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Articular Cartilage

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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Articular Cartilage

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  1. Articular Cartilage

  2. Structure • Hyaline Cartilage • Ends of long bones (1-5 mm thick) • Avascular • Aneural

  3. Function • Synovial Joints • Distribute loads • Allow for movement

  4. Composition Cellular – chondrocytes (10% of volume)

  5. 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%)

  6. Composition • Collagen fibers offer little resistance to compressive forces • Highly organized stiffness and tensile strength

  7. Composition • Isotropic – material properties of substance are same regardless of loading • Hyaline cartilage is anisotropic: • Collagen arrangement • Cross link density • Collagen/PG interaction

  8. Composition

  9. Composition

  10. 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)

  11. Collagen-PG Interaction • Plays direct role in organization of extracellular matrix • Important to mechanical properties  resists compression

  12. AC under Compression

  13. AC under Compression

  14. AC under Compression

  15. AC under Compression • constant load  rapid initial deformation  slow (time dependent)  deformation  equilibrium • 20 to exudation of interstitial fluid

  16. AC under tension

  17. AC under tension

  18. AC under tension • Toe region – alignment of collagen fibers • Linear region – stretching of collagen fibers

  19. AC under tension

  20. Osteoarthritis • Collagen cross link alteration  • fibrillation  • OA  • deterioration of tensile properties of collagen-PG solid matrix • Loosening of collagen network  increased swelling

  21. Synovial Fluid • Lubrication • Reduce Friction • Nutrition

  22. Synovial Fluid • Plasma-like • High in hyaluronate  lubrication to reduce friction • Lubricin – has an affinity for AC - cartilage lubrication

  23. Synovial Fluid • Hyaluronate (HA) – responsible for viscosity of synovial fluid • Resistance to shear forces

  24. Lubrication of Articular Cartilage • Boundary Lubrication • Fluid-Film Lubrication • Hydrodynamic (non-// surfaces) • Squeeze-film ( surfaces) • Mixed Lubrication • Boundary - Fluid-film • Boosted

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

  26. Type of Lubrication Fluid Film • thin fluid film provides greater surface separation • rigid bearings (stainless steel)

  27. Fluid Film Lubrication • Hydrodynamic – a wedge of fluid is formed when non-parallel surfaces slide over each other

  28. Fluid-Film Lubrication Squeeze film – pressure is created in the fluid film by bearing surfaces that are 

  29. Mixed Lubrication

  30. 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)

  31. Type of Lubrication Boundary • high loads • low relative speeds • long duration Fluid-film • low/oscillated magnitude • high relative speeds

  32. 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

  33. 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

  34. Wear of AC • Interfacial - adhesive - abrasive • Fatigue - disruption of collagen-PG solid matrix due to repetitive stress

  35. Cartilage Degeneration • Magnitude of imposed stresses • Total number of sustained stress peaks • Changes in collagen-PG matrix • Changes in mechanical properties of tissue

  36. Cartilage Degeneration • Loosening of collagen network  • PG expansion  • Tissue swelling  • Decrease in stiffness and increase in permeability  • Altered cartilage function

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