# CIM Case: Metal-on-Metal Hip Failure ## Clinical Scenario **Patient:** 58-year-old woman **Presentation:** Right hip and groin pain 7 years after metal-on-metal THR, worsening over 6 months, difficulty walking and ascending stairs **Relevant history:** MoM THR in 2017 for hip dysplasia (36mm head), no prior hip surgery, under annual surveillance, asymptomatic until recently, non-smoker, no other joint replacements **Examination findings:** - Antalgic gait - Tender over greater trochanter and groin - Range of motion limited: flexion 90° (painful), IR 10°, ER 30° - Positive anterior impingement sign - Mild Trendelenburg gait (subtle abductor weakness) - Well-healed lateral wound - No distal neurovascular deficit - No palpable lymphadenopathy - Cardiovascular examination normal --- ## Investigations Provided ### Laboratory Results | Test | Result | Normal Range | Interpretation | |------|--------|--------------|----------------| | Cobalt | 12 ppb | <7 ppb | ↑ Elevated (concerning) | | Chromium | 9 ppb | <7 ppb | ↑ Elevated | | Hb | 125 g/L | 120-150 g/L | Normal | | WCC | 7.2 ×10⁹/L | 4-11 ×10⁹/L | Normal | | CRP | 5 mg/L | <5 mg/L | Normal | | ESR | 12 mm/hr | <20 mm/hr | Normal | | Creatinine | 68 μmol/L | 45-90 μmol/L | Normal (renal function) | | LFTs | Normal | - | Normal (hepatic function) | ### Imaging **Image 1: AP Pelvis and Lateral Hip Radiograph** **Radiological features:** - Large head metal-on-metal THR in situ (36mm) - Cup inclination 55° (steep - outside safe zone of 35-45°) - Cup anteversion appears normal - No obvious osteolysis on plain film - Stem appears well-fixed - No loosening features **Image 2: MARS MRI (Metal Artifact Reduction Sequence)** **MRI findings:** - Large fluid collection posterior to hip joint (5cm pseudotumour) - Significant soft tissue oedema in periarticular region - Gluteal muscle atrophy with fatty infiltration - No definite abductor tendon tear - Marked inflammatory changes around implant - **Features consistent with ARMD (Adverse Reaction to Metal Debris)** --- ## Questions & Model Answers **Diagnosis: Adverse Reaction to Metal Debris (ARMD) with Pseudotumour Formation** **Key Diagnostic Features:** - Metal-on-metal bearing with symptoms - Elevated cobalt (12 ppb) and chromium (9 ppb) levels (both >7 ppb threshold) - Pseudotumour on MARS MRI - Steep cup inclination (55°) causing edge loading - Progressive symptoms despite initially asymptomatic **Essential Investigations:** | Investigation | Purpose | Key Findings | |---------------|---------|--------------| | **Metal ion levels** | Quantify systemic exposure | Cobalt and chromium >7 ppb = concerning | | **Plain radiographs** | Component position, osteolysis | Cup inclination 55° (steep), no osteolysis visible | | **MARS MRI** | Soft tissue assessment | Pseudotumour, muscle atrophy, osteolysis | | **Inflammatory markers** | Exclude infection | CRP/ESR normal (ARMD not infection) | **Additional Investigations to Consider:** | Investigation | Purpose | |---------------|---------| | FBC | Baseline, polycythaemia (cobalt effect) | | Renal function | Cobalt nephrotoxicity | | Liver function | Hepatic effects | | Echo | Cardiomyopathy screening if severely elevated ions | | Thyroid function | Hypothyroidism screening | > **Consultant Tip:** Metal ions >7 ppb + symptoms + imaging changes = failing MoM. This triad mandates revision. **Risk Stratification for MoM Hip Surveillance:** | Risk Level | Criteria | Management | |------------|----------|------------| | **Low Risk** | Metal ions <7 ppb AND asymptomatic AND cup inclination <50° AND no osteolysis AND good implant track record | Annual clinical + X-ray review | | **Moderate Risk** | Any one criterion not met | MARS MRI or ultrasound for soft tissue assessment | | **High Risk** | Metal ions >7 ppb OR symptomatic OR pseudotumour on imaging | Consider revision surgery | **This Patient's Risk Level: HIGH RISK** - Cobalt 12 ppb (>7) - Chromium 9 ppb (>7) - Symptomatic (pain, limited ROM) - Pseudotumour on MARS MRI - Steep cup (55°) **TGA/AOANJRR Surveillance Recommendations:** | Interval | Assessment | |----------|------------| | Annual | Clinical review, symptom assessment | | Annual | Metal ion testing (cobalt and chromium) | | If symptomatic | Cross-sectional imaging (MARS MRI or ultrasound) | | If elevated ions or imaging abnormality | Consider revision | **Metal Ion Interpretation:** | Cobalt Level (ppb) | Interpretation | |--------------------|----------------| | <2 | Normal (non-MoM patients) | | 2-7 | Acceptable for well-functioning MoM | | 7-20 | Elevated, concern for ARMD | | >20 | High, systemic effects possible | > **Consultant Tip:** All MoM patients need lifelong surveillance. Rising ions or new symptoms should prompt imaging. **Risk Factors for MoM Failure:** | Category | Risk Factors | |----------|--------------| | **Patient factors** | Female sex (more sensitive), small native hip size, high activity level | | **Implant factors** | Larger head size (36mm+), certain designs (DePuy ASR recalled), component mismatch | | **Surgical factors** | Cup inclination >50° (edge loading), excessive anteversion, femoral offset mismatch | | **Biological factors** | Metal hypersensitivity, immune response variability | **This Patient's Risk Factors:** - Female - 36mm large head - Steep cup inclination (55°) - most important factor **Pathophysiology of ARMD:** | Mechanism | Description | |-----------|-------------| | **Wear debris generation** | Metal-on-metal articulation releases cobalt-chromium particles | | **Edge loading** | Steep cup causes contact at edge rather than central articulation → massive wear increase | | **Local tissue reaction** | Metal ions cause type IV hypersensitivity response | | **ALVAL** | Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion (histological pattern) | | **Pseudotumour formation** | Fluid collection + necrotic tissue + inflammatory mass | | **Soft tissue destruction** | Progressive muscle atrophy, tendon damage, bone loss | **Edge Loading Explained:** - Normal articulation: Head contacts central cup - Edge loading: Head contacts rim of cup - Cause: Cup inclination >50° or inadequate cup coverage - Effect: 100-1000× increase in wear debris > **Consultant Tip:** Edge loading is the most important mechanism. Steep cups (>50°) should prompt surveillance even if asymptomatic. **Local Effects (ARMD):** | Effect | Clinical Features | |--------|-------------------| | **ALVAL** | Perivascular lymphocytic infiltrate, tissue necrosis (histological) | | **Pseudotumour** | Solid or cystic mass adjacent to hip (this patient: 5cm posterior) | | **Soft tissue necrosis** | Destruction of periarticular tissues | | **Muscle atrophy** | Gluteal muscle wasting, abductor dysfunction | | **Osteolysis** | Bone destruction around implant | | **Joint effusion** | Large fluid collections | **Systemic Effects ("Arthroprosthetic Cobaltism"):** | System | Effect | Mechanism | |--------|--------|-----------| | **Haematologic** | Polycythaemia (early), then anaemia | Cobalt mimics erythropoietin initially | | **Cardiac** | Cardiomyopathy, heart failure | Direct cobalt toxicity | | **Neurological** | Peripheral neuropathy, cognitive impairment, vision/hearing loss | Neurotoxicity | | **Thyroid** | Hypothyroidism | Blocks iodine uptake | | **Renal** | Nephrotoxicity | Direct toxicity | | **Hepatic** | Hepatic necrosis | Direct toxicity | **Cobalt vs Chromium Toxicity:** | Metal | Solubility | Primary Effect | |-------|------------|----------------| | **Cobalt** | Water-soluble | More systemic toxicity (absorbed) | | **Chromium** | Less soluble | More local tissue destruction | **Screening for Systemic Effects (if cobalt >20 ppb):** - FBC (polycythaemia/anaemia) - Thyroid function - Echocardiogram - Renal function - Neurological assessment > **Consultant Tip:** Systemic cobalt toxicity is rare but serious. Screen for cardiac and neurological effects if cobalt very high (>20 ppb). **Indication for Revision:** This patient meets criteria for revision: - Symptomatic (pain, limited ROM) - Elevated metal ions (cobalt 12, chromium 9) - Pseudotumour on MARS MRI - Abductor dysfunction **Preoperative Planning:** | Assessment | Purpose | |------------|---------| | **Exclude infection** | Aspiration if any doubt (can coexist with ARMD) | | **Bone stock assessment** | CT if osteolysis suspected (not visible on plain films) | | **Soft tissue assessment** | MARS MRI extent of pseudotumour, abductor status | | **Systemic workup** | Cardiac, renal, thyroid if high metal ions | | **Patient counselling** | Realistic expectations (worse outcomes than primary THR) | **Surgical Management:** | Step | Details | |------|---------| | **Approach** | Posterior or direct lateral (excise pseudotumour) | | **Remove MoM components** | Both cup and head must be revised | | **Debride pseudotumour** | Complete excision of necrotic tissue | | **Assess abductors** | Repair if torn, may need reconstruction | | **Bone loss management** | Augments, cages, impaction grafting as needed | | **New bearing surface** | Ceramic-on-polyethylene (XLPE) preferred | **Bearing Surface Options for Revision:** | Option | Advantages | Disadvantages | |--------|------------|---------------| | **Ceramic-on-XLPE** | No metal ions, good wear properties | Risk of liner wear (long-term) | | **Ceramic-on-ceramic** | Zero wear, no metal debris | Squeaking, fracture risk, cost | | **Metal-on-PE (CoCr)** | Lower cost, familiar | Some metal debris (acceptable) | | **Dual mobility** | Stability if abductors compromised | Not for young/active patients | **Postoperative Management:** - Monitor metal ion decline (should fall by 50% at 1 year) - Abductor rehabilitation (slow if compromised) - Dislocation precautions if abductors weak - Long-term surveillance > **Consultant Tip:** Early revision before extensive soft tissue destruction has better outcomes. Don't wait for severe damage. **Outcomes After Revision for ARMD:** | Outcome | Rate | |---------|------| | Pain relief | 80-90% improved | | Metal ion reduction | 50% fall at 1 year | | Re-revision rate | 10-15% at 5 years | | Dislocation rate | Higher than primary (5-10%) | | Infection rate | Slightly higher than primary | **Factors Affecting Outcome:** | Factor | Better Outcome | Worse Outcome | |--------|----------------|---------------| | Timing | Early revision (less damage) | Late revision (severe destruction) | | Pseudotumour | Small, cystic | Large, solid, extensive | | Abductors | Intact | Destroyed, fatty infiltration | | Bone loss | Minimal | Extensive osteolysis | | Metal ions | Only mildly elevated | Very high (>20 ppb) | **Counselling Points:** **1. Why This Happened:** - Metal-on-metal bearings have higher failure rate than expected - Steep cup caused edge loading and increased wear - Not necessarily surgical error - implant design issues **2. What to Expect:** - Pain should improve (80-90% get better) - Metal ions will fall over time - Abductor weakness may persist (Trendelenburg) - May need walking aids initially - Dislocation precautions for 6-12 weeks **3. Long-term Considerations:** - Re-revision rate higher than primary THR - Need ongoing surveillance - May have residual stiffness or weakness - Contralateral MoM (if present) also needs monitoring **4. If Severe Abductor Damage:** - May need constrained liner (limits ROM) - Prolonged rehabilitation - Possible permanent limp - Rarely: excision arthroplasty (salvage) > **Consultant Tip:** Be honest about outcomes. Revision for ARMD has worse results than revision for other causes. Early intervention is key. --- ## Key Teaching Points ### Pattern Recognition **This pattern suggests MoM Failure (ARMD):** - Patient with MoM bearing and new/progressive hip symptoms - Elevated metal ion levels (>7 ppb cobalt or chromium) - MARS MRI showing pseudotumour or soft tissue destruction - Risk factors: female, steep cup (>50°), larger head size **ARMD vs Periprosthetic Joint Infection:** | Feature | ARMD | Infection | |---------|------|-----------| | CRP/ESR | Usually normal | Elevated | | Aspirate | Metallic debris, lymphocytes | Elevated WCC, positive culture | | Histology | ALVAL (lymphocyte-dominated) | Neutrophil infiltration | | Onset | Gradual (years) | May be acute or chronic | ### Critical Management Points 1. **All MoM patients need lifelong surveillance** - annual clinical review and metal ion testing 2. **Metal ions >7 ppb = concerning** - warrant cross-sectional imaging 3. **Edge loading is key mechanism** - cup inclination >50° dramatically increases wear 4. **MARS MRI is investigation of choice** - detects pseudotumour and soft tissue damage 5. **Early revision = better outcomes** - before extensive tissue destruction 6. **Never use MoM at revision** - change to ceramic-on-polyethylene or ceramic-on-ceramic --- ## Common Examiner Follow-ups **Q: "What is ARMD and ALVAL?"** | Term | Definition | |------|------------| | **ARMD** | Adverse Reaction to Metal Debris - umbrella term for all local adverse reactions to MoM debris (pseudotumour, necrosis, osteolysis) | | **ALVAL** | Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion - specific histological pattern with perivascular lymphocytic infiltrate and tissue necrosis | Key difference: ARMD is clinical/radiological diagnosis; ALVAL is histological diagnosis. ALVAL represents a type IV hypersensitivity reaction (delayed cell-mediated) to metal ions, unlike polyethylene wear which causes macrophage-dominated osteolysis. --- **Q: "What is edge loading and why does it matter?"** Edge loading occurs when: - Head articulates at rim of cup rather than centrally - Caused by: steep cup inclination (>50°), inadequate cup coverage, impingement Consequences: - Friction and wear massively increased (100-1000×) - Rapid metal debris generation - Accelerated ARMD development Prevention: Cup inclination 35-45° ("safe zone"), adequate anteversion (15-25°). --- **Q: "What bearing surface would you use at revision?"** | Scenario | Recommended Bearing | |----------|---------------------| | Standard revision | Ceramic-on-XLPE | | Young, active patient | Ceramic-on-ceramic | | Abductor deficiency | Dual mobility (stability) | | Major bone loss | Ceramic-on-XLPE with reconstruction | Never use metal-on-metal again. The offending bearing must be completely removed. --- **Q: "What is the DePuy ASR recall and why was it important?"** The DePuy ASR (Articular Surface Replacement) was recalled in 2010: - Hip resurfacing and MoM THR system - Unacceptably high revision rates (5-year: 12-13%) - Design issues causing edge loading - Widespread ARMD Implications: - Led to increased surveillance of all MoM hips - Established metal ion thresholds (>7 ppb) - Demonstrated importance of arthroplasty registries (AOANJRR) - Litigation and regulatory changes --- ## Related Topics - AOANJRR Hip Registry Data - Bearing Surfaces in Arthroplasty - Metal Hypersensitivity - Revision Hip Arthroplasty - Pseudotumour of Hip - Hip Resurfacing