Intervertebral discs Flashcards
1
Q
When does the vertebral columna and disc occur in development?
A
- At week 4 gestation
2
Q
What is each vertebra formed form?
A
- 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
3
Q
What does the notocord form?
A
- 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
4
Q
Can you decribe macroscopically the disc?
A
- The outer annulus fibrosis and the inner nucleus pulposus
5
Q
What is the anatomy of the vertebral disc throughout the spine?
A
- 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
6
Q
What is the annulus fibrosis consist of?
A
- 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
7
Q
What is the nucleus pulposis consist of?
A
- 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
8
Q
How can the disc be divided regions?
A
- 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
9
Q
What does the endplate consist of?
A
- 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
10
Q
Where are nerve fibres found in the disc?
A
- 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
11
Q
What is the blood supply of the disc?
A
- realtively avascular in adult
- vessels lying only on the surface of the annulus
- some may penetrate outer layer
12
Q
How is th disc sustained?
A
- By diffusion and convection of nutrients through the porous central concavity of the end plate and the porous permeable solid matrix
13
Q
Hoe does the disc change with age?
A
- 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
14
Q
What is the function of intervertebral discs?
A
- 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.
15
Q
What is the biphasic phenomenon in intervertebral discs?
A
- 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
