KNES 363 - lecture 7 cartilage: structure and function

3 types of cartilage

hyaline, elastic, fibrocartilage

hyaline cartilage

most prevalent cartilage found in adults, and includes the articular cartilage that cover joint surfaces

elastic cartilage

found in the external ear, eustachian tubes, and epiglottis. more flexible and elastic than hyaline cartilage

fibrocartilage

found in intervertebral disks, meniscus, tendon-bone attachments. can form when hyaline cartilage is damaged

Hyaline can transition into firbro type of cartilage, becomes a bit stiffer, less permeable and stuff and this is a sign of degeneration

composition of articular cartilage

-extracellular matrix (ECM) composed mostly of type II collagen and proteoglycans (mostly aggrecan)

-ECM is produced by cartilage cells known as chondrocytes that live inside lacunae (just like osteocytes)

-cartilage has no blood or nerve supply

Chondrocytes - brief

active metabolically, but there is no cell-to-cell connections or other means of communication with each other

way less abundant than osteocytes, don't have dendrites, no cell to cell communication. This is how its differentiaed from osteocytes. But we do believe them to be mechanically sensitive and metabolically activated through mechanical loading

cartilage - composition by weight

mostly water (like 70%)

chondrocytes are like 2%

and then ECM is like 20%

ECM components

50% proteoglycans (PGs)

chondrocytes

-<5% of the tissue volume

-metabolically active: synthesis and degradation of the ECM

-obtains nutrition from synovial fluid

-soft: mechanical compression is the signal for metabolic activity

-surrounded by a pericellular capsular called chondron

chondrocytes - my notes

Can synthesize the ECM

Very controversial - the chondrocytes are in some way believed to contribute to the degradation of the cartilage. Controversy is around when can chondrocytes be good or bad

Seems like lots of chondrocyte metabolic activity is really important during growth and development but then something happens in old age where the mote metabolically active they are, they can degrade the ECM

Synovial fluid

Need to get synovial fluid through the deep and middle layers. Love to just get mushed in there and that crushing action to force in synovial fluid

Cells are very soft and deform easily so they can more easily measure the mechanical loads that are applied to the cartilage tissue

extracellular matrix (ECM)

-responsible for the mechanical properties of cartilage

-collagen & proteoglycans : 20-35%

-interstitial fluid (mostly water): 65-80%

All the articular cartilage mechanical properties come from the ECM

collagen

50-60% of solid matrix (ECM)

-type 2 collagen - very high tensile stiffness - hard to pull apart

-fibrillar network

collagen - fibrillar network

-high tensile strength

-high tensile stiffness

-influences permeability

Fibrillar network - like roping a bunch of pieces of rope together to make it stiffer and stronger than before = higher tensile strength = higher tensile stiffness

Increase permeability - how easily fluid can move in and out. The tighter the fibrillar network, the harder it is to get fluid in and out of the cartilage

Proteoglycans (PGs)

-25-35% dry weight

-negatively-charged, mutual repulsion

-provide high compressive strength

these are how something (cartilage) that is 70% water can carry loads

2 main functions of PGs

Hydration: they hold onto the water

Repel: swelling effect to get this compressive stiffness

Each proteoglycan is negatively charged adn repel each other, which creates this swelling effect in the articular cartilage

But also cause they are negatively charged they attract water

main proteoglycan

Aggrecan

-tend to form aggregates

interaction between PGs and collagens

-collagens form fibrillar network

-PGs bind to collagen fibrillar network

-water fills this molecular framework

Very clever organization

PGs wrap themselves around the collagen and then water is attracted to the PGs

Need all three of these things for cartilage to carry load

Doesn't just carry compressive, also carries a bit of shear and tensile

In osteoarthrities we may see increases in water content or decreases in the water content depending on the stage and stuff. At end stage we see huge reductions in the fibrillar network and PGs with way less water, no swelling effect

cartilage structure depending on depth

The shape and the orientation of the ECM and collagen fibers change

Depending on depth

cartilage structure

-non-homogenous (heterogenous)

depth-wise variations

-in contents of collagens and PGs

-in the orientation of collagen fibers

-shape and size of chondrocytes

cartilage structure - zones

zone by distance to articular surface

-4 zones (superficial, middle, deep, calcified)

zone by distance to chondrocytes

-3 zones (pericellular, territorial, inter-territorial)

articular cartilage has 4 zones

The top - the chondroctyes themselves are kinda flat horizontal ellipses, then they get more circular the deeper you go

Can be influenced by mechanical loading environment they grow up in?? A good

Hydrostatic stress like diving deeper into the ocean (from the interstitial fluid) gets worse deeper so maybe that's why they become more circular

superficial tangential zone (STZ)

-tangential fibers

-10-20% of tissue thickness

-highest collagen and water content

-lowest PG content

-collagen fibers oriented parallel to surface

-chondrocytes are elliptical with their axes aligned with the surface

Highest collagen content, lowest PG

Weird because you have the highest water content but has less to do with PGs and more to do with higher permeability by being so close to the synovial fluid

middle zone

-40-60% of the thickness (makes up most of cartilage)

-randomly arranged collagen orientation

-chondrocytes are round and randomly distributed

-highest proteoglycan content

deep zone

-30% of the thickness

-collagen oriented perpendicular to the surface

-chondrocytes are arranged in a columnar perpendicular to the calcified cartilage

collagen are vertical, kinda anchored into the cartilage

calcified zone

- a layer of calcified cartilage anchored to underlying subchondral bone

Transitional zone from bone to hyaline cartilage - calcified cartilage

Calcified cartilage

Where mineralized calcified cartilage are

The tidemark marks this

Deep has non-mineralized hyaline cartilage so its different

calcified zone - osteoarthritis

Osteoarthritis:

If calcium starts moving up, then you start losing articular cartilage and it becomes thinner

So in addition to the loss of collagen fibers, decrease in PGs and water, it just also gets removed from more calcified cartilage

which zone has the highest PG proportion

middle zone

cartilage function

-transfers and distributes loads between bones, thereby lowering joint stress

-allows load-bearing surfaces to articulate with very low friction

-it is "not" a shock absorber as is frequently states

Low friction surface because we rotate the joints all the time when we move (that's how we move) so we constantly have the joints sliding, rotating, translating so we don't from friction there (would create energy loss, heat loss and damage)

One of the most important roles

cartilage - a shock absorber?

NOT A SHOCK ABSORBER

Doesn't deform quickly or rapidly or well under quick impact loads. It would help dissipate the energy of a high impact event if it was a shock absorber BUT ITS NOT so it doesn't do this

mechanical properties of cartilage

-inhomogeneous

-biphasic (fluid phase and solid phase)

-viscoelastic (time-dependent response)

-anisotropic (different properties in all directions)

Biphasic

Really just means some of the load is bourn by the water and some by the collagen & proteoglycans (solid stuff)

This means it is viscoelastic - time-dependent responses - some times will take load with water and sometimes with solid

Anisotropy

Global term to say directional dependence

cartilage - inhomogeneous

-depth dependent modulus

-Because the PG content and the collagen vary depending on depth

HIGHER MODULUS THE DEEPER U GO

biphasic behavior

-articular cartilage is considered a fluid-filled biphasic porous permeable material

-fluid-saturated porous material

-two phases: fluid (interstitial fluid) and solid (collagens, PGs, and cells)

It is a sponge

The solid parts are the solid parts of the sponge. The porous material that have holes inside of it

The fluid portion is the water in the sponge

This makes it a biphasic, fluid-saturated material

viscoelastic material

-property of materials that exhibit both viscous and elastic characteristics when deformed

-depends on the internal friction of its component molecules as they slide by one another

-depends on the speed of the movement

whats more viscoelastic - cartilage or bone

Cartilage is much more viscoelastic than bone

Viscoelasticity depends on the international friction and the speed of the movement

In cartilage the internal force is generated by the interaction between the fluid and the solids

viscoelastic materials demonstrate (5)

-creep

-stress relaxation

-effective stiffness load dependent

-hysteresis when load applied

-energy loss during impact

creep test

Creep test

Take material and apply a fixed stress and you hold it there

Then you monitor the deformation response (strain response)

Will see this gradual increase in deformation and then at some point bottoms out at a new equilibrium(this is a sign of a viscoelastic material). If it doesn't illustrate this, its not a viscoelastic material

stress relaxation

constant length and then seeing how much force can be produced

The stress shoots up really really fast then gradually relaxes over time

Viscoelastic materials demonstrate this

cartilage viscoelasticity during loading

-fluid escapes cartilage

-ECM starts to compact

-load borne mainly by fluid phase

-dynamic modulus

cartilage viscoelasticity during equilibrium

-fluid flow stops

-load borne mainly by solid phase

-equilibrium modulus

cartilage viscoelasticity - my notes

luid saturated, permeable material then load it

As the fluid escpaes the cartilages, the ECM starts to prepare to take more of the load. At the initial stages the load is bourn mostly by the water but as the fluid spills out, the ECM starts to take more of the load

Non-linear modulus becasue the stiffness changes over time as the fluid starts to exude out of the material

Then reaches a new equilibrium when the fluid stop exhuding out, the fluid pressure and the internal stress bourn by the cartilage is equalled to the applied stress/loading

cartilage creep

-compressive load

As the fluid exhudes, the cartilage deforms more and more and more

Then slows down over time, new equilibrium state is reached

Fast at the beginning then slows down

cartilage creep - mechanism

-fluid exudation is most rapid initially (initial rapid rate of deformation)

-fluid exudation decreases gradually until it completely ceases

-load is equilibrated by solid matrix and the fluid friction

-at equilibrium: load is balanced by the solid phase alone

when the load is eliminated, cartilage slowly return to its initial volume

cartilage creep - the speed of fluid exudation depends on what

on the permeability of the cartilage which depends on:

-the amount of water (to begin in)

-the amount of PG and collagen

collagen creep - my notes

Fluid escapes because the collagen fibrillar network is only so tight but then also when you start to apply load, the ability for the PGs to hold onto water is only sp strong

So this is why we lose fluid

It would just shoot the fluid up into out joint capsule

cartilage stress relaxation

compressive load

Displace this thing very rapidly, stress shoots up before all the fluid can exude. Its all bourn by the water but then afterwards the fluid starts to exude very slowly out of the material and then the stress starts to decrease

cartilage stress relaxation - mechanism

-quick rise of stress: fluid efflux and PG and collagen compaction

-relaxation: matrix redistribution

-equilibrium: external load is equal to the intrinsic compressive resistance of the solid phase (PG and collagen)

strain rate dependence

Slope of the stress strain curve is different depending on the speed of loading

Faster you load it, the stiffer the modulus

Has to do with the permeability of the articular cartilage

Permeability increases with decreases in strain

Higher the magnitude of loading, the decrease in permeability

permeability

-permeability of articular cartilage decreases with increasing strain

-increasing compaction of the solid matrix at the surface of the cartilage: decreasing the diameter of pores and increasing intrinsic friction

-cartilage becomes more resistant and stiffer

permeability - my notes

Has to do with the permeability of the articualr cartilage

Permeability increases with decreases in strain

Higher the magnitude of loading, the decrease in permeability

Cartilage loves being loaded fast

More of the water phase is loaded, the faster it is loaded

The higher the applied strain, the lower the permeability because the fibers just get all strung together

loading of cartilage

-vary from zone to zone

-related to proteoglycan concentration

-least on the surface and highest in the middle (transitional) zone

major surface function is therefore not to resist compression

Loading (strain)

Lowest on the surface and highest on the middle zone

Major function of the surface is probably not to resist compression, more likely to do with getting fluid into the cartilage to get it deeper in

cartilage is loaded under... (3)

-compression

-tension

-shear

tensile loading

-physiological loading about 15%

-young's modulus 3-10MPa

We can test tensile strength of the collagen fibrillar networl

Physiological loading is dependent on what activity you are doing but in general its right after toe region

tensile loading - toe region

the toe region in the stress-strain curve is due to the reorientation of collagen fibers in the direction of the tension

The collagen fibers at first are randomly oriented - wont carry load until it starts to get stressed

Unravelling the fibers and then at some point they start to carry load

tensile properties

-high anisotropic due to different orientation of collagen fibrils

-strength (parallel) >>> strength (perpendicular)

-collagen fibers become realigned when loaded

-tensile strength of cartilage is far less than tensile strength for tendon or ligaments

Parallel is strongest

Fibers weakest in carrying load in the perpendicular direction