1. Chapter 3 Biomechanics of Articular Cartilage

2. Introduction Articular cartilage is found in synovial joints.
Articular cartilage functions:
Increase load distribution area
Allow movement while reducing friction and wear

3. Articular Cartilage Composition
Articular cartilage is multiphasic:
Matrix of collagen and proteoglycan (25%)
Free interstitial fluid (75%)
Ion phase

4. Collagen Gives a layered character to cartilage Presents 3 zones:
Superficial tangential zone (10% to 20% thickness)
Middle zone (40% to 60% thickness)
Deep zone (30% thickness)
Has a high tensile stiffness and strength

5. Proteoglycan Large protein-polysaccharide molecule
Not homogenously distributed in collagen:
Highest concentration in middle zone
Lowest concentrations in superficial and deep zones
Adds stability and rigidity to ECM

6. Water Most abundant component of cartilage
Most concentrated near articular surface
Contains free mobile cations (e.g. Na+, K+, Ca2+)
Allows movement of gas, nutrients, waste products
Its movement influences cartilage mechanical behavior.

7. Interaction among cartilage components
PG interacts with collagen:
It forms a porous matrix.
This matrix is swollen with water.
When load is applied to cartilage:
Fluid runs outside the matrix.
It protects against excessive stress and strain.

8. Biomechanical Behavior of Articular Cartilage
Cartilage treated as a biphasic material with:
Interstitial fluid phase
Porous-permeable solid phase
Cartilage is a highly stressed material
To understand cartilage response to stress:
Intrinsic mechanical properties must be determined.
Compression, tension, and shear are considered.

9. Biomechanical Behavior of Articular Cartilage (continued)
Intrinsic material properties and resistance to flow of solid matrix define interstitial fluid pressurization.
Interstitial fluid pressurization influences:
Load-bearing capacity
Lubrication capacity 9

10. Lubrication of Articular Cartilage
Lubrication processes limit wear of cartilage
Fluid-film lubrication:
Uses film of lubricant causing a bearing surface
Load is supported by pressure developed in fluid-film
Boundary lubrication:
Surfaces protected by adsorbed layer of boundary lubricant
Prevents surface to surface contact

11. Lubrication of Articular Cartilage (cont’d)
Mixed lubrication
Combination of fluid-film and boundary lubrications:
Temporal coexistence of both at distinct locations
Boosted lubrication: shift of fluid-film to boundary 11

12. Role of interstitial fluid pressurization in joint lubrication
Fluid-film lubrication contribution is transient because of rapid dissipation of fluid-film thickness by joint loads.
When interstitial pressurization is high, friction coefficient is low.
As creep equilibrium is reached, friction coefficient is high.
Effective friction coefficient decreases:
With increasing rolling and sliding joint velocities
With increasing joint load

13. Wear of articular cartilage
Is unwanted removal of material from solid surfaces by mechanical action. Can be:
Interfacial wear:
Bearing surfaces come into direct contact, with no lubricant film separating them.
Fatigue wear:
Accumulation of microscopic damage within the bearing material under repetitive stressing
Wear due to synovial joint impact loading

14. Wear of articular cartilage (continued)
Once collagen-PG matrix is disrupted it can induce:
Further disruption of collagen-PG matrix due to repetitive matrix stressing
Increased “washing out” of PGs due to violent fluid movement and thus impairment of articular cartilage’s interstitial fluid load support capacity
Gross alteration of normal load carriage mechanism in cartilage, thus increasing frictional shear loading on the articular surface 14

15. Hypotheses on the Biomechanics of Cartilage Degeneration
Cartilage failure progression relates to:
Magnitude of imposed stresses
Total number sustained stress peaks
Changes in intrinsic molecular and microscopic structure of collagen-PG matrix
Changes in intrinsic mechanical property of tissue
This is associated with decreased cartilage stiffness and increased cartilage permeability.

16. Functional Tissue Engineering of Articular Cartilage
Cartilage has poor healing capacity.
Tissue engineering is:
Incorporating an appropriate cell.
This cell will grow fabricated tissues.
These tissues will be used for repair and replacement of damaged or diseased tissues and organs. 16