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Articular Cartilage. Structure. Hyaline Cartilage Ends of long bones (1-5 mm thick) Avascular Aneural. Function. Synovial Joints Distribute loads Allow for movement. Composition. Cellular – chondrocytes (10% of volume) . Composition. Extracellular Matrix

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structure
Structure
  • Hyaline Cartilage
  • Ends of long bones (1-5 mm thick)
  • Avascular
  • Aneural
function
Function
  • Synovial Joints
  • Distribute loads
  • Allow for movement
composition
Composition

Cellular – chondrocytes (10% of volume)

composition5
Composition

Extracellular Matrix

  • Organic – collagen (type II) (10-30% of weight) & proteoglycans (3-10% of weight)
  • Water (most abundant component), inorganic salts, glycoproteins, lipids (60 - 87%)
composition6
Composition
  • Collagen fibers offer little resistance to compressive forces
  • Highly organized stiffness and tensile strength
composition7
Composition
  • Isotropic – material properties of substance are same regardless of loading
  • Hyaline cartilage is anisotropic:
      • Collagen arrangement
      • Cross link density
      • Collagen/PG interaction
fluid component
Fluid Component
  • Permits diffusion of gases, nutrients, wastes  SYNOVIAL FLUID
  • Important to the structural organization of collagen  load bearing /mechanical behavior (80% surface / 65% deep)
collagen pg interaction
Collagen-PG Interaction
  • Plays direct role in organization of extracellular matrix
  • Important to mechanical properties  resists compression
ac under compression15
AC under Compression
  • constant load  rapid initial deformation  slow (time dependent)  deformation  equilibrium
  • 20 to exudation of interstitial fluid
ac under tension18
AC under tension
  • Toe region – alignment of collagen fibers
  • Linear region – stretching of collagen fibers
osteoarthritis
Osteoarthritis
  • Collagen cross link alteration 
  • fibrillation 
  • OA 
  • deterioration of tensile properties of collagen-PG solid matrix
  • Loosening of collagen network  increased swelling
synovial fluid
Synovial Fluid
  • Lubrication
  • Reduce Friction
  • Nutrition
synovial fluid22
Synovial Fluid
  • Plasma-like
  • High in hyaluronate  lubrication to reduce friction
  • Lubricin – has an affinity for AC - cartilage lubrication
synovial fluid23
Synovial Fluid
  • Hyaluronate (HA) – responsible for viscosity of synovial fluid
  • Resistance to shear forces
lubrication of articular cartilage
Lubrication of Articular Cartilage
  • Boundary Lubrication
  • Fluid-Film Lubrication
    • Hydrodynamic (non-// surfaces)
    • Squeeze-film ( surfaces)
  • Mixed Lubrication
    • Boundary - Fluid-film
    • Boosted
type of lubrication
Type of Lubrication

Boundary – single layer of lubricant molecules on each bearing surface (lubricin has affinity for AC)

type of lubrication26
Type of Lubrication

Fluid Film

  • thin fluid film provides greater surface separation
  • rigid bearings (stainless steel)
fluid film lubrication
Fluid Film Lubrication
  • Hydrodynamic – a wedge of fluid is formed when non-parallel surfaces slide over each other
fluid film lubrication28
Fluid-Film Lubrication

Squeeze film – pressure is created in the fluid film by bearing surfaces that are 

mixed lubrication30
Mixed Lubrication

Boosted

  • ultrafiltration of synovial fluid thru collagen-PG matrix
  • H2O & electrolytesarticular cartilage (squeeze-film)
  • concentrated gel of HA protein complex coats surfaces (boundary)
type of lubrication31
Type of Lubrication

Boundary

  • high loads
  • low relative speeds
  • long duration

Fluid-film

  • low/oscillated magnitude
  • high relative speeds
wear of ac
Wear of AC

High Impact Loading Wear

Interfacial Wear

Fatigue Wear

results from

results from

results from

Bearing surfaces in direct

contact - no film separating

Microscopic damage 20

repetitive loading

High load w/ insufficient

time for fluid redistribution

 Strain rate - microtrauma exceed reparative process

 Stress

 Strain

Adhesion

Abrasion

Disruption of collagen-PG matrix, PG “wash-out”, alteration of load reducing mechanism

DISRUPTION OF CARTILAGE MICROSTRUCTURE

cartilage degeneration
Cartilage Degeneration

Magnitude & distribution of imposed stress

# of sustained stress peaks

Degenerative changes to matrix

Changes in tissue’s mechanical properties

Swelling & weak tissue destruction by normal forces

 stress conc.

 in load frequency & magnitude

Insult to molecular structure of collagen-PG matrix

Loosening of collagen network, abnormal PG expansion, tissue swelling,  cartilage stiffness,  permeability

Abnormal: Stresses & strains

Mechanoelectrochemical stimuli

ECM remodeling by chondrocytes

OA

wear of ac34
Wear of AC
  • Interfacial

- adhesive

- abrasive

  • Fatigue

- disruption of collagen-PG solid matrix due to repetitive stress

cartilage degeneration35
Cartilage Degeneration
  • Magnitude of imposed stresses
  • Total number of sustained stress peaks
  • Changes in collagen-PG matrix
  • Changes in mechanical properties of tissue
cartilage degeneration36
Cartilage Degeneration
  • Loosening of collagen network 
  • PG expansion 
  • Tissue swelling 
  • Decrease in stiffness and increase in permeability 
  • Altered cartilage function