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 ( %)

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