A 72-year-old woman presents 8 years after total hip replacement with progressive groin pain and a rising serum cobalt level of 12 µg/L. MARS-MRI shows a large pseudotumor around the acetabular component. The most likely diagnosis and the primary implant characteristic responsible are:

• A Polyethylene wear with osteolysis — highly cross-linked polyethylene liner failure
• B Infection — biofilm formation on smooth cobalt-chromium femoral head
• C Adverse local tissue reaction (ALTR) — metal-on-metal articulation releasing cobalt-chromium ions ✓
• D Aseptic loosening — stress shielding from titanium femoral stem

Explanation

Elevated serum cobalt/chromium ions combined with a pseudotumor on MARS-MRI is pathognomonic of adverse local tissue reaction (ALTR) from metal-on-metal (MoM) hip bearings. Cobalt and chromium ions released by tribo-corrosion at the articulation cause a type IV hypersensitivity reaction, leading to necrosis and pseudotumor formation. Polyethylene wear produces osteolysis but not elevated metal ions, and infection would present with elevated inflammatory markers and positive aspiration cultures.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.