Articular Cartilage and Lubrication

Question 1

What makes up hyaline cartilage?

Answer 1

• Cells- CHONDROCYTES
• Extracellularly matrix
  
  • collagen mainly type 2
  • Elastin
• Ground glass
  
  • ** water**
  • **PROTEOGLYCANS and glycosaminoglycans **
    
    • aggrecan
    • Hyaluronan
    • decorin,byglan
  • Glycoproteins
    
    • cartilage oliogomeric protein ( COMP)
    • Cartilage matrix protein (CMP)
  • Degradation enzymes
  • Extracellular ions

Question 2

What is the function of hyaline cartilage?

Answer 2

• To distribute weight bearing forces and reduce friction

Question 3

Describe the properties of ARTICULAR cartilage?

Answer 3

• AVASCULAR
• ANEURAL
• Alymphatic
• Almost non immunogenic

Question 4

What is the main constituent of the extra cellular matrix?

Answer 4

• Water 75%
  
  • Which is held In place by PROTEOGLYCANS (e.g.aggrecan) 10-15%
• Collagen type 2 fibres
• CHONDROCYTES
  
  • ​manufacture and maintain the extracellular matrix

Question 5

Cent you name all the layers of the ARTICULAR cartilage?

Answer 5

1. Superifical Gliding zone
   
   • collagen parallel to surface and CHONDROCYTES- resist shear forces
   • thinnest layer, highest water + collagen concentration
2. Middle Transitional zone
   
   • mixture of oblique collagen fibres + CHONDROCYTES
   • protepglycan concn highest
   • transition between shearing and compression forces
3. Deep Radial zone
   
   • vertical collagen fibres
   • largest part of articular cartilage
   • resists compression
4. ​ TIDEMARK
   
   • • boundary between calcified and uncalcified cartilage
5. Calcified zone
   
   • ​​hydroxyapatite crystals anchor cartilage to bone - barrier to diffusion from blood vessels supply the subchondral bone
   • type X collagen here

Question 6

What is the role of chondrocytes in the middle transitional zone and deep radical zone of articular cartilage?

Answer 6

• Produce all the components of the Extracellular matrix
• no intercellular junctions between chondrocytes, communites of 2 or more cells form chondrons

Question 7

Where is collagen produced?

Answer 7

• Within and outside the CHONDROCYTE
• Polypeptide chains formed from mRNA translation within the rough endoplasmic reticulum.
• Signal peptide is cleaved and modified polypeptide chain from a triple helical molecule.
• Disulphides bonds determine its shape
• Within Golgi apparatus the resultant procollagen is packed into secretory vesicles and released into the ECM via micro tubules.
• Outside the cell the terminal ends of pro-collagen uncoil and cleaved -> Tropocollagen fibrils.
• These combine via cross linkage of LYSINE and HYDROXYLYSINE residues -> COLLAGEN FIBRES

Question 8

What is the type of collagen in ARTICULAR cartilage ?

Answer 8

• Type 2 90%
• Type X is found in calcified zone
• The core of the collagen is formed by II and XII
• **type IX ** found on surface of the fibre

Question 9

What is the role of the PROTEOGLYCANS ?

Answer 9

• Water content of the cartilage

Question 10

What are PROTEOGLYCANS ? How does their structure allow them to function?

Answer 10

• Large hydrophilic molecules containing chains of GLYCOSAMINOGLYCANS (GAG)- chondroitin sulphate and keratin sulphate bound by sugar bonds to a linear core.
• The gag’s have a negative charge from attached carboxyl and sulphate groups- increasing their osmotic pressure

Question 11

What is the most common PROTEOGLYCAN in ARTICULAR cartilage?

Answer 11

• Aggrecan
• which is Heavily GLYCOSYLATED with GAG components such as chondroitin sulphate and keratin sulphate
• aggrecan interacts with hyaluronic acid , stabilised by link protein to form a large proteogylcan aggregate up to 50x10⁶ MW

Question 12

What are the degradation enzymes ?

Answer 12

• Collagenases, stromelysins, gelatinases and membrane -associated metalloproteinases
• which degrade collagen and PROTEOGLYCAN aggregates as part of normal turnover of the matrix

Question 13

What prevented degradation of cartilage by metalloproteinases ?

Answer 13

• Tissue induced metalloproteinases inhibitors
• are acidic polypeptides that prevent degradation by metalliproteinases by binding to the matrix proteins
• the avascular nature of articular cartilage is maintained by TIMPs that inhibit proteinases prodcued by migrating vascular endothelium

Question 14

What is the ion make up in ARTICULAR cartilage?

Answer 14

• Typically- High sodium and potassium ion content
  
  • the sulphate ion on the proteoglycans attract these cations.
• Extra cellular Ca high in calcified zone
  
  • ​low in superifical gliding and deep radical zones

Question 15

What is the role of glycoproteins in the cartilage?

Answer 15

• They are in the ECM and act like ‘glue’ binding various constituent s of the matrix and CHONDROCYTE surface
  
  • ​ E.g. COMP ( which binds to various matrix proteins), CHONDROCALCIN

Question 16

Where do CHRONDOCYTES originate ?

Answer 16

• MESENCHYMAL stem cells
• Majority of cartilage changes to bone thru ENDOCHONDRAL ossification , with growth plate and cartilaginous epiphysis persisting after birth

Question 17

What happens to CHONDROCYTES in osteoarthritis ?

Answer 17

• chondrocytes mimic the events of endochondral bone formation by
• CHONDROCYTE proliferation, hypertrophy and expression of type x collagen , alkaline phosphatase, matrix vesicles and matrix calcification

Question 18

What is the metabolic rate of the CHONDROCYTES in ARTICULAR cartilage?

Answer 18

• Generally very low
• Deep cartilage zones contain chondrocytes with decreased rough endoplasmic reticulum and increased degeneration products

Question 19

What influences CHONDROCYTES ?

Answer 19

• Growth factors - fibroblast growth factor stimulates adult CHONDROCYTES DNA synthesis.
• PTH and Thyroxine stimulate matrix synthesis

Question 20

What is the mania function of ARTICULAR cartilage?

Answer 20

• Joint lubrication
• Shock absorption - distribute joint loads and so reduce stress

Question 21

What is articular cartilage coefficient of friction?

Answer 21

• V low - 0.002
• X30 smoother than most joint replacements!
• lowered by
  
  • fluid-film lubrication
  • elastic deformation of articular cartilage
  • synovial fluid
  • efflux of fluid from cartilage
• increased by
  
  • fibrillation of articular cartilage

Question 22

What are the BIOMECHANICAl properties of cartilage?

Answer 22

• Biphasic material that is Viscoelastic
  
  • which undergoes CREEP, STRESS RELAXATION, HYSTERESIS AND STRESS/strain rate.
  • these occur thru macro molecular and water movement
  • cartilage is freely permeble to water but under high compressive loads water movement is hindered by the frictional drag of moelcules- decreases flow, stiffens allowing greater resistance to higher loads
  • ANISTROPHIC- different mechanical properties depending on direction of the load
    
    • due to collagen arrangements, x links and collagen-PROTEOGLYCAN interactions
    • • in tension cartilage pulled apart, increases water permeability and decreases the compressive stiffness

Question 23

What are the main types of lubrication?

Can you describe the 2 main types in engineering?

Answer 23

• Fluid film
• Boundary
• **Hydrodynamic **
• Squeeze film
• Elastohydrodynamic
• **weeping **
• **Boosted **

​

• Boundary
  
  • involves a MONOLAYER of lubricant molecule absorbed in each surface of the joint.
  • This prevents DIRECT ARTICULAR contact -
  • NB IMPORTANT AT REST/ Under load
• Fluid film
  
  • a THIN LAYER OF FLUID INCREASES THE SEPARATION OF THE 2 Surfaces

Question 24

Describe the types of lubrication?

Answer 24

• HYDRODYNAMIC
  
  • 2 surfaces are at an Angle to each other
  • the viscosity in the resulting wedge of fluid separates the 2 surfaces
• SQUEEZE-FILM
  
  • 2 surfaces are parallel and move perpendicular to each other
  • the viscosity of the incompressible fluid maintains lubricate.
  • As the load increases the layer of fluid is forced out so a thin layer remains to prevent surface contact
• ELASTOHYDRODYNAMIC
  
  • as speed increases the ARTICULAR surface creates a larger surface area when compressed by the fluid.
  • There is less dissipation of the fluid and load sustained for a longer period
  • main lubrication in dynamic movement.
• WEEPING
  
  • as ARTICULAR cartilage of the joint slides under compression, fluid is exuded under and infront of the leading edge of the load , enhancing lubrication.
  • As load decreases water is once again imbibed and the ARTICULAR cartilage reforms it’s shape
• BOOSTED
  
  • the solvent part of the lubricant enters the ARTICULAR cartilage which leaves behind the concentrated hyaluronic acid complex as a lubricant in trapped pools of concentrated synovial fluid.

Question 25

What happens with a Superificial laceration to ARTICULAR cartilage above the TIDEMARK ?

Answer 25

* It doesn't heal as completely **AVASCULAR**

Question 26

What happens to ARTICULAR cartilage lesions that are below the TIDEMARK ?

Answer 26

* Results in **haematoma, fibrin, clot and activation of the inflammatory response**-\> this acts as a **scaffold** for the formation of **fibrocartilage** * Produced by **undifferentiated MESENCHYMAL cells** that have migrated into the defect- this is different from hyaline cartilage as **disorganised bundles of type I collagen** - so not suitable for repetitive load bearing

Question 27

What biochemical changes happens with the aging process?

Answer 27

Water decrease - shrivel as age! Synthetic activity decrease COLLAGEN UNCHANGED PG Content decreases - length of protein core and GAG decrease PG synthesis decrease PG degradation - decrease Chondroitin sulphate- decrease KERATAN SULPHATE INCREASES CHONDROCYTE SIZE INCREASES Chondrocyte no decrease YM INCREASES

Question 28

What effect does infection have on cartilage?

Answer 28

* Direct result of organism itself e.g. chondrocyte protease of staphylococcus aureus or due to host's inflammatory response * polymorphs stimulate production of cytokines and other inflammatory products-\> hydrolysis of collagen and proteogylcans * destruction of cells and release of lysosomal enzymes- collagenases, proteases further injures the joint

Question 29

What happens to the biochemical changes In ARTICULAR cartilage with osteoarthritis?

Answer 29

* **WATER INCREASES**-90% cf n 65-80%\* * this softens articular cartilage-\> **_decreased Young's M_** * **reducing its ability to bear load** * Increased permeability-\> **loss of lubricant-\> increased interfacial wear/ fatigue wear** * -\> disruption of collagen-proteglycan matrix/ leaching out of protoglycan with large fluid movements * **_decrease in collagen but relative conc increase due to loss of proteoglycan_** * **_PG degradation increases significantly_** * rapid repitive high loads with no time for stress relxatn-\> CP matrix damage so Chrondrocytes attempt to restore by \*\* **_SYNTHETIC ACTIVITY INCREASES_** * **_PG synthesis increase_** * **_CHONDROTIN SULPHATE - increase_** * **_Keratan sulphate- decrease_** * **_Enzymes- increase activity_** * **_Matrix subunits - increase levels_**

Question 30

What other biomechanical factors does an increase in permeability and reduction in PG concentration have on the cartilage?

Answer 30

* Loss of lubrication-\> increase interfacial wear Cartilage bearing surface deformation occurs under load with repetitive stressing leading to fatigue wear and accumulative microscopic damage

Question 31

How does fatigue wear occur?

Answer 31

* Thru high stress and low cycle loading or low stress thru high cyclical loading

Question 32

How does fatigue wear cause damage in the osteoarthritic cartilage?

Answer 32

* Disruption of the collagen- pg matrix with increasing age * Leaching out of pg by repetitive large interstitial fluid movements in the Superificial layer, leading to increased permeability and decreased stiffness * Rapid repeat high loading where there is no time for stress relaxation leading to collage matrix damage * Chondrocyes try to compensate by increasing their rate of DNA synthesis, collagen and pg- proliferate and hypertrophy initially but eventually pg decrease and chains become shorter.

Question 33

What macroscopically is seen on the surface if a damaged ARTICULAR cartilage?

Answer 33

* Fibrillations - vertical splits then development of deep fissures

Question 34

What are the treatment options for ARTICULAR cartilage damage?

Answer 34

* Non operative * **physical therapy** * **loading of joint essential** to allow diffusion of synovial fluid and metabolic turnover of cartilage. * IMMOBILSATION -\> cartilage atrophy * Oral visco-supplementation- **GLUCOSAMINE-** amino-monosaccharide sugar, naturally occurring **component of keratan sulphate and hyaluronate** * **​** In vitrio studies have shown the effect of glucosamine in **reversing the inhibition of PROTEOGLYCAN synthesis by IL-1 and suppressing the inflammatory response of neutrophils** -v low risk of side effects ( shellfish allergy) 1500mg/day * INTRA-ARTICULAR visco supplementation- replace lost hyaluronate and improve Viscoelastic properties of ARTICULAR cartilage but-publication bias, poor numbers and Outcome measures

Question 35

What are the surgical options for tx?

Answer 35

* **ABRASION ARTHROPLASTY** = cartilage repair from subchondral bone * turn defect into deep injury- **undifferentiated MESENCHYMAL cells proliferate in the defect-\> fibrocartilage and initially type 2 collagen** but changes to type 1. * Contained defects \< 2cm * **Autograft/MOSAICOPLASTY** * **​**full thickness osteochondral grafts from the least weight bearing periphery of the ARTICULAR surface- _superiormedial margin of the femoral notch are transplanted into the cartilage defect._ * Most benefit **medial compartment knee** cf patella- small lesions \< 2cm square due to donor site risks * **Allograft** - * caderveric cartilage usually frozen with cryoprotectant- glycerol which prevents rupture of the cell membranes during freezing process. * Only some CHONDROCYTES will remain after thawing. * Bony part of graft acts as scaffold therefore import at same size as defect * .\>3cm defect * **Periosteal/PERICHONDRAL MESENCHYMAL stem cell** * Peroiosteum and perichondrium both have sig no of stem cells that could-\> hyaline cartilage. * Periosteum ( cambrial layer) most chondrocytes positioned ) from rib sutured onto bone graft- * type 2 collagen and pg see in this repair * **CHONDROCYTES from MESENCHYMAL stem cells** * MSc located In bone marrow using tissue culture -the stromal cells inc MSC adhere and cultured. * Using periosteum - msc released from cambrium layer using enzymetic tx. * MSC placed in 3 dimensional matrix ( collagen /alginate) they differentiate into CHONDROCYTES . * These continue to differentiated and introduced into collagen gel which is placed into defect. * forms a hyaline cartilage * **Autologous CHONDROCYTE implantation** -ACI * A small amount of cartilage from nwb area harvested, cells grown. * CHONDROCYTE proliferation low in vivo,but in tissue culture CHONDROCYTES no's expand over 4/5 wks, * then suspended in gel carrier and re implanted . * A periosteal/ collage flap sutured over the top. Can't use In patella femoral joint OA if joint space narrowing - as need whole surround cartilage to protected aci * **Maci- matrix induced chondrocyte implantation-** growing cells of cartilage membrane and suspend in 3d hyaluron based scaffold or polymer fleece which is implanted, no need for cover

Question 36

What is the classification system for cartilage defects?

Answer 36

* Outerbridge * 0= normal * 1-softening and swelling * 2-partial defect , * 3- fissure diameter 1.5 cm * 4- exposed subchondral bone

Question 37

What happens to the biochemical changes in aging process of articular cartilage?

Answer 37

* water content **decreases** - "old and wringly" * synthetic activity - decreases * collagen- unchanged * PG content- decrease length of protein core and GAG chains decrease * PG synthesis- decreases * PG degradation- decreases * Chondrotin sulphate decreases * **keratan sulphate- increases** * **chondrocyte size increases** * chondrocyte number decreases * **young's modulus- increases- stiffer**