A 68-year-old woman presents 14 years after total hip replacement with progressive groin pain and a rising serum cobalt level of 38 µg/L. Pelvic X-ray shows eccentric femoral head position within the acetabular cup. MRI reveals a large pseudotumour around the hip. What is the most likely diagnosis and primary causative mechanism?

• A Polyethylene particle-induced osteolysis from a metal-on-polyethylene bearing
• B Late haematogenous infection with Staphylococcus epidermidis
• C Stress shielding-induced aseptic loosening of the acetabular component
• D Adverse local tissue reaction (ALTR) due to metal ion release from a metal-on-metal bearing ✓

Explanation

Elevated serum cobalt and chromium levels combined with a pseudotumour (ALTR/ARMD) are hallmarks of metal-on-metal (MoM) bearing surface corrosion and tribocorrosion. MoM bearings release cobalt and chromium ions causing delayed-type hypersensitivity and necrosis in peri-implant tissues. Polyethylene wear causes osteolysis without pseudotumour formation, and infection would present with systemic signs and elevated ESR/CRP.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.