CORROSION MECHANISMS IN ORTHOPAEDIC IMPLANTS
Electrochemical Degradation | Metal Ion Release | ALVAL | Implant Failure
Four Major Types of Corrosion
Critical Must-Knows
- Corrosion is electrochemical degradation of metal in physiological environment
- Passivation layer (TiO2, Cr2O3) protects implants - breakdown causes corrosion
- MACC (mechanically-assisted crevice corrosion) is key mechanism at modular junctions
- ALVAL (aseptic lymphocyte-dominated vasculitis-associated lesion) from metal hypersensitivity
- MoM hip failures primarily from taper corrosion, not bearing surface wear
Examiner's Pearls
- "Body fluid is corrosive: 0.9% NaCl, pH 7.4, 37°C, proteins
- "Mixed metal couples (CoCr/Ti) have higher galvanic corrosion than matched materials
- "Serum metal ions: Cobalt and chromium levels monitor MoM hips
- "Taper assembly: clean, dry, single impaction reduces fretting corrosion
Critical Corrosion Exam Points
Electrochemical Process
Oxidation-reduction reaction. Metal loses electrons (oxidation: M → M+ + e-) at anode. Electrons flow to cathode where reduction occurs (O2 + 2H2O + 4e- → 4OH-). Passivation layer prevents this.
MACC at Modular Tapers
Mechanically-Assisted Crevice Corrosion is primary failure mode in modular hip implants. Micro-motion disrupts passivation + crevice environment creates low pH = accelerated corrosion.
ALVAL and Metal Hypersensitivity
Type IV delayed hypersensitivity to metal ions. Aseptic lymphocyte-dominated vasculitis causes soft tissue destruction (pseudotumor). Cobalt and chromium ions from corrosion products.
Prevention Strategies
Avoid mixed metals (match CoCr with CoCr), proper taper assembly (clean/dry/single impaction), use highly polished surfaces, consider ceramic heads to eliminate taper corrosion.
Clinical Imaging
Trunnion Surface Microscopy
At a Glance
Corrosion is the electrochemical degradation of orthopaedic implants in the physiological environment (0.9% NaCl, pH 7.4, 37°C, proteins), occurring when the protective passivation layer (TiO₂, Cr₂O₃) breaks down. Four major corrosion types exist: Galvanic (dissimilar metal couples, e.g., CoCr/Ti), Crevice (oxygen depletion in gaps creating acidic pH 3-4), Pitting (localized passivation breakdown from chloride ions), and Fretting (micro-motion disrupting oxide layer). MACC (mechanically-assisted crevice corrosion) at modular taper junctions is the primary failure mechanism in modern hip implants, as micro-motion disrupts passivation within the crevice environment. Metal ion release leads to ALVAL (aseptic lymphocyte-dominated vasculitis-associated lesion), a Type IV delayed hypersensitivity reaction causing pseudotumor formation—serum cobalt and chromium levels monitor MoM hip patients. Prevention strategies include avoiding mixed metal couples, proper taper assembly (clean, dry, single impaction), polished surfaces, and ceramic femoral heads to eliminate head-taper corrosion.
GCPFTypes of Corrosion
Memory Hook:Get Careful, Prevent Failure - know your corrosion types!
CHAMPSFactors Accelerating Corrosion
Memory Hook:CHAMPS accelerate corrosion - control these factors!
ALVALALVAL Features
Memory Hook:ALVAL describes the pathology of metal hypersensitivity reactions!
Overview and Electrochemistry
Corrosion is the electrochemical degradation of metallic materials in their environment. In orthopaedic implants, this occurs when metals are exposed to the aggressive physiological environment: aqueous solution with chloride ions (0.9% NaCl), pH 7.4, temperature 37°C, and organic molecules.
The body is a hostile environment for metals:
- Chloride ions (aggressive anions) attack passivation layers
- Proteins and amino acids affect electrochemical reactions
- Cyclic mechanical loading creates stress and micro-motion
- Oxygen gradients develop in crevices and gaps
Understanding corrosion mechanisms is essential because metal ion release can cause:
- Implant mechanical failure - material loss weakens structure
- Metal hypersensitivity - Type IV delayed hypersensitivity (ALVAL)
- Local tissue toxicity - soft tissue necrosis and pseudotumor
- Systemic metal elevation - unclear long-term effects
Electrochemical Principles
Oxidation at Anode
Metal loses electrons and goes into solution as ions:
M → M^n+ + ne^-
This is the corrosion reaction. Metal atoms become ions in solution (metal ion release).
Reduction at Cathode
Electrons are consumed in reduction reaction:
O2 + 2H2O + 4e^- → 4OH^-
Oxygen is reduced to hydroxyl ions. This completes the electrochemical circuit.
Passivation: The Key Defense
All orthopaedic metals (titanium, stainless steel, cobalt-chromium) rely on a thin oxide layer for corrosion resistance:
- Titanium: Forms TiO2 layer (very stable, rapid self-healing)
- Stainless steel: Forms Cr2O3 layer (chromium oxide)
- Cobalt-chromium: Forms Cr2O3 layer (chromium oxide)
This passivation layer is typically 2-10 nm thick and prevents metal from contacting the corrosive environment. Corrosion occurs when passivation is disrupted.
Why Titanium is Corrosion Resistant
Titanium forms a tenacious TiO2 passivation layer that is extremely stable and self-healing - if scratched, it reforms in milliseconds in the presence of oxygen. This makes titanium highly corrosion-resistant despite being a reactive metal thermodynamically. CoCr relies on Cr2O3 which is less robust.
Principles and Mechanisms of Corrosion
Galvanic Corrosion
Galvanic Corrosion Mechanism
Occurs when two dissimilar metals are in electrical contact in an electrolyte. The less noble (more anodic) metal corrodes preferentially.
Galvanic series in physiological saline (most noble to least noble):
- Platinum (most noble - least corrodes)
- Cobalt-chromium alloys
- Titanium alloys
- Stainless steel 316L
- Magnesium (least noble - most corrodes)
| Metal Couple | Anode (Corrodes) | Cathode (Protected) | Clinical Example |
|---|---|---|---|
| CoCr head / Ti stem | Titanium stem | CoCr head | Modular hip taper junction |
| CoCr / Stainless steel | Stainless steel | CoCr | Mixed implants (rare) |
| CoCr / CoCr | Neither (matched) | Neither (matched) | Preferred combination |
Key principle: The larger the potential difference between metals (farther apart in galvanic series), the greater the corrosion rate of the anodic metal.
This principle explains why galvanic corrosion occurs at modular taper junctions in hip implants.
Crevice Corrosion
Definition: Localized corrosion within shielded areas (crevices, gaps) where oxygen is depleted.
Crevice Corrosion Development
Crevice exists between surfaces (modular taper, plate-bone interface). Initially passive.
Oxygen consumed in crevice faster than it can diffuse in. Oxygen gradient develops - low inside crevice.
Metal oxidation continues: M → M^n+ + ne^-. Hydrolysis: M^n+ + H2O → MOH + H^+. Produces H^+ ions. pH drops to 3-4 in crevice.
Low pH dissolves protective oxide layer. Chloride ions migrate into crevice (charge balance). Acidic chloride environment is highly aggressive.
Corrosion accelerates and becomes self-sustaining. Metal ion release increases. Crevice pH stays low (autocatalytic).
Clinical sites of crevice corrosion:
- Modular taper junctions (head-neck, neck-stem)
- Screw-plate interfaces
- Plate-bone interfaces
- Modular knee tibial tray interfaces
MACC: Mechanically-Assisted Crevice Corrosion
When micro-motion is superimposed on crevice corrosion, the result is MACC:
- Micro-motion continuously disrupts passivation layer
- Fresh metal exposed to aggressive crevice environment
- Mechanical wear debris plus corrosion products
- Synergistic effect: corrosion rate much higher than either alone
MACC is the primary mechanism of modular taper corrosion in hip implants.
MACC at Modular Tapers
MACC is the leading cause of modular hip taper failures. Risk factors: large femoral heads (higher torque), offset head position (increased moment arm), patient factors (high BMI, activity). Prevention: proper assembly technique, avoid mixed metals, consider ceramic heads.
Pitting Corrosion
Definition: Highly localized breakdown of passivation creating small pits that propagate deep into metal.
Mechanism:
- Passivation layer has microscopic defect (inclusion, scratch)
- Chloride ions adsorb at defect site
- Local passivation breakdown creates micro-anode
- Pit propagates inward (autocatalytic like crevice)
- Small surface opening, deep penetration
- Can act as stress concentration site for fatigue crack initiation
Why chloride is aggressive: Cl^- is small anion that penetrates oxide layer and prevents re-passivation.
Clinical relevance:
- Less common than fretting/crevice in orthopaedics
- Can occur on stainless steel more than Ti or CoCr
- Pits act as stress risers → fatigue crack initiation
- Surface finishing reduces susceptibility
Fretting Corrosion
Definition: Combined mechanical wear and corrosion from small amplitude oscillatory motion between surfaces.
Mechanism:
- Micro-motion between surfaces (typically less than 100 microns)
- Passivation layer abraded away mechanically
- Fresh metal exposed to corrosive environment
- Oxidizes rapidly, forms debris
- Debris trapped between surfaces (third-body wear)
- Cycle repeats - synergistic mechanical + chemical degradation
| Feature | Pure Wear | Pure Corrosion | Fretting Corrosion |
|---|---|---|---|
| Mechanism | Mechanical removal | Chemical dissolution | Synergistic mech + chem |
| Motion required | Yes (sliding) | No | Yes (micro-motion) |
| Passivation role | Not relevant | Critical | Repeatedly disrupted |
| Debris | Metallic particles | Oxide/ions | Oxide particles + ions |
| Rate | Linear with cycles | Time-dependent | Accelerated (synergy) |
Clinical examples:
- Modular tapers: Fretting corrosion is dominant wear mode
- Polyethylene backside: Fretting between poly and tibial tray
- Screw-plate interfaces: Micro-motion causes fretting
- Cerclage wires: Fretting against bone or plate
Visual appearance: Black debris (metal oxide particles) characteristic of fretting corrosion.
This completes the description of fretting mechanisms in orthopaedic implants.
Metal Ion Release and Biological Effects
Metal Ion Release Mechanisms
When corrosion occurs, metal ions are released into surrounding tissues and systemic circulation.
| Metal | Normal Serum Level | Threshold for Concern | Source of Elevation |
|---|---|---|---|
| Cobalt (Co) | Less than 1 ppb | Greater than 5 ppb | CoCr implant corrosion, MoM bearing |
| Chromium (Cr) | Less than 1 ppb | Greater than 5 ppb | CoCr implant corrosion |
| Titanium (Ti) | Less than 5 ppb | No established threshold | Ti implant corrosion (rare) |
| Nickel (Ni) | Less than 1 ppb | Greater than 5 ppb | Stainless steel corrosion |
Routes of distribution:
- Local tissues - highest concentration, direct toxicity
- Regional lymph nodes - metal particles transported by macrophages
- Systemic circulation - ions absorbed into bloodstream
- Distant organs - liver, spleen, kidney accumulation
Biological Effects
Local tissue effects:
- Metallosis: Macroscopic metal staining of tissues (black/gray discoloration)
- Aseptic lymphocytic vasculitis-associated lesion (ALVAL): Type IV hypersensitivity
- Pseudotumor: Soft tissue mass with necrosis, not true neoplasm
- Osteolysis: Particle-induced bone resorption
ALVAL Pathophysiology:
ALVAL represents a Type IV delayed hypersensitivity reaction to metal ions:
ALVAL Development
Corrosion releases Co and Cr ions into tissues.
Metal ions bind to proteins, creating metal-protein complexes (haptens).
Antigen-presenting cells present hapten to T-cells. T-cells become sensitized (priming phase).
Continued metal release re-exposes sensitized T-cells.
T-cells recruit macrophages and lymphocytes. Perivascular lymphocytic infiltration. Tissue destruction and necrosis.
Histology of ALVAL:
- Perivascular lymphocytic infiltration
- Aseptic fibrinoid necrosis
- Diffuse lymphocytic infiltrate
- Absence of infection
Clinical presentation:
- Pain (most common)
- Soft tissue mass (pseudotumor on imaging)
- Instability or dislocation (soft tissue destruction)
- Elevated serum metal ions (Co and Cr)
MoM Hip Surveillance
Monitor all MoM hip patients with serum cobalt and chromium levels annually. Threshold for concern: greater than 7 ppb for either metal. MRI for soft tissue evaluation if elevated or symptomatic. Consider revision if progressive elevation or pseudotumor.
Systemic Effects
Unclear long-term consequences:
- Cobalt: Potential cardiomyopathy, thyroid dysfunction at very high levels
- Chromium: Theoretical carcinogenic risk (Cr^6+ form)
- Nickel: Known allergen, hypersensitivity common
Current evidence: Most patients with moderate metal elevation (5-10 ppb) do not have systemic symptoms, but long-term data limited.
This section completes the discussion of metal ion biological effects.
Anatomy of Corrosion Sites
Key Anatomical Sites of Implant Corrosion
Head-Neck Taper Junction
Most critical corrosion site in modern THA
- Morse taper connection between femoral head and stem
- Contact area: 50-150 mm² depending on design
- Crevice geometry creates ideal environment for MACC
- Micro-motion disrupts protective oxide layer
Anatomy:
- Taper angle typically 5°40' (Morse taper)
- Roughness affects seating and corrosion
- Material couple (CoCr/Ti) critical factor
Stem-Sleeve Junction
Modular neck systems
- Additional taper connection for neck modularity
- Double modular systems have two corrosion sites
- Higher failure rates led to some recalls
Key anatomy:
- Neck angle and offset adjustability
- Increased corrosion debris with two tapers
- Greater surface area for metal ion release
Implant Regions Prone to Corrosion
| Location | Corrosion Type | Clinical Significance |
|---|---|---|
| Head-neck taper | Crevice + fretting (MACC) | Primary failure site in modular THA |
| Screw-plate interface | Crevice + fretting | Hardware loosening, pain |
| Poly backside | Fretting + third-body wear | Osteolysis, loosening |
| MoM bearing surface | Tribocorrosion | Metal ion release, ALVAL |
| Stem surface | Pitting (rare) | Usually minor clinical impact |
Exam Pearl
FRACS Viva Point: "Why is the head-neck taper the most important corrosion site?" Answer: Crevice geometry + micro-motion creates MACC environment. The combination of oxygen depletion in the crevice (low pH 3-4), cyclic loading causing micro-motion, and mixed metal couples maximizes corrosion.
Understanding anatomical sites of corrosion helps identify at-risk implants and plan revision surgery.
Classification
Four Major Types of Corrosion
Galvanic Corrosion
Definition: Corrosion when two dissimilar metals contact in an electrolyte
Mechanism:
- Less noble metal = anode (corrodes)
- More noble metal = cathode (protected)
- Electron flow from anode to cathode
Clinical example:
- CoCr head on Ti stem
- Ti is less noble, corrodes at junction
- Mixed metal couples show worse outcomes
Crevice Corrosion
Definition: Localized corrosion within shielded areas
Mechanism:
- Oxygen depleted in crevice
- pH drops to 3-4 (autocatalytic)
- Chloride concentration increases
- Passivation layer dissolves
Clinical example:
- Modular taper junctions
- Screw-plate interfaces
- Under bone cement mantle
Pitting Corrosion
Definition: Highly localized passivation breakdown
Mechanism:
- Chloride ions attack defects
- Small surface pit, deep penetration
- Self-propagating once initiated
Clinical example:
- Stainless steel implants (most susceptible)
- Surface contamination sites
- Rare in Ti and CoCr
Fretting Corrosion
Definition: Mechanical wear + chemical corrosion
Mechanism:
- Micro-motion disrupts oxide layer
- Fresh metal exposed and oxidizes
- Cyclic process with each loading cycle
Clinical example:
- Modular tapers under load
- Polyethylene liner backside
- Screw-plate motion
MACC - The Key Mechanism
Mechanically-Assisted Crevice Corrosion (MACC) combines crevice + fretting corrosion. This is the PRIMARY failure mechanism at modular hip tapers. Micro-motion disrupts passivation within the crevice environment, accelerating metal dissolution.
Classification helps identify specific corrosion mechanisms and guide prevention strategies.
Clinical Implications and Prevention
Failed Metal-on-Metal Hips
Primary failure mode: Taper corrosion, NOT bearing surface wear in most modern MoM failures.
Evidence from retrieval studies:
- Taper shows severe MACC with debris
- Bearing surfaces often well-preserved
- Mixed metal couples (CoCr/Ti) show worse corrosion
- Large head diameter increases taper mechanical stress
Risk factors for taper corrosion:
- Large femoral head diameter (greater than 36mm)
- Increased offset (higher moment arm on taper)
- High patient BMI and activity level
- Mixed metal couples (CoCr on Ti)
- Poor taper assembly (contamination, multiple impacts)
Taper Assembly Technique
Critical steps to minimize MACC:
Rationale:
- Clean and dry: Contaminants prevent full seating and increase micro-motion
- Single impaction: Multiple impacts damage taper surface
- Adequate force: Ensures taper engagement and minimizes micro-motion
- Matched materials: CoCr on CoCr eliminates galvanic component
Prevention Strategies
| Strategy | Mechanism | Effectiveness | Considerations |
|---|---|---|---|
| Ceramic heads | Eliminates metal taper corrosion | Excellent | Fracture risk (historical concern, rare with modern ceramics) |
| Matched metals | Eliminates galvanic corrosion | Good | CoCr on CoCr preferred over CoCr on Ti |
| Dual mobility | Reduces taper mechanical stress | Good | Alternative bearing, not directly addressing corrosion |
| Avoid large heads | Reduces moment arm on taper | Moderate | May increase dislocation risk in some patients |
| Proper assembly | Minimizes micro-motion | Essential | Clean, dry, single impaction - surgeon dependent |
Material selection considerations:
- Titanium stems: Excellent corrosion resistance but weaker at taper when coupled with CoCr
- CoCr stems: Stronger taper but heavier, more stiff
- Ceramic heads: Eliminate taper corrosion but historical fracture concern
- Modern approach: Ceramic head on Ti stem is popular compromise
This completes the prevention strategies discussion.
Investigations
Serum Metal Ion Testing
Cobalt and Chromium Levels
Primary biomarker for MoM monitoring:
- Normal: Less than 1 ppb (μg/L)
- Elevated: 2-7 ppb - increased surveillance
- Concerning: Greater than 7 ppb - further investigation
- Critical: Greater than 20 ppb - revision often indicated
Interpretation caveats:
- Fasting sample (some foods contain metals)
- Trace element tube (metal-free)
- Recent activity may elevate levels
Sampling Protocol
Correct technique essential:
- Fasting morning sample
- Metal-free (trace element) tube
- Avoid contamination from needle hub
- Laboratory with validated assay (ICP-MS)
Frequency:
- MoM hips: Annual minimum
- Symptomatic: At presentation
- Post-revision: 3, 12 months
Imaging for Corrosion-Related Pathology
| Modality | Findings | Role |
|---|---|---|
| Plain radiographs | Osteolysis, implant migration, loosening | First-line, baseline comparison |
| MARS MRI | Pseudotumor, soft tissue necrosis, fluid collection | Gold standard for ALVAL |
| Ultrasound | Fluid collections, pseudotumor | Screening, guided aspiration |
| CT with MARS | Bone detail with metal suppression | Osteolysis assessment |
Exam Pearl
FRACS Viva Point: "What is MARS MRI?" Answer: Metal Artifact Reduction Sequence MRI uses specialized pulse sequences and post-processing to reduce metal artifact, allowing visualization of periprosthetic soft tissues to detect ALVAL, pseudotumor, and fluid collections.
Serum metal ions and MARS MRI are the cornerstone investigations for suspected corrosion-related pathology.
Management
Management Algorithm for Corrosion-Related Pathology
Asymptomatic with Normal Metal Ions
Metal ions less than 2 ppb:
- Annual clinical review
- Annual metal ion levels
- Radiographs every 2 years
- Patient education on symptoms
No intervention required unless:
- Symptoms develop
- Metal ions rise progressively
- Imaging abnormalities appear
Asymptomatic with Elevated Metal Ions
Metal ions 2-7 ppb:
- 6-monthly clinical review
- MARS MRI to assess soft tissues
- More frequent metal ion monitoring
Consider revision if:
- Progressive elevation of ions
- Imaging shows pseudotumor
- Patient high-demand, long life expectancy
Indications for Revision Surgery
| Indication | Strength | Timing |
|---|---|---|
| Symptomatic with Co/Cr greater than 7 ppb | Strong | Elective revision planned |
| Pseudotumor on imaging (Type 2b) | Strong | Semi-urgent revision |
| Progressive osteolysis | Strong | Before bone loss worsens |
| Metal ions greater than 20 ppb | Moderate-strong | Revision recommended |
| Asymptomatic, rising ions 3-7 ppb | Moderate | Close surveillance vs revision |
Delay increases complexity. Early revision before extensive soft tissue or bone destruction improves outcomes. Waiting for severe symptoms often means worse tissue damage.
Management depends on symptoms, metal ion levels, imaging findings, and patient factors.
Surgical Technique
Revision Surgery for Corrosion-Related Failure
Head Exchange (If Stem Well-Fixed)
Indications:
- Taper corrosion with well-fixed stem
- Good bone stock
- No extensive tissue destruction
Key steps:
- Dislocate hip, remove head
- Clean taper thoroughly (dry)
- Inspect taper for damage
- New ceramic head on clean, dry taper
- Or sleeve adapter if taper damaged
Complete Revision
Indications:
- Loose stem
- Severe taper damage/fracture
- Extensive osteolysis
- Pseudotumor with bone destruction
Key steps:
- Extended approach for exposure
- Thorough debridement of necrotic tissue
- Revise acetabulum if affected
- New stem with modular options
- Bone grafting if needed
Soft Tissue Management
| Tissue Finding | Action | Rationale |
|---|---|---|
| Pseudotumor | Complete excision | Remove source of inflammation |
| ALVAL-affected tissue | Thorough debridement | Remove reactive tissue |
| Metallosis staining | Lavage, debride what possible | May not fully remove all staining |
| Necrotic abductors | Debride back to viable tissue | Document for prognosis |
Exam Pearl
FRACS Viva Point: "How do you manage the taper at revision?" Answer: If taper undamaged: clean thoroughly, dry completely, place ceramic head with single firm impaction. If taper damaged: use a sleeve adapter system or revise the stem.
Careful debridement and appropriate component selection are essential for successful revision.
Complications
Complications of Implant Corrosion
Local Tissue Destruction
ALVAL and Pseudotumor:
- Soft tissue necrosis around implant
- Pseudotumor (solid or cystic mass)
- Abductor muscle destruction
- Femoral nerve compression (rare)
Bone involvement:
- Osteolysis from debris
- Periprosthetic bone loss
- May compromise revision fixation
Systemic Metal Toxicity
Cobalt toxicity syndrome:
- Cardiomyopathy (cobalt cardiomyopathy)
- Thyroid dysfunction
- Neurological symptoms (peripheral neuropathy)
- Visual and hearing impairment
Typically with very high levels:
- Usually greater than 50-100 ppb
- May be reversible with revision
- Cardiac monitoring recommended
Complication Severity
| Complication | Incidence | Severity |
|---|---|---|
| ALVAL/pseudotumor | 5-30% of MoM hips | Moderate to severe |
| Aseptic loosening | 5-15% (corrosion-related) | Moderate |
| Systemic toxicity | Less than 1% | Potentially severe |
| Implant fracture | Less than 1% | Severe - requires revision |
| Chronic pain | 10-20% | Variable |
Exam Pearl
Exam Viva Point: "What are the systemic effects of cobalt toxicity?" Answer: Cardiomyopathy, hypothyroidism, peripheral neuropathy, and visual/hearing impairment. Usually occurs with very high serum cobalt levels (greater than 50-100 ppb) and may be reversible after revision surgery.
Complications range from local tissue destruction to rare but serious systemic toxicity.
Postoperative Care
Post-Revision Surveillance Protocol
Metal Ion Monitoring
Post-revision schedule:
- 3 months: Baseline after revision
- 12 months: Assess decline
- Annually: Until levels normalize
Expected trajectory:
- Rapid initial decline (first 3-6 months)
- Gradual normalization over 1-2 years
- May not return to normal in all patients
Clinical Review
Follow-up schedule:
- 6 weeks: Wound check, weight-bearing
- 3 months: Function assessment
- 6 months: Metal ions, radiographs
- 12 months: Comprehensive review
- Annually: Ongoing surveillance
Assess at each visit:
- Hip function (Oxford Hip Score)
- Stability (dislocation precautions)
- Symptoms (pain, clicking)
Rehabilitation Considerations
| Phase | Focus | Special Considerations |
|---|---|---|
| Week 0-6 | Protected weight-bearing, precautions | Extended hip precautions if abductor damage |
| Week 6-12 | Strengthening, gait training | Focus on abductor rehabilitation |
| Month 3-6 | Progressive loading, function | May need abductor brace if weak |
| Month 6+ | Return to activities | Some permanent limitations expected |
Exam Pearl
Exam Viva Point: "How do you follow up a patient after revision for ALVAL?" Answer: Serial metal ion levels (3, 12 months then annual), clinical review for function and stability, imaging if symptoms, and extended hip precautions due to abductor compromise.
Close surveillance ensures early detection of complications and guides rehabilitation.
Outcomes
Outcomes After Revision for Corrosion
Metal Ion Decline
Post-revision trajectory:
- Mean 50-70% reduction at 12 months
- Normalization (less than 2 ppb) in most patients
- Some patients have persistent elevation
- Rate of decline varies
Factors affecting decline:
- Extent of initial metallosis
- Completeness of debridement
- New implant bearing choice
Functional Outcomes
Clinical scores:
- Oxford Hip Score: Significant improvement
- Harris Hip Score: Mean improvement 20-30 points
- Patient satisfaction: 70-85%
Limitations:
- Often do not reach primary THA levels
- Abductor weakness limits function
- Some persistent pain common
Outcome Predictors
| Factor | Better Outcome | Worse Outcome |
|---|---|---|
| Pre-revision tissue damage | Minimal pseudotumor | Extensive destruction |
| Abductor status | Intact abductors | Necrotic/absent abductors |
| Bone stock | Minimal osteolysis | Severe bone loss |
| Timing of revision | Early (before extensive damage) | Late (symptomatic, severe ALVAL) |
| Metal ion levels | Moderately elevated | Very high (greater than 50 ppb) |
Exam Pearl
Exam Viva Point: "What determines outcome after revision for ALVAL?" Answer: Pre-revision tissue destruction (especially abductor status), bone stock, timing of revision (earlier is better), and completeness of debridement. Patients with extensive destruction have worse functional outcomes.
Early revision before extensive tissue destruction optimizes outcomes.
Evidence Base
Gilbert et al - Corrosion at Modular Interfaces in Total Hip Arthroplasty
- MACC (mechanically-assisted crevice corrosion) is primary mechanism at tapers
- Mixed metal couples (CoCr/Ti) show significantly more corrosion than matched materials
- Large femoral heads increase mechanical forces on taper, accelerating corrosion
- Proper assembly technique (clean, dry, single impaction) critical to minimize micro-motion
Langton et al - Blood Metal Ion Concentrations After Hip Resurfacing
- Cobalt and chromium levels elevated 10-100 fold in failed MoM hips vs normal
- Threshold of 7 ppb for either metal associated with increased revision risk
- Levels correlate with wear/corrosion at bearing and taper
- Asymptomatic patients can have elevated levels - routine surveillance needed
Willert et al - Metal-on-Metal Bearings and Hypersensitivity
- First description of ALVAL in failed MoM hips
- Histology shows aseptic lymphocyte-dominated vasculitis, not infection
- Type IV delayed hypersensitivity to metal debris (Co, Cr)
- Soft tissue destruction can be extensive (pseudotumor)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Types of Corrosion
"Examiner asks: Describe the four main types of corrosion in orthopaedic implants and give a clinical example of each."
Scenario 2: ALVAL and Metal Hypersensitivity
"Patient with painful MoM hip has serum cobalt 45 ppb, chromium 38 ppb, and MRI shows large pseudotumor. Examiner asks about pathophysiology and management."
Scenario 3: Taper Assembly Technique
"Examiner asks: You are performing primary THA with a CoCr head on titanium stem. Describe your taper assembly technique and explain the rationale."
MCQ Practice Points
Passivation Layer Question
Q: What provides corrosion resistance to titanium implants? A: TiO2 passivation layer (titanium oxide). This thin (2-10 nm) oxide layer forms spontaneously and is self-healing. It prevents titanium metal from contacting the corrosive environment.
MACC Definition Question
Q: What is MACC and where does it occur? A: Mechanically-Assisted Crevice Corrosion - synergistic combination of micro-motion (disrupts passivation) and crevice environment (low pH, chloride). Primary failure mode at modular hip tapers.
Galvanic Series Question
Q: In a CoCr head on Ti stem taper, which metal corrodes? A: Titanium - Ti is less noble than CoCr in the galvanic series, so Ti acts as anode and corrodes preferentially. CoCr acts as cathode and is protected.
ALVAL Question
Q: What type of hypersensitivity reaction is ALVAL? A: Type IV delayed hypersensitivity (T-cell mediated). Metal ions bind proteins forming haptens, leading to T-cell sensitization and lymphocytic tissue infiltration.
Metal Ion Threshold Question
Q: What serum cobalt or chromium level warrants concern in MoM hips? A: Greater than 5-7 ppb for either metal. Levels above this threshold associated with increased corrosion/wear and revision risk. Normal is less than 1 ppb.
Crevice pH Question
Q: What is the pH inside a crevice undergoing crevice corrosion? A: pH 3-4 (highly acidic) - Metal hydrolysis produces H+ ions and oxygen depletion prevents neutralization. This low pH dissolves the passivation layer.
Fretting Debris Question
Q: What is the characteristic appearance of fretting corrosion debris? A: Black debris - mixture of metal oxide particles (FeO, TiO2, Cr2O3) from repeated disruption of passivation and oxidation. Distinguishes fretting from pure wear.
Australian Context
AOANJRR Data on Metal-on-Metal
MoM THA Outcomes
AOANJRR findings:
- MoM THA has higher revision rates than other bearings
- Large-head MoM (greater than 36mm) especially problematic
- Led to significant decline in MoM use in Australia
- Current MoM use less than 1% of primary THA
Revision rates:
- MoM THA: 15-year revision rate approximately 25%
- Ceramic-on-ceramic: approximately 8%
- Ceramic-on-poly: approximately 6%
Hip Resurfacing
Australian registry data:
- Better outcomes than large-head MoM THA
- Still higher revision than standard THA
- Patient selection critical (young, active males)
- Declining use in Australia
Current indications:
- Young male patients (less than 55 years)
- High activity demands
- Experienced surgeons only
- Limited centres perform resurfacing
Australian MoM Surveillance Guidelines
| Category | Recommendation | Source |
|---|---|---|
| All MoM hips | Annual review minimum | TGA, AOA |
| Symptomatic patients | Metal ions + MARS MRI | TGA |
| Metal ions greater than 7 ppb | Further investigation, consider revision | TGA |
| Large head (greater than 36mm) | Higher surveillance intensity | AOA, AOANJRR |
Exam Pearl
Exam Viva Point: "What does AOANJRR data show about MoM hips?" Answer: Higher revision rates than other bearing couples, especially large-head MoM THA. This led to decline in MoM use to less than 1% of primary THA in Australia. Registry data influenced global practice changes.
Australian registry data significantly influenced worldwide practice changes for MoM implants.
CORROSION MECHANISMS IN ORTHOPAEDIC IMPLANTS
High-Yield Exam Summary
Electrochemistry Basics
- •Corrosion = electrochemical degradation (oxidation-reduction)
- •Anode: M → M^n+ + e^- (metal oxidizes, goes into solution)
- •Cathode: O2 + 2H2O + 4e^- → 4OH^- (reduction)
- •Passivation layer (TiO2, Cr2O3) prevents corrosion
- •Body fluid aggressive: 0.9% NaCl, pH 7.4, chloride ions
Four Types of Corrosion
- •GALVANIC: Dissimilar metals (CoCr/Ti) - less noble corrodes (Ti)
- •CREVICE: Oxygen depletion → pH 3-4 → passivation dissolved
- •PITTING: Localized chloride attack → small hole, deep penetration
- •FRETTING: Micro-motion disrupts passivation + corrosion (synergy)
- •MACC = Mechanically-Assisted Crevice Corrosion (taper failure mode)
ALVAL Pathophysiology
- •Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion
- •Type IV delayed hypersensitivity to Co and Cr ions
- •Metal ions + proteins = haptens → T-cell sensitization
- •Perivascular lymphocytic infiltration, tissue necrosis
- •Pseudotumor = soft tissue mass with necrosis (not tumor)
- •Histology: lymphocytes, no bacteria (NOT infection)
Metal Ion Monitoring
- •Normal: Co and Cr less than 1 ppb
- •Concern threshold: greater than 5 ppb either metal
- •Action threshold: greater than 7 ppb warrants investigation
- •Annual surveillance for all MoM hips recommended
- •Elevated levels: MRI for pseudotumor, consider revision
- •Levels decline 6-12 months after revision (slow)
Taper Assembly (Prevent MACC)
- •CLEAN: No debris on tapers
- •DRY: No blood/fluid contamination (prevents full seating)
- •SINGLE: One firm impaction (multiple damages surface)
- •MATCH: CoCr on CoCr better than CoCr on Ti (no galvanic)
- •CERAMIC: Ceramic head eliminates taper corrosion risk
Prevention Strategies
- •Avoid mixed metals (galvanic corrosion)
- •Proper taper assembly critical
- •Ceramic heads eliminate metal taper corrosion
- •Avoid large heads (reduce taper mechanical stress)
- •Polish surfaces (reduce stress concentrations for pitting)