METAL-ON-METAL HIP COMPLICATIONS
ALTR | Pseudotumor | Metal Ion Toxicity | Surveillance and Revision
ALTR SEVERITY (HART CLASSIFICATION)
Critical Must-Knows
- Cobalt greater than 7 ppb = concerning, warrants further investigation and surveillance
- Cup inclination greater than 55 degrees = edge loading, accelerated wear and ion release
- MARS MRI = Metal Artifact Reduction Sequence for soft tissue assessment
- ALTR = Adverse Local Tissue Reaction (umbrella term for all MoM soft tissue problems)
- ASR recalled 2010 = DePuy ASR hip resurfacing and XL Acetabular System
Examiner's Pearls
- "Pseudotumor does not mean malignancy - it is a sterile inflammatory mass
- "Small head MoM THAs have higher failure rates than resurfacing
- "Metal ions can remain elevated for years after revision
- "Revision for ALTR has poorer outcomes than revision for other causes
Clinical Imaging
Imaging Gallery
Critical Metal-on-Metal Exam Points
Cobalt Ion Threshold
Cobalt greater than 7 ppb is the concerning threshold (MHRA guidelines). Levels greater than 10 ppb require urgent investigation. Chromium follows similar pattern but cobalt is more clinically relevant. Test whole blood, not serum. Annual monitoring mandatory for all MoM hips.
Cup Positioning Risk
Inclination greater than 55 degrees causes edge loading with metal-on-metal contact outside the bearing surface. Combined anteversion matters - aim for inclination 40-45 degrees and anteversion 15-20 degrees. Malposition is the strongest predictor of failure.
MARS MRI Protocol
Metal Artifact Reduction Sequence MRI allows soft tissue visualization around metal implants. Essential for detecting ALTR, pseudotumor, and muscle atrophy. Ultrasound is alternative but operator-dependent. CT less useful for soft tissue but shows osteolysis.
Pseudotumor Pathology
Pseudotumor = benign inflammatory mass, not malignancy. Caused by hypersensitivity reaction to metal debris (Type IV). Histology shows ALVAL (Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion). Can cause extensive soft tissue destruction including abductor damage.
At a Glance: Quick Decision Guide - MoM Complications
| Clinical Scenario | Metal Ion Level | Imaging Findings | Management |
|---|---|---|---|
| Asymptomatic, well-positioned implant | Cobalt less than 4 ppb | No abnormality on imaging | Annual surveillance with bloods |
| Mild symptoms, borderline ions | Cobalt 4-7 ppb | Small fluid collection | 6-monthly surveillance, optimize activity |
| Symptomatic, elevated ions | Cobalt 7-10 ppb | ALTR present on MARS MRI | Consider revision, close monitoring |
| Significantly symptomatic | Cobalt greater than 10 ppb | Large pseudotumor or tissue destruction | Revision surgery recommended |
METALCauses of Metal-on-Metal Failure
Memory Hook:METAL fails when the METAL bearing goes wrong - position, loading, and biological reaction!
CIMMSALTR Investigation Protocol
Memory Hook:CIMMS investigation - Complete the picture before deciding on revision!
SMALLRisk Factors for ALTR
Memory Hook:SMALL things cause big problems - pay attention to these risk factors!
Overview and Epidemiology
Metal-on-metal (MoM) hip bearings were introduced as an alternative to metal-on-polyethylene to reduce wear-related osteolysis and improve longevity, particularly in younger active patients. However, concerns about adverse local tissue reactions (ALTR), elevated metal ion levels, and pseudotumor formation have led to a significant decline in their use and multiple product recalls.
Historical Context
The DePuy ASR Hip Resurfacing System and ASR XL Acetabular System were voluntarily recalled in August 2010 due to higher than expected revision rates. The Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) data showing 6.4% revision rate at 5 years was instrumental in identifying the problem.
Epidemiology
- Peak usage: 2005-2010 before concerns emerged
- Current use: Near zero for primary THA, very limited resurfacing
- Affected population: Estimated 500,000+ patients with MoM hips worldwide
- ASR failure rate: 12-13% at 5 years (recalled devices)
- Well-positioned resurfacing: 95%+ survival at 10 years in experienced hands
Clinical Impact
- Pseudotumor incidence: 1-4% symptomatic, up to 60% asymptomatic on MRI
- Revision burden: Significant healthcare cost and patient morbidity
- Systemic toxicity: Rare but documented cardiac, neurological, thyroid effects
- Legal implications: Major class action settlements globally
- Ongoing surveillance: Lifetime monitoring required for all MoM patients
Pathophysiology and Mechanisms
Understanding ALTR Mechanism
ALTR is caused by a combination of wear-generated metal debris and corrosion products from the bearing surface and modular junctions. This triggers a Type IV hypersensitivity reaction in susceptible individuals, leading to soft tissue destruction, osteolysis, and potentially systemic toxicity.
Tribological Failure
Metal-on-metal bearings rely on fluid film lubrication to separate the articulating surfaces. When this lubrication fails (due to edge loading, malposition, or high activity), metal-on-metal contact occurs, generating wear debris.
Wear Mechanisms in MoM Bearings
| Mechanism | Cause | Result | Clinical Significance |
|---|---|---|---|
| Edge loading | Cup inclination greater than 55 degrees or low coverage | Contact at rim, not center | 10-100x increased wear rate |
| Stripe wear | High range of motion, microseparation | Linear wear pattern on head | Visible on explanted components |
| Trunnionosis | Modular junction corrosion | Metal release from taper | More common with large heads, long necks |
| Optimal lubrication | Well-positioned, matched components | Minimal wear | Low ion levels, good outcomes |
Biological Response
Local Effects (ALTR)
- ALVAL: Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion
- Pseudotumor: Solid or cystic inflammatory mass
- Metallosis: Black staining from metal debris deposition
- Soft tissue necrosis: Destruction of capsule, abductors
- Osteolysis: Bone resorption around implants
Systemic Effects
- Neurological: Cognitive changes, peripheral neuropathy (rare)
- Cardiac: Cardiomyopathy reported at very high levels
- Thyroid: Hypothyroidism in some case reports
- Renal: Metal deposition, uncertain clinical significance
- Hematological: Cobalt affects erythropoiesis
Histological Pattern
ALVAL (Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion) is the characteristic histological finding. It shows perivascular lymphocytic infiltrates, endothelial swelling, and tissue necrosis. This pattern distinguishes hypersensitivity from simple wear debris reaction and is associated with worse soft tissue destruction.
Classification Systems
Hart MRI Classification of ALTR
Based on MARS MRI appearances of periprosthetic soft tissue abnormalities.
| Type | Description | MRI Appearance | Management |
|---|---|---|---|
| Type 1 | Fluid only | Simple effusion, no wall thickening | Surveillance, consider aspiration |
| Type 2a | Fluid with synovitis | Wall thickening, enhancing synovium | Close monitoring, consider revision |
| Type 2b | Solid pseudotumor | Mass lesion, solid component | Revision recommended |
| Type 3 | Mixed/destructive | Complex tissue destruction, muscle atrophy | Urgent revision, expect bone/muscle loss |
Imaging Classification
Type 2b and Type 3 lesions have the worst outcomes after revision. Early detection and intervention before extensive tissue destruction improves revision outcomes significantly. Size of pseudotumor correlates with abductor damage.
Clinical Assessment
History
- Pain: Groin, lateral hip, thigh - often different from arthritic pain
- Swelling: Visible or palpable mass (large pseudotumor)
- Functional decline: Worse than expected after arthroplasty
- Implant details: Date of surgery, implant type (critical)
- Clicking or clunking: May indicate subluxation or impingement
- Neurological symptoms: Numbness, weakness (nerve compression)
Examination
- Gait: Trendelenburg sign (abductor damage)
- Mass: Palpable swelling anterolaterally
- ROM: May be preserved despite significant ALTR
- Leg length: Assess for subsidence or dislocation
- Neurological: Sciatic or femoral nerve involvement
- Skin changes: Rarely, sinus or discoloration
Beware the Asymptomatic Patient
Up to 60% of patients with MoM hips may have asymptomatic ALTR on imaging. Symptoms do not correlate well with severity of soft tissue damage. This is why systematic surveillance with imaging is essential regardless of symptoms.
Differential Diagnosis
| Condition | Distinguishing Features | Investigations |
|---|---|---|
| ALTR/Pseudotumor | Elevated metal ions, characteristic MRI findings | Metal ions, MARS MRI |
| Periprosthetic infection | Raised inflammatory markers, positive culture | CRP, ESR, aspiration and culture |
| Aseptic loosening | Pain with activity, progressive radiolucencies | Sequential X-rays, CT |
| Iliopsoas impingement | Anterior groin pain, pain with hip flexion | Injection test, CT for cup position |
| Trochanteric bursitis | Lateral pain, point tenderness | Clinical diagnosis, ultrasound |
Investigations
Investigation Protocol for MoM Hip Surveillance
Whole blood cobalt and chromium - not serum (serum underestimates levels). Cobalt less than 7 ppb generally acceptable. Cobalt greater than 7 ppb requires further investigation. Cobalt greater than 10 ppb is high risk. Annual monitoring mandatory for all MoM hips.
AP pelvis and lateral hip - assess component position (inclination, anteversion), loosening signs, osteolysis. Cup inclination greater than 55 degrees is concerning for edge loading. Compare with previous films.
Metal Artifact Reduction Sequence MRI - gold standard for soft tissue assessment. Detects fluid collections, pseudotumors, muscle atrophy, osteolysis. Classification guides management. Should be performed if ions elevated or symptoms present.
Alternative to MRI when unavailable or contraindicated. Operator-dependent but can detect fluid and masses. Less sensitive for muscle atrophy and bone changes. Useful for guided aspiration.
Rule out infection before revision. Send for microbiology culture (extended incubation), cell count, and differential. Synovial fluid alpha-defensin if available. Essential in all painful MoM hips.
Metal Ion Interpretation
| Cobalt Level (ppb) | Interpretation | Action Required |
|---|---|---|
| Less than 2 ppb | Normal/optimal | Annual surveillance |
| 2-4 ppb | Acceptable | Annual surveillance |
| 4-7 ppb | Borderline elevated | Increased surveillance, consider imaging |
| 7-10 ppb | Elevated - concerning | MARS MRI required, 6-monthly review |
| Greater than 10 ppb | High - significant risk | Urgent imaging, consider revision |
| Greater than 20 ppb | Very high - systemic risk | Revision recommended, systemic assessment |
Metal Ion Testing Points
- Whole blood, not serum - serum underestimates by 50%
- Cobalt more clinically relevant than chromium for ALTR
- Chromium:Cobalt ratio - very high chromium suggests trunnionosis
- Unilateral MoM - compare to population norms
- Bilateral MoM - interpret with caution, levels additive
Management Algorithm
Lifetime Surveillance for MoM Hips
All patients with MoM hip implants require lifelong surveillance regardless of symptoms.
Surveillance Schedule
Clinical assessment, metal ion levels (Co and Cr), plain radiographs if any concerns. Document symptoms, examination findings, implant function.
Many centers recommend baseline MARS MRI for all MoM patients to establish soft tissue status. Repeat if symptoms develop or ions rise.
6-monthly review if Cobalt greater than 7 ppb, new symptoms, or abnormal imaging. More frequent if rapidly rising ions or progressive symptoms.
Progressive ALTR on imaging, Cobalt greater than 10 ppb with symptoms, functional decline, neurological compromise, or component failure.
Surveillance is Lifelong
Metal ions can rise years after implantation. ALTR can develop in previously asymptomatic patients. There is no "safe" time to stop monitoring. Document surveillance in writing to patients.
Surgical Management
Key Surgical Principles for MoM Revision
Preoperative planning:
- Review all imaging including MARS MRI
- Map pseudotumor location and extent
- Assess bone stock on CT if osteolysis suspected
- Rule out infection with aspiration
- Counsel patient regarding expected outcomes
Intraoperative principles:
- Extended approach - may need extensile exposure for pseudotumor excision
- Complete debridement - remove all necrotic tissue and metallosis
- Pseudotumor excision - thorough removal of reactive tissue
- Tissue sampling - multiple samples for microbiology and histology
- Bone grafting - address osteolytic defects
- Bearing change - ceramic-on-polyethylene preferred
- Stability assessment - anticipate instability from tissue loss
Bearing Choice
- Ceramic-on-polyethylene: Preferred for revision
- Ceramic-on-ceramic: Alternative but squeaking risk
- Avoid MoM: Never use MoM for revision
- Dual mobility: Consider if instability risk high
Constraint Consideration
- Standard: If soft tissues preserved
- Dual mobility: Abductor weakness or tissue loss
- Constrained liner: Severe instability risk
- Assess intraoperatively: Trial stability before final
Evidence Base and Key Studies
ASR Hip Recall and Registry Data
- ASR revision rate 12-13% at 5 years significantly higher than other THAs
- Cup inclination greater than 55 degrees strongly associated with failure
- Small ASR cups (less than 50mm) had higher failure rates
- AOANJRR data instrumental in identifying problem leading to recall
Metal Ion Thresholds and Clinical Outcomes
- Cobalt greater than 7 ppb associated with significantly higher risk of ALTR
- Cobalt greater than 10 ppb indicates high probability of soft tissue damage
- Chromium:Cobalt ratio greater than 1.5 suggests trunnion corrosion contribution
- Whole blood testing more accurate than serum
MARS MRI for ALTR Detection
- MARS MRI detects ALTR in up to 60% of asymptomatic MoM patients
- Solid pseudotumors (Type 2b) predict worse outcomes after revision
- Fluid-only lesions may be observed with serial imaging
- Muscle atrophy visible on MARS MRI correlates with functional outcome
Revision Outcomes for Pseudotumor
- Revision for ALTR has 15-25% re-revision rate at 5 years
- Outcomes worse than revision for aseptic loosening or instability
- Earlier revision before extensive tissue loss improves outcomes
- Dual mobility cups reduce dislocation rate after ALTR revision
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Symptomatic MoM Patient
"A 52-year-old man presents with progressive groin and lateral hip pain 6 years after a metal-on-metal hip resurfacing. He was previously very active but has noticed increasing difficulty with walking. Examination shows Trendelenburg gait and limited internal rotation. Investigations show Cobalt 15 ppb and Chromium 12 ppb. Plain films show a well-fixed implant with cup inclination of 52 degrees. What is your diagnosis and management plan?"
Scenario 2: Asymptomatic MoM Surveillance
"A 48-year-old woman with bilateral ASR hip resurfacings performed in 2008 attends for routine surveillance. She is asymptomatic with excellent function. Metal ions show Cobalt 5 ppb and Chromium 4 ppb. X-rays show cup inclination of 48 degrees on right and 58 degrees on left. How do you manage this patient?"
Scenario 3: Systemic Cobalt Toxicity
"A 65-year-old man with a MoM THA from 2009 presents with progressive hearing loss, visual disturbance, cognitive decline, and peripheral neuropathy. Cardiology has found a new cardiomyopathy. Metal ions show Cobalt 180 ppb. What is your diagnosis and emergency management?"
Australian Context
AOANJRR and MoM Hip Surveillance:
The Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) has been pivotal in identifying problems with metal-on-metal hip arthroplasty. Registry data was instrumental in the ASR recall and continues to provide essential outcome data.
Key AOANJRR Findings:
- ASR revision rate: 6.4% at 5 years (2010 data that triggered recall)
- MoM THA vs resurfacing: Small head MoM THA has worse outcomes than resurfacing
- Revision for ALTR: Higher re-revision rates than revision for other causes
- Current MoM use: Near zero for primary THA in Australia
- Surveillance coverage: All Australian patients with MoM hips should be in surveillance programs
Australian Surveillance Guidelines:
The Australian guidelines align with international recommendations:
- Annual clinical review for all MoM patients
- Annual whole blood cobalt and chromium levels
- Cross-sectional imaging (MARS MRI preferred) if ions greater than 7 ppb or symptomatic
- Baseline imaging recommended for all ASR patients
- Lifelong surveillance - no endpoint for monitoring
Recalled Devices in Australia:
Major MoM devices recalled or withdrawn in Australia:
- DePuy ASR (2010): Hip Resurfacing System and XL Acetabular System
- DePuy Pinnacle MoM (2013): MoM articulation option withdrawn
- Various other MoM options voluntarily withdrawn by manufacturers
TGA Reporting:
All MoM complications should be reported to the Therapeutic Goods Administration (TGA). Revision surgeries captured by AOANJRR.
Medicolegal Considerations:
Major class action settlements in Australia related to ASR hips. Documentation of surveillance, counseling, and informed consent is essential. Patients should receive written confirmation of surveillance plans.
Fellowship Exam Relevance:
The Orthopaedic Viva frequently tests:
- Metal ion interpretation and thresholds
- MARS MRI classification of ALTR
- Indications for revision
- Surgical principles for MoM revision
- Bearing choice for revision
- AOANJRR data and recall history
- Surveillance protocols
- Systemic toxicity recognition
METAL-ON-METAL HIP COMPLICATIONS
High-Yield Exam Summary
Metal Ion Thresholds
- •Cobalt less than 2 ppb = optimal
- •Cobalt 2-7 ppb = acceptable, annual surveillance
- •Cobalt 7-10 ppb = concerning, MARS MRI
- •Cobalt greater than 10 ppb = high risk, consider revision
- •Cobalt greater than 20 ppb = systemic risk, revise
Cup Position Risk
- •Inclination greater than 55 degrees = edge loading
- •Optimal inclination 40-45 degrees
- •Optimal anteversion 15-20 degrees
- •Malposition strongest predictor of failure
ALTR Classification (Hart)
- •Type 1: Fluid only - surveillance
- •Type 2a: Synovitis - consider revision
- •Type 2b: Solid pseudotumor - revise
- •Type 3: Destructive - urgent revision
Investigation Protocol
- •Whole blood (not serum) metal ions
- •Plain X-rays for position/loosening
- •MARS MRI for soft tissue
- •Aspiration to rule out infection
- •Annual surveillance lifelong
Revision Principles
- •Early revision preserves soft tissue
- •Complete pseudotumor excision
- •Ceramic-on-polyethylene bearing
- •Consider dual mobility for instability
- •Never MoM for revision
Key Facts
- •ASR recalled August 2010
- •AOANJRR data key to identifying problem
- •Pseudotumor = benign inflammatory mass
- •ALVAL = histological pattern
- •Small head MoM worse than resurfacing