Intervertebral discs

Question 1

When does the vertebral columna and disc occur in development?

Answer 1

• At week 4 gestation

Question 2

What is each vertebra formed form?

Answer 2

• 2 adjacent sclerotomes ( mesodermal cell layer)
• the caudal portion of each sclerotome segment binding to cephalic half of subjacent sclerotome, forming the precartilaginous vertebral body

Question 3

What does the notocord form?

Answer 3

• A sturcture derived from the endothelial germ layer, regresses in the region of the vertebral body, enlarges in the disc to -> Nucleus pulposus
• this then becomes surrounded by circular fibres of the annulus fibrosis

Question 4

Can you decribe macroscopically the disc?

Answer 4

• The outer annulus fibrosis and the inner nucleus pulposus

Question 5

What is the anatomy of the vertebral disc throughout the spine?

Answer 5

• Become larger caudally
• Cervical spine
  
  • disc thicker in anterior portion- contribute to lordosis
• Thoracic spine
  
  • uniform in height
  • thicken caudally ( possibly allowing increased mobility)
• Lumbar spine
  
  • discs thicker anterior portion max at L5/S1
  • combined with vertebral shape contribute to lumbosacral angulation
  • **largest disc at L4/5- the most avascular disc

Question 6

What is the annulus fibrosis consist of?

Answer 6

• Densely packed layers of TYPE 1 Collagen = Lamellae
• this give it form and tensile strength, along with fibroblast or fibrocyte like cells
• Layered fibres orientated at about 30 degrees to the horizontal, the direction alternating with each layer, enabling the disc to resist both distraction and shear forces
• a larger fibrocartilaginous inner annulus fibrosis layer is found more centrally, containing chondrocytes and a less dense predominant type 2 collagen
• attached to both anterior and posterior longitudinal ligaments
• attached to vertebra by Sharpey’s fibres

Question 7

What is the nucleus pulposis consist of?

Answer 7

• lies mostly centrally
• Type 2 collagen in a mucoprotein gel rich in polysaccharide and proteogylcan matrix ( consiting of central hyaluronan filaments and mutliple aggrecan molecules, satbilised by a link protein)
• this gives it visco-elasticity, stiffness and resistance to compression through its interaction with water
• the concentration of type 2 increases towards the nucleus propulsis
• collagen V, VI and XI present
• high porportion of collagen I and 2 is x linked with pyridinoline residues - vital to maintain cohesiveness

Question 8

How can the disc be divided regions?

Answer 8

• the outer third and inner 2/3rds
• the outer third, the disc is anchored to the vertebra by Sharpey’s fibres -> a ring apophysis
• the inner 2/3 layers curve into and form the fibrocartilaginous component of the vertebral end plate covering the superior and inferior surfaces if the disc

Question 9

What does the endplate consist of?

Answer 9

• Hyaline cartilage in children/young adults
• calcified bone in elderly
• it has no fibrillar connection with the collagen of the vertebral subchondral bone, making it susceptible to horizontal shear forces

Question 10

Where are nerve fibres found in the disc?

Answer 10

• Outer ring of annulus only
• dorsally from the sinuvertebral nerve -(a branch of the spinal nerve as soon as it formed in the intervertebral foramen) and ventrally form the sympathetic chain

Question 11

What is the blood supply of the disc?

Answer 11

• realtively avascular in adult
• vessels lying only on the surface of the annulus
• some may penetrate outer layer

Question 12

How is th disc sustained?

Answer 12

• By diffusion and convection of nutrients through the porous central concavity of the end plate and the porous permeable solid matrix

Question 13

Hoe does the disc change with age?

Answer 13

• Nucelus changes
  
  • gradual loss of cells and proteoglycans
  • -> decrease in water/proteoglycan content and fibrous replacement ( from 3rd decade)
  • proportion of non aggregated proteoglycans and non-collagenous proteins increases
  • size of aggregan molecules and conc of fucntional link protein decreases
• -> annulus becomes fibrotic ( firm & white not soft & translucent)
• -> fissures and cracks develop in annulus with loss of orientation of the collagen fibres
• loss of volume, shape, microstructure ->. abnormal loading -> facet joint degeneration adn weakens disc-> risk of disc herniation

Question 14

What is the function of intervertebral discs?

Answer 14

• redistribute compressive load
• Resist tensile, rotational and shear forces
• whilst faciliating smooth motion in an otherwise rigid spine
• annulus fibrosis= resist tensile, rotational and shear forces
• Nucleus pulosis- viso-elastic properties to allow to absorb load and maintain height by a combination of the hydrostatic pressure in the interstitial fluid, the donnan osmotic pressure ( repulsive forces between fixed negatively charge on proteoglycans and also forces arising from freely mobile interistitial counter-ions )and loose framework of porous-permeable collagen-proteoglycan matrix
• these properties are also in the inner annulus adn permit large deformations in repsonse to load, creating intradiscal fluid flow that dissipate energy and visco-elastic creep.

Question 15

What is the biphasic phenomenon in intervertebral discs?

Answer 15

• relates to hoop stresses generated during compression in the outer layers of the annulus in comparison with the inner layers which deform and act as shock absorbers
• During prolonged periods of axial loading the interistitial water is squeezed out of the discs causing a decrease in height and therefore a bulging of the annulus. when loading is ceased, during sleep, the disc height is restored by the in flow of water back into the discs.
• most disc prolapse happen in the mane when the disc is loaded in an upright posturing

Question 16

What are the symptoms of annular tears due to?

Answer 16

• Contents of nucleus irriating the innervated outer layers of the annulus, secondary to a tear in the inner layers
• the inital tear is thought to be due to a sudden increase in intradiscal pressure
• subsequent symptoms are due to fluctations in pressure

Question 17

What is disc herniation?

Answer 17

• When the nucleus pulposis herniated thru a defect in the annulus fibrosis
• Often occurs at the insertion of the outer annulus to the vertebral body in the cervical/lumbar regions where stresses and motion greatest
• -> buldging of the annulus or herniation of the nuclear material, causing spinal-cord, thecal or nerve root compression

Question 18

What often happens to most herniated discs?

Answer 18

• 90% resolve /pain free at 3 months

Question 19

What is degeneration of the disc due to ?

Answer 19

• Decrease in the amount of nutrition reaching the cells and removal of waste products
• with aging arterial blood supply declines with the onset of adulthood, compounded by calcification of the cartiaginous end plates, accumulation of the degraded matrix molecule and decrease in matrix water concentration
• all of which interfere with nutrient convection and diffusion
• other factors
  
  • physical activity
  • scoliosis
  • vibration
  • diabetes
  • smoking
  • arterial disease

Question 20

What does age related degeneration due to the disc?

Answer 20

• with aging arterial blood supply declines with the onset of adulthood, compounded by calcification of the cartiaginous end plates, accumulation of the degraded matrix molecule and decrease in matrix water concentration
• disruption of collagen fibrils ( due to imparied formation, increase cross linkage and increased denaturation)
• annulus delamination
• all decrease the ability of the disc to recover from deforming and results in more solid behaviour
• End plate damaged too- with thinning and microfx from preventing the disc from maintaining its hydrostatic pressure -> increased fluid exudation on loading