Joint surfaces covered with 2-4 mm of hyaline cartilage
Well suited to withstand rigors of joint environment, capable of handling cyclic loading
Distinct from bone
Devoid of blood supply, nerves
Cellular density less than any other tissue
[ ( slide credit: @Jastifer2010Cartilage ) ]
Articular cartilage
Millions of cycles required in a lifetime
Cartilage allows activity while minimizing wear to the joint
surface
[ ( slide credit: @Jastifer2010Cartilage ) ]
Friction at articular surfaces
Exercise and cartilage wear (literature example)
Critique the abstract?
The effect of long-term exercise on canine knees was studied to
determine whether an increased level of lifelong weight bearing
exercise causes degeneration,or changes that may lead to degeneration,
of articular cartilage. Eleven dogs were exercised on a treadmill at 3
km/hr for 75 minutes 5 days a week for 527 weeks while carrying
jackets weighing 130% of their body weight. Ten control dogs were
allowed unrestricted activity in cages for the 550 weeks. At the
completion of the study all knee joints were inspected for evidence of
joint injury and degeneration. …
Articular cartilage surfaces from the medial tibial plateau were
examined by light microscopy, the cartilage thickness was measured,
and the intrinsic material properties were determined by mechanical
testing. No joints had ligament or meniscal injuries, cartilage
erosions, or osteophytes. Light microscopy did not demonstrate
cartilage fibrillation or differences in safranin O staining of the
tibialarticular cartilages between the two groups. Furthermore, the
tibial articular cartilage thickness and mechanical properties did not
differ between the two groups. These results show that a lifetime of
regular weight bearing exercise in dogs with normal joints did not
cause alterations in the structure and mechanical properties of
articular cartilage that might lead to joint degeneration.
Thoughts???
Fibrocartilage - enthesis
The enthesis is the connective tissue between tendon or
ligament and bone.
There are two types of entheses:
Fibrous entheses and
Fibrocartilaginous entheses
In a fibrous enthesis, the collagenous tendon or ligament
directly attaches to the bone.
In a fibrocartilaginous enthesis, the interface presents
a gradient that crosses four transition zones:
Tendinous area displaying longitudinally oriented fibroblasts
and a parallel arrangement of collagen fibers
Fibrocartilaginous region of variable thickness where the
structure of the cells changes to chondrocytes
Abrupt transition from cartilaginous to calcified
fibrocartilage—often called ‘tidemark’ or ‘blue
line’
Bone
(text adapted from Wikipedia, 2020)
Sidebar - Tendon healing (and its relationship to fibrocartilage)
Left: Normal supraspinatus tendon insertion site in a rabbit
Right: Fibrovascular (IF) scar tissue 4 weeks after repair
Note the repaired tendon (scar tissue) is much less organized
@Rodeo2007
[ ( slide credit: @Geeslin2016Cartilage ) ]
Perichondrium
Dense connective tissue that covers cartilage (except articular
cartilage of joints.)
Contains blood, nerve supply, lymphatics.
Contains collagen fibers, fibroblasts
[ (slide credit: @Jastifer2010Cartilage) ]
Articular cartilage composition, microstructure
Typical total thickness: 2-4 mm
Chondrocytes: cartilage cells
Extracellular Matrix (ECM)
Predominant component is water
Type II collagen, Proteoglycans
Charged molecules, ions
Fractional composition varies by zone
Avascular, aneural, low metabolic rate
[ ( slide credit: @Geeslin2016Cartilage ) ]
Chondrocytes
Chondrocytes (cartilage cells)
Sparsely distributed cells in articular cartilage
5% of wet weight; Less than 10% of tissue volume (water is largest by weight)
Manufacture, secrete, and maintain organic component of extracellular matrix (ECM)
Chondrocyte Distribution in Articular Cartilage
Superficial tangential zone (10-20%)
Chondrocytes oblong, parallel to articular surface
Middle Zone (40-60%)
Chondrocytes round
Deep Zone (30%)
Calcified Zone
Chondrocytes arranged in columnar fashion
Subcondral Bone
between calcified and non-calcified tissue
[ ( slide credit: @Jastifer2010Cartilage ) ]
Articular cartilage zones
@Ulrich-Vinther2003
[ ( slide credit: @Geeslin2016Cartilage ) ]
Organization of Cartilage
@Bartel2006
Fig (a) Three phases of cartilage: porous matrix (made up of the
proteoglycan aggrecan and collagen fibers), water, and charged
ions associated with the proteoglycans.
Fig (b) The molecular structure of the proteoglycan monomer (top
and aggregate (bottom). (from @Mow1991)
Slightly simplified view of Extracellular (porous) matrix
Organic matrix
Composed of dense framework of type II collagen fibrils enmeshed
in concentration of proteoglycans (Proteoglycan)
Collagen (15-22% of wet weight)
Proteoglycans (4-7% of wet weight)
60-85% water content, inorganic salts, other proteins, glycoproteins, and lipids
Type I: skin, tendon, vasculature, organs, bone (main component of the organic part of bone)
Type II: cartilage (main collagenous component of cartilage)
Type III: reticulate (main component of reticular fibers), commonly found alongside type I
Type IV: forms basal lamina, the epithelium-secreted layer of the basement membrane
Type V: cell surfaces, hair, and placenta
Note:
Type II in Hyaline cartilage, Type I in Fibrocartilage
Collagen
Most abundant protein in the body
Think of it structurally as a rope
Provides fibrous ultrastructure in cartilage
Tropocollagen is basic biological unit of collagen
Composed of 3 α chains coiled in left hand helices
α chains coiled around each other in right hand triple helix
Form tropocollagen molecules
Cross links formed between tropocollagen molecules
high tensile strength
[ ( slide credit: @Jastifer2010Cartilage ) ]
Collagen Structure
[ ( slide credit: @Jastifer2010Cartilage ) ]
Structure and Arrangement of Collagen in Articular Cartilage
Collagen inhomogeniously distributed in three zones
Superficial tangential zone (STZ)
Zone with highest concentration of collagen
Middle zone
Collagen fibers randomly distributed and farther apart
Deep zone
Randomly layered fibrils of collagen to accommodate the high
concentration of proteoglycans and water
Pattern of collagen fibril arrangement related to tensile stiffness
and strength characteristics
Note correspondence between collagen and chondrocyte arrangement.
[ ( slide credit: @Jastifer2010Cartilage ) ]
Strength of Collagen
Strong in tension
Weak in compression (high slenderness ratio: length/width)
[ ( slide credit: @Jastifer2010Cartilage ) ]
Proteoglycans
Proteoglycans another major component of extracellular matrix
A compound consisting of a protein bonded to glycosaminoglycan
groups, present especially in connective tissue.
Complex macromolecules
Proteoglycans (Continued)
Important for providing compressive strength
Attract water, ions
Building blocks
Central protein core with polysaccharide chains
Aggrecans: large aggregating proteoglycans with >100 sidechains
Aggregate macromolecule: aggrecans bound to Hydroxy Appetite
[ ( slide credit: @Geeslin2016Cartilage ) ]
Proteoglycans (Continued)
Large protein-polysaccharide molecules that exist as either
monomers
or as
aggregates
Proteoglycan aggregation promotes immobilization of the
Proteoglycan’s within the collagen meshwork (adding structural
rigidity to the extracellular matrix of articular cartilage)
[ ( slide credit: @Geeslin2016Cartilage ) ]
Proteoglycan Aggregate
Note the terms well used in supplement marketing!
[ ( slide credit: @Jastifer2010Cartilage ) ]
Water, ions
Water
Accounts for 70-90% of wet weight (most abundant component of
cartilage)
H2O allows movement of gases, waste products, nutrients to
chondrocytes (in cartilage which is avascular)
Ions
Cations (Na\(^+\) and Ca\(^{2+}\)) are attracted to negatively
charged proteoglycans for electrical neutrality
Mechanical influence
Fluid flow is important contribution
Joint lubrication
[ ( slide credit: @Geeslin2016Cartilage ) ]
In summary:
Collagen and Proteoglycans
Form structural components that support mechanical stresses
applied to cartilage
Together with water determine biomechanical behavior of
cartilage
Collagen and proteoglycans interact to form a porous
composite fiber-reinforced organic solid matrix that is
swollen with water
Collagen-Proteoglycan solid matrix and interstitial
fluid protect against high levels of stress and strain
developing in the ECM when articular cartilage subjected to
external loads
[ ( slide credit: @Jastifer2010Cartilage ) ]
Biomechanical Loading of Articular Cartilage
Forces at joint surface vary from zero to several times body weight
“Contact” area varies in a complex manner; typically only several
square centimeters
Potentially high pressures/stress
Think of cartilage behavior with load as biphasic (solid component
and water) … wet sponge?
[ ( slide credit: @Jastifer2010Cartilage ) ]
Viscoelasticity
Cartilage is viscoelastic
Hysteresis
Strain rate dependent on time
Creep
Stress relaxation
[ ( slide credit: @Jastifer2010Cartilage ) ]
Impact of arrangement of collagen on mechanical properties
Superficially:
Tangential orientation of collagen resists shear as
joint surfaces move past each other
[ ( slide credit: @Jastifer2010Cartilage ) ]
Middle:
High water content, high Proteoglycan content
With early load, water moves to joint space and participates
in lubrication
With late load, negative charge of Proteoglycan molecules
begin to repulse each other and offer resistance to
compression
Fairly isotropic
[ ( slide credit: @Jastifer2010Cartilage ) ]
Mechanical properties
Anisotropic - differ with direction of loading (may be associated
with zonal arrangement of collagen)
Heterogeneous (due to varying collagen, proteoglycan,
and water content)
Poroelastic solid – Biphasic
Flow of interstitial fluid through the matrix and resistance of
matrix to the flow (and is dominant in dynamic response)
Intrinsic: flow-independent behavior of
collagen-proteoglycan matrix
Intrinsic properties are not representative of in vivo
behavior, however, they affect fluid flow
[ ( slide credit: @Geeslin2016Cartilage ) ]
Material Properties of Articular Cartilage
“Split lines” - surface collagen fiber pattern; functionally related
to tensile strength (and highest modulus aligned with split lines)
Referred to in @Bartel2006 p133
[ ( slide credit: @Jastifer2010Cartilage ) ]
Uniaxial tensile test
Typical stress-strain behavior for articular cartilage subjected to
tensile loading at a low constant strain rate.
Low constant strain rate
Values higher than “equilibrium” due to dynamic nature of test
Some fluid flow occurs (Model does match/accommodate fluid flow
present in physiologic loading)
This dynamic Young’s modulus (from linear region) is ~ 40-400 MPa
[ ( @Bartel2006 Ch 4.2. ) ]
[ ( slide credit: @Geeslin2016Cartilage ) ]
Equilibrium stress-strain behavior
Can be determined from steady-state response of tensile creep
loading with no fluid flow
Linear up to strains of 15%
Modulus ~ 4-10 MPA depending on specimen location in joint
[ ( slide credit: @Geeslin2016Cartilage ) ]
Intrinsic compressive properties
Confined compression creep test: is a uniaxial strain test.
Chamber restrict deformation except along axis of loading
an applied constant stress
fluid can escape through porous surface of test fixture
creep occurs and eventually equilibrium (steady-state is reached when fluid flow stops)
Experiment is repeated over various stresses and respective strains
and aggregate modulus \(H_A\) is obtained (typically 0.3-1.3 MPa)
Depends on repulsive electrical charges of Proteoglycan,
increases with increased Proteoglycan content (next slide)
Unrelated to collagen content (structurally) although collagen
constrains separation of Proteoglycan, resulting in internal
tensile stresses
Intrinsic compressive properties
Early load results in fluid efflux
Late load results in increased charge density due to negative
Proteoglycan side-chains
Note “Permeability” is another factor which can be quantified
- Resistance to flow
[ ( adapted from @Jastifer2010Cartilage @Geeslin2016Cartilage ) ]
“Creep” manifestation in cartilage confined compression tests
Constant load applied (in the test)
Deformation is not instantaneous, as it would be in a single-phase
elastic material such as a spring.
Displacement of the cartilage is a function of time, since the
fluid cannot escape from the matrix instantaneously
Initially, the displacement is rapid. This corresponds to a
relatively large flow of fluid out of the cartilage.
As the rate of displacement slows and the displacement
approaches a constant value, the flow of fluid likewise slows.
Equilibrium takes several thousand seconds
Note this is very different from classical metal creep
[ ( adapted from @Jastifer2010Cartilage ) ]
Intrinsic compressive properties
Dependence of equilibrium compressive aggregate modulus for human
patellar cartilage on hyaluronic acid content (a surrogate measure of
proteoglycan content).
[ ( slide credit: @Geeslin2016Cartilage ) ]
Water content and compressive modulus
Dependence of equilibrium compressive aggregate modulus for human
patellar cartilage on water content. @Armstrong1982
Less water means less proteoglycan per unit volume? (Also a
function of aging.)
Pure Shear
Small torsional displacements of cylindrical samples (which
produce pure shear), result in no volume change of the cartilage
to drive fluid flow.
The interstitial fluid (water) has low viscosity and does not make an
appreciable contribution to resisting shear.
Therefore, the resistance to shear is due to the solid matrix.
Tests of cartilage in shear show that the matrix behaves as a
viscoelastic solid
[ ( slide credit: @Jastifer2010Cartilage ) ]
Shear
Flow-independent shear rigidity of cartilage likely due to collagen fibers
Proteoglycan likely interact to support matrix
[ ( slide credit: @Geeslin2016Cartilage ) ]
Shear: collagen and modulus
Positive correlation between the magnitude of the dynamic shear
modulus (as measured in dynamic viscoelastic tests) and collagen
content for bovine cartilage
Shear Stress
High poison’s ratio, thus normal load leads to large lateral
displacement relative to bone, thus high shear stresses at
cartilage/bone interface
Not intuitive but joints aren’t normal, and thus see shear stress
[ ( slide credit: @Jastifer2010Cartilage ) ]
Stress relaxation
@Mow1984
Material subjected to constant deformation
High initial stress
Progressive decrease in the stress required to maintain deformation
(Mow 1977)
[ ( slide credit: @Geeslin2016Cartilage ) ]
Compression and permeability (of cartilage plug)
As you compress the tissue, you do two things:
Close the space for water to flow through by compacting tissue
and Increase charge density thus slowing flow.
[ ( slide credit: @Jastifer2010Cartilage ) ]
Creep and stress relaxation
Functionally:
Stress Relaxation
Nonlinear phenomenon
@DeLee1994
[ ( slide credit: @Geeslin2016Cartilage ) ]
Permeability
Permeability, κ, is the ease of fluid flow
Flow velocity proportional to pressure gradient
\(10^{-15}\) to \(10^{-16} m^4/Ns\)
As cartilage is compressed (and fluid flows from cartilage) the
permeability decreases (with increased strain), and stiffness
increases which prevents further loss of fluid
[ ( slide credit: @Jastifer2010Cartilage ) ]
If a pressure difference of 210,000 Pa (about the same pressure as
in an automobile tire) is applied across a slice of cartilage 1 mm
thick, the average fluid velocity will be only \(1\times10^{-8}\) m/s
Cartilage - putting it together
As you stand
Complex interaction of stress in the cartilage matrix and pressure
in the fluid.
Cartilage stops itself from “bottoming out” because its permeability
decreases with increased strain, and it’s stiffness increases.
Cartilage has to be thought of as a dynamic structure (see example
4.2)
[ ( slide credit: @Jastifer2010Cartilage ) ]
Clinical Correlate
Arthritic cartilage has a lower modulus and increased permeability
(higher water content, lower proteoglycan content).
This leads to greater and more-rapid deformation than normal.
Theoretically, these changes may influence the metabolic activity of
the chondrocytes, which are known to respond to their mechanical
environment (mechanotransduction).
[ ( slide credit: @Jastifer2010Cartilage ) ]
Clinical Correlate
Hypothesis
Cartilage thought to fail in tension/shear at surface that
creates a fissure that propagates
Fact
Femoral head arthritis far more likely than talus arthritis (contradiction?)
[ ( slide credit: @Jastifer2010Cartilage ) ]
Clinical Correlate
Old cartilage fails earlier than young cartilage
[ ( slide credit: @Jastifer2010Cartilage ) ]
Clinical correlates
Return to this figure:
Disruption of the collagen fibril meshwork allows the
Proteoglycan to expand, increases \(H_2O\), decreases
Proteoglycan
Less repulsive electrostatic forces to resist loading
Decrease in cartilage stiffness, increase in matrix permeability
In osteoarthritis, decreased Proteoglycan and increased \(H_2O\)
- Greater deformation
- Altered mechanotransduction?
[ ( slide credit: @Geeslin2016Cartilage ) ]
Surgical repair options
Microfracture - 2 years
Microfracture
Results in fibrocartilage repair
[ ( slide credit: @Geeslin2016Cartilage ) ]
Microfracture - 2 years
The Surgical Procedure: Microfracture
“The microfracture procedure is done arthroscopically. The surgeon visually assesses the defect and performs the procedure using special instruments that are inserted through three small incisions on the knee. After assessing the cartilage damage, any unstable cartilage is removed from the exposed bone. The surrounding rim of remaining articular cartilage is also checked for loose or marginally attached cartilage. This loose cartilage is also removed so that there is a stable edge of cartilage surrounding the defect. The process of thoroughly cleaning and preparing the defect is essential for optimum results.”
“Multiple holes, or microfractures, are then made in the exposed bone about 3 to 4mm apart. Bone marrow cells and blood from the holes combine to form a "super clot" that completely covers the damaged area. This marrow-rich clot is the basis for the new tissue formation. The microfracture technique produces a rough bone surface that the clot adheres to more easily. This clot eventually matures into firm repair tissue that becomes smooth and durable. Since this maturing process is gradual, it usually takes two to six months after the procedure for the patient to experience improvement in the pain and function of the knee. Improvement is likely to continue for about 2 to 3 years.”
“Is the new tissue that forms after the microfracture identical to
the original articular cartilage?”
“No, the new tissue is a”hybrid" of articular-like cartilage
plus fibrocartilage. Experience shows that this hybrid repair
tissue is durable and functions similarly to articular
cartilage."
Harvest of viable hyaline cartilage from non- weight bearing
surface, transfer to defect
Fresh frozen allograph
Fresh frozen allograph (cadaver, with viable chondrocytes)
MRI demonstrates incorporation, isointense cartilage signal
Bedi et al. J Bone Jt Surg Am 2010.
[ ( slide credit: @Geeslin2016Cartilage ) ]
Evidence based medicine
Grades of Recommendation for Cartilage Repair Procedures
*A = good evidence (Level-l studies with consistent findings) for or
against recommending intervention, B = fair evidence (Level II or III
studies with consistent findings) for or against recommending
intervention, C = poor-quality evidence (Level-IV or V studies with
consistent findings) for or against recommending intervention, and 1 =
there is insufficient or conflicting evidence not allowing a
recommendation for or against intervention.
Bedi et al. J Bone Jt Surg Am 2010.
[ ( slide credit: @Geeslin2016Cartilage ) ]
Future directions in cartilage repair
Chondral scaffolds
Osteochondral scaffolds
Platelet rich plasma, mix of growth factors
Mesenchymal stem cells
Isolated growth factors
Quantitative MRI
[ ( slide credit: @Geeslin2016Cartilage ) ]
Chondral scaffolds
Chondral scaffolds
E.g. juvenile, adult
Fibrin glue
Autologous chondrocyte implantation
Ex vivo expansion
Implantation under periosteal patch or other matrix
[ ( slide credit: @Geeslin2016Cartilage ) ]
Scaffold methods
Biocartilage (Arthrex) (Hirahara Sports Med Arthrosc 2015)
“…dehydrated allograft cartilage ECM scaffold and can stimulate
autologous cellular interactions. The ECM is made up of type II
collagen, proteoglycans, and cartilaginous growth factors, which
are components of native cartilage”
“…consists of allograft articular cartilage from donors younger
than 13 years old that has been cut into approximately 1-mm
cubes. It is applied to cartilage lesions in a monolayer and held
in place with the use of fibrin sealant” (particulated juvenile
allograft)
Neocart (Histogenics) (DeBerardino Sports Med Arthrosc 2015)
“3-dimensional type-I collagen scaffold seeded with autologous
chondrocytes”
