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
 Organic – collagen (type II) (10-30% of
weight) & proteoglycans (3-10% of weight)

Water (most abundant component), inorganic
salts, glycoproteins, lipids (60 - 87%)
Composition


Collagen fibers offer little resistance to
compressive forces
Highly organized  stiffness and
tensile strength
Composition


Isotropic – material properties of
substance are same regardless of
loading
Hyaline cartilage is anisotropic:



Collagen arrangement
Cross link density
Collagen/PG interaction
Composition
Composition
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


Plays direct role in organization of
extracellular matrix
Important to mechanical properties 
resists compression
AC under Compression
AC under Compression
AC under Compression
AC under Compression


constant load  rapid initial deformation
 slow (time dependent)  deformation 
equilibrium
20 to exudation of interstitial fluid
AC under tension
AC under tension
AC under tension


Toe region – alignment of collagen
fibers
Linear region – stretching of collagen
fibers
AC under tension
Osteoarthritis





Collagen cross link alteration 
fibrillation 
OA 
deterioration of tensile properties of
collagen-PG solid matrix
Loosening of collagen network 
increased swelling
Synovial Fluid

Lubrication

Reduce Friction

Nutrition
Synovial Fluid



Plasma-like
High in hyaluronate  lubrication to
reduce friction
Lubricin – has an affinity for AC cartilage lubrication
Synovial Fluid


Hyaluronate (HA) – responsible for
viscosity of synovial fluid
Resistance to shear forces
Lubrication of Articular Cartilage


Boundary Lubrication
Fluid-Film Lubrication



Hydrodynamic (non-// surfaces)
Squeeze-film ( surfaces)
Mixed Lubrication


Boundary - Fluid-film
Boosted
Type of Lubrication
Boundary – single layer of lubricant molecules on
each bearing surface (lubricin has affinity for
AC)
Type of Lubrication
Fluid Film
 thin fluid film provides greater surface
separation
 rigid bearings (stainless steel)
Fluid Film Lubrication

Hydrodynamic – a wedge of fluid is formed
when non-parallel surfaces slide over each
other
Fluid-Film Lubrication
Squeeze film – pressure is created in the
fluid film by bearing surfaces that are 
Mixed Lubrication
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 Lubrication
Boundary
 high loads
 low relative speeds
 long duration
Fluid-film
 low/oscillated magnitude
 high relative speeds
Wear of AC
Interfacial Wear
Fatigue Wear
High Impact
Loading Wear
results from
results from
Bearing surfaces in direct
Microscopic damage 20
High load w/ insufficient
contact - no film separating
repetitive loading
time for fluid redistribution
Adhesion
Abrasion
results from
 Strain rate microtrauma exceed
reparative process
 Stress
 Strain
Disruption of collagen-PG matrix, PG “wash-out”, alteration of load reducing mechanism
DISRUPTION OF CARTILAGE MICROSTRUCTURE
Cartilage Degeneration
Magnitude &
distribution of
imposed stress
 stress conc.
# of sustained
stress peaks
 in load
frequency &
magnitude
Degenerative
changes to matrix
Swelling &
weak tissue
destruction by
normal forces
Changes in tissue’s
mechanical
properties
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 AC


Interfacial
- adhesive
- abrasive
Fatigue
- disruption of collagen-PG solid matrix
due to repetitive stress
Cartilage Degeneration




Magnitude of imposed stresses
Total number of sustained stress peaks
Changes in collagen-PG matrix
Changes in mechanical properties of
tissue
Cartilage Degeneration





Loosening of collagen network 
PG expansion 
Tissue swelling 
Decrease in stiffness and increase in
permeability 
Altered cartilage function