Study Notes on Articular Cartilage Biomechanics

Articular Cartilage Biomechanics

• Composition & Structure

  • Biphasic nature: solid framework (collagen, proteoglycans, chondrocytes) and fluid phase (water, ions).
  • Collagen provides tensile strength, forming a mesh.
  • Proteoglycans attract water and resist compression.
  • Zonal heterogeneity affects mechanical properties from surface to deep layers.

• Viscoelasticity

  • Viscoelastic materials show time-dependent responses under constant load or deformation.
  • Major responses:
  1. Creep: Initial rapid deformation followed by slow increases until fluid flow ceases.
  2. Stress Relaxation: Stress peaks during deformation, then relaxes to equilibrium.

• Permeability

  • Articular cartilage is ~80% porous, affecting fluid flow and resistance.
  • Anisotropic and heterogeneous; permeability decreases with depth and increases with compression.

• Response Stages

  • Initial loading causes fluid to move out; collagen aligns for stiffness.
  • Large strains lead to fluid loss, damaging collagen.
  • Cyclic loading dissipates energy, potentially causing irreparable damage.

• Mechanical Properties

  • Young's modulus reflects non-linear, anisotropic stress-strain responses.
  • Stiffer under compression/tension than shear; varying stiffness based on collagen orientation and depth.

• Swelling Behavior

  • High swelling pressure from negatively charged proteoglycans attracts water.
  • Dynamics of water influx and collagen resistance reach equilibrium, affecting load distribution and cartilage function.

• Disease Effects

  • Osteoarthritis alters swelling behavior, permeability, and load-bearing capacity, compromising joint health.