Lubrication of Articular Cartilage

• Synovial joints subjected to enormous range of loading conditions

• Cartilage typically sustains little wear

• Implication: Sophisticated lubrication process required

Joint Lubrication

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

Lubrication Processes for Articular Cartilage

Boundary Lubrication (dominant for low loads)

• 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

Boundary Lubrication (dominant for low loads)

• 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

Lubrication Processes for Articular Cartilage

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

Lubrication Processes for Articular Cartilage

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

Schematic of Hydrodynamic Lubrication

Schematic of Hydrodynamic Lubrication

Lubrication Processes for Articular Cartilage

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)

Schematic of Squeeze-film Lubrication

Schematic of Squeeze-film Lubrication

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

Asperities in Articular Cartilage

Lubrication Processes for Articular Cartilage

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

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

Modes of Mixed Lubrication

2. 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

Variation of Lubrication Processes for Articular Cartilage

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

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

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

Dynamic Relationship between Vertical Load and Hip Joint Lubrication

Dynamic Relationship between Vertical Load and Hip Joint Lubrication

Two Types of Wear of Articular Cartilage

• Interfacial
• Fatigue

• Interfacial - due to interaction between bearing surfaces
  • Adhesion wear - surface fragments from bearing surfaces in contact with each other adhere and are torn away
  • Abrasion wear - soft material is scraped by hard material (opposing surface or loose particles)
• Fatigue wear
  • due to accumulation of microscopic damage within the bearing material under repetitive stress; not from surface-to-surface contact
  • Bearing surface failure from repeated application of high loads over short period of time or repetition of low loads over long period of time

• Effective joint lubrication makes interfacial wear unlikely under normal articular cartilage conditions
• Interfacial wear may occur in impaired or degenerated synovial joint

Potential Methods for Articular Cartilage Degeneration

• Magnitude of imposed stresses
• Total number of sustained stress peaks
• Change in the collagen-Proteoglycan matrix
• Change in mechanical properties of the tissue

Articular Surface of Cartilage

Cartilage Mechanics

Wolff’s Law

“Bone in a healthy person or animal will adapt to the loads it is placed under” -Late 1800’s

1960’s

Mechanotransduction

• Any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity.
• This form of sensory transduction is responsible for a number of senses and physiological processes in the body, including proprioception, touch, balance, and hearing. the basic mechanism of mechanotransduction involves converting mechanical signals into electrical or chemical signals.

(text adapted from wikipedia, 2020)

according to wolff’s law, a typical bone, e.g. the tibia has a security margin of about 5 to 7 between typical load (2000 to 3000 μStrain) and fracture load (about 15000μStrain).

Mechanotransduction in soft tissue

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mechano&part=A1859

• This same effect is seen in Tendons, Ligaments, Muscle, Bone, and to lesser extent Cartilage