A 68-year-old woman, 8 years post left total hip arthroplasty with a 28 mm metal-on-polyethylene bearing, presents with progressive groin pain and a markedly elevated serum cobalt (42 µg/L) and chromium (38 µg/L). MRI MARS (Metal Artifact Reduction Sequence) shows a large pseudotumor with soft tissue necrosis. The most likely diagnosis and primary mechanism of failure is:
- A Aseptic loosening due to cement interface fatigue
- B Periprosthetic joint infection with Propionibacterium acnes
- C Dislocation with fibrous ankylosis
- D Adverse local tissue reaction (ALTR) from metal ion release due to taper corrosion at the head-neck junction ✓
Explanation
Elevated serum cobalt and chromium, a pseudotumor on MRI MARS, and a metal-on-polyethylene bearing all point to taper corrosion (trunnionosis) at the modular head-neck junction causing adverse local tissue reaction (ALTR). This mechanism is distinct from classic metal-on-metal bearing wear; corrosion products trigger a Type IV hypersensitivity-like reaction causing necrosis and pseudotumor formation. Aseptic loosening and infection do not produce this pattern of elevated metal ions and pseudotumor. Management requires revision with cobalt-chrome head removal and bearing exchange.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.