A patient with a metal-on-metal total hip arthroplasty (MoM THA) presents 5 years post-surgery with hip pain, a soft-tissue mass on MRI, and elevated serum cobalt (18 μg/L) and chromium (9 μg/L) levels. What is this complication and the appropriate management?
- A Periprosthetic joint infection — two-stage revision with antibiotic spacer
- B Heterotopic ossification — treat with NSAIDs and radiation
- C Stress shielding with osteolysis — bisphosphonate treatment
- D Adverse local tissue reaction (ALTR/ARMD) to cobalt-chromium ions — MRI-guided monitoring; revision arthroplasty if tissue destruction or worsening ✓
Explanation
Adverse local tissue reaction (ALTR), also termed ARMD (adverse reaction to metal debris), is unique to MoM bearings. CoCr nanoparticles and ions cause a macrophage/lymphocyte-mediated (Type IV hypersensitivity-like) pseudotumour and local tissue necrosis ('metallosis'). MHRA and AAOS guidelines recommend annual MRI (metal artefact-reduced sequences) and annual cobalt/chromium ion monitoring for symptomatic MoM THAs. Revision to ceramic or non-MoM bearing is recommended when ion levels are persistently elevated (>7 μg/L for cobalt), symptoms are present, or MRI shows fluid collections or soft-tissue destruction. PJI would show elevated CRP/ESR and fever.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.