Wear & Corrosion

Principles

• Factors that determine wear:
  • The material Friction coefficient
  • The material surface finish
  • The material hardness
  • The material molecular weight (the higher the better)
  • The load applied
  • The sliding distance/cycle, depending on the head diameter and arc of motion
  • The number of cycles that occur over time
• Wear measurement is difficult, as the actual wear is volumetric
• Realistically, it is measured as a linear wear; the smallest radius from the head center to acetabular rim
• Normal wear rate = 0.1 mm/year
• Friction wear
  • The coefficient of friction is constant for any two surfaces regardless of their size
    • The normal joints coefficient ? 0.008-0.02
    • Metal-on-metal joints tested in vivo ? 0.8
    • Metal on HDPE ? 0.02 (ceramic-on-ceramic/ceramic-on-PE also is low)
    • Metal-on-ceramic produces a high coefficient of friction metal wear
  • Frictional moment is related to congruency and size of the ball and socket
  • Metal on metal surfaces show adhesive wear and seizing
  • Metal on plastic surfaces show low friction and less wear
• Corrosion of various types are dependant on the pH and oxygen tension in the region
• Metal wear debris may lead to local inflammation, scaring and occasionally toxic or allergic reactions
• In THR, abrasive and adhesive mechanisms are the most important
• In TKR the most important is the fatigue type of wear (subsurface lamellation)
• Dissimilar metals immersed in solution may cause galvanic corrosion with accelerated corrosion of the more reactive metal. Not a clinical problem with the newer metals

Wear Debris

• Increased modularity results in increased wear and may lead to a path for the passage of wear debris distally
• Unknown significance of metallic debris relative to polyethylene, as 50 million particles of polyethylene produced in 12 months normal wear of a THR, compared to about 3 – 4 million metallic particles

Types

• Adhesion: local binding of bumps or asperities can cause small pieces of the softer material to be torn loose
• Abrasion: large asperities on the harder material can plow debris from the softer material
• Transfer: a film of the softer surface may be transferred to the harder surface, changing the nature of the interface. Such transfer films are unstable
• Fatigue: local surface roughness of the hard side may not cause abrasion, but produce sufficient subsurface stress concentration in the softer material to produce fatigue failure
• Third body wear: trapping of debris within the moving interfaces produces local stress concentrations that result in a combination of abrasive and fatigue wear
• Back side wear: between polyethylene and the metal backing
  

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