Lecture 10: Articular Cartilage

What are the different types of cartilage?

What are the different types of cartilage?

• Elastic

• Hyaline

• Fibrous

What are some examples of where elastic cartilage would be found?

• Epiglottis

• Larynx

• External Ear

T/F: Elastic cartilage has the most extracellular matrix compared to the other two types of cartilage

True! (most elastic = most ECM)

Hyaline cartilage is the primary component of ________ cartilage

Hyaline cartilage is the primary component of articular cartilage

Where would you typically find hyaline cartilage?

Lines most diarthrodial joints (except TMJ)

Where would you typically find fibrous cartilage?

• Intervertebral Discs

• Pubic Symphysis

• Meniscus

Fibrous cartilage has the least ________ and the most _________.

Fibrous cartilage has the least ECM and the most collagen

When does articular cartilage begin to develop?

Around the 5th week of gestation

What stem cells produce cartilage?

Mesenchymal stem cells

Mesenchymal stems cells producing cartilage are _________.

Mesenchymal stem cells producing cartilage are pluripotent (meaning that they are able to differentiate — able to “become anything”)

What is another term for limb development?

Endochondral ossification

Long bones start as…

Articular cartilage

What are the steps of limb development (endochondral ossification)?

1. Cartilage scaffolding forms

2. Chondrocytes form at the articular and transient cartilage at the ends of opposing bones

3. Chondrocytes undergo hypertrophic differentiation in the middle of the limb bud

4. Limb bud layers mesenchyme over layer of ectodermal cells forming a ridge (apical ectodermal ridge)

What does AER stand for?

Apical ectodermal ridge

AER is critical in the process of…

• Allowing the mesodermal cells to replicate

• Protecting the cells allowing for limb growth

How is the AER controlled?

• Controlled 3-dimensionally

  • Proximal → Distal

  • Anterior → Posterior

  • Dorsal → Ventral

What is the zone of polarizing activity?

Controls the anterior → posterior (formation of the digits)

Zone of polarizing activity is controlled from what gene?

Sonic hedgehog gene

What is the main role of articular cartilage?

To cover articular surfaces of diarthrodial joints (except the TMJ)

What are the four functions of articular cartilage?

• Distribute joint load over a wider area

  • Cushioning / shock absorption

• Allow relative movement of the opposing joint surface with minimal friction

• Improve fit of articular surfaces

• Provide lubrication of articular surfaces

T/F: Articular cartilage is avascular

True! (once it gets past tide mark)

T/F: Articular cartilage has lymph drainage

False! (no lymph drainage)

T/F: Articular cartilage is innervated

False! (articular cartilage is not innervated)

T/F: Articular cartilage has very few cells (chondrocytes)

True!

What happens to articular cartilage as we age?

It breaks down

If a microtrauma occurs to articular cartilage, is it painful?

No (articular cartilage is not innervated → therefore partial tear, etc. wouldn’t necessarily cause pain)

If a macrotrauma occurs to articular cartilage, is it painful?

It may be painful (e.g., complete ear that goes to the bone can cause pain)

Chondrocytes account for less than ___% of the tissue volume in articular cartilage

Chondrocytes account for less than 10% of the tissue volume in articular cartilage

What is the function of chondrocytes?

Manufacture, secrete, organize, and maintain the organic component of the ECM

What type of collagen is most abundant in the ECM of articular cartilage?

Type II collagen

Type II collagen has a large concentration of…

Proteoglycans

Collagen makes up ___% of the wet weight in articular cartilage

Collagen makes up 15-22% of the wet weight in articular cartilage

Articular cartilage has (more/less) collagen compared to ligament and tendon.

Articular cartilage has less collagen compared to ligament and tendon

Proteoglycans make up ___% of the wet weight in articular cartilage

Proteoglycans make up 4-7% of the wet weight in articular cartilage

Water (and inorganic salts + proteins) make up ___% of the wet weight in articular cartilage

Water (and inorganic salts + proteins) make up 60-85% of the wet weight in articular cartilage

What force does cartilage handle the best?

Compressive forces

What is the tidemark?

The transition point from calcified to non-calcified cartilage

Collagen in articular cartilage is __________ distributed and _________

Collagen in articular cartilage is heterogenous distributed and layered

What are the three “zones” of collagen in articular cartilage?

• Superficial Zone

• Middle Zone

• Deep Zone

Discuss the arrangement of collagen in the superficial zone.

Densely packed fibers randomly woven in parallel planes to the articular cartilage

The superficial zone accounts for ___% of the dry weight

The superficial zone accounts for 85% of the dry weight

T/F: The superficial zone of articular cartilage has the lowest concentration of proteoglycans

True!

What force does the superficial zone resist?

Resists the tensile component of the compressive load

The collagen fibers in the superficial zone are _______ to the joint surface

The collagen fibers in the superficial zone are parallel to the joint surface

Discuss the arrangement of the collagen in the middle zone.

There is greater distances between the collagen; thickest

The middle zone accounts for ____% of the thickness of the collagen

The middle zone accounts for 40-60% of the thickness of the collagen

The middle zone has the greatest concentration of…

Proteoglycans (what is attracting H2O)

The middle zone is also known as…

The workhorse of cartilage

Discuss the arrangement of collagen in the deep zone.

The fibers begin to come together to form larger, radially oriented fiber bundles (perpendicularly oriented to the subchondral layer)

In the deep zone, the fibers cross the __________ and insert on the…

In the deep zone, the fibers cross the tidemark and insert on the calcified cartilage and subchondral bone

What is a proteoglycan?

Protein polysaccharide with GAGs attached

What is the function (or purpose) of aggrecan?

Aids in structural stability (by attracting water)

How does aggrecan attract water?

Aggrecan has an affinity for hylauronic acid which attracts water

What does aging cause at molecular level of CT?

Aging causes the water content and carbohydrate/protein ratio to decrease

(leads to individual being less able to withstand compressive forces)

T/F: Water content in articular cartilage is the most abundant (~80%)

True

What cations does the water in articular cartilage contain?

Na+, K+, and Cl-

What does the high water content of articular cartilage allow for? Why is this important?

The fluid allows for waste, gases, and nutrients to flow back and forth in the cartilage

Important due to articular cartilage being avascular

The movement of water with loading is crucial for….

Joint lubrication (Dr Evans — “motion is lotion”)

What is deformation controlled by in articular cartilage? How?

Proteogylcans

1. Fluid flows out of collagen →

2. Increased concentration of PGs →

3. Increased Negative Chart →

4. Attraction of Water → (stiffens the tissue)

What is the function of biglycan and decorin (“the other PGs”)?

Improves the structural integrity and provides additional stiffness

Compressive force will lead to little _______ or _______ changes —> cells are _________ and no _________ damage to the tissue is observed

Compressive force will lead to little deformation or volume changes → cells are protected and no mechanical damage to the tissue is observed

The ability of PGs to resist compression comes from….

1. The Donnan Osmotic Swelling Pressure associated with the tightly packed anionic group

2. The bulk of stiffness of the collagen-PG complex

What is “weeping”?

Fluid exudation that occurs from compressive loads

During “weeping”, we can lose ___% of the original fluid content.

During “weeping”, we can lose 50% of the original fluid content

T/F: Articular cartilage tissue is very permeable

True

What is permeability?

The measure of ease with which fluid can flow through a porous material

Permeability is inversely proportional to…

The frictional drag of fluid flow

It implies the amount of force required to get fluid moving (through the tissue)

Permeability of a tissue changes with…

• Changes in magnitude

• Rate of compression loading

These changes are important for joint lubrication

When load and rate increases, what happens to permeability?

Permeability decreases exponentially

When load and rate decreases, what happens to permeability?

Permeability increases

What happens when the yield point is reached in articular cartilage?

Failure occurs quickly (fracture patterns do vary)

Why is the toe region in articular cartilage small?

Because it is not reflective of de-crimping

T/F: The toe region of articular cartilage is small and non-linear

True

Once an individual’s articular cartilage tissue reaches the plastic region, what occurs very soon after?

Fracture point

T/F: Articular cartilage deformation is always seen with failure

True

Creep, stress relaxation, and hysteresis are all (more/less) in articular cartilage than in tendon/ligament.

Creep, stress relaxation, and hysteresis are all less in articular cartilage than in tendon/ligament.

In articular cartilage, it takes ______ to reach equilibrium on creep and stress relaxation

In articular cartilage, it takes longer (HOURS) to reach equilibrium on creep and stress relaxation.

What is happening when articular cartilage is trying to reach equilibrium for creep and stress relaxation?

Fluid is redistributing and allowing the load to spread out over a greater surface area

(More/Less) reps are required for hysteresis to reduce?

More reps are required for hysteresis to reduce

Is the “lag” (area between hysteresis curve and typical curve) in articular cartilage bigger or smaller than ligament/tendon

Area between the curves is NOT any bigger than ligament than ligament/tendon

What is occurring during stress relaxation in articular cartilage?

“Weeping” occurs initially followed by fluid redistribution

What is occurring during creep in articular cartilage?

Only “weeping” occurs

How long does it take for equilibrium to be reached for stress-relaxation and creep in articular cartilage?

Takes hours to reach relative equilibrium

Rate of time-dependent behaviors are dependent on…

Fluid exudation rate (it is related to the permeability of the tissue)

What is responsible for tension restriction in articular cartilage?

Collagen

If a shear force is applied and there is no change in volume, what does this mean?

No change in volume = no interstitial fluid flow

Which layer of articular cartilage resists tension?

Superficial Layer

(when you push down/compress → superficial layer is streched out, causing tension)

Rank how articular cartilage deals with tension, shear, and compression forces from best to worst?

1. Compression (handles it “best”)

2. Tension

3. Shear (handles it “worst”)

What plays a significant role in lubrication?

Synovial Fluid (“biofilms”)

What is the fluid-film interaction?

Thin film of fluid that creates a surface separation (“like oiling”)

How does the fluid-film interaction develop?

Probably develops as a result of weeping of articular cartilage

What is the “first responder” in lubrication of compressive loads?

Fluid-film layer

The fluid-film layer partially supports…

Compressive loads (especially fast-rates of loading)

Fluid-film layers can carry ______ loads for _______ durations

Fluid film layers can carry high loads for short durations

Over time, the fluid-film layer _______ with pressure application. What does this lead to?

Over time, the fluid-film layer thins out with pressure application.

This leads to high load areas being unprotected

The fluid-film layer is important for....

• Reducing Friction

• Nutrition of articular cartilage (most important)

T/F: The coefficient of friction for the fluid-film layer is just as good as the boundary layer

False! (coefficient of friction is not as good as the boundary layer)

What is the boundary-layer of lubrication?

A mono-layer of lubrication that is directly on top of articular cartilage

• Consists of hyaluronan (or hyaluronic acid)

• Considered a “concentrated fluid”

What type load, speed, and duration does the boundary layer handle?

• High load

• Low speed

• Long duration

What does the boundary layer prevent?

Prevents surface-to-surface contact and eliminates most of the surface wear (that would occur from daily use)