8.8 Lubrication of Articular Cartilage

  • Synovial joints subjected to enormous range of loading conditions

  • Cartilage typically sustains little wear

  • Implication: Sophisticated lubrication process required

8.8.1 Joint Lubrication

  • Amazing engineering feat
  • Coefficient of friction of cartilage on cartilage somewhere around 0.001!!!!
  • Compare to Teflon on Teflon = .04

8.8.2 Lubrication Processes for Articular Cartilage

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8.8.3 Boundary Lubrication (dominant for low loads)

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  • Surfaces of cartilage protected by a layer of boundary lubricant
    • Direct surface-to-surface contact is prevented
    • Most surface wear eliminated
    • Lubricin (glycoprotein) - a synovial fluid constituent - responsible for boundary lubricant
      • Adsorbed as monolayer to each articular surface
      • Able to carry loads (normal forces) and reduce friction

8.8.4 Boundary Lubrication (dominant for low loads)

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  • Primarily depends on chemical properties of lubricant
    • Function largely independent of physical properties of lubricant (e.g., viscosity) and bearing material (e.g., stiffness)
    • In contrast to fluid-film lubrication
  • Also functions under high loads at low relative velocities, preventing direct contact between surfaces

8.8.5 Lubrication Processes for Articular Cartilage

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8.8.6 Fluid-film Lubrication

  • Thin film of lubricant separates bearing surfaces
  • Load on bearing surfaces supported by pressure developed in fluid-film
  • Lubrication characteristics determined by lubricant’s properties
    • Rheological properties (i.e., everything flows… but rate matters)
      • Viscosity and elasticity
    • Film geometry
    • Shape of gap between surfaces
    • Speed of relative motion of two surfaces

8.8.7 Lubrication Processes for Articular Cartilage

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8.8.8 Hydrodynamic Lubrication

  • Occurs when 2 nonparallel rigid bearing surfaces move tangentially with respect to each other and are lubricated by a fluid-film
    • Wedge of converging fluid formed
  • Lifting pressure generated in wedge by fluid viscosity as the bearing motion drags fluid into gap

8.8.9 Schematic of Hydrodynamic Lubrication

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8.8.10 Schematic of Hydrodynamic Lubrication

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8.8.11 Lubrication Processes for Articular Cartilage

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8.8.12 Squeeze-film Lubrication

  • Occurs when weight bearing surfaces move toward each other (normal-normal)
  • Wedge of converging fluid formed
  • Pressure in fluid-film result of viscous resistance of fluid that acts to impede its escape from the gap
  • Sufficient to carry high loads for short durations (eventually contact between asperities in bearing surfaces)

8.8.13 Schematic of Squeeze-film Lubrication

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8.8.14 Schematic of Squeeze-film Lubrication

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8.8.15 Articular Cartilage Asperities and Lubrication

  • Articular cartilage not perfectly smooth; asperities
    • Fluid film lubrication in regions of cartilage non-contact
    • Boundary lubricant (lubricin) in areas of asperities
  • Low rates of interfacial wear suggests that asperity contact rarely occurs in articular cartilage

8.8.16 Asperities in Articular Cartilage

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8.8.17 Lubrication Processes for Articular Cartilage

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8.8.18 Modes of Mixed Lubrication

  • Combination of fluid-film and boundary lubrication
  • Temporal coexistence of fluid-film and boundary lubrication at spatially distinct locations
  • Joint surface load sustained by fluid-film and boundary lubrication Most friction in boundary lubricated areas; most load supported by fluid-film

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8.8.19 Modes of Mixed Lubrication

  1. Boosted lubrication
    • Shift of fluid-film to boundary lubrication with time over the same location
    • Articular surfaces protected during loading by ultrafiltration of synovial through the collagen-Proteoglycan matrix

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8.8.20 Modes of Mixed Lubrication

  1. Boosted lubrication (continued)

    • Solvent component of synovial fluid passes into the articular cartilage during squeeze-film action yielding a concentrated gel of HA protein complex that coats and lubricates the surfaces
    • As articular surfaces approach each other, difficult for HA macromolecules to escape from gap between surfaces

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8.8.21 Variation of Lubrication Processes for Articular Cartilage

  • Elastohydrodynamic Lubrication
    • associated with deformable articular cartilage
    • pressure from fluid-film deforms surfaces

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8.8.22 Comparison of Hydrodynamic and Squeeze-film Lubrication under Rigid and Elastodynamic Conditions

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8.8.23 Elastohydrodynamic Lubrication

  • Beneficial increase in surface areas
    • Lubricant escapes less rapidly from between the bearing surfaces
    • Longer lasting lubricant film generated
    • Stress of articulation lower and more sustainable
  • Elastohydrodynamic lubrication greatly increases load bearing capacity

8.8.24 Dynamic Relationship between Vertical Load and Hip Joint Lubrication

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Support phase

  • Initial load on hip at heel contact likely supported by hydrodynamic lubrication
  • As load continues, fluid is squeezed between articular surfaces and is supported more by squeeze-film lubrication

Swing phase

  • Small vertical load on hip articular cartilage supported by hydrodynamic lubrication

8.8.25 Dynamic Relationship between Vertical Load and Hip Joint Lubrication

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  • at Time = start
    • Load on hip supported by squeeze-film lubrication
  • at Time = 3 minutes
    • Over time fluid-film may be eliminated and surface-to-surface contact may occur
    • Surfaces protected by thin layer of ultrafiltrated synovial gel (boosted lubrication) or by the adsorbed lubricin monolayer (boundary lubrication)