Articular Cartilage
Basic Sciences
Articular Cartilage: Location
Articular cartilage covers the joint
surfaces:
•Bottom of the femur
•Top of the tibia
•Back-side of the patella
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
0.0025.
Articular Cartilage: Anatomy
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
Photo of knee joint
cross section.
The white tissue is the
articular cartilage covering
the distal femur (top) and
proximal tibia (bottom)
bone surfaces.
Articular Cartilage: Histology
By volume: 1% chondrocytes and 99% matrix
•Chondrocytes produce collagen, proteoglycans, etc.
as needed
•Release enzymes to break down aging components.
Articular Cartilage: Chondrocytes
 Chondrocytes vary in size, morphology and arrangement
 Changes with depth at tissue
Articular Cartilage: Chondrocytes
Articular Cartilage: Chondrocytes
Cells from surface to deeper levels:
•Flatter Æ Rounder
•Scattered Æ Organized into columns
•Below tidemark, cells are encrusted
with apatite salts (bone)
Articular Cartilage: Composition
Note that articular (hyaline) cartilage has the
highest proportion of water and also the
highest proteoglycan content
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 %)
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
Proteoglycans (15%)
 Each subunit
consists of a
combination of
protein and sugar:
Long protein chain
ÂSugars units
attached densely in
parallel
Proteoglycans (15%)
 Subunits are attached at
right angles to a long
filament
 Produce a
macromolecules: the
proteoglycan aggregate
Proteoglycans (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.
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Mechanical Behavior
Lateral
Condyle
COMPRESSIVE AGGREGATE
MODULUS (MPa)
POISSON’S RATIO
PERMEABILITY COEFFICIENT
(10-15m4/N•s)
Patellar
Groove
0.70
0.53
0.10
1.18
0.00
2.17
Tensile Force
³ Toe region: collagen
fibrils straighten out and
un- “crimp”
³ Linear region that
parallels the tensile
strength of collagen fibrils:
collagen aligns with axis
of tension
³ Failure region
Tensile Force
A.) Random
alignment of collagen
fibrils
B.) Histogram of
measurements made
from micrograph
show distribution of
fibril alignment
Shear Force
Creep behavior
Creep of a viscoelastic material
under constant
loading
Stress rise during a
ramp displacement
compression of a
visco-elastic
material and stress
relaxation under
constant
compression
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
porouspermeable
collagenproteoglycan
solid matrix
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
Permeability
Permeability decreases in an exponential manner as
function of both increasing applied compressive
strains and increasing applied pressure
Compressive force
Copious exudation of fluid at start but the rate of
exudation decreases over time from points A to B to C
Compressive force
At equilibrium, fluid flow ceases and the load is
borne entirely by the solid matrix (point C)
Compressive force
Creep behavior of collagen-proteoglycan matrix is
analagous to an elastic spring
Creep behavior
Creep of a viscoelastic material
under constant
loading
Stress rise during a
ramp displacement
compression of a
visco-elastic
material and stress
relaxation under
constant
compression
Stress Relaxation
The sample is compressed to point B and then maintained over time
(points B to E)
Stress Relaxation
Increase stress to reach end of compressive phase
Stress Relaxation
Fluid redistribution allows for relaxation phase (points B to D) and
matrix deformation
Stress Relaxation
Equilibrium is reached at point E