# Metal-on-Metal Hip Arthroplasty Failure ## Clinical Scenario Patient with MoM hip presenting for surveillance or with symptoms ## Questions & Model Answers **Low Risk:** - Cup abduction <50° - No osteolysis - No loosening - Good track record - Metal ions <7ppb **Low Risk Management:** Annual follow up **Moderate Risk:** Any of the above - worth investigating with MARS MRI or ultrasound **Bloods:** Metal ions in blood - >7ppb is the cut off **Imaging:** MARS MRI shows effusion and pseudotumour on T1 weighted images connecting with joint **Consideration:** Possibility of infected pseudotumour - Positioning - abduction >50° - Larger head size - Women - Obesity - Age >65yo - Haematologic - microcytic anaemia - Hepatic necrosis - Renal toxicity - Cardiac toxicity - Neurotoxicity Surgical revision options: 1) Modular head/liner exchange - if implants well-fixed, exchange MoM bearing to ceramic-on-polyethylene (CoP) or ceramic-on-ceramic (CoC), 2) Complete revision - if loosening, osteolysis, or component malposition, revise acetabular and/or femoral components, 3) Pseudotumour debridement - thorough excision of inflammatory tissue essential for success, send for culture (exclude infection). Key surgical considerations: 1) Posterior approach allows better access for pseudotumour excision, 2) Soft tissue damage may be extensive - assess abductors, 3) Dual mobility cup if abductor compromise, 4) Bone loss from osteolysis - may need augments or structural allograft, 5) Avoid ceramic heads if trunnion corrosion (risk of ceramic fracture). Outcomes after revision: 80-85% satisfactory results at 5 years, worse outcomes with larger pseudotumours, muscle damage, and delayed intervention. **Key Points:** - Modular exchange if implants well-fixed - Complete revision if loosening/malposition - Thorough pseudotumour debridement essential - Consider dual mobility if abductor damage Prognosis after revision: Outcomes vary based on extent of disease at revision. Favourable factors: Early intervention, small pseudotumour, intact abductors, no bone loss, well-positioned revision components. Unfavourable factors: Large pseudotumour, abductor deficiency (leads to instability), significant bone loss, trunnionosis, delayed diagnosis. Post-revision monitoring: 1) Metal ions - should decline after revision, persistent elevation concerning for retained debris or new corrosion, 2) Clinical follow-up - pain, function, hip stability, 3) Imaging - X-rays at 6 weeks, 3 months, 12 months, then annually, repeat MARS MRI if symptoms recur. Outcomes data: 5-year revision rate after MoM revision ~10-15%, dislocation rate higher than primary THA (5-10%), especially with abductor damage. Prevention: For patients with well-functioning MoM hips, continued surveillance per MHRA/TGA guidelines with metal ions and imaging as indicated. **Key Points:** - Early intervention = better outcomes - Abductor damage = higher dislocation risk - Metal ions should decline post-revision - Continued surveillance even after revision ## Key Learning Points - Metal ion threshold is 7ppb - Cup abduction >50° is a major risk factor - MARS MRI is the imaging of choice - Pseudotumour may be sterile or infected - Metal toxicity affects multiple organ systems