1. Articular Cartilage
Basic Sciences

2. Articular Cartilage: Location
Articular cartilage covers the joint surfaces:
Bottom of the femur
Top of the tibia
Back-side of the patella

3. Articular Cartilage: Primary Functions
Transmits applied loads across mobile surfaces
Lines the ends of bones
surfaces roll or slide during motion
Hyaline cartilage is fluid-filled wear-resistant surface
It reduces friction coefficient to

4. Articular Cartilage: Anatomy
Photo of knee joint cross section.  The white tissue is the articular cartilage covering the distal femur (top) and proximal tibia (bottom) bone surfaces. 
1. Quadriceps femoris
2. Patellar tendon
3. Suprapatellar bursa
4. Patella
5. Joint cavity
6. Infrapatellar fat pad
7. Tibia
8. Meniscus
9. Articular cartilage
10. Femur

5. Articular Cartilage: Histology
By volume: 1% chondrocytes and 99% matrix
Chondrocytes produce collagen, proteoglycans, etc. as needed
Release enzymes to break down aging components.

6. Articular Cartilage: Chondrocytes
Chondrocytes vary in size, morphology and arrangement
Changes with depth at tissue

7. Articular Cartilage: Chondrocytes

8. Articular Cartilage: Chondrocytes
Cells from surface to deeper levels:
Flatter  Rounder
Scattered  Organized into columns
Below tidemark, cells are encrusted with apatite salts (bone)

9. Articular Cartilage: Composition
Note that articular (hyaline) cartilage has the highest proportion of water and also the highest proteoglycan content

10. Articular Cartilage: Composition
Components are arranged in a way that is maximally adapted for biomechanical functions
Chondrocytes (~ 1%)
Collagen (15%) (Type II in articular cartilage)
Proteoglycans (15%)
Water (70 %)

11. Collagen (15%)
Creates a framework that houses the other components of cartilage
Majority is Type II collagen
Provides cartilage with its tensile strength
Look at Ligament & Tendon notes for structure of collagen fibers

12. Proteoglycans (15%)
Each subunit consists of a combination of protein and sugar: Long protein chain
Sugars units attached densely in parallel

13. Proteoglycans (15%)
Subunits are attached at right angles to a long filament
Produce a macromolecules: the proteoglycan aggregate

14. Proteoglycans (15%)

15. Proteoglycans (15%)
Each sugar has one or two negative charges, so collectively there is an enormous repulsive force within each subunit and between neighboring subunits
This causes the molecule to extend stiffly out in space
This property gives articular cartilage its resiliency to compression
The negative charges make the molecules extremely hydrophilic and cause water to be trapped within
It is used during biomechanical or lubricant activity.
Water functions as the "shock absorber" in cartilage, lubricates and nourishes the cartilage.

16. Mechanical Behavior 0.70 0.53 0.10 0.00 1.18 2.17 Lateral Condyle
Patellar Groove
COMPRESSIVE AGGREGATE MODULUS (MPa)
0.70
0.53
POISSON’S RATIO
0.10
0.00
PERMEABILITY COEFFICIENT
(10-15m4/N•s)
1.18
2.17

17. Tensile Force
Toe region: collagen fibrils straighten out and un- “crimp”
Linear region that parallels the tensile strength of collagen fibrils: collagen aligns with axis of tension
Failure region

18. Tensile Force A.) Random alignment of collagen fibrils
B.) Histogram of measurements made from micrograph show distribution of fibril alignment

19. Shear Force

20. Creep behavior
Creep of a visco-elastic material under constant loading
Stress rise during a ramp displacement compression of a visco-elastic material and stress relaxation under constant compression

21. Compressive force
Rate of creep is determined by the rate at which fluid may be forced out of the tissue
This, in turn, is governed by the permeability and stiffness of the porous-permeable collagen-proteoglycan solid matrix

22. Permeability
Articular cartilage shows nonlinear strain dependence and pressure dependence
The decrease of permeability with compression acts to retard rapid loss of interstitial fluid during high joint loadings

23. Permeability
Permeability decreases in an exponential manner as function of both increasing applied compressive strains and increasing applied pressure

24. Compressive force
Copious exudation of fluid at start but the rate of exudation decreases over time from points A to B to C

25. Compressive force
At equilibrium, fluid flow ceases and the load is borne entirely by the solid matrix (point C)

26. Compressive force
Creep behavior of collagen-proteoglycan matrix is analagous to an elastic spring

27. Creep behavior
Creep of a visco-elastic material under constant loading
Stress rise during a ramp displacement compression of a visco-elastic material and stress relaxation under constant compression

28. Stress Relaxation
The sample is compressed to point B and then maintained over time (points B to E)

29. Stress Relaxation
Increase stress to reach end of compressive phase

30. Stress Relaxation
Fluid redistribution allows for relaxation phase (points B to D) and matrix deformation

31. Equilibrium is reached at point E
Stress Relaxation
Equilibrium is reached at point E