Disc Anatomy  

Function 

• Spinal motion 
• Stability 
• Links adjacent vertebrae 
• 25% of spinal column height 

Biomechanics 

• Converts axial load to hoop stress on the Annulus 
• Water shifts out of the nucleus pulposis into hoop stress on the annulus 

Composition 

• Hyaline cartilage attaches it to vertebral bodies 
• Annulus Fibrosis 
  • Outer ring 
  • Type 1 collagen oblique, water, proteoglycan 
  • High collagen: proteoglycan ratio 
  • Flexible enough for motion 
  • Fibroblast cells 
• Nucleus Pulposis 
  • Central 
  • Type 2 collagen, water, proteoglycan 
    • 88% water 
  • Hydrophilic matrix gives height to disc 
  • Compressible 
    • b/c polysaccharide and water (aggrecan) 
    • Viscoelastic matrix distributes force smoothly to annulus and end plates 
  • Low collagen: proteoglycan  
  • Chondrocyte-like cells 

Blood Supply 

• Avascular 
• Capillaries terminate at endplates 
• Nutrition via diffusion though pores in endplates 
• Annulus is not porous 

Innervation 

• Dorsal root ganglion gives sinuvertebral nerve: superficial fibers of annulus 
  • No nerve fibers beyond this 
• Sensory transmission neuropeptides 
  • Substance P, calcitonin, VIP, CPON 

Biomechanics 

• Disc 
  • Viscoelastic 
    • Creep – allows deformity over time 
      • Slow permanent deformity of a solid material due to long term exposure to force that still may be below the yield strength 
    • Hysteresis 
      • Energy absorption with repetitive axial compression 
        • This property decreases with time 
• Stresses 
  • Annulus fibrosis 
    • Highest tensile stresses 
  • Nucleus pulposis 
    • Higher compressive stresses 
  • Intradiscal pressure position dependent 
    • Lowest supine 
    • Medium standing 
    • Maximum sitting flexed forward 
    • Increases with weight farther from body 
• Stability 
  • Extension is most stable loading mode 

Pathoanatomy 

• Disc Herniation 
  • Spontaneous increase in: 
    • Osteoprograin 
    • IL1 beta 
    • RANKL 
    • PTH 
  • Disc aging- loss water, conversion to fibrocartilage 
    • Decrease 
      • Nutritional transport 
      • Water 
      • Absolute number of viable cells 
      • Proteoglycans 
      • pH 
    • Increase 
      • Keratin sulfate: chondroitan sulfate 
      • Lactate 
      • Degradative enzyme activity 
      • Fibroblast-like cells density (these only exist in annulus) 
    • No change in absolute quantity of collagen 
    • Neovascularization from outer annulus 
     

Elements of Intervertebral Disc 

• Nucleus Pulposis 
  • Type 2 cartilage 
  • Histology – oval, chondrocyte-like cells 
  • 90-63% water 
• Annulus 
  • Lamellar concentric 
  • Type I cartilage 
  • Histology – elongated, fibroblast-like cells 
• Cartilage Endplates 
  • Hyaline cartilage 

Aging/ Degeneration 

• Similar cascade as disc degeneration 
  • More senescent cells that non-HNP 
  • Higher matrix metalloproteinases 
  • Connective tissue 90-63% water 
  • Age related degeneration 
  • Secondary biomechanical changes 
  • Very little extrinsic influence on progression of degenerative changes 
  • Recent evidence (Mike Ford) – extrinsic factors ie: jobs/sports have no role in degeneration 
• Vascular 
  • Pediatric Direct blood supply in cartilaginous endplates 
  • Vessel recession at age 1 
  • Adult – no blood supply 
  • Adult – nutrient diffusion through central porous concavity of endplate 
    • DDD with decreased permeability of cartilage endplate 
• Nucleus matrix 
  • Water decrease 
  • Proteoglycan decrease 
  • More fibrous 
  • Fissures 
  • Blood vessel in-growth through fissures 
• Endplate 
  • Sclerosis (from fissuring nucleus) 

Neural Elements 

• Dural/ arachnoid (tethers)/ pia mater 
• Root arrangement 
  • Cephalad lateral 
  • Motor ventral 
• Naming 
  • Disc naming the same 
    • Disc is named after the body above it 
    • C6 disc is below C6 vertebra 
  • Root naming changes thus disc herniation – root mismatch  
    • Cervical 
      • Root exits cephalad to pedicle for which it is named 
      • C5 root exits above C5 pedicle 
      • C8 root exits above T1 pedicle so naming changes here 
    • Thoracolumbar 
      • Root exits caudal to pedicle for which it is named 
      • Disc is caudal to the vertebral body for which it is named 
      • L4 root exits caudal to L4 pedicle 
      • L4 central herniation affects L5 nerve root 

Regarding the intervertebral discs, all of the following are true EXCEPT? 

Aggrecan is the most common proteoglycan 

Type I collagen predominates in the nucleus pulposis  – Type II 

Remnant notochordal cells decrease with aging 

FINAL ANSWER: B