Metal on Metal Hip Arthroplasty

Main properties of MOM:

Good - Fracture toughness - Hard
- Low surface roughness
- Small size particle generation (50-100nm)
- Low biologic activity
- Macrophage need 0.1 – 0.5 micrometer to be most active
- Self-Healing
- Small scratches polished out with articulation
- Very Resistant to abrasive wear
- Osteolysis not a major problem

Bad - Produce greater number of particles
- Especially during initial 2 years
- Bedding in period after which particle volume plateaus
- This is greater in implants with greater radial clearance

Wear mode of MOM Articulations

• Adhesive
  • Overcome by use of alloys with greater carbide (molybdenum)

Lubrication in MOM

• Predominantly Fluid film lubrication exists in MoM bearings
  
• In past were equatorial bearing
  • High frictional torque, jamming, high failure
    
• No fluid able to seep into articulation and lubricate
  
• When the cup and head are manufactured from the same cast
  • E.g. original BHR
    
• If pure polar bearing
  • Much lower frictional torque
    
  • Excessive point contact, stress concentration and wear
    
• Ideal is Mid-Polar contact
  • Needs a cup head clearance of 90-200 micrometers
    
  • i.e. a size mismatch

Problems:

• Metal ion dissemination into the blood stream
  
• Potential for carcinogenesis
  
• Occurrence of mettalosis > ALVAL > pseudotumours

MoM soft tissue problems

• Metallosis
  • Macroscopic staining of the soft tissues
    
• ALVAL
  • Aseptic Lymphocytic Vasculitis Associated Lesions
    
  • Histological diagnosis
    
  • May be present with or without metallosis
    
• Pseudotumour
  • Cystic/Solid mass lesion
    
  • Often has a synovial like bio-membrane
    
  • Secretes collagenase, IL-1 and TNF alpha and beta
    
  • Therefore can cause osteolysis chemically & mechanical destruction due to size
    
  • Presentation
    • Pain
    • Mass
    • Dislocation
    • Nerve compression
      
  • Diagnosed by imaging or macroscopically
    
  • Usually but not always symptomatic
    
• ARMD
  • Adverse Reaction to Metal Debris
    
  • An umbrella term for all of the above
    
  • All or only one may be present
    
  • Metallosis does not necessarily progress to Pseudotumour formation
    
  • Main metal found in tissues is Chromium III
    
  • Molybdenum, Cobalt present sporadically

Aetiology

• Two main theories:
  1. Wear debris related cytotoxicity
     
  2. Hypersensitivity reaction

Wear related Cytotoxicity

• Caused by component malposition & edge loading
  
• Primarily increased cup inclination
  
• Previously thought to occur in a dose dependent local cytotoxic response
  
• But, retrieval analysis on the ASR shows that a true vasculitis may occur
  
• ALVAL can occur in presence of
  • Normally aligned components
    
  • Non MoM articulations
    
• Therefore, this is a spectrum where at the most benign end, there is a normal physiological response to metal debris which occurs in all Hip Replacements
  
• Problems occur when the physiologic response is heightened
  • Excessive metal debris
  • Patient related factors – genetics, hypersensitivity etc.

Hypersensitivity

• True Type 4 Hypersensitivity reactions have been reported but are rare
  
• Hypersensitivity is thought to be overstated

Local Effects of Metal Debris

• Metal wear produces higher volume of smaller particles
  
• These particles has a higher surface area than MoP debris
  
• This makes it more biologically active
  
• Metal ions especially Cobalt cause cell death once ingested by macrophages
  
• Inhibit osteoblastic activity
  
• Contributes to the cascade of biological events that result in osteolysis
  
• Cause a local immune response of uncertain aetiology

Systemic Effects of Metal Debris

• Circulating metal ion levels increase in MoM articulations
  
• They vastly increase if the implants are loose or malpositioned
  
• No proven effect on organ function despite reports of seeding in solid organs
  
• Metal ions are excreted in urine therefore renal failure is a contraindication

Carcinogenesis

• Chromium IV is related to causing lung cancer
  
• Meta analysis of over 1 million person years follow up (Onega study)
  • No attributable increase in cancer found although some cancers increased and some decreased in frequency
    
  • But we need to be vigilant because
    
  • Follow up is relatively short (11yrs)
    
  • Malignancy may take up to >30 yrs to develop

Teratogenicity

• Committee on Mutogenicity have reported that internal exposure to metal ions can cause genotoxicity
  
• Animal studies have shown effect of teratogenicity
  
• Metal ions can cross placenta
  
• No evidence thus far shown of teratogenicity in humans
  
• But, implications are severe therefore avoid in young females

Incidence

• Dependent on implant type
  
• Less in Resurfacings
  • More in female resurfacings – smaller sizes/head-neck ration
    
• Asymptomatic incidence estimated at 4%
  
• Symptomatic incidence between 1-5%

Risk Factors

• Implant positioning
  • Positive correlation by higher inclination angles and revisions with– edge loading
    
  • In vitro studies confirm the increase in metal wear debris
    
  • XS anteversion proposed as a cause also but not proven
    
  • 4X reduction in pseudotumours when alignment within 10 deg of recommended
    
  • 40 deg inclination and 20 deg anteversion angles in resurfacing
    
• Implant size
  • Smaller sized resurfacing implants have increased wear rate due to impingement
    
  • Smaller component sizes in resurfacing shown to increase pseudotumour incidence
    
• Gender
  • Increased Pseudotumour formation in women with MoM articulations
    
  • Mainly for above reasons

The ASR

• Non-hemispheric cup to increase ROM
  
• Resulted in reduced head coverage, edge loading, wear osteolysis and ALVAL
  
• Much more technically demanding
  • Implant in the correct position

Resurfacing vs THR

• THR MoM implants shown to have higher levels of serum metal ions
  
• Trunion wear and neck cup impingement implicated as cause
  
• Trunion wear made worse by large THR bearings

Specific Hip Resurfacing Complications

• Femoral Neck Fracture (2%)
  • Risk factors – patient and surgical dependent factors
    
  • Females
    
  • Poorer quality bone
    
  • Smaller head Size
    
  • More cement penetration and bone necrosis
    
  • Smaller Head size
    
  • Neck impingement on cup
    
  • Higher risk of notching
    
• Superior neck notching
  
• Varus positioning
  
• Less Surgeon experience – learning curve

Post operative AVN Avascular Necrosis

• Exacerbated by
  • Posterior Approach
    
  • Thermal necrosis of head
    
  • Cylindrical reaming of hard bone
    
  • Deep cement penetration
    
  • Predisposes to neck fracture which is how it usually presents

ARMD

• Umbrella term for MoM soft tissue problems

When Should THR Components be revised?

• Some papers show evidence of progression of metallosis
  
• Early revision for pseudotumour shown to have better outcome than late revision
  
• Likely due to less soft tissue destruction
  
• Early revision recommended for these reasons

Results of Revision

• Worse for Pseudotumour than for any other cause of failure
  
• Worse if revision delayed

How Long Should MoM Hip Replacements be followed up?

• MHRA Guidelines from report published in June 2012
• Resurfacing
  • Symptomatic
    • MARS scan
    • Consider revision if MARS positive and metal ions elevated
      
  • Metal ion levels
    • If >7 ppb then repeat - If rising and still >7ppb consider revision
      
  • Follow up yearly for life
    
  • Asymptomatic
    • Follow up as per local protocols
      
    • No need for investigation
      
• MoM THR with head size <36mm
  • Exactly same as resurfacing
    
• MoM THR head size >36mm
  • All should have metal ion levels
    
  • All should be followed up yearly for life regardless of symptoms
    
  • MARS if blood ion levels elevated or symptomatic
    
  • Revision if serial levels >7ppb, patient symptomatic or destructive lesion
    
• ASR
  • All should have MARS and metal ion levels regardless of symptoms
    
  • All followed for life
    
  • Revision according to same criteria above
    
  • 7 parts per billion = 119nmol/L cobalt and 135 nmol/L Chromium do same