cartilage structure and mechanical behaviour

Question 1

role of PGs

Answer 1

repel each other with negative charge plus attract positive charges leads to donnan osmotic pressure, influx of water, keeps cartilage hydrated.
keeps cartilage pre-stressed

Question 2

combined function of collagen fibres and PGs

Answer 2

negative charge between PGs leads to a repulsive force causes tissues to swell
cartilage is pre-stressed
tension in the collagen fibres ensures it doesn’t swell too much and maintains structure,
low permeability water doesn’t flow in easily - difficult to move water in cartilage - the high water content and low permeability means there is a high comrpessive strength

Question 3

features of cartilage

Answer 3

avascular, aneural, alymphatic, hypocellular, 0.5-5mm thick

Question 4

collagen structure

Answer 4

triple alpha helix
rope like structure, good tensile strength, cross-linked for stability and immobolize PGs in mesh
arranges into sheets that form arcs

Question 5

components of articular cartilage

Answer 5

• collagen II, chondrocyte, aggrecan, PGs
  water and electrolytes
  hierarchical structure

Question 6

PGs

Answer 6

negatively charged, core protein with one or more covalently attached sugars, backbone of GAGs

Question 7

Aggrecan

Answer 7

negatively charged protein, most common GAG, aggregates on hyaluronan,

Question 8

types of cartilage

Answer 8

hyaline/articular - most commonly nose ribs etc

elastic cartilage - epiglottis, eustachian tubes, contains elastin protein

fibrocartilage - meniscus etc, can form as a result of pathology to the articular cartilage

Question 9

chondrocytes

Answer 9

engine of the cartilage
responsible for synthesis, modification and assembly of PGs.
Synthesis and secretion of collagens
degradation and matrix turnover
balance of anabolic and catabolic activity
controlled by growth factors, balance disturbed under pathology.
chondrocytes live a long time
don’t continually renew/regenerate - VERY LITTLE CELLULAR TURNOVER

Question 10

cartilage gets nutrition?

Answer 10

yes, through diffusion, from synovial fluid in the synovial joints, only very small moelcules can diffuse,
convective transport for larger molecules,
pressure causes larger molecules to be forced into cartilage

Question 10

layers/structure of articular cartilage

Answer 10

moving down get a lower density of cell and the in deep zone there is a columnar arrangement
Arrangement is.
1. Super rich superficial tangential zone
a. Collagen fibres are aligned with articular surface in the superficial tangential zone
b. Become more randomly organised
c. By the time they reach the deep zone going into subchondral bone they are aligned perpendicular to the subchondral bone)
2. Middle zone
3. Deep zone
4. Calcified zone (right next to bone surface)
5. Tide mark (lies between calcified zone and subchondral bone layer)

By putting a greater water content of PGs in the deep zone, are encouraging positive ions to be drawn into the structure to bring as much water deep into the cartilage as possible.

Question 10

cartilage function

Answer 10

support, elasticity, bear load and impact, spread load, friction lubrication, deforms when impacted, excellent wear characteristics

Question 11

what affects cartilage mechanical properties

Answer 11

compression
tension
shear
timescale
permeability
pressure and charge denisty
hydration

Question 12

compressive stiffness in cartilage

Answer 12

NO EFFECT OF COLLAGEN CONTENT
all because of GAGs
aggregate equilibrium modulus
measure of stiffness of the solid matrix, independent of fluid flow, highly loaded regions are stiffer in compression and have higher PG content

Question 13

tension in cartilage

Answer 13

• load taken by collagen
• not affected by PGs
  superficial stiffer than middle and deep
  not loads of loading in tension in cartilage - mainly compressive
  cartilage fibres increasingly align, once aligned start taking strain.

Question 14

shear loading in cartilage

Answer 14

• more common than tensile
  under rapid loading may occur due to differences in compliance between bone and cartilage
  uncommon in adults, common in chidlren due to structural differences between cartilage and bone.
  no volumetric change tehrefore no fluid flow,

Question 15

time effects in cartilage

Answer 15

cartilage is biphasic - fluid + solid (water + ecm)
when apply pressure, fluid leaves and enters synovial space

Question 16

rate dependent cartilage behaviour

Answer 16

elastci behaviour - rapid loading, no fluid flow
stiffens with increase strain
it takes time to push water out of cartilage and its not very permeable therefore not a lot of fluid flow so stiffer when apply load quickly.

Question 17

permeability

Answer 17

determines flow of fluid under load within (in and out) of cartilage
cartilage permeability is very low
varies by zone
highest at superficial zone, lowest at deep zone
permeability varies with tissue deformatino
as cartilage is compressed its permeability decreases.

AS TISSUE COMPRESSED POROSITY DECREASES AND DENSITY OF NEGATIVE CHARGE INCREASES - MORE COMPRESSED –> MORE DIFFICULT TO SQUEEZE OUT FLUID

Question 18

PRESSURE AND CHARGE DENISTY

Answer 18

compression affects PG netwrok
1. forces negative charges closer together
2. forces water out with dissolved positive ions

Question 19

hydration

Answer 19

water content increase - less stiff + more cartilage
osteoarthritic cartilage has more water
As water increases, cartilage becomes less stiff, more permeable, increased pore size, water can move in, too much water means porosity increased water can move in and out easily increases degradation.
Don’t know if osteoarthritis has degraded so has more water or has more water so is degraded but they are connected.

Question 20

testing cartilage

Answer 20

INDENTATION
UNCONFINED COMPRESSION
CONFINED COMPRESSION
unconfined - water comes out of sides as well as top
confined - piston has porosity so water can flow through more accurately