and crevice corrosion at the modular head-neck trunnion junction ✓
• C Corrosion of the titanium acetabular shell screws
• D Oxidative degradation of the polyethylene producing inflammatory debris

Explanation

Elevated cobalt with minimal polyethylene wear points to trunnionosis, mechanically assisted crevice corrosion and fretting at the modular head-neck junction. Risk factors include large femoral heads, mixed-alloy couples, male sex, high body mass index, and high offset. It can produce an adverse local tissue reaction identical to that seen after metal-on-metal arthroplasty. Liner wear-through would show eccentric polyethylene thinning, and screw corrosion would not selectively raise cobalt over titanium.

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

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Written and medically reviewed by the StethoPrep medical team.