articular cartilage

where is articular cartilage found?

articulating surfaces of synovial joints

another name for articular cartilage

hyaline cartilage

articular cartilage protects synovial joints from?

compression & shear

cartilage is essential for?

absorbing impact energy/force

articular cartilage distributes impact forces over?

a larger area

articular cartilage provides additional time to?

transfer momentum

at rest, articular cartilage is?

swollen

to absorb impact energy, articular cartilage?

slowly leaks fluid

articular cartilage reduces compressive damage by?

distributing contact forces over a wider area; less stress

articular cartilage reduces shear by?

providing a smooth & well-lubricated surface

abrasion

material removed from surface as if rubbed by sandpaper

adhesion

surfaces stick/hook/bind together at one location and then separate by tearing at other locations

articular cartilage minimizes friction through?

• fluid pressurization

• lubricating protein attaches to surface

• smooth surface

articular cartilage characteristics

• ~1-6mm thick

• avascular

• no lymph drainage

• not innervated

• very few cells

composition of articular cartilage

• chondrocytes

• extracellular matrix (type II collagen)

chondrocytes are responsible for?

making and maintaining the extracellular matrix

what does the collagen network trap?

the proteoglycans

what do the proteoglycans trap?

the fluid content

poisson effect

compression on one axis causes tension on other axes

does collagen resist compression along its length?

NO, it crimps or folds

how does collagen indirectly resist compression?

resists tension on other axes

the structure of what defines functionally distinct zones of articular cartilage?

collagen

superficial zone

densely woven collagen fibers parallel to articular surface; resists tensile component of compressive load

middle zone

more space between randomly oriented collaged fibers

deep zone

fibers bundle vertically and anchor into bone below; cross tidemark into calcified cartilage and subchondral bone

how many layers at birth?

one, layers develop in response to forces

what are essential to the structure & function of articular cartilage?

proteoglycans

what charge are GAGs?

negative charges to repel each other, creating space to trap fluid

most important PG for articular cartilage

aggrecan - form larger structural units by binding to hyaluronic acid

other PGs that contribute to structure & stiffness?

biglycan & decorin

what reduces the sugar/protein ratio which reduces the capacity to trap fluid?

aging

what is also essential to the function of articular cartilage?

fluid content

why is it essential for the fluid to move when loaded?

• nutrient exchange

• shock absorption

• joint lubrication

articular cartilage is what kind of structure?

porous-permeable structure

porous

can hold a lot of fluid

low permeability

very large frictional drag forces resist fluid flow

what deforms PGs?

compression

what has to happen before PGs can deform?

fluid has to get out of the way

what happens when the load is removed?

GAGs repel each other, creating space & negative swelling pressure to pull fluids back in

what contributes to resisting compression in articular cartilage?

• interstitial fluid pressurization

• stiffness of collagen-PG matrix

• Donnan osmotic swelling pressure from crowded ionic groups

what do frictional forces cause in articular cartilage?

shear

what is the first responder to compressive loads?

fluid film lubrication (large, fast, & brief loads)

what is the backup when & where the fluid film gets too thin?

boundary layer lubrication (longer & slower loads)

viscoelastic behavior of articular cartilage

creep & stress relaxation

what does the rate of time-dependent depend on?

fluid exudation rate

how long does it take creep & stress relaxation take to reach relative equilibrium

hours

what does structural damage that expands the pores due to time-dependent behavior?

speeds up

stress relaxation

waiting for fluid movement to decrease fluid pressure

creep

once the swelling goes down, there is less resistanceto relative motion

when are the points measured on a stress-strain curve?

after stress-relaxation finishes

mechanism of failure?

generally tensile failure of secondary axes

types of breakdown

• traumatic mechanical breakdown

• progressive mechanical degeneration

• enzymatic breakdown

any mechanism of failure will?

increase permeability, making it more susceptible to all other forms of damage

articular surface can?

fray, crack, or split

inferfacial wear (superficial layer)

• abrasion

• adhesion

• crack, fray, split

• increase permeability & softening of cartilage

middle layer is more susceptible to?

fatigue failure

what does failure generally start with (middle layer)?

disruption of randomly oriented collagen fibrils

anything that increases permeability impairs what?

the best defense against sudden impacts - makes articular cartilage more susceptible to failure

what causes cells to become more susceptible to cell death?

compression

partial thickness tear

does not penetrate the subchondral bone; cells from synovium may try to promote healing

cracks will fill with what in a partial thickness tear?

fibrous scar tissue

full thickness tear

tear reaches bone; some bleeding; chance for partial repair

cracks will fill with what in a full thickness tear?

fibrocartilage

what is the leading cause of cartilage injury in children & adolescents?

acute trauma

two factors in children/adolescents that cause acute trauma?

obesity or lack of activity

what trauma does sport specialization cause?

acute trauma

what other injuries are involved with acute trauma?

ligamentous, meniscal tears, patellar dislocations, etc.

chondral lesions

injuries to the articular cartilage

who is more likely to have injuries to articular cartilage that do NOT extend to the subchondral bone?

adults

chondral lesions present as?

pain that increases with weight bearing, swelling (effusion & edema), loss of ROM, locking or catching, and instability

osteochondral lesions

any injury that includes damage to both articular cartilage & subchondral bone

osteochondritis dissecans

a specific type of osteochondral lesion where a necrotic piece of subchondral bone breaks off with the cartilage that covers it; thought to involve a combination of contributing factors

why are injuries more likely to extend to bone in juveniles?

bone are still maturing & cartilage hasn’t differentiated into non/calcified layers yet

osteoarthritis involves what kind of inflammation?

chronic inflammation

what degrades collagen?

matrix metalloproteinase

what degrades aggrecan?

aggrecanase

what is the degenerative cycle of osteoarthritis?

mechanical weakness → excessive strains —> mechanical damage —> inflammatory response —> more mechanical weakness

mechanical theories of what starts the process of degenerative joint disease?

• overloading from a single insult or cyclic fatiguing

• less than ideal articular surface interaction

• recovery can't outpace ongoing damage that results from loss of structural integrity

chemical theories of what starts the process of degenerative joint disease?

hormone, proteinase, enzyme, etc.

any issues with the matrix will what?

reduce the resistance to fluid flow

loss of protection will shift more load where?

subchondral bone

the bone will respond by? (after loss of protection)

getting stronger through calcification

calcification makes bone what?

less able to absorb energy, reflecting more load back to articular cartilage

why is it very heard to heal articular cartilage

tissues get stuck in a cycle of degeneration

clinical signs & symptoms

pain & stiffness in joints that worsen w/ activity & are relieved by rest: pain worsens & stiffness improves

prolonged OA s/s

deformity & reduced ROM

what do ACL injuries cause with articular cartilage?

near or long-term problems

ACL injury increases the likelihood of OA by?

3-fold

treatments may include?

• NSAIDS

• steroids

• chondroitin sulfate

• hyaluronic acid injections

• glucosamine

• platelet-rich plasma injections

• bone marrow aspirate concentration

• exercise

NSAIDS

short-term use

steroids

always make the tissue worse; reduce pain & inflammation; decrease collagen synthesis & decrease up-take of glycine; kill chondrocytes

chondroitin sulfate

provides mild pain relief & functional improvement when taken as a supplement; trying to counteract decreased sugar/protein ratio of aggrecan with aging

hyaluronic acid injections

direct injection into joints; thicken synovial fluid & form larger groups of aggrecans

glucosamine

targets proteoglycans & lubricating glycoproteins

platelet-rich plasma injections

hoping activated platelets will release growth factors; improve function of chondrocytes

bone marrow aspirate concentration

concentrated stem cells from iliac or tibial bone marrow; stem cells become new chondrocytes

exercise

w/out load, atrophy occurs; cyclic compressive forces

cyclic compressive forces lead to?

• better diffusion of nutrients to articular cartilage

• thicker articular cartilage

• more matrix synthesis

exercise prescription

3-5x/wk min; slow rates of low load; 15-20 min per session; weight bearing cyclic exercise

synovectomy

removal of inflamed synovium